HomeMy WebLinkAbout1989-10-11Stormwater Management Planning Notes I► , 4 ma
Page 1 l it J i (
I . Stormwater mgmt can' address 5 categories of problems
10 flooding
2, pollutant washoff
3. erosion
4. groundwater recharge
5, stream channel preservation
II. Problems not commonly perceived in stormwater ping
14 Detention structures can be counterproductive. In
approximately, the lower one -third of a watershed, detention
merely delays peak flows until the time that the upstream
surges have reached the lower one - third, thereby increasing
flooding.
2. Detention structures designed for a single frequency storm
event provide inadequate protection, doing nothing to minimize
flooding from smaller events. Therefore, multiple detention
designs make more sense (ie for several different design
storms).
38 Because of number 1 above, there often exists a need for
coordination of stormwater management among multiple
municipalities governing different parts of the same
watershed.
44 Where storage of runoff is anticipated (ie retention or
detention), we need knowledge of runoff volumes, not just peak
rates. Therefore stormwater ordinances should require
generation of hydrographs in such situations. If storage is not
to be used, then analysis that predicts peak rate of flow is
sufficient.
50 For purposes of administration and review, there is an
advantage in restricting analyses to one standard method (eg
the SCS TR55 method). This makes all comparable and assures
that the municipal reviewer is qualified for reviewing what is
presented. For the same reason, it makes sense to require a
standard design for submittals and analyses of what happens
when a drainage system is overtopped.
6. Limiting peak flow causes an increase in the duration of
bankfull flow for storms larger than the design storm.
• However, because the major component of erosion occurs
�- - during bankfull flow, merely limiting peak flows may
considerably increase erosion.
Stormwater Management Planning Notes
Page 2
III. Site specific detention structures are "last choice"
1. These structures are heavy on:. expense, maintenance, safety
concerns, administration problems, and enforcement. In
addition, proper analysis requires watershed -wide analysis
and planning.
2. Therefore, non - structural or "soft" solutions may be more
appropriate, eg:
a. limit development in stream corridors
b. require large -lots in the most severe environments
co ponding behind culverts (mini- detention)
d, controlled flooding of undeveloped or recreational areas
e. use natural storage capacity of ditches & swales that are
designed to slow runoff.
3. Regional detention structures often are more economical and
effective than site - specific ones. (The municipality might
install and maintain the structure, while developers might
contribute to a fund for it in proportion to each development's
net effect on runoff.)
d L
®i
CHAPTER TWO: Local watersheds and stormwater management
One of the more difficult tasks facing the legislators of
environmental regulations is the creation of laws that can be
widely applied to a wide range of very specific issues. It is
unrealistic to expect that one set of laws will be applicable to
every passible type of development activity on every possible
site. Arid yet this is exactly what drainage regulations attempt
to do. The common conception of "justice" requires legislation
to treat individuals equally under the law, but land is not
created equal and if legislation is truly going to protect
environmental values then it must resolve this inherent disparity
between the dissimilarity of environments and equal treatment
under the law.
There are two avenues toward the resolution of this disparity.
One would be to expand the content of the legislation to include
special provisions for every possible permutation of site
development. Although some of the legislation examined in this
report attempts to do just that, these regulations are simply too
ponderous to be seriously considered in Tompkins County. The.
other avenue is to maintain a high degree of specificity, but to
customize local policy decisions to local conditions. Tompkins
County has several unique geologic and physiographic
characteristics which should automatically call into question the
-applicability of stormwater- management law and practice imported
from the Piedmont region or Florida.
This chapter discusses some of the hydrological characteristics
and the extent of stormwater induced problems in Tompkins G unity
that were revealed through research and an informal survey of
local government officials, engineers, and practitioners. This
discussion is presented to highlight local runoff problems, make
a general assessment of the effectiveness of different Hest
Management Practices in dealing with those problems, and suggest
what legislative measures might be most effective.
Watershed Characteristics of Tompkins County
Stream profiles and watershed sizes:
Cayuga
Lake is
the
largest
of
the
Finger
Lakes.
This
lake,
of
glacial
width.
origin,
Almost
is
all
40
of
miles long
the streams
arid
in
averages
the
County
over.a
drain
mile
into
in
Cayuga
Lake.
in the
Towns of
"Valley
east -west)
The exceptions
Owasco
Newfield,
Heads
and
Lake
Glacial
drain
drainage,
Darby,
to
to this
and
Moraine"
the Susquehanna
1 ie
and
Caroline
(a
lin
the
major
the
Outlying
which
Town
lie
drainage
basin
of
portions
t� �
Groton,
ions
south
divide
the
s• tuth.
which
of the
of the
running
is
Of
STANDARDS FOR STORMWATER MANAGEMENT IN TOMPKINS COUNTY, NEW YORK:
An analysis of current regulation and proposals for the future
evolution of stormwater management Policy in a heterogeneous area
Of upstate New York.
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41
the watersheds feeding
Creek, and Taughannock
County. Fall Greek is
its headwaters located
Creek also drains from
Taughannock. Creek from
Cayuga Lake, _only the Fall Creek, Salmon
watersheds d� � not lie wholly within the
the largest watershed in the County with
near Sempronius in Cayuga County. Salmon
the north from Cayuga County, and
Schuyler- County to the West.
Most of the major streams in the county that drain to Cayuga Lake
have cut deep ravines into the bedrock where the land dips
sharply towards the lake and have significant waterfalls in their
gorges. Further up the watershed, away from the lake, these
creeks are more typical of waterways developed in rolling
topography with the largest stream, Fall Creek, having a well
defined floodplain. In their upper reaches these creeks exhibit
the dendritic drainage pattern typical of streams that have
formed in homogenous materials. Just before they drain to the
lake, these streams assume a more flattened profile. A profile
typical of -the larger streams in Tompkins County is represented
by that of Cascadilla Creek and illustrated in Figure 10.
In the northern half of the county much of the land slopes
towards the lake with a r=ounded convex profile. Here, there are
many small parallel streams that exhibit profiles similar quite
different from streams in most parts of the world (see Figure
The significance of this stream profile is obvious to the lei
observer: these streams run straight downhill, are deeply cut
into the bedrock, and run very fast. Almost no sediments
accumulate on their channel bottoms. The headwaters of these r
streams are often in the wet and swampy areas that occupy the
crests of the convex slopes surrounding-Cayuga Lake. Because
many of these streams have such small drainage areas there may be
little or no base flew in the stream during dry periods, but they
may nonetheless carry considerable flows during storms. Because
these channels are so deeply cut many of them. draining the convex
slopes along the east and west shores of the lake have a large
capacity to handle increased flaws without flooding. This fact
should have some bearing on future stormwater management policy
making.
�40®
4 2
d
the watersheds feeding
Creek, and Taughannock
County. Fall Greek is
its headwaters located
Creek also drains from
Taughannock. Creek from
Cayuga Lake, _only the Fall Creek, Salmon
watersheds d� � not lie wholly within the
the largest watershed in the County with
near Sempronius in Cayuga County. Salmon
the north from Cayuga County, and
Schuyler- County to the West.
Most of the major streams in the county that drain to Cayuga Lake
have cut deep ravines into the bedrock where the land dips
sharply towards the lake and have significant waterfalls in their
gorges. Further up the watershed, away from the lake, these
creeks are more typical of waterways developed in rolling
topography with the largest stream, Fall Creek, having a well
defined floodplain. In their upper reaches these creeks exhibit
the dendritic drainage pattern typical of streams that have
formed in homogenous materials. Just before they drain to the
lake, these streams assume a more flattened profile. A profile
typical of -the larger streams in Tompkins County is represented
by that of Cascadilla Creek and illustrated in Figure 10.
In the northern half of the county much of the land slopes
towards the lake with a r=ounded convex profile. Here, there are
many small parallel streams that exhibit profiles similar quite
different from streams in most parts of the world (see Figure
The significance of this stream profile is obvious to the lei
observer: these streams run straight downhill, are deeply cut
into the bedrock, and run very fast. Almost no sediments
accumulate on their channel bottoms. The headwaters of these r
streams are often in the wet and swampy areas that occupy the
crests of the convex slopes surrounding-Cayuga Lake. Because
many of these streams have such small drainage areas there may be
little or no base flew in the stream during dry periods, but they
may nonetheless carry considerable flows during storms. Because
these channels are so deeply cut many of them. draining the convex
slopes along the east and west shores of the lake have a large
capacity to handle increased flaws without flooding. This fact
should have some bearing on future stormwater management policy
making.
�40®
4 2
r J�
r
1 qo0
1300
12,00
(loo
loco
j00
Sao
700
boo
J
14oD
IL
Q MoD CASGAp t LLA GRIc L� K
rl floe
too*
100
tOo
700
{�" < too
c' Soo
y! 400
LL.
iSoo
14e0
13e0
1200
1100
1000
goo
j Soo
s
(0 00
Soo
4100
0 1 2 3
• FIGUIZ6 10. SE.LE0 5 -rREAM PROM 5
rt� F✓
pP.Y5 �R�Nc say p/�Y 1� cmf K W V
(AFTER MOR.15AWA, 1960 p. I21)
Ci7
2
0
5
7 8 9 to
m11-Es
g
MI I-E.S
5
43
i
Soils
Sail types will have an effect on what particular management
practices are most suitable for a given site, and should also
have some bearing on future regulatory decisions. The General
Soil l Map of the County (U. S. D. A. SCSI 1565) shows that the more
highly urbanized areas surrounding Ithaca are generally
unsuitable for stormwater infiltration practices due the presence
of shallow bedrock and low soil permeability. Therefore, runoff
regulation �n that requires the retention of starmwater to control
both the peak rate and volume of runoff to the predeveloped
condition in order to most accurately maintain the natural flaw
regime will be difficult to achieve. In this circumstance,
extended detention will prove to be the best engineering
alternative. Where infiltration of runoff is impossible and the -
site is too small to allow for surface detention, management of
peak flows will necessitate some sort of underground resevoir,
which will likely be complicated by the shallow depth to bedrock
and a high water table. When regulations and site conditions
begin to dictate such complex and expensive management solutions
developers will be forced to reconsider- their site selection.
Starmwater management regulations will effect patterns of
development. This effect should be considered in the formulation
of these regulations so that other planning goals are riot
compromised by starmwater planning goals.
The Town Line
Municipal boundaries crass many watersheds which can create
significant political obstacles to management of runoff on a,
watershed--wide basis. The most confused situation lies within
the City of Ithaca which receives runoff from Caroline, Dryden,
Danby, The? Town of Ithaca, Enfield, Newfield, and Cayuga Heights.
The Town of Ithaca is similarly hemmed in by several different
municipalities. None of the major watersheds in the county lie
within the boundary of one palitical'unit, bat certain
municipalities can exert significant control on specific
watersheds. The upper reaches of both Fall and Cascadilla Creeks
lie largely in the Town of Dryden; Buttermilk Creek is in Danby,
The Cayuga Inlet in Danby and Newfield; the headwaters of Enfield
Creek are shared by Newfield and Enfield; Caroline contains the
uppermost reaches of Six -Mile Creek, but the watershed is
distributed between Dryden, Danby, and the Town of Ithaca. Since
the City arid Town of Ithaca end up taking the brunt of upstream
management decisions it is to be'expected that these
municipalities would have evolved the most restrictive sty �rmwater-
policies, but these policies are of little value in protecting
ing
development or environment within these rnaj� �r stream corridors
when riluch of the watershed is beyond their- contr� mil.
44
le
4�.
The Town and Village of Lansing are located such that they are
riot impacted significantly by ricor do they in turn impact ether
municipalities. Although Salmon Creek drains a large area to the
north, h, hm� Est stormwater problems in the Town and Village of
Lansing are confined to the small drainage basins that drain
directly to Cayuga Lake and lie almost wholly within their
jurisdictions. A few of these watersheds drain from the Town to
the Village and in these drainages coordination of management
efforts is important. In both of these ,jurisdictions the
watershed areas are small, and therefore, relatively small
developments can have significant impacts. Rut, for the same
reasons, stormwater management practices can be much more
effective.
As Cayuga Heights is nearly fully developed, concern for runoff
management within the Village is not great. Of particular
interest to the Village is the effect of future development in
the Northeast neighborhoods of Ithaca which drain to Cayuga
Heights since many of the bridges and culverts in the Heights are
already overloaded in larger storm events.
Dryden is also experiencing an increase in development and is _
rightly concerned about impacts on runoff. As no other
municipalities exert upstrearil . development pressures on the Town,
Dryden can develop management policies that concentrate on local
impacts and without concern that a municipality upstream will
thwart their efforts. Other outlying Towns -- Caroline, Danby,
Ulysses, Newfield, Enfield, and Groton- -are riot likely to
consider runoff regulation for their own communities since
development pressures are low. Nevertheless, all should
recognize that a development in cone of these outlying areas
contributes ,just as much erosion and pollution to the county's
rivers and streams as does a development in Lansing. Being
located in a rural area does not absolve one of responsibility
for the maintenance of the county's natural resources.
The scope of the problem in Tompkins County
There is no question that problems with flooding, pollution,
= groundwater depletion, erosion, and sedimentation are fund
throughout the county. To determine whether or not these
problems are due to increases in runoff from developed areas and
whether a stormwater management program would be an effective
means of correcting these problems is difficult at best. First
of all, most of the problems due to increased runc of f are riot
entirely new, they are simply the acceleration or magnification
of processes that occur naturally. Second, problems due to
development may not be apparent for marry years unless a major
storm occurs and even then problems may show up 'far downstream
from their source. Third, problems associated with stormwater
are the additive result of many separate roan -made impacts coupled
to natural processes over time and is therefore it is nearly
• 45
i
impossible to ascribe a problem to one particular act ion. from Qvwe the lack of data from historical conditions ins prevents us fry �m
being able to accurately measure what portion of the problem is a
natural phenomenon and what portion can be attributed to man.
St i 11, it is a fact that development does amplify these problems.
And by l eking a 1i
development, waters
existing problems i
management efforts
Major floods
t
he
t
m
1
d
i
9
e more carefully at patterns of
characteristics, and the magnitude of
s possible to suggest where stormwater
ht best be made.
The last major flood in Tompkins County happened in October of
19810 At: the peak. of flooding parts of Dryden were under 4 feet
of water and flows in Fall Creek and Virgil Creek washed out
several bridges in the Town of Dryden and the Town of Ithaca.
Six Mile Creek overtopped bridges at German Cross and Burns Road
and caused extensive damage to several structures that were
undermined by the flood flews. In the City ;of Ithaca, Tioga
Steet bridge over Cascadilla Creek was ruined, approximately 2
feet of water buried sections of the Fall Creek neighborhood, and
many of the structures adjacent to these streams subject to the
tremendous erosive forces of the flood were damaged.
During this flood, 5.08 inches of rain fell in 24 hours (U. S. D. A.
SCSI 1989)8 This quantity of rainfall is roughly equivalent to
the 24 -h• lur, 100 -year storm of 5.4 inches as given by the U.S.
