FENWICK AREA DRAINAGE STUDY
INTRODUCTION
Background
The
Village of Hastings-on-Hudson has been plagued by flooding problems in an area
east of
In
view of a continuing controversy over the need for and extent of improvements,
the Village governing body sought technical help through the issuance of a
Request for Proposals in May 2002 (see Appendix A). This was followed with further requirements
in a document issued in September 2002 (also Appendix A).
The
accepted proposal was presented to the Trustees in July 2002 (see Appendix B),
and was followed by a supplementary document in September 2002 (also Appendix
B). A presentation was made before the
Trustees in September 2002, which established the intended procedures and
technical investigations which would ensue as the “modus” for the study.
The
challenges of this study have resulted from the following controversies and
concerns:
a)
Complaints from property owners heavily damaged by
localized ponding, versus the lesser concern of property owners that are
minimally disturbed during similar storms.
b)
Major concerns by many property owners about
potential environmental damage that might be caused by significant stream or
culvert improvements.
c)
Apprehension by taxpayers over the incurring of large
capital expenditures for localized problems.
Description of Area and Its Problems
The
affected area lies in the southeasterly portion of the Village (see Plate
1). In general, this is a fully
developed residential neighborhood, adjacent to major thoroughfares, such as
Ground
relief is considerable, with changes from Elevation 107.0 near the
The
waterway, and the affected areas, are localized (see
Plate 2), yet there are portions of the drainage area extending into the upland
slopes for almost ½-mile above the most severely flooded locations. The waterway itself is an open channel for
the most part, with culverts only under roadways. At the source, near
Reports
indicate the following flood conditions (see greater detail in later sections):
Depths of ponding to 30 inches in roadway, basement problems, overland
flow through yards.
Yard flow, and a reported 12-inch depth at the low
Wide overland flow through yards, with depths to 6 inches, minimal
flooding at
Flow confined to culverts, minimal
damage.
Over-bank flooding in undeveloped
area owned by Village.
Stepping
Stones Area –
Overland
flow through yards, surcharge from dam’s backwater curve, (and from channel
inadequacy), and some basement flooding.
Stepping
Stones Dam to
Minor nuisance,
some driveway flooding.
* * * *
As will be developed in later sections of this report, the area in
greatest need of relief lies at the low
Prior Studies
Reports
on the area’s problems have been published for almost 20 years.
In
October 1978, a Flood Insurance Study was prepared for the U.S. Department of
Housing & Urban Development, Federal Insurance Administration. The only waterways given detailed study in
the Village were the
In
June 1985, the District Conservationist, James B. Cropper, provided a brief
report (see Appendix D) to the property owner of Stepping Stones. He noted the overland flooding, the
submergence of the driveway, siltation in the pond behind the dam, and erosion
of the banks. He pointed out the deleterious effects of upstream development
and waterway enclosures. Solutions were
presented in the form of: a) deepened channelization, to a 12-foot width and
3-foot depth, with gabion bank walls; b) a culvert of similar dimensions
under the driveway; and c) lowering of the dam to accommodate the 100-year
flood condition. Pragmatically, the
author stated that “the cost of all this…to save the pond may be more than you
wish to bear”. Also,
that the property owner must weigh “how important the pond’s visual appeal is
to you.” No action was taken
subsequent to this report.
In
November 1986, a letter report was prepared by Hazen and Sawyer,
- Regrading of
the Ravensdale /
- Retention basin in
3.
Culvert and channel improvements downstream of
The report also indicated that an
assessment program could be undertaken to offset a portion of the program’s
capital costs. The suggested measures
were not undertaken.
In
1997, a series of letter reports were issued by a White Plains engineer, Mr.
William H. Bruder, P.E. (see Appendix D), which
concentrated on problems at Ashley Road.
Under his direction, the blockage and partial collapse of the existing
stone culvert was repaired. Also, a sloped
grating was placed on the upstream side of the roadway, to reduce the entry of
debris into the culverts.
Surveys Performed
Frequent
field forays were conducted for this study, to examine existing facilities,
environmental conditions, evidence of flooding, utilities, tree identities, and
for visual evaluations of reports by others.
Instrument
surveys were used to establish roadway elevations, topography in
Existing
mapping proved to be of variable value.
Tax maps and the zoning map were helpful in establishing property lines
and saturation development. However, the
existing drainage map, and the sanitary sewer map, both lacked detail and were
incomplete in several locations. The
eventual release of County-prepared topographic maps proved to be of most
value. This mapping provided excellent
base maps (see plates), and permitted a reduction in the detailed surveys
originally contemplated.
