HomeMy WebLinkAbout15 - Bowman Ave Dam Flood Mitigation StudyProject Report
Flood Mitigation Study
Bowman Avenue Dam Site
City of Rye and Village of Rye Brook
Westchester County, N.Y.
Flood Control Improvements
Blind Brook at Bowman Avenue Dam Site
Prepared By:
CHAS. . SELLS, I NC,
555 Pleasantville Road
Briarcliff Manor, NY 10510
March 12, 2008
Final Project Report
mood mitigation Study
Bowman Avenue Dam Site and Lower Pond
TABLE OF CONTENTS
Executivesummary.........................................................................................................................1
introduction......................................................................................................................................2
Description of Existi.q Co.�ditions..........................................................................................
Bowman Avenue Dam Site and Lower Pond............................................................................. 6
AlternativesAnalysis.......................................................................................................................8
Methodology............................................................................................................................... $
Alternati v e Descriptions and Initial Calculation Results.......................................................... 1 U
No -Build Altemative............................................................................................................ 1 U
Upper Pond Resizing Alternatives........................................................................................ 11
Oritice Optimization Alternatives......................................................................................... 13
Raising the bowman Avenue Vam Alternatives.................................................................. 15
Combined Upper Fond Resizing and Uritice Optimization Alternatives ............................. 18
Modifications to the Lower Po,,d Altemati v es..................................................................... 20
Preferred Alternatives....................................................................................................................22
Methodology............................................................................................................................. 22
AnalysisResults........................................................................................................................ 25
Alteinati v e A: Optimizing the Oritice Opening at the Dam ................................................. 25
Alternative B: Optimizing the Orifice Openi.�g and Maximizing the Upper Pond .............. 26
Alternative C: Optimizing the Orifice Opening, Maximizing the Upper Pond and
oredging 2 -feet of pediment Material from upper Fond ..................................... 29
UpstreamImpacts..................................................................................................................... 35
Conclusion and Reconhueudations................................................................................................37
LiS i Or 1+ iGURES
Figure1- Location Map.................................................................................................................. 3
_ figure 2 - study Area...................................................................................................................... 5
t igure 3 - Upper Fond 'I opography................................................................................................ /
Hgure 4 - Maximizing Upper Fond - No Dredging...................................................................... 3U
Figure 5 - Maximizing Upper Pond with Dredgi..g...................................................................... 31
Figure 6 - 10-Y ear Water Surface Elevations............................................................................... 32
t{igure / - 5U -Year Water surface Elevations............................................................................... 33
t igure Zi - 1 UU- Y ear W ater Surtace Elevations............................................................................. 34
BIBLIOURAPHY
Find Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
EXECUTIVE SUMMARY
This project involves a feasibility analysis of various flood damage reduction measures at the
Bowman Avenue Darn site and Lower Pond. This initiative is consistent with the City of Rye's
(City) Flood Mitigation Plan dated November 2001 to which the City identified conceptual level
improvements at the Bowman Avenue Darn site and Lower Pond as being part of a
comprehensive plan to provide downstream flood control. This study will assess the feasibility,
costs and benefits associated with these conceptual flood control alternatives, It is the intent of
this report to aid the City in implementing meaningful flood mitigation measures and to provide
documentation necessary for securing Hazard Mitigation Grant Program (HMOP) funding.
The Bowman Avenue Dram property is located within the Village of Rye Brook immediately
upstream of I-287. The site is the only regional Mood control facility owned and operated by the
City. Originally constructed in the 1900's, the dam and the Upper Pond were used for ice
production. In 1941, the dam collapsed and was rebuilt. The existing darn is a reinforced
concrete gravity dam founded on ledge rock. Currently the darn has low-level outlet with a fixed
orifice opening of 15 -feet wide by 2.5 -foot high.