Weather ]Bureau (Hershfield, 1961). More significantly, 4.4
inches of rain fell over one six hour- period and was largely
responsible for the 7 million dollars of damage experienced in
Tompkins County. This quantity of rain was far and above the
previous record of 2.9 inches in 6 hours set during the famous
flood of 1935. Other storms responsible for widespread flooding
in the county happened in 1964, 19697 19721 and 1975. In 1972,
Hurricane Agnes dropped a total of more than 11 inches of rain on
the County over several days. This resulted in some flooding,
but most of the damages in this area were in the floodplain areas
of the large Chemung River to the south. July 111 1976 brought
4.25 inches of rain in a 24 -hour period (slightly smaller than a
Weather Bureau 25 year storm), causing localized flooding of some
roads and significant damage to some properties. The "flood of
record ", July 81 1935, saw 7.9 inches of rain fall in 24 hours.
Four or more feet of water covered many areas of downtown awn Ithaca
and the l owlying areas of surrounding communities.
Co unity -wide sty �rmwater managment regIAlat i� ins of new devel opment
will have little effect on the flooding effects of rna.j� ter- sty arm
events such as these. The majority of the damages resulting from
ma j, =,r storms is confined to the fl �dplair�s of the larger -
waterways in the County. Because these areas are flat, and are
otherwise easier to develop that the st_;rrounding highlands, the
fact that they may be subject to an occasional flood is not
46
-4
_ s
sufficient
i
cause
to
deter
development.
The flooding
of
floodplains
is a
natural
condition
that
is nearly
impossible
to
counteract
through
stormwater
management
controls
practiced
at
the
individual
site
level
particularly
if
one considers
the
extent
of
urbanization
in
the
county. This
point
is well
r:
•
illustrated in the following examination �n of a major flood control
project that is under- way in the county.
The Virgil Creek Watershed Plan
The Town and Village of Dryden suffered more than 35 million
dollars worth of damage in the 181 flood. The Virgil Creek
Watershed Project report predicts that Dryden endures almost
$400,000 worth of damage from storms as small as those of a 5
year return frequency (U. S. D. A. SCS, 19897 pg. 837)a This damage
is largely the consequence of poor planning, common throughout
the world, that has allowed substantial building within the
natural floodplain. In an effort to control these problems, the
Soil Conservation Service, working with Tompkins and Cortland
Counties and the local communities have put together a, plan to
locate a regional detention facility on Virgil Creek just to the
west of the County line in the Town of Dryden. This facility has
been designed to control the flow of Virgil Creek to reduce 97%
of the potential damages in Dryden due to flooding from storm
events up to and including the 100 year storm. The plan calls
for the construction of a detention pond capable of storing 2175
acre feet of water.
$Mme quick calculations show that this one project dwarfs arty
mitigation measures likely to be instituted on a site level.
Page c of the Virgil _Creek _Watershed_Pr_o,j.ect report identifies
596 acres of land in the watershed as "Other" than agricultural,
forest, grass, or croplands. If this area (just a bit less than
one square mile) is the extent of urbanization in this watershed
then each acre of developed land would have to provide 3.65 acre -
feet of water detention in order to provide the same protection
that the proposed project would supply. A more typical figure
for detention alight be on the order of .05 to . 1 acre -feet of
detention per acre of land under a regulation that required
detention to control the peak rate of flow for a 100 year storm
(Aitken and Goyen, 1982, pg. 179, Urbonas and Glidden, 1982, pg.
189 -93). Since detention on the more typical scale is only one-
thirtieth of the volume provided by the Virgil Creek project
there is little hope that local stormwater detention practices
could prevent regional flooding in this watershed. But in
smaller watershed areas the effect of site level deve 1 opment on
the hydrol _'g is regime becomes more evident. Therefore, it should
be possible to identify watersheds where runoff management to
minirnize damage from major storm events could be successfully
implemented. As of yet this has not been done.
Other Watersheds
47
I
t
t
r
i
i
i
i
1
i
r
i
It is pr- es!tmed that the Virgil Creek Project will confer- some
protection from flooding on Fall Creek downstream of the Virgil
Creek confluence. The level of protection provided-by this
improvement to development further downstream has some bearing ors
future stormwater management decisions in this important
watershed, and should be determined. Upstream from this
confluence flooding of Fall Creek will not be abated by the
Virgil Creek Project and the need for future control of runoff
upstream of Freeville cannot be dismissed.
In the City of Ithaca, the Cayuga Inlet Flood Control Channel
provides an improved dr-ainageway for the several streams that
enter the lake at its southern end. By deepening, widening, and
straightening Inlet Creek, this channel is designed to contain
flood waters within its banks to limit overbank flow and the
subsegent inundation of properties located in the floodplain.
During the 1981 flood there was little flooding in the West End
area or to the south of Ithaca; presumably partially due to this
structure and partially due to the fact that the heaviest rains
were to the east of Ithaca.
Both six Mile Creek and Cascadilla Creek have inflicted serious
damage in t e Town and City of Ithaca during past major- storm
events. six Mile Creek is the second largest watershed in the
County. Because of its size, site scale management efforts aimed
at the control of major flood's will be largely unsuccessful. The
may be of a size
'Cascadilla Creek watershe , on the other hand,
where stormwater controls to limit flooding may have an effect.
A comprehensive watershed study would have to be undertaken to
confirm this. Both of these watersheds will see significant
residential and institutional development" in the future. For
this reason these watershed basins need to consider some sort of
flood god management program.
Buttermilk, Enfield, and the other creeks that feed InletICleek
have less potential for damage because the floodplain Of
Creek is riot heavily developed, and because of the flood
tempering provided by the Flood Control Channel. These
watersheds will also not be as subject to intense urbanization
since hells of the area is parkland, very steep, and not well
serviced by utilities. The Taughannock Creek-watershed is both
large and rural which allows it some immunity from damage due to
increases in storrawater- flows. The limited development this area
is likely to see in the future would suggest that the Town � �f
Ulysses need riot institute rigorous controls or, peak rates of
runoff.
The Owas ake Inlet was heavily flooded ire 781. But, Simi Iar
to the situation in Virgil Creek, the damage Sustained car, be
attributed to poor flood plain management ar�d runoff control
w� mould have little effect on d� �wnstrearn flooding.
�c ding. Salmi ter, Cr -eel <.
48
r.
k
i
k
E
has some potential for flooding, but with little development in
the watershed, runcif f control to prevent major floods i n this
area does riot make sense. A similar argument applies to those
•se
lands in the southern part of the County that drain to Cayuta,
Catatcank, and Owego Creeks eventually leading to the Chemung
River. These headwater areas have some potential for negative
effects on downstream properties, but again these areas are so
lightly developed that the control of runoff at the site scale
for flood mitigation purposes would simply be .a waste of time and
money.
The other
Gulf Creek
Creek, Wil
little cor
events and
unrestrict
Priorities
small watersheds t
Twin Glens, Will
low •w Creek., etc., t
no floodplain are
would riot be subj
ed runoff from dev
Although it is beyond the
statements about the magni
urbanization will have in
watersheds, these generali
significance of those impa
hat drain di
i ams Brook,
hat have dee
not sever1y
ect to subst
e1opment.
scope of thi
tude of impa
each of the
nations can
cts on major
rect
Ind'i
ply
irapa
ant i
ly to Cayuga Lake:
an Creek, Glenwood
incised channels with
cted by large storm
al damage due to
s study to make definitive
cts that future
County's different
be made about the relative
flood events:
1. Damages from major floods are concentrated in
the floodplains of the streams. Watersheds
that have substantial development within the
flood plain are the most vulnerable to the
effects of urbanization.
Some of the County° s most sensitive
watersheds have or will have large scale
f1pod control structures that can effectively
mitigate the increase in runoff effects of
development which should have bearing on
runoff control policy in these areas.
Sala11 watersheds with f1oodplains are more
susceptible to the effects of development
than are larger watersheds.
4. Watershed areas with no floodplain and deeply
incised channels are generally not subject to
extensive damages from flooding.
�. Cayuga Lake, by virtue of its size, has the
capacity to assimilate all the additional
runciff likely to be the result of development
in the forseeable future,
Is
MM
W
1
0
i
I
R
y
l�
E. The control of damages from major storm
events is best done through floodplain
management efforts that restrict development
I and alteration of areas naturally subject
to flooding.
Site scale stormwater management efforts to control major storm
events will be of little use in Tompkins County. Priorities
should be placed on the further study of the Co=unty "s watersheds
to evaluate the potential benefits of large regional control
efforts such as the Virgil Creek Project as opposed to
legislation that enforces a piecemeal approach. The following
list indicates what watersheds should be given priorities for
further study.
Priority 1: Watersheds that have significant
development within the floodplain with rice flood central
measures. Cascadilla Creek, Six Mile Creek, Fall Creek
above the confluence of Virgil Creek.
Priority: Watersheds with flood plain development
and some flood control measures. The Cayuga Inlet/
Enfield Creek/ Buttermilk Creek complex, Fall Creek.
Priority 300 Watersheds with little floodplain
development. Salmon Creek, Taughannock Creek.
Priority 4: The several minor watersheds that drain
directly to the lake have almost no potential for major
flood damage since their flood plains are so small, but
will be. shown to benefit from stormwater management
efforts to control localized flooding from smaller
storm events.
JJ
A
•
E,
14
Localized flooding
• On a less dramatic scale 7 flooding of driveways, yards,
baserents, roads, and other manmade structures causes significant
inconvenience and, in some cases, substantial damage during
small, more frequent storm events. Localized flooding is defined
here as flooding problems on or adjacent to a development site
Plus problems further downstream amplified by that development.
In the past, stormwater management planning was aimed only at the
prevention and control of adjacent flooding and remains as the
sole concern of local governments. Regulations were aimed at
establishing the upper landowner's responsibilities for
prevention of nuisance flooding of the downstream landowner.
Since this is an easily understood and clear problem, regulatory
solutions are well developed and often thorough for preventing
problems with the immediate neighbors, Problems typically arise
when continued development upstream eventually overwhelms some
drainage structure or system further downstream, Problems like
this are riot yet common in Tompkins County, but the potential
increases with each new development.
In Tompkins County one can expect a 2.7 inch, 24 -hour rain once
every two years. Once in ten years we can expect to get 4.9
inches of rain in one day. Storms of this magnitude do not lead
to widespread flooding, but can result in localized flooding and
damages. Culvert capacities can be overwhelmed resulting in
• flooding across roads, drainage ditches can overflow and back up
into basements, streambanks can be eroded causing property
damage, and considerable amounts of flotsam get left about.
Ponding of water in yards, puddling of parking .lots and
driveways, curbs and sidewalks under water are all commi.n
accurances during the bigger storms experienced every few years.
Usually these storms only create a temporary inconvenience and
minor damage, but there are occasional instances where these
problems are ,chronic and do present more than . an inconvenience.
Conversations with local municipal engineers highlighted several
specific problems: the piping of groundwater in utility trenches
causing subgrade problems, drainage swales that didn't work
properly either due to poor design or lack of rnaintainence,
undersized culverts, and ineffective land grading around
structures were all mentioned as causing local flooding. Of
these problems, undersized culverts and other drainage structures
were of greatest concern, since these were problems that were
potentially property threatening and clearly the responsibility
of the municipality. Many of these problems can and will. be
corrected. Others will probably be left
to p as permanent monuments
��o�r planning and design. And although there does not seem to
be an extensive history of problems directly due to poorly
planned runoff, all of the engineers interviewed four this study
are becoming i-ricreasingly concerned with the potential for%
• pr oblems and t finely reqUire that substantial supporting
51
34
information accompany plans for development so that fUtUr� e
problems can be avoided.
Since few problem areas were revealed by these interviews it was
not possible to correlate problems to particular site features.
Still there are several factors that can influence a particular
site's potential to have or cause problems. Much of the County
is underlaid by soils that have moderately to very poor
permeability with little cover over bedrock. This makes many
areas, particularly flat ones, susceptible to wet conditions much
of the year. Development of these flatter areas often includes
improvements designed to remove water from the soil as well as
divert runoff. In these areas the potential for increases in
runoff from development are high because of the substantial
changes made to runoff timing by drainage improvements. Since
many of these areas do not have the well defined drainageways
that the more sloping sites do, development usually entails the
alteration of existing channels to increase capacity and often
the inadvertent rerouting of flows. This can lead to problems or,
adjacent properties.
The building of reads is particularly disruptive to natural
drainage patterns. New roads running across the slope act like
dikes and tend to channelize and concentrate storm flows. On
flat sites these problems are coupled with the difficulties
encountered attempting to install culverts of sufficient capacity
under these roads, since there is often little clearance to be
had between the road base and the bottom of these shallow
channels without building the base of the road up well above the
surrounding ground level. Areas prone to these difficulties
include the Northeast neighborhoods in Ithaca, many areas of the
Town of Lansing, much of the West Dryden area, and the upper
elevations of South Hill, Snyder Hill, Mount Pleasant, and West
t Hill. Problems are not always evident in these areas, but their
potentials are high.
In areas on the sideslopes of Cayuga Lake and the hills
surrounding Ithaca there are usually well defined drainage
channels that have large capacity. In these areas, diversions
upstream of structures are usually necessary to protect
buildings, roads, and ether edifices. These diversions can have
unfavorable results on neighboring properties. For the most
part, flooding problems on slopes are less prevalent unless there
is significant development on the lands upslope. The Village of
Cayuga Heights is one of the areas subject to local flooding
problems that can be squarely blamed an upstream development. On
the other hand, where sideslope development drains to flatter
areas below there is a good potential for flooding at the toe of
the slope where the water velocity drops and tends to "back. up ".
In the developed floodplain of Ithaca, where there are extensive
underground stcorm sewers, backups due to overload and clogging
.J LM
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i
are considered routine. Often the existing storm sewer system
• was riot designed with a secondary route for overland flew in the
event of backup, or subsequent development has violated the
secondary route to the point where it no longer functions. Since
future connections to these overwrought systems are limited., new
development in these areas may have to consider alternate means
of storrnwater routing or perhaps even the retention of excess
flows of runoff. But, as was shown earlier, the detention of
storm flows in the lower portion of a watershed may actually
exacerbate flooding problems.
1.
V'
I I
In the City
of
Ithaca solutions
to
flooding problems
will lie
either in
the
enlargement
of the existing
storm
system or in
the
construction
of
extensive
detention
facilities
upstream.
Considering
the
tinning characteristics
of
the
watersheds in
question,
it is
probable
that upstream
detention
of flows will
riot be successful
either.
The.City
of Ithaca
has recently
commissioned
a
study of a
South Hill
watershed
that will
hopefully
shed
some light
on this problem.
Whatever
the
solution,
it is
going to
require more
capital
than is going
to be
provided
by developer
contributions
and the
expenditure
of
general public
revenues will
be required
to-make
improvements.
Other municipalities
may
share the
responsibility
for problems
in
the City,
but
political boundaries
limit their
participation
in
the resolution
of these problems.
Priorities"
Although localized flooding is not yet a large problem in the
County there are some areas where problems are becoming evident
and many more areas where the potential for future problems is
great. Areas of primary concern ar,e the urbainizing areas on the
higher elevations surrounding Ithaca. Further development in
these areas without runoff regulation can result in both
immediate and future problems for neighbors adjacent to the
project and for drainage structures downstream. General
guidelines for the identification of potential problem areas are
as follows:
i
1. Projects that involve high density of
development and /or the development of large
tracts of land are the most 1 ikel'y to have
immediately realized impacts.