Input
from the Department of Public Works provided identification of many utility
locations. Superintendent Michael
Gunther was most cooperative in this endeavor.
With his aid, and with pavement markouts by the utility companies, the pertinent
pipelines were located and identified.
In several locations, culvert conditions were visually examined for
obstructions or collapse.
Soil
borings were obtained at critical locations along relief sewer routes, or at
important intersections (see Plate 12).
Of
most value, however, was the input from local residents. On three occasions
(see Appendix E), groups of property owners appeared on site (frequently in the
rain) to advise as to observed flooding conditions, property damage, culvert
maintenance, and environmental concerns.
Through these voluntary contributions, much information was obtained,
lessening the need to make observations during storm conditions. Further, the expressed opinions were helpful
in balancing the varying needs and concerns of each neighborhood.
Study Limitations
It
should be recognized that the purposes of this study are to identify the
optimal solutions for the flooding problems of the area. Field data is of an accuracy to permit
comparison of alternatives, and to evaluate construction variables for estimating
purposes. The results are not intended
to be sufficient for design purposes, but only to provide criteria, costs, and
preliminary sizing.
Mapping
also presents limitations. The 5-foot
contours on the County topographic maps permit alternative comparisons, but
would not be sufficient for design or construction purposes.
Cost
estimates must also be viewed as study level values. To provide a level of comfort, contingency
allowances are made (see tables), and prices are projected forward with an
allowance for modest inflation.
Agencies
Contacted
In
addition to local and County contacts, the following
offices were contacted:
Soil
Conservation District (
For
information on soils, permit requirements, and file data.
For wetlands data, environmental requirements (SEQRA), and guidance.
For wetlands data.
For highway data, drainage
information, etc.
TECHNICAL EVALUATION
Existing
Conditions
Observations
and reports (Appendix E) indicate the following storm related conditions:
Along Fenwick Road (See Plate 3) -
Water coursing along The Fenway, along Branford Road, through Dan Rile
Park, and from roadway areas to the east, all collect at the low point on
Fenwick Road. A swale located across
Branford Road (See Plate 3) – Considerable flood water enters
Branford Road from the Fenway area, collects at a low spot opposite #40
Branford Road (Leviss) then courses across this same
property and through the rear of other properties until it reaches the
stream. There are no curbs to control this
flow, and a
Ashley Road (See Plate 4) -
The culvert under Ashley Road is a 3 -foot x 4-foot box culvert which
was recently repaired, and which is guarded by a trash rack on the upstream
side which requires frequent cleaning.
No significant flooding is reported at this road, as long as the rack is
kept clear. (It was noted during
inspection that a 6-inch overflow pipe
connects the box culvert to a sanitary sewer; this illegal connection should be
dye tested, and plugged if necessary).
Ashley Road to Farragut Avenue (See Plate 4) – Some
flooding is reported along the westerly parts of #2 Ashley Road (Jackson), but
there appears to be little significant damage, except for occasional basement
flooding.
South of
Farragut Avenue
Stepping Stones Area (See Plate 5) - Flooding occurs across the Stepping
Stones driveway and to an area immediately north, in part because of an
undersized 36-inch culvert under the driveway.
In addition, there is flooding from the driveway south to the existing
masonry dam (reference report in Appendix D).
There is also considerable siltation of the pond area north of the
dam. Flooding is reported in several
basements along the west side of
South of Stepping Stones (See Plate 5) - The stream below the existing dam is
narrow and deep, with limited capacity.
However, flooding is localized in areas immediately adjacent to the
stream banks. A small shed forms an
encroachment over the channel in the rear yard at
Below
Drainage Areas and Flows
The
small stream that is the subject of this study is fed by a significant drainage
area (see Plate 1). To the north, the
area extends more than ½ mile to
At
the
Drainage Area (acres)
This
Study * Hazen & Sawyer Conservationist **
Stepping
Stones 181 158 174
Discrepancies
in the reported drainage areas may be the result of differing contour
information, storm drain routing, or better data available to this report. However, the discrepancies are not beyond the
normal degree of study error, and the data used in this report is clearly more
conservative.
For
this study, estimates of storm flow were calculated using the broadly accepted
criteria established by the U. S. Dept. of Agriculture, Soil Conservation
Districts, in their Technical Release #55 (TR-55), published in 1975. This booklet was used in conjunction with
published Soil Maps (used to determine soil porosity and the degree of surface
runoff), and the Village’s Zoning Map (used to indicate types of area
development). There was also the need to
observe density of development, impervious areas, and the extent of grassed or
pervious areas.