Based on aerial photographs from 1925, the Bowman Avenue Darn site has changed
considerably. Over the past 75 -years, the Upper Pond has been significantly reduced in size due
to siltation. It has been estimated that the Upper Pond is approximately one-quarter its original
size. Up until 1976, the Lower Pond diel not exist. It was formed as a result of the abandonment
of a quarry operation at the site, The Lower Pond was not designed nor does it currently
function as a flood control measure.
Several alternatives were investigated as part of this analysis. Each alternative was compared
based on its benefit in terms of relative flow reduction and lowering of downstream water
surface elevations versus overall cost and impacts.
The preferred alternative, From a short-term perspective, consists of the installation of an
automated sluice gate at the bowman Avenue Dam. An automated sluice gate has the ability to
vary the outlet opening, thus providing the optimum orifice size for the flow rate in the stream.
The sluice gate would be automatically controlled based on water surface elevations measured at
a gauge mounted at the dam. Based on our analysis, this alternative provides the most cost-
effective means to reduce water surface elevations downstream. For example, during the 100 -
year design storm, it has been determined that the water surface elevation at Highland Road
would be reduced by approximately 1 -foot. The budgetary construction cost for this alternative
is estimated at $1 - $2 million. This alternative will not result in upstream impacts.
Other alternatives, including maximizing the storage potential of the Upper Pond in conjunction
with the sluice gate, resulted in a further reduction of downstream water surface elevations. The
budgetary construction cost for this alternative is estimated at $10 - $15 million. However, it
should be noted that the cost/benefit of this alternative heavily relies on the limit of rock
excavation and the presence of contaminated material. Further subsurface investigation
including rock probes and soil testing is necessary.
Page] of38 it fl,I i.I �.I'4r
Fi►►al Project Report
mood mitigation Study
Bowman Avenue Dam Site and Lower Pond
INTRODUCTION
Pursuant to the request of the City of Rye a.,d the Village of Rye Brook; Chas. H. Sells, Inc.
(Sells) has prepared this Flood Mitigation Study for Blind Brook. The scope of this study is to
evaluate various flood damage reduction measures at the tsowman Avenue uam Site and Lower
Fond so as to reduce downstream tlooding specifically in the reach between 1-28'/ and 1-95.
The proposed project, Bowman Avenue Dam site ana Lower Yo..d, is located in the northern
portion of the City of Rye and at the southern limit of the Village of Rye Brook, Westchester
Lounty, New York (see rigure 1). the project site is bounded by tsowman Avenue to the north;
1-28'/ to the south; and the Fye Fiage Flaza and Koanoke Avenue to the east (Latitude 410
00'10" North by Longitude "/30 41' 16" West). the total project area, including the Upper ana
Lower Ponds, is appro,imately 35 -acres.
DESCRIPTION OF EXISTING CONDITIONS
the watershed of Blind Brook is located within the corporate entities of the town of ureenwich
in Lonnecticut, the City of Fye, the Town/ Village of Harrison, and the Villages of Fye Brook
ana Fortchester in New York.
The watershed area of Blind Brook was delineated on and measured from the uSGS quadrangle
sheets for ulenville, U l —N Y and Mamaroneck, N Y -l; l as shown in Appendix A. the
measured area at the downstream end of the study area, the U.S.O.S. CJauge 01300000 just
aownstream of 1-95_ is 9.6 mit.
examination of the quadrangle sheets indicates that the streambed is moderately sloping, with an
average slope of U. / percent upstream of the Bowman Avenue llam and U.1 L percent upstream
of 1-y5.
The quadrangle sheet also shows that the watershed is suburban in the upper and middle third,
and urbanized in the lower third and eastern part of the watershed. the topography of the
watershed is gently rolling and lightly wooded hills in the upper portion, and less hilly and
partially cleared in the lower part. h'or the most part; between Westchester Avenue ana 1-95, the
floodplain is wide when compared to the stream channel. Most of the development presently in
the floodplain is comprised of low to medium density residential and office uses.