Older development downstream will typically ,
be the first point where problems arise since.
the oldest pieces of the drainage system are
usually the most undersized and the rnost
overloaded.
Some areas served with undersized syst erns
with no provision for over capacity flow may
53
have to consider expensive alternatives to
conventional st c orm conveyance practices.
4. The capacity of the existing storm system,
natural or manmade, and the capacity
necessary for the future is the basis upon
which many decisions about stormwater
regulation should be made. Drainage analysis
should always include an assessment of the
effect of development on downstream
structures.
5. Runoff control BMPs are expensive to �
implement, partiCIAlarly so in light the fact i
that most of these practices are land
intensive. Careful consideration of the
costs of these controls versus the benefits �
obtained should be completed before �
legislation is effected.
Prevention of localized flooding has to be approached on a ,
project by project basis. It is appropriate for municipalities
to ask the developer- to provide analysis of the effect of the
proposed project on drainage. But it is up to the municipality
to establish some equitable basis for the evaluation of the
proposal and also to inform the developer of problems that
already exist. In most instances it will be up to the developer-
to mitigate the negative effects of his activities, but there
'will be instances where it would be advantageous to both the
developer and the municipality to mitigate the problem. elsewhere
than on the development site or for the developer to ccentrol more
runoff than wo=uld be strictly required by the effects of his own
development activities. Some provision far the sharing of
responsibility between developer and the community is a necessary
component of a progressive stormwater management plan.
If the municipality is to require that the developer control
runoff on his site so as to protect downstream properties the
developer will have some expectation that the municipality will
protect his property from future development as well. Thus it is
imperative that the municipality have a clear- vision of what that
future development might be and be sure that any drainage
improvements planned now are coordinated with the potentials for
the future.
_r
54
• Pollution
In Tompkins County it may be found that the control of stormwater
runoff quality is more important than the control of.strormwater
g1_tantity. A 'lack of data and the difficulty of obtaining:
information renders it difficult to determine the impact of
storrnwater runoff on the pollutant loading of local waterways.
Over the years, there has been the occasional study done on the•
pollution levels in the major streams feeding the Lake, but it is
impossible to establish the necessary controls to make it
possible to compare data gathered 20 years ago to data gathered
today. _
The Tompkins County Planning Office has been conducting °research
on the pollution of the waters that feed Cayuga Lake.under'a
grant from the State for the Aquatic Vegetation Management
Program (Skaley, personal communication). Recent information on
pollutant loads indicate that none of the streams studied are
subject to significant. pollution problems. Comparisons between
data assembled in 1972 -74 and more recent information indicate
that the levels of phosphorous, potassium, and nitrogen have not
changed significantly over that time.
Another study undertaken to quantify the change in total lake
biomass (the sum total weight of, all living things"- primarily
•vegetation) over time seems to indicate that the total biomass
has actually decreased-in recent years. This may indicate that
the nutrient loadings (potassium, phosphorous, and nitrogen) of
these streams have actually decreased over this time. It also
appears that the stream sediment loads.may have decreased since
the early 1970's as well. One theory for this effect is that the
net decrease in acreage of active farmland and improved
agricultural practices have reduced runoff and erosion from
agricultural, land. It.is presumed that this reduction has been
Offset to some extent by the increase in urbanization.
If this information is correct, then there would be little need
for concern, over the net effect of stormwater -born pollutants,
but this oversimplifies the problem. First. of all, runoff from
furban lands contains several pollutants -- hydrocarbons, heavy
metals, and chlorides- -that have not been monitored in these'
previous studies. Second, the testing methodology used in these
Studies does not account for the "first flush" phenomenom and the
fact that primary impacts of storrnwater pollutants are shock
loads that happen only over a very short period of time.
Furthermore, there are no established criteria for evaluating the
impact of short but intense increases in pollutantl levels. It
stands to reason that pollutants that cannot be tolerated for
long periods of time might be tolerated at high levels for short
an10unts of time, and conversely that pollutants norroial ly harmless
• at Small c� �ncent rations can be quite poisonous when increased.
55
i
Until there is coati
time and some criter
danger Of these ' leve
walue,in assessing t
problem.
n Li
is
Is
he
ous m ni
establi
any dat
magnitu
taring of pollutant levels aver
shed for assessing the potential
a collected will be of questionable
de of the storrnwater: Pollution
In parts of the country where water- pollution problems compromise
drinking water supplies and preclude the recreational use of
streams, lakes, and ponds it has been discovered that pollutants
carried by storrnwater runoff 'are far and away' the most
significant contributor to the problem. In Tompkins County our -
concerns should also-be to protect the beneficial uses of our -
waters. Nearly all of the waters in the county provide for some
recreational or other valuable use: fishing, swimming, wading,
wildlife Ihabitat, or domestic water supply. It is fortunate that
Pollution problems are few.
Fart of this luck can be attributed to the fact that Cayuga Lake
has an enormous capacity to assimilate pollutant and sediment
loads and perhaps it can handle even greater loads in the future.
But in local streams it may be important to actively preserve
present benefits. Enfield Creek, TaUghannock Creek, and
Buttermilk Creek, as parts of the State park System, must not be
allowed any degradation from pollution. Fall Creek, Six Mile
Creek, Cascadilla Creek, Lick Brook, Coy Glen and many other
streams offer the'residents of the County rnany opportunities for
both passive and active recreation. Six Mile and Fall Creeks are
sources of the public water supply. The Cayuga Inlet, Salmon
Creek, Virgil Creek and Fall Creekl' are some of the finest cold
water fisheries in New York State. And all of the smaller
streams have recreational and ecological values that should be
preserved as well.
There is no question that stormwater runoff contributes something
to the pollutant and sediment loadings of creeks and streams.
Although Cayuga Lake eventually receives most of these materials,
and may have the assimilative capacity to receive more, can the
public tolerate any increase in the pollution of these creeks and
streams? The question is not necessarily one of the magnitude of
the problem, but whether any increases in pollution of these
important resources is acceptable.
Priorities:
Since there is no hard data to assess the magnitude of the
Pollution problem in the 'county a cautious approach to the latent
passibility of further pollution 'of the County's waters from
storrnwater runoff is advisable. These general points should be
kept in mind in the evaluation of a development ro �Jsal
P P-
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•
1. Pollutant washoff from roads and parking lots
add the largest component of pollutants to
stormwater runoff.
c. Industrial and commercial areas can present. a
potential for significantly higher
accumulations of pollutants than do
residential areas.
Municipal housekeeping practices: frequent
storm sewer flushing and street sweeping, and
limiting the use of salt for deicing, must
also be part of the pollution abatement
strategy. The residential, commercial, and
agricultural use of pesticides, herbicides,
and fertilizers must be held to a minimum.
Legislation may not yet be appropriate, but
public education is certainly a valid part of
the pollution control program.
4. The placement and monitoring of proposed and
existing on -site sewage treatment - facilities
should be given due attention throughout the
County,
5. Best Management Practices for the control -of
pollutant washoff are not prohibitively
expensive on most sites particularly if they
are designed in addition to other stormwater
runoff quantity or erasion controls.
Until more is known about the impacts of the pollutants that are
washed into local streams by runoff it woUld be prudent to
institute some controls for pollution abatement in the County's
most sensitive watersheds. The sensitivity to pollution of the
County's streams might be listed like this:
Priority In Taughannock Creek, Buttermilk Creek, and
Enfield Creek are important not only as recreational
resources but as the basis of the important tourism
component of the County economy,
Priority The
especially from t
with standard wat
Creeks should be
they bath offer m
residents of the
Public water
hose po 1 1 ut an
er treatment
protected for
any add it iona
CO Unt y.
Supply must be
is that are not
practices. Six
this reason an
1 recreational
protected,
easily removed
Mile and Fall
d the fact that
uses to the
Priority At a minimum, our fishable streams should be
protected from pollutant washoff from industrial and
commercial land uses.
5 7
W
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Priority 4: Many of the small
well removed from areas of act
drain directly to Cayuga Lake
lot residential development ca
increase in pollution from thi
harm,
Erosion and Sedimentation
er watersheds that are ei
i ve recreational use or~ t
and are only subject t� � 1
n probably accept the sma
s land use witho=ut apprec
ther
hat
arge
11
iable
This report concerns itself with two major- sources of erosion
closely related to stormwater runoff. The first is the commonly
encountered erosion from development sites typically manifested
in the milk chocolate colored streams just downstream of any
development project during a good rain, and the second is the
natural erosion of stream channels which is greatly accelerated
by increases in runoff.- Both of these problems are quite common
around the county, but they are not necessarily a problem in
every watershed,
Erosion from Construction Sites
Erosion of materials from a construct -ion site is most clearly a
problem where the ensuing sedimentation either precludes another
beneficial use or puts unreasonable stresses on the environment.
Sedimentation typically occurs where sediment laden waters are
slowed and lose the energy necessary to keep the particles of
- sand, silt, and clay- in suspension, In Tompkins County this
`--typically means Cayuga Lake.
The south end of Cayyga Lake has been Slowly filling in with
eroded materials from upstream sources for tens of thousands of
years. Certainly man's activities have hastened this process,
but it could be argued that it will be an awfully long time
before we fill the lake in. Other alluvial landforms include
Taughannock. Park Point (Taughannock Creek), Myers Point (Salmon
Creek), and Portland Point (Gulf Creek).
Further upstream, sediments fill in behind the constructed
impoundments at the Ithaca City Resevoirs and Beebe Lake on Six
Mile and Fall Creeks respectively, Eventually these sediments
must be removed i f he function of these resevoirs is to be
preserved.
But sediment laden waters cause more problems than simply the
filling in of lakes and resevoirs. Sediments in the water can
alter the light levels reaching the bottoms of streams and lakes
which changes the pattern and type of vegetation that grows in
these waters. In turn. this effects ether plant and animal life
in these environments. Sediments deposited over stream and lake
beds can destroy vegetation and animal life, cover fish spawning
areas, and otherwise disrupt the water environment as well as
JG
make the
waters unpleasant
also
for
recreational
use.
In Tompkins
carried
County
marry streams
run fast
enough to
have.
some
immunity to
siltation
over much
of their
swales,
lengths,
but where
the
waters
slaw
enough
to allow dep�
Est ion
of
sediments
- -in
pools,
eddies, and
system.
flat stretches
-- sedimentation
materials
is concentrated
the
and
can quickly_
of these
turn these
segments
of water
into biological
more
wastelands.
and increases
in flooding.
The
costs
of
maintaining
One
must
also
consider
that many
eroded
materials
carried
by
stormwater
runoff
end
up being deposited
in ditches
and
swales,
in culverts
and sewers,
and in other
parts
of
the storm
drainage
system.
These
materials
decrease
the
efficiency
of these
systems
resulting
in
more
sedimentation
and increases
in flooding.
The
costs
of
maintaining
these storm
systems
are
increased
by.
erosion
and sedimentation.
Clay and silt particles can also serve as a vehicle for the
transportation of many pollutants. Nutrients, pesticides, and
heavy metals (Jeung, 1978, pg. 80) all have a tendency to adsorb
onto sail particles and thus become concentated in areas subject
to sediment deposition. Little information exists on haw these
concentrations may effect the ecology of the county's lakes and
streams, but the knowledge that these pollutants can became even
more concentrated in organisms further up the food chain would
suggest that some caution be exercised in this regard.
Stream Channel Erasion
•The erasion and transportation of sediments is a natural f=unction
of streams. The character of the strear0 s channel, banks',
floodplain, and the biological community that live within this
zone are adapted in response to the natural fluctuations of
stream flaw working on the soils and underlying bedrock over marry
hundreds or thousands of years. In all cases, this adaptation is
largely a manifestation of the steepness of slope and the
durability of the underlying geology which determines how the
erosive processes will farm that stream's character. In Tompkins
County these physical. conditions are highly variable ranging from
waterfalls cascading through steep walled gorges cut into solid
rock, to meandering flows lazily traversing a broad floodplain.
As discussed previously, research on stream channel shape and
size has been-shown to be directly related to the quantity,of
water passed by the channel from a storm size that one would
expect once every 1.5 years. But this relationship between
channel size and storm flow is not universal and is generally
applicable only to streams that are hydrologically "old" with
meandering channels and well developed floodplains. In the more
urbanized areas of Tompkins County streams do not have these
characteristics. Stream channels in the hills surrounding Ithaca
have steep sided valleys, channel characteristics determined by
bedrock geology rather than energy dynamics, and high gradients
• m'are characteristic of "Young" streams. These channels are
59
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virtually carved in solid rack, have no floodplain, and are
therefore not subject to the same changes in channel shape or
profile from increased flows reported in these ether studies.
At the same time there are portions of these same creeks, further
upstream,, where small increases in the erosive farce of the water
can cause tremendous increases in erasion of the streambank.
This phenomenom is very common where the streams make the
transition to the "young" valley below from the "old" floodplain
above. It is in this in- between zone of the stream profile le where
r many streams gain most of their sediment load. For instance,
there area several paints along Fall Creek where stream meanders
are actively cutting into unstable slopes, contributing huge
j amounts of sediment to the streamflow. Erosion control in these
critical locations is necessary to protect downstream dams from
siltation and upslope roads and structures from undermining.
Municiapalities spend many thousands of dollars installing
Lgabions and riprap along streambank.s to control these problems,
Many of the smaller streams that would change considerably from
increases. in runoff due to development do not run year- round.
When water levels fluctuate between raging torrents in the spring
to muddy sloughs in the summer it becomes difficult to say that
the effects of development will cause this environment to suffer.
Intermittent streams present a very different habitat than do
perennial streams and it therefore may be questioned as to
whether the same management practices to protect environmental
values are appropriate in each case. Certainly the public
benefits presented by intermittent streams are less obvious than
those of perennial streams and for that reason are of less
concern.
Priorit iel.s:
Erosion from construction sites is universal and causes problems
all the way down the watershed. This should not be tolerated
anywhere in ,Tompkins County. Proper land management and erasion
control is not expensive n=or difficult to achieve. For the most
part, there is little or no regulation of construction activities
to reduce erosion and subsequent sedimentation in the County.
Local contractors are often unconcerned with erosion and have
knowledge of only the most basic control methods. As long as
comprehensive erosion control plans are not required of the
developer, erosion will only receive attention that is both too
little and too late. In terms of total benefit to the public,
erosion control legislation may have a better- payback than
stormwater control. Erasion is better understood, easier to
control, and easier to administrate than is runoff. Runoff
control legislation without erosion control is extremely short
sighted. As has been pointed out, all runoff control-practices-
are
negatively impacted by erosions
�, 6c�
•The problem of accelerated strearnbank erosion from increases in
storrawater flows is not commonly recognized in Tompkins County,
Where it is a problem there are two alternatives for mitigation,
One approach is to prevent increases in flows from smaller, more
frequent storm events capable of creating bankfull flaw
conditions by detaining storm flows upstream. The other
alternative is to stabilize streambanks at points of high erosion
pot ent i a 1.
The places where streambank erosion is causing problems in the
county do not generally coincide with areas of intense
development. Therefore, it would be difficult to blame these
problems on upstream development and it would be unlikely that
stormwater controls used to maintain the natural flew condition
would lessen the problem. More effective management of the
problem would be to use structural measures to reinforce specific
problem areas although this is far from a perfect solution.