A
tabulation of flow data is provided (Table 1) for the critical points along the
waterway, and for storm frequencies of 2 year, 5 year, 10 year, 25 year, 50
year, and 100 year. As an illustration
of the data, the reported flow at
For
comparison with prior studies, the following tabulation is presented for a
10-year frequency storm:
________Peak
Flows (cfs) _____
This
Study Hazen & Sawyer Conservationist *
Stepping
Stones 354 510 375
These discrepancies were discussed
with the author of the Hazen & Sawyer report (D. Walrath). In his opinion, our assumptions are
reasonable and the results may be accepted for purposes of this study.
* * * * *
*
*
Note that better mapping was made available for this study,
than could be obtained for the other studies.
** A further comparison of interest lies in the estimated storm flows for a 100-year frequency event. The Conservationist provided a flow estimate of 660 cfs at the Stepping Stones property, which may be compared with the estimate for this study of 674 cfs.
Basic to these studies is the
perceived need to divert extraneous flows away from the seriously impacted
areas. Therefore, the following remedies
are included in all of the following evaluations:
a.
Flow along
b.
Flow along
c.
The existing pavement swale at
Hydraulic Gradients
Hydraulic modeling of the area was
accomplished with the aid of stream cross sections, culvert data, roadway
elevations, and field observations. This
information was the raw data for computer modeling using the widely accepted U.
S. Army Corps of Engineers (1979) software, developed by their
The basic assumption in this study,
for existing channel roughness, was an “n-value” of 0.05. Other factors were used to offset the lack of
precise contour information and detailed cross-sections. The final model was successfully tested
against empirical data reported for peak storm flooding, starting at a reported
water level in the
ALTERNATIVES STUDIED
Basic Criteria
The
basic concern, of course, is the reduction (or elimination) of flooding and the
resultant damage to property, inconvenience to the traveling public, and
hindrance to emergency vehicles.
Depending on the storm frequency, damage in the pertinent areas varies
from very light minor ponding… to impassable local water depths, side yard
flooding, and basement seepage.
Therefore, solutions must vary with the area under study.
The
existing channel capacity is inadequate for all but the lightest rainfall. The open channel cross-section is minimal,
and the stream banks are overgrown in many areas. Typical (average) cross-sections are as
follows:
Above
Above
Below
Below
Above
Stepping Stones dam….. 3.0 ft. wide, 3.0 ft. deep
Below
Stepping Stones dam….. 5.0 ft. wide, 5 to 6 ft. deep
Basic
to all approaches is the need to minimize flow entering the basin. To this effect, flow along
To
the extent feasible, consideration has been given to solutions which would
provide flood reduction through the use of relief storm sewers. Routing of storm flow around the most
severely affected areas was planned, and then evaluated as to economic
feasibility.
Maximum
and minimum projects are given consideration herein, phased wherever
possible. The final evaluation gives
consideration to the physical, financial, and local acceptability of each plan.
During
development of this study, the following essential criteria were found basic to
the project’s feasibility:
a)
Project costs must be kept to a minimum, considering
the Village’s fiscal restraints and taxpayer concerns.
b)
Deep local flooding in
c)
Damage to in-situ landscaping, trees, and even
natural underbrush, must be minimized or avoided.
d)
Downstream flooding must not be worsened.
e)
Objectionable yard flooding should be reduced, and
basement flooding diminished.
Channelization
The
Request for Proposals (Appendix A) included an evaluation of channel
improvements.* Accordingly, the following design
criteria were established, assuming:
Channel dimensions –
Trapezoidal
(T), grassed, with side slopes of 1:2 (B = Base Width, D = Depth) Rectangular
(R), gabion
walls (B = Width, D= Depth)
Channel
Slope expressed in feet
per 100 feet of length.