As documented in numerous previous studies, (see Bibliography for listing), the Blind Brook
Watershed is subject to trequent tlooaing throughout its entire length. A combination of a
narrow channel, obstructed flows, vegetative growth in stream banks, constricted bridge
openings, low banks, sedimentation in tidal reaches, years of wetland filling, and floodplain
encroachment are considered the primary cause of the flooding.1
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C'I•I'Y OF RYE FLOOD MITIGATION MEASURES FIGURE NO.
- - PROJECT LOCATION MAP I
Final Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and tower band
There are estimated to be approximately 1,500 strictures located within the entire Blind brook
100 -year floodplain. It is estimated that 20% of the properties in the City of Rye are within a
FEMA designated flood zone.2 This study will focus on flooding conditions occurring within the
segment of Blind Brook south of 1-287 and north of I-95. This segment is roughly 135 -acres in
size with approximately 140 structures located within the 100 -year floodplain. See Figure 2 for
the Study Area.
There are known records quantifying extreme floods events on Blind Brook. Hurricane Agnes in
June 1972 produced the largest flow (2,320 cfs) ever recorded at the gauge and the 'September
1975, Hurricane Eloise, discharge was slightly smaller (2,280 cfs), These storms produced
extensive damage to buildings, yards, and streets. Flooding in the Blind Brook watershed
resulted in substantial damage especially between Purchase Street and Highland Road.
,According to the preliminary 2006 Flood Insurance Study, "the areas subject to flooding are
immediately upstream of road culverts where constrictions cause backwater. The most severe
problems on Blind Brook occur at Bowman Avenue, Westchester Avenue, Lincoln Avenue and
Brookside Way culverts".
Indian Village is documented as having a high concentration of repetitive loss claims.
According to National Flood Insurance Program (NFIP) data there have been 273 repetitive loss
claims in the City of Rye since 1978. The total of these claims exceeds $4.5 million with an
average claim exceeding $16,000,' according to the Army Corps of Engineers (ALOE)
approximately 1/3 of these claims occur within Indian Village.
The most recent storm event was the April 15, 2007 Nor'easter. This storm yielded roughly 8"
of rain over a 24-hour period and was classified as a 100 -year event. The City of Rye incurred
severe damage to both private property and public facilities. A summary of damages is provided
in 'fable 1.
]bid... 4
3 Ibid., p. 19
Page 4 of 38 . E 1\ I-:_ IL IN I I I Irl ,.
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Finan Project Report
,Mood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
TABLE. I
April 15, 2007 Nor'caster
Sunimaq of 1)aniages
Damage Description
Total Cost
Private Property
Minor Damage$4,691,670
Moderate Damage
- $20,863,350
Major Damage
$57,675,620
Total Private Property Damage
$$3,230,640
Public Properly
Debris Removal
$24,560
Elm Place .Detaining Wall
$1,032;000
Emergency Services
$128,160
Theodore Fremd Retaining Wall
$880,000
Locust Avenue Firehouse
$153,840
Parking Paystation
$12,490
Total Public Property Damages
$2,231,050
Grand Total
$851461,690
Aeac T_ding to CEDAR damage report for Westchester County. Damage amounts are based on building assessed values (minor -
17A, moderate -44%, major— 63%)
"According to FEMA PAPA forms prepared by the City of Rye
Bowman Avenue Dam Site and Lower Pond
The Bowman Avenue Dam represents the only flood control structure on Blind Brook and is
owned and operated by the City of Rye. Originally constructed in the early 1900's, the darn and
the upstream pond were used for ice production. In 1941, the dam collapsed and was rebuilt.
The existing dam is a reinforced concrete gravity dam founded on ledge rock. The dam is 119
feet long by 13 Feet high (measured to the spillway). The dam was constructed with a 15 -foot
wide by 11.5 -foot high outlet at the bottom of the dam and a 20 -foot wide by 2 -foot high
spillway at the top. Currently the dam has an orifice opening of 15 -feet wide by 2.5 -foot high
due the presence of a fixed timber gate. Based on a visual inspection, the dam appears to be in
overall fair condition with fine random cracks with efflorescent stains. The dam is not listed on
the New York State Department of Environmental Conservation Dam Safety Inventory list.