Stream channel erosion is a natural phenomonom that can be very
difficult to control,
For Tompkins County the most effective method of lessening the
impacts of development on streams is to maintain a natural buffer
area of vegetation between development and the stream channel.
The width of this buffer is dependent upon the intensity of the
development and the sensitivity of the stream corridor, but
generally accepted standards are in the vicinity of 20 to 100
feet (although the argument for a comprehensive stream corridor
protection program as proposed by Foster (1989) should be
carefully considered in lieu of an arbitrary setback). The
County's largest streams have some protection offered by
floodplain regulations that limit development within the
floodplain, but these laws do little to protect this natural
buffer. Development proposals on smaller streams may or may not
be subject to review by the D. E. C. , but, at this t irne, such
review is limited.
The evaluation of the effect of any development on stream erosion
must be based on considerations of the effect of that development
on the flood hydrograph of the stream in relation to the stream's
physical parameters of profile and geology that influence its
capacity for erosion. Once again, the effect of that development
has will be Much greater in a smaller watershed basin. Small
perennial streams with earthen banks in areas under intense
development pressure are rnl:: ist likely to suffer fry w degradation
due to channel erosion. In Tompkins County this is not a common
situation, but the protection of all stream corridors from
development activities is a worthwhile goal.
Groundwater recharge
Although there are many residents in the county who experience_
• problems with their well wcter supply, the evidence that
E,1
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constructed improvements limiting infiltration of runoff is
causing these problems is hardly convincing. In most parts o
the country the agricultural, engineering, and standard
construction practices used to intercept and direct shallow
groundwater flows to surface conduits could be shown to have a
much more pronounced effect on groundwater recharge than does
building and paving.
Widespread problems due to excessive runoff from developed areas
do not exist in Tompkins county. Indeed, there exist potentials
for problems, and their are problems that may exist that have not
yet been discovered, but without distinct problems, the
formulation of appropriate management goals is not a simple task..
Efforts to control erosion and sedimentation and efforts to limit
Pollutant discharges in Our lakes and streams have clear and
tangible benefits. Efforts to control flooding, lessen
streambank erosion, and enco=urage groundwater recharge are not s•
clearly beneficial. Therefore the highest priority for the
county stormwater management program is to do the research
necessary to determine the true magnitude of the runoff related
problems experienced in the county and develop the correct
management program from those findings. Because extent of the
runoff problem in the county is not great there is time to do the
needed research and analysis before problems do get out of hand.
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CHAPTER THREE: Stormwater management regulation in Tompkins
County
Current regulation in Tompkins County
The
present
restrictions
status
to
of
erosion
within the
and
drainage
regulation
At
this time,
in
Tompkins
County
Towns of
is difficult
and Dryden,
and the
to
pin
dawn since
Ithaca
little
of
in this program.
what is
Other than
enforced
is actually written down. In order to participate in the
National Flood Insurance Program, a municipality must adopt
certain
restrictions
to
control
development
within the
floa.ndp1ain.
At
this time,
only
the
Towns of
Ithaca
and Dryden,
and the
City
of
Ithaca
participate
in this program.
Other than
these
particular
restrictions
only
the Towns
of Dryden
and Ithaca
have
a
distinct
set of
written
drainage
regulations
anywhere in
their
town
laws.
In the Town of Ithaca, Section 2E of the Subdivision Regulations
states that drainage improvements must be approved by the Town
Engineer with minimal runoff computed by the rational method with
a rainfall rate of .5 inches per hour. As stated, this comes out
to be the equivalent of a two -year, one hour storm. This
regulation also contains a few sentences that require
construction plans if a diversion of the natural stream channel
is planned. Furthermore, it states that detention facilities or
other structural measures can be required where the average grade
exceeds 7% (which is much of the Town) to control the velocity of
runoff flows. _
It is not uncommon to find that the written regulation differs
considerably from actual practice. Presently the Town of Ithaca
Engineering department policy is that the runoff characteristics
of new development shall have "no increase in runoff per unit
t ime ". In other words, storm drainage for new development must
be designed to prevent any increase in rate of flow at.any point
of the hydrograph - -a much more difficult engineering feat than
simply limiting the peak rate of flow. Drainage plans are to be
based on a design storm acceptable to the Town Engineer with
calculation done using either the Rational or SCS methodology.
If uniformly enforced, this requirement would necessitate the
construction of some sort of retention or infiltration structure
on every development site. And although this regulation is quite
strict in terms of what it requires of the developer- it does not
directly address the problems that it attempts to mitigate. Thus
while this regulation is effective at achieving the ultimate
management goal, no alteration of the natural runoff condition,
this achievement is realized only through considerable expense.
Arid this expense may preclude the achievement or eq Ua 1 l y
important, but not as carefully legislated, planning goals such
63
1,
I
M
N
as the co 6centrat i� �n of development t� � limit the extensi _in of
municipal services and the encouragement of affordable hcsusing
deve l c ipmerit . The current regulatory stance in the Town is due
partially fronm pressures exerted by the City cof Ithaca
downstream. The City stormwater system is subject t•, f1• iding
from re 1 ate i ve 1 y m i nc it storm events and simply cannot accept
additional runooff from further development upstream. At this
time the Town and City of Ithaca are in need of a major
stormwater planning effort so as not ton compromise i ether planning .
goals with shortsighted attempts to address the runoff issue.
In Dryden, the Subdivision Regulations require that new drainage
facilities be sized to-accommodate the 25 year storm event based
on a pc otent ial for increased runoff from both the propo used
develcipraerit and the remainder of the Upstream watershed under a
hypothetical fully developed condition. In addition, a study
must be done to evaluate the effect con downstream drainage
structures. If the increase in runciff from the propcised
development would overload any downstream structure during the S-
year storm event, the project carnet proceed until 'the Town Bayard
resolves how the undersized structures will be replaced.
By itself,' this regulation should provide adequate protection
from the effects of localized flooding. The requirement that
structures be sized for the maximum potential development in the
watershed, if followed, would result in developers downstream
incurring costs for drainage structures to serve future
(
development upstream, which doesn't distribute these costs
`— equitably.; There is also � a potential problem when the
overloading of downstream structures dines c,ccur. It is unlikely
that the Town Board is going to bend over backwards t,=o resolve
downstream problems so that upstream development can proceed.
What typically seems to Occur is that the development proposal is
modified cir withdrawn once a discussion of extensive downstream
improvemerits is triggered. Over the short run, the paragraphs in
the Dryden Subdivision regulation dealing with drainage are
fairly effective in offering some protection to both the
developeriand the Town if properly administrated, but address
only the issue of localized flooding.
Since the Town of Dryden dines not have a municipal engineering
staff, the ordinance states that the Soil Conservation Service
will review the developer's plans. In reality, the SCS hasn't
actually done any review of plans for the Town; review is done by
a consulting engineer. However, in the past three or four years,
the regulation actually practiced has evolved into this: no
increase in the peak rate' of runoff,. based con the `5 -year event.
The Train's engineering consultant. wants to see the SCS TO
method used for the deterrrnination' of runciff and has required that
structures be designed for larger storm events when prudent.
G4
Tills inforroal reyl_li.ati •n, r,l increase in the peal:. rate f rl_rrl ff
• �J l t �l any l y s l s Gased C n the �J -year' evert � by the ` l -, met hod 1 5 a
core me ir, Unwritten law throughout the ccll_lnty. In add it icin 'c c, the
T• awn � .f Drv_ den; The Town of Ithaca, the 'T, �wr, 1_1f Lansing, and the
Village cif Lansing all share similar bask r'eq�_lirerncr,ts. But
l-jhereas these municipal it ies share a basic requirement for the
initial review o1= drainage plans, what is acceptable toz.1 the
administrator Cif the regulaticin varies. CCinsiderably frCir,l pr'Dject
t I project even within one municipality. in interviews with
=.every 1 p'ract i t i c iners there were ciz mime in cc imp l a i nt s cif st irnlwat er
r,lai'lallef,lent c iverki l 1, where reviewing engineers made ,_lnreasCYlaGle
demands cin pr.Ijects which in some cases made prcijects unfeasible.
Other c 1bject ic lns were raised when the requirements for sty rrowater
C IZI r,tr •l were applied blindly with Oi_lt any ccansiderat io'n for the
fact that siprosp pro o•ject s were sc , small c or situated in such a way
that the impacts of increased flews would be next tc, ncithing.
un the either side of the cclin, reviewing engineers complain cif
inadequate drainage reps arts, disregard c of po tent i-al problems
beycind these "cifficially" recognized by the written regulations,
and the fact that there aren't any c• Ir,lnlc inly accepted standards.
CI-sme Of these engineers also expressed discclrjifclr't with the role
of establishing policy for the municipality--a concern for
pc itent ial professional _anal 1 iabi 1 ity.
In the rest cif the county there are few regul at is er,s and r,• � r,lear,s
•to enforce the regulations if they existed. I'n these sitl_lat is ins, �
drainage is almost entirely the responsibility of the developer
and is considered and built in the basis cif regional knowledge
and seat of the pants design. This is n• at necessarily
inappr 1priate since commor, sense will UsUal ly result in
r'eascinable drainage practice fc sr lcocal i zed prevention of flooding
where land values are not sc s high as to preclude g• id .j l_ld gernent .
There still exists sc ime concern for ere os i ran and pollutant
washoff, but i•n mare rLlral areas municipal expenditure tc,
adr,linister the regulation cif these impacts may simply be t� cl
great. This is where a case could be made that the reguiaticin of
stcirrowater and erosic in be administered at the cc lunty level of
government.
•
New st orr,lwat er management reg l_l l at i c ins are being considered
thrc l,_lgh� �l_lt the c• il_lrlty in efforts by individual rounicipyl it ies tc
formal ize their Previously unwritten pcisit is ins and t• i clarify
exact lv what will be r'egl_lired cif the developer. To this end, all
t he parties concerned S upper Ort scime, sc in of regulation. F1: 1rr,ly I
performance, submittal? LAnd review regl_lireroents s:i_ol_lld help tc,
�tr'raroe n the perr,litting pr'CtCC:dI_lre a4nd w_ l ' 'pie yppreci�`ed by
bcsth the developer arid the reviewing enoineer. The <se regu:.ations
i re r sor,le I(=` 'j yi prat ect icc rl to tt",E -� uC C.c cpE'r an d tht
bJ
W
1"111- Unici pal i.ty by clarifying respi= insibiIities
standards against which perfclrmance can be
By stating requir�eofents up fr c irit, drainage
(appropriately) c insidered at the onset c1f
as an a,Fterthc,, -tght.
Future regI_rllation in Tompkins County
and establishing
accurnately meas� r� ed.
issues w i l l be
the pr,clject instead ot.
In April cof this year the Department of Envirmonmental
�Ccinservat i cirr released its "St or%mwat er Management Guidelines for
New Development ". AlthO Ugh these guidelines specifically state
that they "fare not fired and inflexible rules to be
applied. .'.without considering the particular facts and
c i r^cumst ances of a part i cu 1 a1" project" ( New York State,
Department of Env i r� inment a 1 Canser - vat i c on, 1990 pg . 2),j it is
likely that many municipalities in the state will adapt these
gt_tidel ines ;as standard practice. These gl-ridel ines are concise
and give a 'thcir�_lUgh overview of ccontempotrary stormwater
ni aria gement practices and how they might best be effected, but
these guidelines d• � no.-it address the need f clr� watershed -wide
starmw- at-er._riian gement_ planning., nor do these guidelines address
fundamental municipal cortcerns: adrninistr�aticin, en.fc--jrcement,
submittal requireriterlts, safety, and maintenance. For communities
to adapt these guidelines without same cc insiderat iori of these
other issues _is inviting a host cof futui^e pr�oblerns. It is
instructive to examine the pcic_,sible scenarios that might develop
if this or the ether, proposed drainage reglclativris were, adapted.
ec
At this
prcicess
time
of
cclnsideririg
the Tcgwris
of Ithaca,
new stclrrnwater
Lansing,
Management
and Dryden
regulat
are in
iclns.
the
The
Town
cif, Ithaca,
in
the roiddle
cif revising
the
Town
piaster
Flan,
is
ccirisidering
the
-Adoption
of a
separate
stc
lrrirwater
c ird i
that
year
add iticin
narice
wciuld
design
or� i
effectively
storm.
tc their-
g i na l
l y
s1_tbdivisicln
draft
The
ed "
r- equine
Town
by Lancaster
all development
of Lansing
reg_ulaticins
is
C� IAnt y
to
looking-at
that would
in Pennsylvania
detain
the
a simple
r- equine
1(.-)-
that
drainage
reports
be
filed
and
that consider-at
is in
be given
to
the
downstream
effects
of
the
25
year stc_rm.
The Town
cif
Dryden
is
in the
r,eg 1_t
prc
1 at i c ins.
loss
of
Mo Dst
revising
recent
their
1 y,
master
the Dryden
plan
Planning
and
the
Beard
land
presented
use
a prc
iposal
to
institute
5 -acre
zoning in
a large
part
iori of
the
Town
prob
considered
fl .-W
l ems
the
( Debt
"Site
plArpcose
,, 199C-00
Plan
cif
Review"
mitigating
The recent
r�unc iff
implementation
process in
and water
the City
sIApply
c of
of
a separately
Ithaca,
soc-or
to be
followed
by
the
Towns
of
Ithaca and
Dryden,
will
also
focus
rn' rre
attention
con
the
t
llAnciff
issue.
In April cof this year the Department of Envirmonmental
�Ccinservat i cirr released its "St or%mwat er Management Guidelines for
New Development ". AlthO Ugh these guidelines specifically state
that they "fare not fired and inflexible rules to be
applied. .'.without considering the particular facts and
c i r^cumst ances of a part i cu 1 a1" project" ( New York State,
Department of Env i r� inment a 1 Canser - vat i c on, 1990 pg . 2),j it is
likely that many municipalities in the state will adapt these
gt_tidel ines ;as standard practice. These gl-ridel ines are concise
and give a 'thcir�_lUgh overview of ccontempotrary stormwater
ni aria gement practices and how they might best be effected, but
these guidelines d• � no.-it address the need f clr� watershed -wide
starmw- at-er._riian gement_ planning., nor do these guidelines address
fundamental municipal cortcerns: adrninistr�aticin, en.fc--jrcement,
submittal requireriterlts, safety, and maintenance. For communities
to adapt these guidelines without same cc insiderat iori of these
other issues _is inviting a host cof futui^e pr�oblerns. It is
instructive to examine the pcic_,sible scenarios that might develop
if this or the ether, proposed drainage reglclativris were, adapted.
ec
Stormwater Management Planning Notes
Page 1
OCT 9 Jim
;t
I. Stormwater mgmt can address 5 categories of problems
1. flooding
2. pollutant washoff
3. erosion
4. groundwater recharge
5. stream channel preservation
II. Problems not commonly perceived in stormwater ping
1. Detention structures can be counterproductive. In
approximately the lower one -third of a watershed, detention
merely delays peak flows until the time that the upstream
surges have reached the lower one - third, thereby increasing
flooding.
2. Detention structures designed for a single frequency storm
event provide inadequate protection, doing nothing to minimize
flooding from smaller events. Therefore, multiple detention
designs make more sense (ie for several different design
storms).