Roughness coefficient
assumed at n = 0.025
Storm Flow Criteria
Selected
for 1-year, 10-year, 25-year, and 100-year frequencies.
|
Reach |
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Storm Frequency |
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1-Year |
10 Year |
25-Year |
100-year |
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T |
R |
T |
R |
T |
R |
T |
R |
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B = 0 ** |
B = 2.5 |
B = 2 |
B = 5 |
B = 3 |
B = 6 |
B = 4 |
B = 8 |
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D = 1.8 |
D = 2.6 |
D = 2.4 |
D = 3.1 |
D = 2.5 |
D = 3.2 |
D = 2.8 |
D = 3.2 |
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(Slope = 2.94) |
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B = 0 ** |
B = 3 |
B = 2 |
B = 6 |
B = 3 |
B = 8 |
B = 5 |
B = 10 |
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D = 2.1 |
D = 2.8 |
D = 2.6 |
D = 3.0 |
D = 2.8 |
D = 3.0 |
D = 2.9 |
D = 3.0 |
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(A10Slope = 2.75) |
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B = 0 ** |
B = 3.5 |
B = 3 |
B = 8 |
B = 4 |
B = 10 |
B = 7 |
B = 12 |
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D = 2.2 |
D = 2.7 |
D = 2.8 |
D = 2.9 |
D = 3.0 |
D = 2.9 |
D = 3.1 |
D = 3.2 |
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(Slope = 1.54) |
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B = 0 ** |
B = 3 |
B = 3.5 |
B = 8 |
B = 4 |
B = 9 |
B = 6 |
B = 12 |
|
Steeping
Stones |
|
|
D = 2.1 |
D = 2.8 |
D = 2.6 |
D = 2.8 |
D = 2.8 |
D = 2.9 |
D = 3.0 |
D = 2.9 |
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(Slope = 2.67) |
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Stepping
Stones to |
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B – 0 ** |
B = 4 |
B = 6 |
B = 10 |
B = 8 |
B = 12 |
B = 10 |
B = 15 |
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D = 2.5 |
D = 2.9 |
D = 2.7 |
D = 3.0 |
D = 2.8 |
D = 3.1 |
D = 3.1 |
D = 3.2 |
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(Slope = 1.77) |
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Examination of the above
requirements, even for a 10-year frequency, by comparison with existing
conditions, indicated the following:
1.
The needed size to carry design storm flows would
exceed existing channel dimensions even for the 2-year storm. This would require extensive excavation and
channel re-shaping.
2.
Construction activities would require access widths of
15 to 30 feet.
3.
The environmental damage would far outweigh the gains
in capacity.
Accordingly,
the alternates chosen for study were retained largely in the public
rights-of-way, and not through developed private property.
* Costs of channelization can be furnished if needed. But the general use of new channelization is unlikely.
** Triangular
Selection
of Design Frequency
As originally proposed,
consideration was to be given to the 2-year, 5-year, 10-year, 50-year, and
100-year return frequencies.
It was quickly realized that
planning for the extremes would not be fruitful. At the 2-year and 5-year levels, the cost of
infrastructure would far outweigh the benefits derived. There would be little effect on storm
frequencies exceeding these levels.
At the other extreme, costs for
50-year and 100-year infrastructure were found to be excessive. Indeed, the pipe sizes at certain locations
could not be accommodated without major utility relocations.
Detailed studies were first
performed using flows at 25-year frequency.
However, construction and capital costs were found to be
prohibitive. Therefore, the major sizing
of drainage pipes was limited to a 10-year storm frequency, with 25-year
capacities only at culverts. (It should be noted that larger storms would
surcharge the system somewhat, but conditions would be much improved over
existing conditions.)
Alternate
This
initial plan included the following basic elements:
a)
Curbing along
b)
Ponding in Dan Rile Park
c)
Relief storm drain along
d)
Partial removal of wall at
e)
Additional drain piping through the side yard of
f)
Enlarged culvert across
g)
Curbing along
The
primary intent of this plan would be diversion of major storm water along
During
lesser storms (say 2-year to 5-year frequency), runoff would initially be
stored in a detention pond behind the proposed berm (see Plate 3) in
With
increased storm runoff, flow would pass over a side overflow weir, into the
relief drain, discharging at
To provide
capacity for a 25-year storm, the relief sewer would have to be 54-inch
diameter reinforced concrete pipe. As
depicted herein (Plate 3), the bypass would be 42-inch diameter, to handle a
lesser 10-year storm. Borings indicate the presence of rock at 7-foot depth
(Plate 12), which adds to the cost of this approach. Energy dissipation would be accomplished with
a drop manhole upstream of the culvert.
To reduce
flooding in
Inlets are
planned, under this approach, at the intersection of The Fenway and
Costs
under Alternate A are estimated at $484,200 for construction, and $604,600 for
capital costs. (Table
1A).