As noted in previous reports, the upstream pond, referred to in this report as the Upper Pond, has
decreased in size over the years due to heavy siltation. It is difficult to accurately determine the
overall reduction in storage capacity. Sells surveyed the Lipper Pond and determined that its
existing reservoir capacity is 145 acre-feet as measured from the normal pool elevation to the
crest of the dam at elevation 57.3 feet. The daturn used for the field survey was NAVD-88. See
Figure 3 for existing topography of Upper Pond.
Downstream of the darn is the Lower Pond of Blind Brook, which also serves as the confluence
with East Branch Blind Brook. The Lower Pond, originally used as a quarry, was abandoned in
1976 and subsequently flooded to form the pond. The I -acre peninsula along the northern shore
Page 6 of 38
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CHAS. H. SELLS, Ilv C
CITY OF RYE FLOOD MITIGATION MEASURES
.
„ ,
BOWMAN AVENUE DAM RESERVOIR - EXISTING CONDITIONS
[��EN03
Final Project Report
Mood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
of the pond has been formed as a result of dumping within the last 25 -years. The maximum
depth of the pond is 30 -feet as determined by soundings performed by Sells.
The lower Pend provides minimal storage capacity for flood control purposes; it was not
designed to do so. There is no man-made outlet control at this location. The water level in the
pond is controlled by a "natural" overflow located immediately downstream of the Lower Pond
approximately 300 feet upstream from the 1-287 bridge.
ALTERNATIVES ANALYSIS
An initial evaluation was made for 24 different alternatives plus the no -build scenario. This
evaluation was based on whether the alternative could provide meaningful flood mitigation in
terms of flaw reduction, Flow rates were computed at the following three locations for each of
the alternatives:
• Downstream of the Bowman Avenue Darn
• Downstream of the 1-287 Bridge
• Downstream of the 1-95 Bridge
From this evaluation, four preferred alternatives (including the no -build) were identified. The
preferred alternatives, discussed later in the report, are evaluated based on the level of mitigation
they could achieve in terms of water surface elevation reduction.
The initial alternatives can be divided into six general categories of work:
1. No -build -- existing conditions
2. Resizing the Lipper Fond
3. Modifying the Opening of the Outlet Orifice on the Bowman Avenue Darn
4. raising the Top of the Bowman Avenue Dam
5. A Combination of Resizing the Upper Pend and Modifying the Orifice
6. Modifications to the Lower Pond
Methodology
Within each of the six above -referenced general categories, sub -alternates were analyzed for
potential mitigation. In the initial evaluation each of the alternatives was compared using flood
routing calculations. Flood routing calculations are used to establish inflow/outflow rates for a
variety of reservoir volumes and outlet openings. The results are discharge rates for the streaxn.
Even though this methodology only provides for rates and not water surface elevations, it was
selected for the initial calculations since it does provide a means to evaluate the magnitude of
mitigation an alternative could provide with a relatively simple calculation. From these results a
short list of alternatives that shove meaningful mitigation potential can be established for more
detailed analysis. The detailed analysis, which is described in the "Preferred Alternatives"
section, produces water surface elevations on Blind Brook within the study area.
The Bowman Avenue Dam is a flood control structure and its efficiency during various
frequency storms depends on the difference between rates of inflow and outflow. As the water
Page 8 of 38 t I I;1S. I L "i",I.I �. I N1 .
Final Project Deport
Flood Mitigation Study
Bowman Avenue Darn Site and tower Pond
level at a control structure rise, so does its flow rate through the structure. To analyze the effects
of a control structure a stage/discharge curve is established that computes the outgoing flow rate
for various water levels (stages). For a given reservoir site the reservoir storage capacity is
constant and the spillway stage/discharge curve is variable depending on the type and size of the
spillway and outlet and how they are operated. Stage/discharge curves (attached in the Appendix
13) were developed at the dam based on topographical ficld survey performed by Sells in August
2007.