30 Because of number 1 above, there often exists a need for
coordination of stormwater management among multiple
municipalities governing different parts of the same
watershed.
4. Where storage of runoff is anticipated (ie retention or
detention), we need knowledge of runoff volumes, not just peak
rates. Therefore stormwater ordinances should require
generation of hydrographs in such situations. If storage is not
to be used, then analysis that predicts peak rate of flow is
sufficient.
5. For purposes of administration and review, there is an
advantage in restricting analyses to one standard method (eg
the SCS TR55 method). This makes all comparable and assures
that the municipal reviewer is qualified for reviewing what is
presented. For the same reason, it makes sense to require a
standard design for submittals and analyses of what happens
when a drainage system is overtopped.
6. Limiting peak flow causes an increase in the duration of
bankfull flow for storms larger than the design storm.
However, because the major component of erosion occurs
during bankfull flow, merely limiting peak flows may
considerably increase erosion.
E
r-
Stormwater Management Planning Notes
4 ; c� Page 2
r "
I11. Site specific detention structures are "last choice
1. These structures are heavy on: expense, maintenance, safety
concerns, administration problems, and enforcement. In
addition, proper analysis requires watershed -wide analysis
and (planning.
2. Theriefore, non - structural or "soft" solutions may be more
appropriate, eg:
a. limit development in stream corridors
b. require large -lots in the most severe environments
C, ponding behind culverts (mini- detention)
d, controlled flooding of undeveloped or recreational areas
e. use natural storage capacity of ditches & swales that are
designed to slow runoff.
3. Regional detention structures often are more economical and
effective than site - specific ones. (The municipality might
install and maintain the structure, while developers might
contribute to a fund for it in proportion to each development's
net effect on runoff.)
I
•
0 CHARTER TWO: Local watersheds sheds and storrnwater management
i
One of the more difficult tasks facing the legislators of
environmental regulations is the creation of laws that can be
widely applied to a wide range of very specific issues. It is
unrealistic to expect that one set of laws will be applicable to
every pi zissible type of development activity on every Possible
site. And yet this is exactly what drainage regulations attempt
to do. The common conception of "justice" requires legislat ion
to treat individuals equally under the law, but land is not
created equal and if legislation is truly going to protect
environmental values then it must resolve this inherent disparity
between the dissimilarity of envircinments and equal treatment
under the law.
There are two avenues toward the resolution of this disparity.
One would be to expand the content of the legislation to include
Special provisions for every Possible permutation of site
development. Although some of the legislation examined in this
report attempts to do just that, these regulations are simply too
ponderous to be seriously considered in Tompkins County. The.
other avenue is to maintain a high degree of specificity, but to
custornize�- ,local Policy decisions to local conditions. ions. T� �mpkins
. County has several unique geologic and physiographic
characteristics which should automatically call into question the
appl icabi 1 ity of storrnwater management law and pract ice imported
from the Piedmont region or Florida.
This chapter- discusses some of the hydrological characteristics
and the extent of storrnwater induced problems in Tompkins County
that were revealed through research and an informal survey of
local g. �vernment officials, engineers, and Practitioners. This
discussion is presented to highlight local runciff problems, make
a general assessment of the effectiveness of different Best
Management Practices in dealing with these problems, and suggest
what legislative measures might be rnc,st effective.
Watershed Characteristics of Tompkins County
Stream profiles and watershed sizes:
Cayuga Lake is the largest of the Finger Lakes. This lake, of
glacial origin, is 40 miles long and averages over .a mile in
width. Almost all of the streams in the County drain into Cayuga
Lake. The exceptions to this lie 'in the T,Dwn of Groton, which is
in the Owasco Lake drainage, and the = outlying portions of the
Towns of Newfield, Danby, and Carc. line which lie so=uth cif the
"Valley Heads Glacial Moraine" (a ma.j =r- dr ainage divide running �' east -west) and drain to the Slusquehanna basin to the south. Of
STANDARDS FOR STORMWATER MANAGEMENT IN TOMPKINS COUNTY, NEW YORK:
An analysis,of current regulation and proposals for the future
evo 1 ut i s =in of st ormwat er management policy in a heterogeneous area
of upstate New Yorke
I
F.rnIe- -$aY 1 c5
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FIGURE 9 - wATFER5NED ANo MM*IJCAPAI.L p0L)NoPARl1✓5 -
OF TOMPKlt15 C0U1olT`(, w
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41
F
I
the watersheds feeding Cayuga Lake, only the Fall Creel {., Salmon ®�
Creek, and Taughannock watersheds do not lie wholly within the
County. Fall Creek is the largest watershed in the County with
its headwaters located near SemprarIlUs in Cayuga County. Sa1miDr,
Creek also - drains from the north from Cayuga County, and
Taughannock Creek. from Schuyler County to the West.
Most of the or streams in the county that drain t• � Cayuga Lake
� maj m
have cut deep ravines into the bedrock where the land dips
sharply towards the lake and have significant waterfalls in their
gorges. Further up the watershed, away from the lake, these
creeks are ;more typical of waterways developed in rolling
topography !,with the largest stream, Fall Creek, having a well
defined floodplain. In their upper reaches these creeks exhibit
the dendritic drainage pattern typical of streams that have
formed in homogenous materials. Just before they drain to the
lake, these streams assume a more flattened profile. A profile
typical of the larger streams in Tompkins County is represented
by that of Cascadilla Creek and illustrated in Figure 10.
In the northern half of the county nn_tch of the land slopes
towards the lake with a rounded convex profile. Here, there are
many small iparallel streams that exhibit profiles similar quite
different from streams in most parts of the world (see Figure
IO).
The significance of this stream profile is obvious to the ® �
observer: these streams run straight downhill, are deeply cut
into the bedrock., and run very fast. Almost no sediments
accumulate 'on their channel bottoms. The headwaters of these
streams are often in the wet and swampy areas that occupy the
crests of the convex slopes surrounding -Cayl_tga Lake. Because
many of these streams have such small drainage areas there may be
little or rio base flow in the stream during dry periods, but they
may nonetheless carry considerable flows during st orrns. Because
these channels are so deeply cut marry of them draining the convex
slopes along the east and west shares of the lake have a large
capacity to handle increased flows without flooding. This fact
should have some bearing on future storrnwater management policy
icy
making.
i
i 4
t`
1 400
1300
17000
1100
1 000
100
Sao
700
boo
ill 1500
140D
(300
Q 12oD CASGAA t LLA CR.L.�.K
J
=�-�� • � 1100
ell 100 --�,
Ar`- °fo0
n G eo
,y Soo
11!
0 ( 2 3 4 S G
1500
j};• 1400
1300
1200
1100
to00
4100
Soo
700
600
3 00
s} 00
P�1GUR� 10.. SE.LLG�Z,p STKEAM PIZ.orlLt�S
DMS WeMAAC N) DAN cr-E.E•K l W V
(AFTER MOFU5AWA, 1960 p.lZI)
0
S
M I L�5
7 8 9 to
Mlj•eI S
g
Ml �S
5
43
I
i
i
0
Ell,
® Sails 0
Soil types will have an effect or, what particular management
practices are most suitable for a given site, and should also
have some bearing ors future regulatory decisions. The General
Soil Map or the County (U. S. D. A. SCS, 1965) shows that the more
highly urbanized areas surrounding Ithaca are generally
unsuitable for stormwater infiltration practices due the presence
of shallow bedrock and law soil permeability. Therefore, r ^Unoff
regt_ilat ion that requires the retention of storrnwater to control
bath the peak rate and volume of runoff to the predevelciped
condition in order to most accurately maintain the natural flaw
regime will be difficult to achieve. In this circumstance,
extended detention will prove to be the best engineering
alternative. Where infiltration of runoff is impossible and the
site is too small to allow for surface detention, management of
peak flows will necessitate some sort of underground resevoir,
which will, likely be complicated by the shallow depth to bedrock
and a highlwater table. When regulations and site conditions
begin to dictate such complex and expensive management solutions
developers will be forced to reconsider their site selection.
Stormwater� rjlanagemerjt regulations will effect patterns of
development. This effect should be considered in the formulation
of these regulations so that ether planning gc-lals are riot
compromised by stormwater planning goals.
® The Town Line
Municipal boundaries cross marry watersheds which can create
significant political obstacles t� � management of runc iff on a.
watershed -wide basis. The most confused situation lies within
the City of Ithaca which receives runljff from Caroline, Dryden,
Danby, The Town of Ithaca, Enfield, Newfield, and Cayuga Heights.
The Town of Ithaca is similarly hernmed in by several different
muniCipalities. None of the major- watersheds in the county lie
within the; boundary of one political unit, bat certain
municipalities can exert significant control on specific
watersheds;. The upper reaches of both Fall and Cascadilla Creeks
lie largely in the Town of Dryden; Buttermilk. Creek. is in Danby,
The Cayuga; Inlet in Danby and Newfield; the headwaters of Enfield
Creek are ;shared by Newfield and Enfield; Caroline contains the
uppermost reaches of Six -Mile Creek, but the watershed is
distributed between Dryden, Danby, and the Town of Ithaca. Since
the City and Town of Ithaca .end up taking the brunt of upstream .,
management; decisions it is to be' expected that these
munici pal ilties would have evolved the mast restrictive stormwater
policies.) ;but these pc Il icier are of little value ire protecting
d eve 1 oprnPnt or ert`l l rariffl•?nt within these riln_) ! �r stream corridors
when much cif the watershed is beycind their cc Introl.
41+
I
i
I
The
Town and
Village
is
nearly
fully
of
Lansing
are
located
runoff
such that
within
they
are
not
impacted
significantly
is
not great.
Of particular
by
nor
do
they
in
turn
impact
other
municipalities. Although Salmon Creek drains a large area to the
north, most st y armwat er problems in the Town and Village of
Lansing are confined to the small drainage basins that drain
directly to Cayuga Lake and lie almost wholly within their
Jurisdictions. A few of these watersheds drain from the Town to
the Village and in these drainages coordination of management
efforts is important. In both of these jurisdictions the
watershed areas are small, and therefore, relatively small
developments can have significant impacts. But, for% the same
reasons, stormwater management practices can be much more
effective.
As Cayuga
Heights
is
nearly
fully
developed,
concern
for
runoff
management
within
the
Village
is
not great.
Of particular
r.
interest to the Village is the effect of future development in
the Northeast neighborhoods of Ithaca which drain to Cayuga
Heights since many of the bridges and culverts in the Heights are
already overloaded in larger storm events.
Dryden is also experiencing an increase in development and is �
rightly concerned about impacts on runoff. As no other
municipalities exert upstream development pressures on the Town,
Dryden can develop management policies that concentrate on local
4t <:
impacts and without concern that a municipality upstream will
' thwart their efforts. Other outlying Towns—Caroline, Danby,
Ulysses, Newfield, Enfield, and Groton- -are not likely to
consider runoff regulation for their own communities since
n development pressures are law. Nevertheless, all should
r,ecogniZe that a development in one of these outlying areas
contributes just as much erosion and pollution to � the co unt y' s
rivers and streams as does a development in Lansing. Being
r'- located i n a rural area does not absolve one of responsibility
for the maintenance of the county's natural resources.
0
-. The scope of the problem in Tompkins County
fir:
- There is no question that problems with flooding, po 1 1 ut i on,
groundwater depletion, eros i on, and sedimentation are found I
throughout the county. To determine whether or not these
problems are due t� � increases in r�_�n� off from devel� sped areas and
whether a stormwater management program would be are effective
means of correcting these problems is difficult at best. First
of all, most of the problems due to increased runoff are not �
entirely new, they are simply the acceleration or magnification �
g of processes that occur natLWally. Second, problems due to �
development may not be apparent for many years unless a major
storm occurs and even then problems may show Up 'tar downstrearA
fry =Im their source. Third, problems assn= iciated with sty irmwater
are the additive result of many separate man -made impacts coupled
•to natural processes over time and i S therefore it is nearly
J4
imp_Issible to ascribe a problem to one particular action. Last,
the lack of data fr Im historical conditions prevents us from
being able to--, accurately measure what p_irt i� �n of the pry 1blem is a
natural phenomenon and what portion can be attributed t.c' man.
Still, it is a fact that develc ipment does amplify these problems.
Arid by looking a little more carefully at patterns of
development, watershed characteristics, and the magnitude of
existing problerns it is possible to suggest where storrlwater
management efforts might best be made.
Major floods
The last major flood in Tompkins County happened in October of
1981. At; the peak of flooding parts of Dryden were under 4 feet
of water and flows in Fall Creek. and Virgil Creek washed out
I several bridges in the Town of Dryden and the Town of Ithaca.
Six Mile Creek overtopped bridges at German Cross and Burns Road
and caused extensive damage to several structures that were
undermined by the flood flows. In the City ;of Ithaca, Tioga
Steet bridge over Cascadilla Creek was ruined, approximately
feet of water buried sections of the Fall Creek neighborhood, and
many of the structures adjacent to these streams subject to the
tremendous erosive farces of the flood were damaged.
During this flood, god, 5. U8 inches of rain fell in 24 hours (U. S. D. A.
•
SCSI 1980. This quantity of rainfall is roughly equivalent to
c
the 4 -hour, storm 100 -year strm of 5.4 inches as given by the U. S.
Weather Bureau (Hershfield, 1961). More significantly, 4.4
inches of rain fell over one six hour period and was largely
responsible for the 7 million dollars of damage experienced in
Tompkins County. This quantity of rain was far and ab=ove the
pr�evicaus record of 2.9 inches in 6 hours set during the famous
flood of 19356 Other storms responsible for widespread flooding
in the county happened in 1964, 19691 1974.2, and 1975. In 1972,
Hurricane Agnes dropped a total of raore than 11 inches of rain are
the County over several days. This resulted in some f l ceding,
but most of the damages in this area were in the floodplain areas
of the large Chemung River- to the south. July 11, 1976 brought
4.:�5 inches of rain in a :_4 -ham sir period (slightly smaller than a
Weather Bureau ;�5 year storm), causing localized flooding of some
reads arid significant damage to scime properties. The "flC 1d of
record ", July 8, 19.35, saw 7.9 inches of rain fall in 04 hours.
Four or more feet of water covered many areas of downtown Ithaca
and the 'l owlying areas cot surrounding commUnit ies.
Co urit y -wide st ormwat er managment re g i_t l at ions- of new .development
will have little effect c are the flooding effects of major storm
events such as these. The majority of the damages resulting from
major storms is confined t1.1 the fly _�dplair�s
of the larger
waterways in the C_Writy. Because these areas are flat, and are
c 7
otherwise easier t•.. develop that the s�_;rr���_inding highlands,
fact that they may be subject to an ciccas-ilzinal flood is rlc it
i
i
46
9
I_.
sufficient
cause
to �
deter
devel,
rpmerit.
The flooding
1:11--
fli.-(aZidplains
under
way
in
is a
natural
condition
that
is nearly
impossible
tr.1
counteract
thro=ugh
sty
irmwater-
management
controls
practiced
at
the
individual
site
level
particularly
if
one considers
the
extent
of
urbanization
in
the
county. This
point
is
well
illustrated
in the
following
lowing
examinat
ion of a major flood control
project
that
is
under
way
in
the
county.