Alternate B –
Under this
alternative, a comprehensive plan was developed to include solutions for all
known areas of flooding damage. The
elements of Alternate B include the following:
a)
Curbing along
b)
Ponding in Dan Rile Park.
c)
Relief drain along
d)
Partial removal of wall at
e)
Additional drain piping through the side yard of
f)
Enlarged culvert across
g)
Curbing along
h)
Berm construction at the south end of the municipal
property, creating an impoundment north of Stepping Stones. (Plate 8).
i)
Bypass storm drain along
This comprehensive plan would prevent
ponding at
Operations at
Costs under Alternate B are estimated at
$1,040,400 for construction, and $1,294,600 for capital costs. (Table 1B).
Alternate C – Minimum Project (ref. Plate 10)
This alternative was
developed with the view of eliminating the worse known problems, and at a
reduced cost for the project. Following
are the elements included:
a)
Curbing along
b)
Ponding in Dan Rile Park.
c)
Partial removal of wall at
d)
Additional drain piping through the side yard of
e)
Enlarged culvert across
f)
Curbing along
g)
New storm drain from
h)
New drainage facilities in
i)
Curbing along
Many elements of this plan are identical with the previously described improvements. But the expensive relief sewers are eliminated to reduce expense.
As before, detention at
The proposed inlets and
drain in
Costs under Alternate C are estimated at $436,200 for construction, and $544,000 for capital costs. (Table 1C).
Alternate D – “Barebones” Plan (See
Plate 11)
A question
was raised as to the effect of an extremely limited project.
Under this
alternate, only the following work would be included:
a)
Curbing along
b)
Partial removal of the wall at
c)
Enlarged culvert across
d)
Curbing along
e)
Curbing along
In all matters, this
alternate creates only partial solutions.
More important, the existing ponding in
Costs under Alternate D are
estimated at $124,300 for construction, and $154,100 for capital costs. (Table
1D).
Other
Considerations
The importance of confining flow to
The intersection of
Further east, along
Similarly, it is important to hold
surface runoff along
Environmental
Concerns
In reviewing elements of this study,
cautions have been expressed about the landscaping, shrubberies, and even the
major trees found along the stream banks.
Property owners throughout the area prefer to sustain a flood effect,
rather than to disturb the plantings and underbrush.
Channels are generally undersized
and overgrown. The Hazen & Sawyer
report (Appendix C) offered the solution of channel improvements throughout the
area, but apparently did not recognize the property owner reactions to such a
devastating approach.
Instead of channel work, this study
concentrates on diminishing of flow from fringe areas, culvert enlargements,
and detention. This approach minimizes
environmental effects, if not providing for full storm water protection.
Other environmental concerns are
common to any construction project.
There are inevitable nuisances to be expected from noise, dust, and
traffic blockage. The proper remedies
lie in strictly enforced technical specifications, wherein measures for noise
and dust control are stressed. Traffic
problems can best be minimized through cooperation with the police department,
and the assignment of traffic control officers.
Stepping
Stones Dam
Much attention has been given to the
Stepping Stones dam, with the following conclusions:
a)
The backwater effect from the dam is pronounced,
affecting flood levels at several upstream properties. But the backwater surface dissipates, even at
100-year frequencies, a short distance upstream of the Stepping Stones
driveway, in the Village property.
b)
The effect of the dam is not only to establish flood
levels, but also to create a stilling pond which encourages siltation. Currently, the silt levels diminish the
pond’s usefulness for swimming or for fish life.
c)
Reduction of the dam height would benefit the
fronting properties and would eliminate siltation. But, the pond would disappear (except during
storms). More importantly, the equipment
needed to shift the huge blocks comprising the dam, would severely damage the
landscaping and stream environment.
d)
The affected property owners have stressed their
preference for an undisturbed condition, and a willingness to accept current
levels of flooding. However, they have
stressed their objection to any upstream improvements which would increase stream
flows.
e)
The culvert under the Stepping Stones driveway should
be cleaned frequently, to minimize flooding.
f)
The Stepping Stones property owners should be advised
of the following long term remedies:
1.
Dam reconstruction to provide for a manually (or
motorized) operated sluice gate, to permit passage of storm flows. Cost for such a gate would be in the order of
$25,000.
2.
Installation of a level controlling gate, similar to
that manufactured by the Waterman Co.
Cost of a small unit would be in the order of $55,000.
In view of the
expressed preference of the Stepping Stones property owners, and of the
property owners upstream of the dam, it would appear preferable for the Village
to defer any action in this area. If
flooding is to be reduced, it should remain the dam owners’ responsibility.
The
opinion rendered to the property owner by the District Conservationist
(Appendix D) should be reiterated… “The cost of all this … to save the pond may
be more than you wish to bear….”