The hydraulic features of the Bowman Avenue barn site contain several components, where
depending upon the water level, flow can occur. The manner in which each of these features
handle flow and interaction between them in different flow conditions makes it a complex
system. In our analysis we took into consideration a number of factors that included.
• irregular stream bed was approximated as a weir (allows the discharge of normal flaw),
• An approximately 15 foot orifice with a varying height of 2.5 feet to 11.5 feet,
• 12 -foot long principal spillway at elevation 55.3,
• An approximately 99 foot long dam crest at elevation 57.3, and
• Overflow channel extending north cast towards Bowman Road (consisting mostly of
processed asphalt fill).
It should be noted that not all of the outlet area is effective (i.e. controls the amount of flow).
There is a substantial amount of bedrock extending into the upstream and downstream channel.
Therefore, smaller effective areas were used in the stage/discharge calculations for a variety of
outlet openings.
During storm events where flows start to exceed approximately 1,450 efs (between the 2- and 5 -
year design storm), the water overtops the crest of the dam and starts to flow in the overflow
channel. It then rejoins the main channel of Blind Brook just downstream of the darn. Thus in
the majority of the storm events (under existing conditions) the dam and the overflow
combination control the discharge downstream of Bowman loam.
The flood routing was performed using the National Resources Center's (NRC) WinTR-20,
Version 1 software. This software is the windows version of the original DOS based TR -20
model developed by the NRC, formerly known as the Soil Conservation Center.
The software forecasts the rate of surface water runoff and watercourse flow rates based on
several factors. The input data includes information on land use, soil types, vegetation,
watershed areas, times of concentration, rainfall data, storage volumes, and hydraulic capacities
of the hydraulic structures. The computer model predicts the amount of runoff as a function of
time, including the attenuation effect due to dams, lakes, large wetlands, and floodplains. Runoff
rates during specific rainstorms may vary due to different assumptions concerning soil moisture,
water levels in ponds, snowmelt, and rainfall patterns. The input data for rainfalls with statistical
recurrence frequencies of 2, 5, 10, 25, 50 and 100 -years were obtained from the U.S. Weather
Bureau Technical Papers. The National Weather Service developed four synthetic storms to
simulate rainfall patterns around the country. For analysis in Westchester County, the Type ill
Page 9 of 38 C1L1:S.11.SL, l.l
Final Project ,Deport
Flood Mitigation Study
Bowman Avenue Dam Site and tower Pond
rainfall pattern with 24-hour duration is valid, Typically, the TR -20 methodology overestimates
the peak discharges for all storm events.
The available TR -20 model data is included in the 1979 Flood Insurance Study backup
information that Sells obtained from FEMA, c/o Michael Baker, Jr., Inc. The backup data
includes drainage area delineations, RunofT Curve Numbers and times of concentration for each
sub watershed, and the model schematic. Although this data is from the 1970's and might not
represent existing conditions, including the extent of natural and manmade changes that have
occurred in the watershed, in our professional opinion for the purpose of determining
inflow/outflow rate at the Bowman Avenue Dam, the available data is valid. This is the same
data that was used in the April 2007 ALOE report.
In order to calibrate our results, a few modi lications of the old model were performed. These
include;
• To analyze the effect of storage at the Lower Fond site, the subbasin located north
of I-287 was divided into sub -watersheds representing smaller portions of the
total area, Based on Sells field survey data in the vicinity of the dam, the structure
data was also updated.
* The entry of routing coefficients x and m (in the Att-Kin routing procedure.) in
lieu of reach crass -section rating data is no longer accepted by the newest version
of WinTR-20. All reaches in the old data used cross sections instead of routing
coefficients. Therefore, the analysis is based on cross section ratings developed
for the 2007 FEMA Flood Insurance Study (FIS) model were used.