The Virgil Creek Watershed Flan
The Town and Village of Dryden suffered m� ire than 3 million
dollars worth of damage in the 181 flood. The Virgil Creek
Wat er_s_h_ed__Pr_o lect report predicts that Dryden end eras almost
$40o,000 worth of damage from storms as small as those of a 5
year return frequency (U. S. D. A. SCS, 1989, pg. 83)M This damage
is largely the consequence of poor planning, common throughout
the world, that has allowed substantial building within the
natural floodplain. In an effort to control these problems, the
Sail Conservation Service, working with Tompkins and Cortland
Counties and the local communities have put together a plan to
locate a regional detention facility ors Virgil Creek just to th`e
west of the County line in the Town of Dryden. This facility has
been designed to central the flaw of Virgil Creek to reduce 97/,
of the potential damages in Dryden due to flooding from storm
events up to and including the 100 year storm. The plan calls
for the construction of a detention pond capable of storing 2175
acre feet of water.
Berne quick calculations show that this one project dwarfs any
mitigation measures likely to be instituted on a site level.
Page c of the Virgil _Creek _Water_shed_Rro,Zect report identifies
596 acres of land in the watershed as "Other" than agricultural,
forest, grass, or croplands. If this area (just a bit less than
one square mile) is the extent of urbanization in this watershed
then each acre of developed land would have to provide 65 acre -
feet of water detention in order to provide the same protection
that the proposed project would supply. A more typical figure
for detention might be on the order of .05 to . i acre -feet of
detention per acre of land under a regulation that required
detent itDr, to control the peak rate of flow for a 100 year storm
(Aitken and Go yen, 198, pg. 179, Urbonas and Glidden, 1982", pg.
189 -9?,) . Since detention on the more typical scale is only one-
thirtieth of the volume provided by the Virgil Creek project
there is little hope that local stormwater detention practices
could prevent regional flooding in this watershed. But in
smaller watershed areas the effect of site level development on
the hydrologic regime becomes more evident. Therefore, it should
be Possible to iderstify watersheds where rUrICIff management to
minimize damage from rna_jor stCrrm events could be successfully
implemented. As of yet this has not beer, done.
Other Watersheds
47
1
r
r
IE,
b.
El�� I
®
It is presumed that the Virgil Creek Rrt �.ject will confer some
protection from flooding on Fall Creek downstream of the Virgil
Creek confluence. The level of protection provided by this
improvement to development further downstream has some bearing or,
future stormwater management decisions in this ireportant
watershed, and should be determined. Upstream from this
confluence flooding of Fall Creek will not be abated by the t
Virgil Creek Project and the need for future control of runciff
upstream of-' Freev i 1 1 e cannot be dismissed.
In the City of Ithaca, the Cayuga Inlet Flood Control Channel
provides an improved drainageway for the several streams that
enter the lake at its southern end. By deepening, widening, and
straightening Inlet Creek, this channel is designed to contain
flood waters within its banks to limit overbank. flow and the
subsegent inundation �n of properties located in the floodplain.
During the 1981 flood there was little flooding in the West End
area or to the sloth of Ithaca; presumably partially due to this
structure and partially due to the fact that the heaviest rains
were to the east of Ithaca.
/Both Six Mile Creek and C'ascadi l la Creek have inflicted serious
damage in the Town and City of Ithaca during past major storm
events. Six Mile Creek is the second largest watershed in the
® Cc�l_�nty. Because of its size, site scale management efforts aimed le'
at the control of major floods will be largely unsuccessful. The
RCascadilla Creek. watershe , or, the other hand, may be of a size
where storrliwater controls to limit flooding may have arl effect.
A comprehensive watershed study would have to be undertaker) to
confirm this. Bath of these watersheds will see significant
residential and institutional development- in the future. For
this reason these watershed basins need to consider some sort of
flood management program.
Buttermilk, Enfield, and the ether creeks that feed Inlet Creek
have less potential for damage because the floodplain of Inlet
Creek is riot heavily developed, and because of the flood
tempering provided by the Flood Control Channel. These
watersheds will also riot be as subject to intense urbanization
since much of the area is parkland, very steep, and riot well
serviced by utilities. The Tal_ighannc -�ck Creek- watershed is bath
large and rural which allows it scime imr.fl inity from damage due to
increases in storrnwater flaws. The limited development this area
is likely to see irl the future would suggest that the Town of
Ulysses need riot institute rigorous controls an peak rates of
rurlclf f.
/ The uwasc�LLak-e_1riIet was heavi I f1i -j oded irl 5 8i. BI_tt, similar
to the situation in Virgil Creek, the damage sustained earl be
attributed to poor- flood plain rnanagernent and runoff control
® would have little effect or, downstr( =ar11 t1t -i odirlg. Salmon C-4 eek
14-8
has some pl =1tent ial for fly lding, but with little development in
[_ the watershed, runof f coat ri_� 1 t o prevent ma.j , �r floods i n this
area does not rnake sense. A similar argument applies to those
lands in the southern part Of the Cc:runty that drain to Cayuta,
Catat� ink, and Owego � Creeks eventually leading to the Chemung
River. These headwater areas have some pttential for negative
effects on downstream pryperties, but again these areas are so,
lightly developed that the control of runoff at the site scale
for flood mitigation purposes would simply be a waste of time and
money.
' The other small watersheds that drain directly to Cayuga Lake:
°_;,
-� Gulf Creek, Twin Glens, Williams Brook, Indian Creek, Glenwood
Creek, Willow Creek, etc., that have deeply incised channels with
little Or no f l oodplain are not Beverly impacted by large storm
events and would not be subject to substantial damage due to
unrestricted runoff from development.
_l
11:
Priorities
;.: Although it is bey ,nd the scope of this study to make definitive
statements about the magnitude of impacts that future
5 urbanization will have in each Of the County's different
... watersheds, these generalizations can be made about the relative
significance of these impacts on ma. :
9 P major flood events
. 1. Damages f
the flood
that have
flood pla
effects o
rorn major floods are concentrated in
plains of the streams. Watersheds
sUbstantial development within the
in are the mast vulnerable to the
f Urbanization.
Some of the County" s most sensitive
watersheds have or will have large scale
flood control structures that can effectively
mitigate the increase in run91ff effects of
development which should have bearing on
:. runoff control policy in these areas.
e.
.3.
Small watersheds with f 1 ood A l a i ns are more
susceptible t• � the effects of development
than are larger watersheds.
4. Watershed areas with n o fly ndplain and deeply
incised channels are generally ri�-.it subject to
extensive damages from flooding.
b. Cayuga Lake, by virtue of its size, has the
capacity to assimilate all the additional
runi_iff likely t• o be the result Of development
in the fc_srseeable future.
•
49
® E. The control of damages from major storm
events is best done through floodplain
management efforts that restrict development
in and alteration of areas naturally subject
to flooding.
Site scale stormwater- management efforts to control major storm
events will be of little use in Tompkins Co=unty. F'riorit i e s
should be placed on the further study of the County's watersheds
to evaluate the potential benefits of large regional control
efforts such as the Virgil Creek project as opposed to
legislation that enforces a piecemeal approach. The following
list indicates what watersheds should be given priorities ies for
further study.
Priority 1: Watersheds that have s
development within the floodplain w
�.' measures. Cascadilla Creek, Six Mi
above the confluence of Virgil Cree
ignificant
ith no flood control
le Creek., Fall Creek
ka
Priority c: Watersheds with flood plain development
and some flood control measures. The Cayuga Inlet/
Enfield Creek./ Buttermilk Creek complex, Fall Creek.
priority Watersheds with little floodplain
® development. Salmon Creek, Taughannock Creek.
Priority 4: The several minor- watersheds that drain
directly to the lake have almost no potential for major
flood damage since their flood plains are so small, but
will be shown to benefit from stormwater management
efforts to control localized flooding from smaller
storm events.
J��
i
• Localized flooding
On a less dramatic scale, flcloding of driveways, yards, is
basements, reads, and other manmade structures causes significant
inconvenience arid, in some cases, substantial damage during
small, more frequent storm events. Localized flooding is defined
here as flooding prob 1 ems can or adjacent to a development site
plus problems further downstream amplified by that development.
In the past, storri7water m aria gernent planning was aimed only at the
prevention and control of adjacent f 1 c iod i ng and remains as the
sole concern of local governments. Regulations were aimed at
establishing the upper landowner's responsibilities for
prevention of nuisance flooding o f the downstream landowner,
Since this is an easily understood and clear problem, regulatory
solutions are well developed and often thorough for preventing
problems with the immediate neighbors. Problems typically arise
when continued development upstream eventually overwhelms some
drainage structure or system further- downstream. Problems like
this are not yet common in Tompkins County, but the potential
increases with each new development.
In Tompkins County one can expect a A2.7 inch, i4 -hour rain once
every two years. Once in ten years we can expect to get 9
inches of rain in one day. Storms of this magnitude do riot lead
to widespread flooding, but can result in localized flooding and
damages. Culvert capacities can be overwhelmed resulting in
•flooding across reads, drainage ditches can overflow and back up
into basements, streambanks can be eroded causing property
damage, and considerable amc iunts of flotsam get left about.
Ponding of water in yards, puddling of parking lots and
driveways, curbs and sidewalks under water are all e immon
occurances during the bigger storms experienced every few years.
Usually these storms only create a temporary inconvenience and
miner damage, but there are occasional instances where these
problems are chronic and do present me are than an inconvenience.
Conversations with local municipal engineers highlighted several
specific problems: the piping of groundwater in utility trendies
causing sUbgrade problems, drainage swales that didn't work
properly either due to poor design car lack of maintainence,
undersized culverts, and ineffective land grading around
structures were all mentioned as causing local flooding. Of
these problems, undersized Culverts and other drainage structures
were of greatest ccincern, since these were problems that were
potentially property threatening and clearly the responsibility
of the municipality. Many of these problems can and will be
corrected. Others will probably be left as permanent monuments
tc• pc r planning and design. Arid although there does not seem to
be an extensive histcory of problems directly due to poorly
planned runoff, all of the engineers interviewed for this study
are becoming increasingly cc ricerned with the pt_teritial for
pro iblems and rclut inely require that substantial supporting
J 1
® ir,formation accompany plans for development s• that future ®�
problems car, be avoided.
Since few problem areas were revealed by these interviews it was
not possible to correlate problems to particular site features.
Still there are several factors that can influence a particular
site's potential to � have or cause problems. Much of the County
is underlaid by soils that have moderately to very po or
permeability with little cover over bedrock. This makes many
areas, particularly flat ones, susceptible to wet conditions much
of the year. Development of these f 1 at t er, areas often includes
improvements designed to rem eve water from the Soil as well as
divert runoff. In these areas the potential for increases in
runoff from development are high because of the substantial
changes made to runoff timing by drainage improvements. Since
many of these areas do not have the well defined drainageways
that the more sloping sites do, development usually entails the
alteration of existing channels to increase capacity and often
the inadvertent rerouting of flaws. This can lead to problems on
adjacent properties.
The building of roads is particularly disruptive to natural
drainage patterns. New reads running across the slope act like
dikes and tend to channe1ize and concentrate storm flows. On
flat sites these problems are coupled with the difficulties
® encountered attempting to install culverts of sufficient capacity
under these roads, since there is often little clearance to be
had between the road base and the bottom of these shallow
channels without building the base of the road up well above the
surrounding ground level. Areas prone to these difficulties
include the Northeast neighborhoods in Ithaca, many areas of the
Town of Lansing, much of the West Dryden area, and the upper
elevations of South Hill, Snyder Hill, Mo=unt Pleasant, and West
Hills Problems are not always evident in these areas, but their
potentials are high.
In areas ors the sideslopes of Cayuga Lake and the hills
surrounding Ithaca there are usually well defined drainage
channels that have large capacity. In these areas, diversions
upstream of structures are usually necessary to protect
buildings, reads, and ether edifices. These diversions carp have
unfavorable results on neighboring properties. For the most
part, flooding problems ors slopes are less prevalent unless there
is significant development on the lands upslope. The Village of
Cayuga Heights is one -of the.areas subject to local flooding
problems that car, be ,squarely blamed ors upstream developments Ors
the other- hand, where sideslope development drains to flatter
areas below there is a good potential for� flooding at the toe of
the slope where the water velocity dr•,ps and tender to "back up ".
In the developed fl1 Dadplain 9-.1f Ithaca, where there are extensive
underground stare; seviers, backups due t� � overload and clogging
C .
JG
are considered routine. Often the existing storm sewer system
r� was not designed with a secondary roi_�te for overland flew in the
event of backup, or subsequent development has violated the
secondary route to the po p i nt where it rio
longer funct i ons. Since
f Ut ure connections to these overwrought systems are limited, new
development in these areas may have to consider alternate means
of storrnwater routing or perhaps even the retention of excess
flews of runoff. But, as was shown earlier, the detention of
storm flows i n the lower portion of a watershed may actually
exacerbate flot,•ding problems.
In the City of Ithaca solutions to flooding problems will lie
either in the enlargement of the existing storm system or in the
construction of extensive detention facilities upstream.
Considering the timing characteristics of the watersheds in
question, it is probable that upstream detention of flows will
not be Successful either. The City of Ithaca has recently
commissioned a study of a South Hill watershed that will
hopefully shed some light on this problem. Whatever the
solution, it is going to require more capital than is going to be
a,. provided by developer contributions and the expenditure of
general public revenues will be required to make improvements.
a; Other municipalities may share the responsibility for problems in
the City, but political boundaries limit their participation in
the resolution of these problems.
' F'riorit ies:
F�
�FY
.« Although localized flooding is not yet a large problem in the
County there are some areas where problems are becoming evident
_ and many more areas where the potential for future problems is
rzr great. Areas of primary concern are the urbainiZing areas on the
higher elevations surrounding Ithaca. Further development in
these areas without runoff regulation can result in both
immediate and future pr• •b l erns for neighbors adjacent to � the
project and for drainage structures downstream. General
guidelines for the identification of potential problem areas are
as follows:
1. Projects that involve high density of
s- development and /or the development of large
}
tracts of land are the most likel -y to have
immediately realized impacts .
s
c. Older development downstream will typically
be the first point where problems arise since .
the oldest pieces of the drainage system are
tsua11y the most ut dersi�ed and the most
overloaded,
Some areas served with undersized systems
• with no provision fOr over capacity flew may
have to ci::%nsider expensive alternatives to
conventional storm conveyance practices.
4. The capacity of the existing storm system,
natural or manmade, and the capacity
necessary for the future is the basis upon
which many decisions about stormwater
regulat i� �rs should be made. Drainage analysis
should always include an assessment of the
effect of develiipment on downstream
structures.
v. Runoff control BMF's are expensive to ti
implement, particularly SO in light the fact
c
that raost of these practices are land
intensive. Careful consideration of the y
casts of these controls versus the benefits
obtained should be completed before �
legislation is effected.
Prevention of localized floiding has to be approached on a ,
project by project basis. It is appropriate for municipalities
to ask the developer to provide analysis of the effect of the
proposed project on drainage. But it is up to the municipality
to establish some equitable basis for the evaluation of the
proposal and also to inform the developer of problems that
already exist. In most instances it will be up to the developer ®'
® to mitigate! the negative effects of his activities, but there
�'wi l l be instances where it would be advantageous to both the
developer and the municipality to mitigate the problem elsewhere
than on the development site or for the developer to control more
runoft than wo=uld be strictly required by the effects of his own
development: activities. Some provision-for the sharing of
responsibility between developer and the community is a necessary
component of a progressive stormwater !Management plan.