EVALUATION OF ALTERNATIVES
Comparison of
Flood Relief
The most comprehensive
plan lies in Alternate B. By use of a bypass drain along
An effective, but
somewhat lesser solution would be the
The approach shown in
Alternate C has the benefit of reducing the worst flooding effects, at
The inclusion of
Alternate D is intended to illustrate the limits of minimization. Scant relief would be secured from flooding,
and there would be increased discharge downstream for lack of storage at the
head of the watershed.
Comparison of
Costs
As would be expected, the most
comprehensive program carries the greatest cost, and the least program would
require the least cost.
Following is a summary of the
estimated construction and capital cost (for details, see Tables 1A thru 1D):
Alternate A Alternate B Alternate C Alternate
D
(
Bypass) Bypass) Project) Project)
Construction Costs $484,200 $1,040,400 $436,200 $124,300
Capital Costs $604,600 $1,294,600 $544,000 $154,100
Comparison of Benefits
Least
benefit is derived from Alternate D, since minimal improvements are planned for
any affected areas. There will be some
diminution of flooding by diversions on
Alternates
A and B provide significant benefit by diverting flows around the most
seriously affected areas. Alternate B
adds flood relief in the
Alternate
C benefits the
Comparison of Environmental Damage
Sensitivity
to the environment in this drainage basin has been stressed from the initiation
of the project. Most of the riparian
owners have expressed concern about their landscaping being damaged by any
flood control project.
Accordingly,
solutions herein have generally avoided extensive channel improvements, or
on-site disturbance. Projects are mostly
planned in roadways, or on public property.
The
notable exception lies in the property at
Common to all projects
are the problems of noise, dust, and traffic blockage. The smallest project (D) would therefore have
far fewer such problems than the largest project (B). But with proper specification control, the
selection of a project in this study need not be based upon environmental problems.
Need for Easements
In
some projects, land acquisition may be a controlling factor in the selection of
a best alternative. In this study,
however, the need for real estate acquisition is minimal.
The
need for easements at
a)
Detailed surveys, and
preparation of large scale plans.
b)
Final design, considering all environmental factors,
access, specification controls, restoration requirements, etc.
c)
Consultation and negotiation with property owners.
d)
Preparation of “metes and bounds” descriptions,
easement language, title search, etc.
e)
Final acquisition, by consent agreement or (as a last
resort) by condemnation.
Since the
affected property owners have been consulted as part of this study, it is hoped
that a harmonious easement acquisition will be possible.
Phasing
Alternate A requires a commitment to
the
Alternate B is the most
comprehensive project, requiring a commitment to the expensive
Alternate C can be deferred, in
part, by postponing the
Alternate D is in itself a phasing
operation, and no deferrals are possible.
Overall
Evaluation
Using the criteria
outlined above, a possible means of comparison would be the following matrix:
Effect
Minimum
Moderate Maximum
Alternate A
Flood relief X
Costs X
Environmental Effect X
Property Benefits X
Alternate B
Flood Relief X
Costs X
Environmental Effect X
Property Benefits X
Alternate C
Flood Relief X
Costs X
Environmental Effect X
Property Benefits X
Alternate D
Flood Relief X
Costs X
Environmental Effect X
Property Benefits X
Assuming
that maximum cost and/or minimum benefit would weigh against acceptance, Alternates A, B,
and D fail by comparison. Alternative C offers moderate
flood relief and cost, but with at least moderate benefit to properties.
Therefore (in terms of the Village’s
2002 Request for Proposals), the selection of Alternate C will
provide the “maximum practical level of protection” against the flooding of
homes and streets along the main drainage channel. The use of 10-year frequency storm flows for
the primary infrastructure is most effective, considering the facilities made
available (such as overflow weirs) for higher frequency storms.
FINANCING
Capital
Costs and Amortization
Contacts were made with
various agencies to determine the availability of grants or loans for the
project. Unfortunately, no aid appears
to be available for such projects.
Currently, municipal projects are at
an extremely low financing cost.
Year Location Term Amount Average Interest Rate
2003 Refinanced
Bonds 11 years, $1,790,000 2.65% interest
2002 Four
Municipal Projects Short Term $
827,000 1.55% interest
2001-3 Library – Antic’n.
Notes 1 year $1,400,000 1.32%
to 3.20% interest
Construction costs are reported in
this study (see Tables 1A to 1D) based on an assumed construction period for
early 2004. Since project design remains
preliminary, a contingency allowance of 10% is allowed to cover unforeseen costs.