• The Fast Tributary located north of Hutchinson Parkway was not studied in detail
and cross section data was not available in the new FIS. Therefore, the FIRMA
cross-section information was supplemented with Sells field survey in this area.
Output generated by WinTR-20 models for each alternate studied in detail are attached in the
Appendix B.
Alternative Descriptions and Initial Calculation Results
No -wild Alternative
The no -build alternative reflects current topographic conditions in the Upper and Lower ponds,
as well as the current outflow configurations. Additionally, flow rates were computed using the
existing topography with the darn removed so as to show the effect of the existing detention and
flood control provided by the dam. As shown in Table 2, the flow rates in the condition where
the darn is removed increase the greatest in the lower design year storms, However, as the
design storm frequency decreases from 25 -year to 100 -year the difference. in discharge rates with
and without the dam approach each other. This occurs since the dam is greatly overtopped with:
higher flows and looses its effect as a control structure. The flaw rates generated under the no -
build alternative (See Table 2) will be used as a basis for comparison with the ether alternatives
to provide an indication of the mitigation potential of the proposal.
Page 10of3$
Final Project Report
Flood Mitigation Study
Bowman Avenue Clam Site and Cower Pond
TABLE 2: Discharges - No -Build (efs)
No Dain
2 -Year Storm
DIS Bowman Dam 397 854
457
DIS 1-267 565 1106
541
DIS 1-45 681 1148
467
5 -Year Storm
DIS Bowman Dam
1253
1592
339
DIS 1-267
1473
1999
526
DIS 1-95
1413
2061
648
10 -Year Storm
DIS Bowman Dam
1768
1984
216
DIS 1-287
2088
2479
391
DIS 1-95
2012
2564
552
25 -Year Storm
DIS Bowman Dam
2800
2845
5
DIS I-287
3396
3495
99
DIS I-95
2994
3410
416
50 -Year Storm
D/S Bowman Dam
3755
3663
-92
D/5 1-287
4546
4498
-8
DIS 1-95
4844
4906
62
100 -Year Storm
D/S Bowman Dam
4322
4215
-107
D/S 1-287
3162
5118
-44
D/S 1-95
5621
5646
25
Upper Pond Resizing Alternatives
The alternatives analyzed under this general category examined the effects of increasing the
storage volume of the Upper Pond. This would be accomplished by excavating along the hanks
of the pond, in particular the north side, combined in some instances with dredging of the pond
bottom to remove siltation. The bottom of the pond itself was taken as an average elevation of
approximately 39.0. Due to the softness of the siltation and muck present in the bottom of the
pond area, a field survey with exact elevations could not be performed. Four scenarios of
increasing the storage were examined.
Alt. 1. The Upper Pond was created the early 1900's when the Bowman Avenue Darn was
constructed. Since that time the pond has silted up and been filled in. The extent to
which the volume has changed is difficult to determine since there are no record
plans for the original darn and pond area. However, an estimate of the original limits
of the pond was taken from Figure B,1 of the Technical Memorandum Evaluation of
the Bowman Avenue Dam Site, prepared by Harza Engineering Company, October
2000. This alternative considers excavating around the pond to the 1925
configuration without any dredging of any material in the pond itself, keeping the
Page 11 of 38 t 111 t �. 11.81:1 I .ti9. IN,
Final Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
bottom elevation at approximately 39.0. The total volume of material to be removed
is 36,000 cubic yards (CY).
Alt. 2. The second alternative includes excavating to the 1925 configuration and dredging
the bottom of the pored area by 2 feet to elevation 37.0. The total volume of material
to be removed is 53,000 CY.
Alt. 3. This alternative looked at maximizing the volume behind the pend by excavating up
to Bowman Avenue without any dredging of the pond. The total volume of material
to be removed is 160,000 CY.
Alt. 4. The last alternative in this category used the maximized volume in Alternative 3 and
included the dredging of 2 feet of the pond area. The total volume of material to be
removed is 190,000 CY.