If the municipality is to require that the developer control
runoff an his site so as to protect downstream properties the
developer will have some expectation that the municipality will
protect his property from future development as well. Thus it is
imperative that the rilunicipality have a clear vision of what that
future development might be and be sure that any drainage
i mprOvement s planned now are coordinated with the potentials for
the future.
0
• Pollution
In Tompkins County it may be frJund that the control of sty irrnwater
run iff quality is more important than the control of .strorrnwater
quantity. A 'lack of data and the difficulty of ,-obtaining
information renders it difficult tri determine the impact of
stormwater runoff or, the pollutant loading of local waterways.
Over the years, there has been the occasional study done on the
pollution levels in the major streams feeding the Lake, but it 'is
impassible t• � establish the necessary controls to make it
possible to compare data gathered 220 years ago to data gathered.
today.
The Tompkins County Planning Office has been conducting ing research
on the pollution of the waters that feed Cayuga Lake ,_ender a
grant from the State for the Aquatic Vegetation Management
Program (Skaley, personal communication). Recent information or,
pollutant loads indicate t hat none of the streams studied are
subject to significant pollution problems. Comparisons between
data assembled in 1972 -74 and more recent information indicate
that the levels of phosphC-1r1= 11A57 potassium, and nitrogen have not
changed significantly over that time.
Another study undertaken to quantify the change in total lake
biomass (the surii total weight of all living things -- primarily
': •
vegetation) over time seems to indicate that the total bior,iass
has actually decreased in recent years. This may indicate that
the nutrient loadings (potassium, phosphorous, and nitrogen) of
these streams have actually decreased over this time. It also
appears that the stream sediment loads may have decreased since
the early 1970's as well. One theory for this effect is that the
net decrease in acreage of active farmland and improved
agricultural practices have reduced runoff and erasion from
.' agricultural land. It is presume that this reduction has been
offset to some extent by the increase in urbanization.
If this information is correct, then there would be little need,
for concern, over the net effect of stormwater -barn pollutants,
but this oversimplifies the problem. First of all, runoff from
'_.
urbar, lands contains several po 1 1 ut ant s-- hydrocar;bons, heavy
metals, and chlorides--that have not beer, monitored in these
previous studies. Second, the testing methodology used in these
Y studies does not account for the "first flush" phen�_�menom and the
`= fact that primary impacts of stOrrnwater pollutants are shock
s:•::
IL-lads that happen only over a very short period of t ime.
Furthermore, there are no established criteria for evaluating the
rt. impact of short but intense increases in p• I11,_itar,t levels. It
r stands to reason that pr -ll i,_itants that can, -it be tolerated for
long perii::lds of time might be tolerated at high levels for short
ar,lID IAnt s of time, and conversely that pollutants normZk 1 1 y h arr,l l es5
F
• at small c�: Incent rat l Ir,s car, be quite po l s 1nous whet, l nc 'eased.
i
s
JJ
a
i
® Until there is cant inuc lus monitoring of pollutant levels over -
time and some criteria established for assessing the potential
danger of those levels any data collected will be of questionable
value in assessing the magnitude of the stormwater pollution
prob 1 ern.
E�
In parts of the country where water pollution problems compromise
drinking water supplies and preclude the recreational use of
streams, lakes, and ponds it has been discovered that pollutants
carried by starmwater runoff are far and away the most
significant contributor to the problem. In Tompkins County our
concerns she uld alsca be to protect the beneficial uses of our-
waters. Nearly all of the waters in the county provide for some
recreational or other valuable use: fishing, swimming, wading,
wildlife habitat, car domestic water supply. It is fortunate that
pollution problems are few.
Fart of this luck can be attributed to the fact that Cayuga Lake
has an enormous capacity to assimilate pollutant and sediment
loads and perhaps it can handle ever, greater loads in the future.
But in local streams it may be important t� � actively preserve
present benefits. Enfield Creek, Taughannock Creek, and
Buttermilk Creek, as parts of the State Park System, must not be
allowed any degradation from pollution. Fall Creek., Six Mile
Creek, Cascadilla Creek, Lick Brook, Coy Glen and many other
streams offer the residents of the County many opportunities for
both passive and active recreation. Six Mile and Fall Creeks are
sources of the public water- supply. The Cayuga Inlet, Salmon
Creek, Virgil Creek and Fall Creek are some of the finest cold
water fisheries in New York State. And all of the smaller
streams have recreational and ecological values that should be
preserved as well,
There is no question that starmwater runoff contributes something
to the pollutant
and
sediment
loadings
of
creeks
and streams.
Although
and may have
public tolerate
Cayuga
the
Lake
assimilative
any increase
eventually
receives
capacity
in the pollution
most
tc r receive
of
of
these
more,
these
materials,
can the
creeks and
streams?
The question
is not
necessarily
cane
of
the magnitude
of
the problem,
but
whether
any
increases
in
pollution
of
these
important:
resclurces
is
acceptable.
Priorities:
Since there is no hard data to assess the magnitude of the
pol lut ion problem in the 'county a cant iclus approach to the latent
possibility of further pollution Of the Count s waters from
sti= irrnwater runoff is advisable. These general points she auld be
kept in mind in the eval uat ic-In of a development prc 1pc'sal.
J F�
0
Ell
•
•
LJ
1. Pollutant washoff from rcDads arid parking lots
add the largest cizimponent of pC it lutants to
storniwater runoff.
Industrial and commercial areas can present. a
potential for significantly higher
accumulations of pollutants than dr.
residential areas.
3. Municipal housekeeping practices: frequent
storm sewer flushing and street sweeping, and
limiting the use of salt for deicing, must
also be part of the pi::11lution abatement
strategy. The residential, commercial, and
agr�icultl_Wal use of pesticides, herbicides,
and fertilizers must be held to a minimum,
Legislation may not yet be appropriate, but
public education is certainly a valid part of
the pollution control program.
4. The placement and monitoring of proposed and
existing on -site sewage treatment - facilities
should be given due attention through gut the
County.
5. Best Management Practices for the control of
pollutant washoff are not prohibitively
expensive on most sites particularly if they
are designed in addition to other stormwater
runoff quantity or erosion controls,
Until more is known about the impacts of the pollutants that are
washed into local streams by runoff it would be prudent to
institute some controls for pollution abatement in the County's
most sensitive watersheds. The sensitivity to pollution of the
County's streams might be listed like this:
Priority In Taughannock Creek, Buttermilk Creek, and
Enfield Creek are important not only as recreational
resources but as the basis of the important tourism
component of the County economy.
Priority `:
The
public
water
supply must
be
protected,
especially
from
thlDse
p� �l
lutants
that are
not
easily
removed
with standard
water
treatment
practices.
Six
Mile
and
Fall,
Creeks should
be
protected
for
this In
and
the
fact
that
they both offer
marry
additional
,real
recreational
uses
to
the
residents of
the
C• aurit%r
Priority At a minimum, _lur fishable streams should be
protected from pollutant washl.-Iff f7rizimt industrial and
coril.merc i a 1 land uses.
5!
2
I
® Rric,rity_4: Many Of the smaller watersheds that are either
well remaDved from areas of active recreational use car that
drain, directly to Cayuga Lake and are only subject tcI large
lot residential development can probably accept the small
increase in pollution from this land use without appreciable
harm.
Erasion and Sedimentation
This report concerns itself with two major sources of erasion
closely related to stormwater- runoff. The first is the commonly
encountered erosion from development sites typically manifested
in the milk chocolate colored streams just downstream of any
development project during a go••d rain, and the second is the
natural erosion of stream channels which is greatly accelerated
by increases in runoff. Both of these problems are quite common
around the county, but they are not necessarily a problem in
every watershed.
Erosion from Construction Sites
Erasion
problem
benefici
Sediment
slowed a
sand, si
'typica11
of materials from a construc
where the ensuing sedirnentat
a1. use car puts unreasonable
at:ion typically Occurs where
nd lase the energy necessary
lt, and clay in suspension.
y means Cayuga Lake,
tion site is most clearly a
ion either precludes another
stresses on the environment.
sediment laden waters are
to keep the particles of
In Tompkins County this
The south end of CaUyga Lake has been slowly filling in with
eroded materials from upstream sources for tens of thousands of
years. Certainly man's activities have hastened this process,
but it could be argued that it will be an awfully long time
before we fill the lake in. Other alluvial landforms include
Taughannock Park Point (Taughannock Creek), Myers Point (Salmon
Creek), and Portland Point (Gulf Creek).
Further upstream, sediments f i l l in behind the constructed
impoundments at the Ithaca City Resevoirs and Beebe Lake can Six
Mile and Fall Creeks respectively. Eventually these sediments
must be rerii ved i t he t unct i can of these resevo i rs is to be
preserved.
But sediment laden waters cause more problems than simply the
filling in of lakes and resevoirs. Sediments ir, the water can
alter the light levels reaching the bottoms of streams and lakes
which charges the pattern and type Of vegetation that grows in
these waters. In turn, this effects Other plant and animal life
in these env i rc inment s. Sediments deposited aver- stream and lake
beds can destroy vegetation and animal life, ccIver fish spawning
® areas, and cctherwiso disr!:pt the water envircanrn4?rit as well as
J�i
make the
waters unpleasant
for
recreational
use.
In Tompkins
County
marry streams
run fast
enough
to
have, some
immunity
to
siltation
over much
of their
lengths,
but where the
waters
slow
enough
to allow depi.- Ist
il-Jr,
of
sediments
- -in paa -Ils,
eddies,
and
flat stretches-
- sedimentation
is
concentrated
and
can quickly
turn these
segments
of water
into
biological
wastelands.
One must also cons i d er that marry erod
stormwater runoff end up being deposi
in culverts and sewers, and in other
system. These materials decrease the
resulting in more sedimentation and i
costs of maintaining these storm syst
and sedimentation.
ed rnat er
ted in d
parts of
efficie
ncreases
erns are
i
i
r
i
als carried by
tches and swales,
the storm drainage
cy of these systems
in flooding. The
ricreased by erosion
Clay and silt particles can also serve as a vehicle for the
transportation of many pollutants. Nutrients, pesticides, and
heavy metals (Jeung, 1578, pg. 80) all have a tendency to adsorb
onto soil particles and thus become concent at ed in areas subject
to sediment deposition. Little information exists on haw these
concentrat ions may effect the ecology of the county's lakes and
streams., but the knowledge that these pollutants can become even
more concentrated in organisms further up the food chain would
suggest that some caution be exercised in this regard.
Stream Channel Erosion
. • The erasion and transportation of sediments is a natural function
of streams. The character of the stream's channel, banks,
t floodplain, and the biological community that live within this
one are adapted in response to the natural fluctuations of
stream flaw working on the soils and underlying bedrock over many
hundreds o fir' thousands of years. In all cases, this adaptation is
largely a manifestation of the steepness of slope and the
durability of the underlying geology which determines how the
erosive processes will farm that stream's character. In Tompkins
' County these physical conditions are highly variable ranging from
waterfalls cascading through steep walled gorges cut into solid
rock, to meandering flows lazily traversing a bread floodplain.
As discussed previously, research on stream channel shape and
size has been-shown to be directly related to the quantity. of
water passed by the channel from a storm size that one would
expect once every 1.5 years. But this relationship between
channel size and stoorm flew is not universal and is generally
applicable only to streams that are hydrologically "old" with
meandering channels and well developed fly- „odplains. In the more
urbanized areas of I c- lrnpklns County streams do not have these
characteristics. Stream channels in the hills surrounding Ithaca
have steep sided valleys) channel characteristics determined by
bedrock geology rather than energy dynamics, and high gradients
• rn -Y'e Char`aCter'lst iC of "y,- +uriq" ea111S.. These channels are
0
I
LI
N
r
E
virtually carved in solid rack, have nc, fli Ddplain, and are
therefore riot subject to the same changes in channel shape or O
profile fry rn increased flows reps art ed in these other studies.
At the same time there are portions ions of these same creeks, further
upst rears., where small increases in the erosive farce of the water
i can cause trerner,do�_ts increases in erasion of the strearnbar,k..
This phent- amenorn is very cornmor, where the strearns rnake the
transition to the "y ,_ng valley below fry ern the "old" flolzidplain
above. It is in this in- between zone of the strearn profile where
many strearns gain most of their sediment load. For instance,
there are several points along Fall Creek where strearn meanders
are actively cutting into unstable slopes, contributing huge
amounts of sediment to the streamflow. Erosion control in these
cr i t i ca 1 1 Deaf
ions ins i s necessary to protect downstream darns frarn
! siltation and Upslope roads and structures frarn undermining.
j Municiapalities spend many thousands of dollars installing
gabions and ripr,ap along streambanks to control these problerns.
Many of the smaller- strearns that would change considerably from
increases in runoff due to development d• � riot run year - round.
When water levels fluctuate between raging torrents in the spring
to muddy sloughs in the surnmer it becornes difficult to say that
the effects of development will cause this environment to suffer.
Intermittent streams present a very different habitat than do
perennial streams and it therefore may be questioned as to
whether the same management practices to protect environmental
values are appropriate in each case. Certainly the public 40
benefits presented by intermittent strearns are less obvio=us than
those of perennial streams and for that reason are of less
concern.
Priorities:
Erasion frarn construction sites is universal and causes problerns
all the way down the watershed. This should not be tolerated
anywhere :in .Tornpkins County. ,Proper- land management and erasion
control is not expensive nor- difficult to achieve. For the most
part, there is little or no regulation of construction activities
to reduce erosion and subsequent sedimentation in the County.
Local contractors are often unconcerned with erasion and have
knowledge of only the most basic control methods. As long as
comprehensive erosion control plans are riot required of the
developer, erosion will only receive attention that is both to
little and too late. In terms of total benefit to the public,
erosion cc�r,tr,c•1 legislation may have a better payback. than
st Orrnwat er control. Erosion is better understood, easier t
cont ro17 and easier to � administrate than is runoff. Runes off
control legislation without erosion control is extremely short
sighted. As Has beer, painted out, all runoff contr Il practices
are negatively impacted by ergs i
0
The problem of accelerated streambank. erosion from increases in
storrawater flows is not commonly recognized in Tompkins Cclunty.
Where it is a problem there are two � alternatives for mitigation.
One approach is to prevent increases in flows from smaller, more
frequent storm events capable of creating bankfull flew
conditions by detaining storm flaws upstream. The other
alternative is to stabilize strearnbanks at paints of high erasion
potential.
The places where streambank. erosion is causing problems in the
county do not generally coincide with areas of intense
development. Therefore, it would be difficult to blame these
problems an upstream development and it would be unlikely that
st orrawat er controls Used to maintain the natural flow cc end i t i c in
would lessen the problem. More effective management of the
problem would be to use structural measures to reinforce specific
problem areas although this is far from a perfect solution.
Stream channel erosion is a natural phenom� �n cim that can be very
difficult to control.
For Tompkins County the most effective method of lessening the
impacts of development on streams is to maintain a natural buffer
area of vegetation between development and the stream channel.