In several of the projects, allowance
is made for real estate costs.
Primarily, this involves the acquisition of easements. Estimates of cost are based on recent costs
for easements over residential properties, with allowances for fees, filing,
etc.
Other costs must be added, in order
to determine the total project cost:
Engineering and
Surveys @ 11%
Construction Phase
Monitoring @ 9%
Legal,
Advertising, Bonding, etc. @ 3%
Administrative and
Accounting @ 1%
Assuming equal annual payments*, an
interest rate of 2%, and a 20-year amortization, the anticipated annual costs
may be evaluated as follows:
Total
Capital Cost** Annual Amortization
Alternate A …………………. $ 604,600 $37,000
Alternate B
……………………………...$1,294,600 $79,200
Alternate C …………………. $ 544,000 $33,300
Alternate D
……………………………...$ 154,100 $ 9,400
* Actual amortization may incur higher initial debt service, and lower final debt service. The “equal annual payment method” is provided for ease of project comparison.
** Note that allowance may be needed for “interest during construction”, say 1.5% for one year, depending on the method of financing.
Assessments
The Village is authorized, by State
statute, to assess properties for special benefits derived from a publicly
financed project. The special benefits
may be evaluated in several ways:
a)
By stream frontage.
b)
By property valuation before and after project
construction.
c)
By evaluation of damages from flooding which are
eliminated by the project.
It
may also be possible to establish an “improvement district”, and to assess all
properties within the district for benefits derived.
After some study, it becomes
apparent that an assessment program in the affected area would have enormous
technical and political barriers. There
is no known method of assessing drainage projects which is not subject to
challenge. For example, stream frontage
is not a suitable criterion since the benefits derived (or the damage
eliminated) has little bearing on the length of waterway.
More important, there is a claim
from many property owners that the flooding causes little damage, even under
extreme storms. Moreover, the damage
which could be caused by construction of improvements may well outweigh the
benefits to property value.
Accordingly, it is recommended that
the project be considered of “general benefit” to the community, rather than of
“special benefit”, thus avoiding the need for local assessments.
Phasing of Alternate C
Assuming the selection of Alternate
C by the Village Board of Trustees, the financing burden can be eased through a
multi-year program, as follows:
§
Initial
Culvert enlargement at
Curbing along
Berms and curbs
along Ravensdale Rd. & Farragut
Pkwy.
Partial wall removal at
Detention pond at
§
Interim
Improvements along
Facilities through
§
Near Future
Drains and inlets along
Enlarged detention at
It is urged that the overall design be accomplished
initially, in order to insure an integrated ultimate project. Phasing can then be accomplished through
contract subdivisions.
It should be realized, however, that
phasing will add to the project cost for several reasons: a) engineering and
bidding expenses are repeated for each phase; b) contractor mobilization and
demobilization costs are repeated for each phase; and c) soft costs (e.g.
insurance and bonding) would be increased with multiple contracts. Therefore, for maximum economy, the project
should be instituted as a whole, without phasing.
Further, phasing will not eliminate
some of the major problems. For example,
if only the “Initial” project is undertaken, problems will continue at
* Detention volume
would be reduced to match the current ponding volume
at the low spot in
OTHER MATTERS
Permits
The State Department of
Environmental Conservation has advised that no wetlands permits will be
required, since the project site is not located within a “Protected Fresh Water
Wetland”.
The U.S. Army Corps of Engineers has
advised that there are no wetlands requirements within their jurisdiction.
The State Department of
Transportation indicated that it will require a permit for any curb or berm placement along Farragut
parkway. No extensive difficulties are
anticipated.
The State Department of
Environmental Conservation may require an application under the State
Environmental Quality Review Act (SEQRA). A decision on the review will be made upon
completion of contract documents.
The Village Planning Department
should receive notice of the impending improvements.
Collateral Matters
A concern was expressed by the
property owner at
Storm drain maps in the area are
incomplete and may also need correction.
New mapping would be desirable, preferably using GIS procedures.
Similarly, maps of the sanitary
sewer system need updating and clarification.
In addition, the Village should consider a “cursory” *infiltration/inflow
study, to reduce sewer flows during storm periods.
Consideration should be given to the
adoption of a zero runoff ordinance, limiting surface discharge from newly
installed impervious areas. This would
ultimately provide seepage pits, or other means of restricting surface runoff,
for the future benefit of the waterways.
The importance of maintenance, for
drainage pipe, culverts, and inlets, must be emphasized. At least monthly, and after storms
(particularly in the fall), there must be inspection and cleaning to insure
full capacity for the next storm. Of
particular importance is the bar rack just north of
There is a blocked concrete culvert
at the most downstream crossing of
* “Cursory” study only requires night observation of sewer flows during wet weather, using prescribed guidelines.
More intensive television observation is
then required only in the most critical areas.
CONCLUSIONS AND ACKNOWLEDGEMENTS
Conclusions
Based
upon the matters described before, the conclusions of this study may be summarized
as follows:
a)
The Village authorized an investigation of the
flooding along an unnamed stream, extending from
b)
Flooding is worst in the
c)
Prior studies were performed by the District
Conservationist at the Stepping Stones area; by the consulting firm of Hazen
& Sawyer for the entire basin; and by a
d)
Surveys were prepared, borings were drilled, and
mapping obtained from various sources, in preparation of this study. Of greatest value was the input of local
residents.
e)
A detailed survey was made of existing flood
conditions under various storm frequencies (see text). The condition is considered tolerable in all
areas except
f)
Hydraulic calculations were prepared to determine
drainage areas, and estimated tributary flows at various storm frequencies. Gradients were cross-checked with reported
flood levels to insure accuracy of the modeling.
g)
Consideration was given to channelization,
but the environmental degradation would be excessive; therefore, improvements
are considered (for the most part) in public rights-of-way.
h)
In reviewing design criteria, the optional storm
return frequency was selected as 10 years for pipe lines, and 25 years for
culverts.
i)
The following alternative plans were studied in
detail:
A.
upstream flooding.
B.
Farragut
Avenue Bypass - Utilizing a relief sewer
along Farragut Avenue to reduce flooding from Fenwick Road to Farragut Avenue;
also a relief sewer in the Neppherhan Avenue area.
C.
Minimum
Project – Limited improvements, sufficient to reduce the worst flood
conditions.
D.
“Barebones”
Plan – An extremely limited project, creating only partial solutions.
j)
Evaluations indicate the importance of confining flow
along
k)
The Stepping Stones dam creates flooding to just
north of the Stepping Stones driveway.
The best solutions would be either lowering of the dam, or utilizing
level controls, any of which would be costly to the property owners, and
damaging to the environment. No public
improvements are planned in this area.
l)
In comparing the alternates, it may be noted that
Alternates A and B are the most comprehensive, but are heavy in capital cost,
perhaps beyond acceptable municipal limits.
Conversely, Alternate D accomplishes little, and worsens downstream
conditions.
m)
Overall, Alternate C appears to present reasonable
flood relief, at moderate cost. If
desired, phasing is possible. Therefore,
Alternate C is the recommended approach.
n)
Capital costs for Alternate C are estimated at
$544,000, and the annual amortization would be $33,300.
o)
Assessment financing is not recommended, in view of
the great difficulty in establishing “special benefits”.
p)
Permit requirements should not be onerous. Application will be required to the State
Department of Transportation, and to the State Department of Environmental
Conservation (re. SEQRA approval).
q)
Other matters are offered for consideration:
§
Improved drain and sewer mapping.
§
Disconnecting of illicit sewer connection.
§
Cursory infiltration / inflow study.
§
A “zero run-off” ordinance
should be considered.
§
Culvert maintenance must be stressed, including the
blocked concrete culvert under
Acknowledgements
Successful
completion of this study would have been impossible without cooperation and
input from the following parties:
- David Walrath, former Trustee
- Michael Gunther, Supt. of Public Works
- Neil P. Hess, Village Manager
- Michael Holdstein, Village Trustee
- Susan Maggiotto, Village Clerk
- Rafael Zaratzian, Village Technology Director
- Area Residents – who appeared on several
occasions (on rainy days) offering information.
- Board of Trustees - providing input and comment
at several public meetings.
RECOMMENDATIONS
- The
study should receive scrutiny and input from the Board of Trustees and
Village officials.
- The
conclusions of the study should be publicized, and made available for
public comment.
- Official approval should be given to the
selected project, and capital funding should be authorized.
- Preparation
of detailed design and bid documents should be authorized, with phasing as
desired.
- Final
cost estimates should be prepared, and bond anticipation notes should be
funded.
- Easement
acquisition should be negotiated.
- State approval should be obtained, from the
Dept. of Transportation for work in
- Bids
should be obtained, and an acceptable low bid should be awarded.
- Construction
should be completed, with appropriate field supervision.
- Separate consideration should be given to
improved utility mapping, and to infiltration/inflow studies.