The width of this buffer is dependent upon the intensity of the
development and the sensitivity of the stream corridor, but
generally standards are in the vicinity 9 Y acce ted P y of 20 to 100
feet (although the argument for a comprehensive stream corridor
R,
' protection program as proposed by Faster (1985) should be
=' carefully considered in lieu of an arbitrary setback.). The
County's largest streams have some protection offered by
t t
floodplain regulations that limit development within the
';'. floodplain, but these laws dc• little to protect this natural
' buffer. Development proposals on smaller streams may or may not
' be subject to review by the D. E. C. , but, at this time, such
review is limited.
The evaluation of the effect of any development on stream erosion
must be based on considerations of the effect of that development
on the flood hydrograph of the stream in relation to the stream's
physical parameters of profile and geology that influence its
capacity for erosion. Once again, the effect of that development
has will be much greater in a smaller watershed basin. Sma11
perennial streams with earthen banks in areas Linder- intense
development pressure are most likely to suffer from degradat i cin
due to channel erasion. In Tompkins County this is not a common
Situation, but the protection of all stream corridors from
development activities is a worthwhile g� gal.
Groundwater recharge
Although there are marry residents in the eraunty whc� er. per ience.
•problems with their well water supply, the evidence that
E' 1
•
constructed improvements limiting infiltration of runiff is
caUsing these problems is hardly convincing. In most parts cif
the county the agricultural, engineering, and standard
construction practices used to intercept and direct shallow
groundwater flows to surface conduits could be shown to have a
much more pronounced effect on groundwater recharge than does
building and paving.
---------------------------------- - - - - --
Widespread problems due t. � excessive runoff from developed areas
do not exist in Tompkins Co=unty, Indeed, there exist potentials
for proble =ms, and their are problems that may exist that have not
yet been discovered, but without distinct problems, the
formulation of appropriate management goals is not a simple task.
Efforts to control erosion and sedimentation and efforts to limit
Pollutant discharges in our lakes and streams have clear and
tangible benefits. Efforts to control flooding, lesser,
strearnbank erosion, and encourage groundwater- recharge are riot s•
clearly beneficial. Therefore the highest priority for the
county storrnwater management program is to do the research
necessary to determine the true magnitude of the runoff related
problems experienced in the county and develop the correct
management program from those findings. Because extent of the
runesoff problem in the county is not great there is time to do the
needed research and analysis before problems do get out of hand.
C�
I
• CHAPTER THREE: Stormwater management regulation in Tompkins
County
•
k ;c
:t
+FL
Current regulation in Tompkins County
The
present
status
of
erosion
and
drainage
regulation
in
Tcompkins
County
is difficult
to
pin
down since
little
of
what
is
enforced
is actually written dawn. In iz order- to participate in the
Nat it.- final Flood Insurance Pr• igrarn, a municipality must adopt
certain res
fl oodplain.
and the Cit
these parti
have a dist
their town
tricti
At t
Y of I
cu1ar
inct
1 aws.
ons to control development
his time, only the Towns o
thaca participate in this
restrictions only the Town
et of written drainage reg
within the
f Ithaca and Dryden,
program. Other than
s of Dryden and Ithaca
ulations anywhere in
In the Town of Ithaca, Section 26 of the Subdivision Regulations
states that drainage improvements must be approved by the Town
Engineer with minimal runoff computed by the rational method with
a rainfall rate of .5 inches per hour. As stated, this comes out
to be the equivalent of a two -year, one hour storm. This
regulation also contains a few sentences that require
construction plans if a diversion of the natural stream channel
is planned. Furthermore, it states that detention �n faci 1 it ies or
ether structural measures can be required where the average grade
exceeds 7% (which is much of the Town) to control the velocity of
runoff flows.
It is not uncomriuin to find that the written regulation differs
considerably from actual practice. Presently the Town of Ithaca
Engineering department policy is that the runoff characteristics
of new development shall have "no increase in runoff per unit
time ". In other wards, storm drainage for new development must
be designed to prevent any increase in rate of flew at any point
of the hydrograph - -a much more difficult engineering feat than
simply limiting the peak rate of flew. Drainage plans are to be
based an a design storm acceptable to the Town Engineer with
calculation done using either the Rational or SCS methodology.
If uniformly enforced, this requirement would necessitate the
construction of some sort of retent itDr, or infiltration structure
ors every development site. Arid although this regulation is quite
strict in terms of what it requires of the developer it does not
directly address the problems that it attempts to mitigate. Thus
while this regulation is effective at achieving the ultimate
management goal., no alteration of the natural runoff cored l t l On,
this achievement is realized c only through considerable expense.
Arid this expense may preclude the achievement f equally
important, but rn<<t as carefully legislated, planning goals sUch
GJ
A
4,
as the clDricentrat icir, rif develc iprnerit to � 1 imit the exterisi�
-r. c 1f
® municipal Services and the ericcluragement of affordable hclusing O
develc ipment. The current regulatory stance in the Town is due
partially fr-cm pressures exerted by the City of Ithaca
diDwnstream. The City stormwater system is subject to--, fl 1odi-rjg
from relatively mine it storm events and simply cannot accept
additional ri_inc iff from further develc ipmerit upstream, At this
time the Town and City of Ithaca are in need c 1f a ma.3. �r
storrnwater planning effort sca as not to:, c r�ipr�_�rnise � ther planning
goals with shortsighted attempts to address the rurl'Dl T- issue,
r--
In Dryden, the Subdivision Regulations ins require that new drainage
facilities be sized to acco ri mcidate the "5 year stclrm event based
on a potential ial for increased runoff from both the proposed
development and the remainder of the upstream watershed under a
hypothetical fully developed condition. In addition, a study
must be done to evaluate the effect con do �wristream drainage
struct1_tres. If the increase in runoff fri.-Im the proposed
development would overl� mad any downs
trearn structure during the 5-
year sty terra event, the project cannot proceed until the Town BC ,ard
resolves how the undersi Zed structures will be replaced.
By itself, this regulation should provide adequatre protection
from the effects of localized flooding. The requirement that
structures be sized for the maximurn potential development in the
watershed, if followed, would result in developers downst rears
incurring costs for drainage structures to serve futi_tre
development upst ream, which doesn't t d i st r i but e these costs
equitably, There is also a potential problem when the
overloading of downstream structures does occur. It is unlikely
that the Town Board is going i ng t o bend aver backwards t.c l resolve
downstream problems so that upstream development can proceed.
What typically seems to occur is that the develcipment proposal is
mcadified or withdrawn once a discussion of extensive downstream
improvements is triggered. Over the short run, the paragraphs ire
the Dryden Subdivision regulation dealing with drainage are
fairly effective in offering some protection to both the
developer and the Town if properly administrated, but address
only the issue of localized flooding,
Since the Town of Dryden does not have a municipal engineering
staff, the ordinance states that the Soil Conservation Service
will review the develc iper' s plans. In reality, the SCS hasn't
actually done any review of plans for the Town; review is done by
a consulting engineer. However, in the past three or four years,
the regulaticon actually practiced has evc -11ved into this: n 1
increase in the peak rate of ri_rn if f,. based _on the `51 -year event.
The Town's er,gineerir�g c= r�st_rltant. wants to see the SCS TR -5E
method used for the determination of r�_�no1 -f and has required that
structures be designed Fr-ir larger storm everts when prudent,
r � - I=, rate _ t r, _l rl - t
I;11E i;`Ir r'rllc +l r'i;yti rl� rl_ ir)C) ^ear` .rl L (_ r _a „ t
'vJl ..�1 clriaiJ i ba_� -ed Orl the `J -year' cVerl� DV LIE• .._,1.,� fllBtil1=1C 15 a
C'1- 11IlrllC'rl UrlL+Jr�1tter1 law thro1_tghci1_tt the C1- 11_trlty. :r1 add1ti_rl tct the
I 4_� Dr'Vder, The Town ! � i I �rl <dCa, '�i-Ie � �_�WrI �_� I Lansing, arid the
IIaq_a csf Lansing all share similar' batS1C 'regl_tir,onlerlt5. But
Lei ler-e 0%s these rlll_niclpali"Gies Share a basic regl_tl'r,erllerit for the
initial )"eView -if d-rainace. plans, what is acceptable tcl the
adrllInist- )"atclr' i -i file r'eriu1aL ICgrl Var1eS CC,rlS1i_e'r' ably TrCtrn GY -•C (.]E•Ct
ti-I pl''C eCt even within -1rle rn(_triiclpalit` /. ire 1rlt2r'V1eWS with
°eVera 1 pr'aCt it i hers there were cc(rornon c Ertl p 1 a i rlt -f St cir'rnwat e'r
roanaoernent C1Ver'k i 11 Where Y eV I 2 1 rig erlc i veers made unreasonabl e
derndYlGS ore pi ^lzi j ect's which in scime cases made prCi j 2iCt S 1_tnTeaS i b l e.
Other -;b ject icIr)s were raised where the reoqui) ^ernerlt=_. for Stor'nlwater^
cCrltr 1 were applied blindly with 1_tt arty c nsiderat i -n f -r the
fact that stnrni? Dry j ect S were so small l 1 r' Sit 1_lat ed in SilCh a Way
that the impacts of incredSed f1ciWs w1--luld be next tc, ncithirlg.
On the ether
side
Cif
the
c• -lire,
reviewing
erlgineers
ccinlplain
of
inadequate
dr,airlage
rep
lets,
disregard
of
potential
pr,oblerlls
beyond those
and the
fact
"officially"
that
there
areril
rec1-
ignited
t
any
by
cornrnciniy
the
written
accepted
r-ect_tl�ltiClrls,
starldar�ds.
Same cif
these
erlgineers
a1sc,
expressed
diaccirnfC11
t
with
the
rclle
of establ
ishir,Q
p!
l
icy
for
the
rnt_tnicipal
ity
- -a
concern
IF
-1
potential pr�(ifessivrlal liability.
In the rest Df the ccit_tnty there are few r,egl_tlaticlns and no meads
tC enfC rce the regl_tlat iclns if they existed. Ire these sitl_tat ionr7
drainage is almost entirely the respi- Jr,sibility if the develClper
and is cclnsidered and built ciri the basis of regiclr,al k:rlctwledge
and seat of the pants design. This is rlc it necessar i 1 y
irlapprropriate since cc irlr)lc ire sense will usually result in
re�� =_.C1nable drainage practice for lcicai ized prevent iorl of f1ciciding
where land values are not scl high as t1-_1 preclude gc od j I_td genlent .
There still exists sor)le cclncerrl for^ eras i Cln and pci 1 1 I_tt arlt
washcITT, but in mare rural areas municipal experlditurt e ti--1
administer the regt_Llat icin of these impacts nlay simply be t� ct
great. This is where a case ccll_tld be made that the rreq_t_tlaticln of
=_.tornlvJate'r, and erosion be adrnirlister-ed at the cc il_trlty level of
government.
New st C irrnwat er marlagernent reg I_t l at i l Drls are being cans i dered
thr'ol_tghc -l1_tt the cclurlty lrl eTTor'ts b% individual municipalities t0
Torrnal ize their prevIC11_tSly 1_lnwrltterl pc isit ions and tc, clarity
exactly What will. be r�equir ed of the devei! 1pe'r'. TcI this end, all
the pdr't lE'S CCtrlcer`ned =_upper't S Ulle SOr't oT regltlat i! �rl. Fctr'rna1
p E• r T 1_rt` rn 0A n c e, s t_t b rn i t t- a l, and r e'1 1 e W r',_� g l_l l r c rn a Tel t e- s i_ I_l l d it (.21P t
�tl ^�,_irnll Tel e tilC' perfliltting prCCedl_iY'e a)'IC w -_�1 Cie app)'eciated ny
Lo i % h the i 1 e - _� o n -. - t-• s. ;� :^ e g 1_l i. a t i 1_1 n =.
tie devel_F f drld the r'f_,ill_.L'J1 erl0ir!C• T��e _
cif C� CST Ter t -true lc =gal pi, e o t tor, t the arld the
•
E,
t�.
rrlr_lnicipal i.t_; by clar%if_: irrc -resrorrsibi 1 it ies
® standard acai'rrst WI- i c h oerformance can be
B11 stating rer _ lr enlents l front, r i rE drainage
(apprc1priate1y) cor•,sidered at the 1_-1rlset
a arr a-ftorth1- 1r_lght.
Fr_ltr_lre regulatir.in in Tompkins County
aYld est
accurate
issues w
the pri -r.J
ab
1 _✓
il.
ec
1 i s 1-1 1 r'r g
nleasr_tred. o
1 be
t instead -f
i't this time ti -re Towns of Itl -raca, Lansing, and Dryden are in the
process C1 c= C'risldeY`lrrg new st- irrrlwater rnanagernent regr_llatl ns.
The (-own -it Ithaca, in the middle -f revlSing the I twn (ylaster
P11 an, is (_ r'�_ider'lrig the ado: ptit -- G1 a separate stoY'rrlWater
ordinance ar i g i rra 1 1'y draft ed ' by Lancaster County in Flenns_y 1 van i a
that wor_lid effectively require all devel opment t1_1 detain the 10-
year design st - irrrl. The Ti Dwrr 1f Lansing is looking at a simple
add it i rn tizi their subdivision req -ullat it -rrrs that would require that
drainage reps art be filed a-rid that c insiderat i in be given tt -, the
d[ Dwnstream effects -11" the i -5 year st, 'rrrl. The T Iwrr -Jf Dryden is
in the process -.if revising their rrlas•ter, plan and their land use
r eg r_l l at i rrs. M 1st 'recently, the Dryden Planning Beard pre=sented
a p'r,OpOs81 t• I institute 5 -acre zoning in a large pe rt ion of the
TIDWrr fctr the pr_lrpcise zir mitigating runiziff and water supply
problems (Debt -J, 1963()) . The recent implementation c1f a separately
considered "Site plan Review" process in the City of Ithaca, s• ion
to be f• rl lr -Iwed by the Ti -1wrrs of Ithaca and Dryden, will alsizi fl Doss
rocnre attention r -in the rr_lrn_iff issue.
In April 917 this year the Department of Environmental
- Cclnservat i orr released its "St _rrrrlwat er Management Guidelines f it
New Development ". Although these guidelines specifically state
that they "are not fixed and inflexible rules tiz, be
appl ied.:. with r_lt c• insidering the particular facts and
circumstances of a part icr_llar project" (New York State,
Department -if Environmental Conservation, 199 Cr pg. G) , i t i s
likely that marry municipalities in the state will adopt these
guidelines as standard practice. These guidelines are c•- incise
and give a thorr.lugh t Dverview cif cl-. 1rrtemporar,y stornlwater
management practices and how they might best be effected, but
these guidelines dr., not address the need f1 -1r watershed -wide
st rrnlw_t_er. _ rn�rna ernerrt planning., rrc-Ir d� � these guidelines address
fundamental municipal concerns: administration, en.for,cerile-rrt,
Sr_lbrrlittal requirements, sarety, and maintenance. Ft-.r communities
to adCopt these gui.del ines• without ss :xrle cr insiderat icin cif these
1 -1ther issues is inviting a h� ist 1 -i rutr_lr ^e pry rblerrls. It iS
irrstrllctivcm- t 1 eXarrline the pC,Ssible scenaricis that alight devel1 -10
if this -�r the 1ther^ or.ipt -sped drainage regulations were, ad pted.
•
i