Flow rates for the four locations listed above were computed for each of the design storms and
compared to the existing conditions flow rates. Table 3 shows the outcome of those
computations and the resulting difference in flow rate.
As can be seen the peak flow reductions resulting from excavating to the 1925 contours are
relatively shall when compared to the total flaw and are therefore Alternative 1 and 2 were
dismissed from further consideration. As shown in Table 4, Alternatives 3 and 4 produce the
largest percent reductions at the 1-287 and 1-95 culverts for the 5- to 10 -year storms.
The costs associated with the Upper Pond resizing mainly stem from excavation and will be
discussed later in the report. The area designated for excavation, between the pored and Bowman
Avenue, will include both unclassified excavation and rock excavation. City officials have
indicated that portions of this area had been filled in with construction material in the past. It is
Page 12 of 38 - -- 1:11 \S.11.SI-TI I;, IN(.
2 -Year Storm
DIS Bowman Dam
397
383
-14
380
-17
341
-56
332
-65
DIS 1-287
565
544
-21
535
-30
526
-39
503
-62
D/S 1-95
691
658
-23
637
44
655
-26
592
-89
5 -Year Storm
DIS Bowman Darn
1253
1114
-139
1085
-168
678
-575
605
-648
D/S 1-287
1473
1303
-170
1269
-204
803
-670
729
-744
DISI -95
1413
1272
-141
1243
-170
1100
-313
1037
-376
10 -Year Storm
D/9 Bowman Ilam
1768
1665
-103
1644
-124
1288
-480
1230
-538
D/9 1-287
2088
1958
-130
1934
-134
1508
-580
1440
-648
1D/1% 1-95
2012
1878
-134
1852
-160
1454
-5s8l
1395
-617
25 -Year Storm
D/S Bowman Dam
28000
2755
-45
2745
-55
2509
-291
2474
-32
D/S 1-287
3396
3319
-77
3306
-90
2967
-429
2918
-478
D/S 1-95
2994
2922
-72
2907
-37
2625
-369
2587
-407
50 -Year Storm
DIS Bowman Dam
3755
3749
-6
3747
.8
3645
-110
3627
-128
DIS 1-2137
4506
4489
-17
4485
-21
4293
-213
4262
-244
D/S 1-95
4844
4798
46
4795
-49
4497
-347
4442
-402
I00 -Year Storm
DIS Bowman Dam
4322
4320
-2
4319
-3
4255
-67
4245
-77
D/S 1-287
5162
5140
-22
5134
-28
4990
-172
4967
-195
DISI -9s
56211
5585
»36
5577
-441
5370
-251
5341
-240
As can be seen the peak flow reductions resulting from excavating to the 1925 contours are
relatively shall when compared to the total flaw and are therefore Alternative 1 and 2 were
dismissed from further consideration. As shown in Table 4, Alternatives 3 and 4 produce the
largest percent reductions at the 1-287 and 1-95 culverts for the 5- to 10 -year storms.
The costs associated with the Upper Pond resizing mainly stem from excavation and will be
discussed later in the report. The area designated for excavation, between the pored and Bowman
Avenue, will include both unclassified excavation and rock excavation. City officials have
indicated that portions of this area had been filled in with construction material in the past. It is
Page 12 of 38 - -- 1:11 \S.11.SI-TI I;, IN(.
Final Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
also evident that ledge rock is present throughout the arca, which can be costly to remove. A
second consideration is the removal of contaminated material. There is the potential for a low
level of contamination, mainly what would be associated with untreated runoff from impervious
surfaces such as roads and parking facilities, in any dredged materials. Cost estimates for the
excavation are based on conservative assumptions for the amount of rock and contaminated
material.
Storm EN,eifl/Locafiao
2 -bear Storm
Red.
Red.
Red.
Red.
D/S 1-287
3.7%
5.3%
6.9%
11.0%
DIS I-95
3.4%
6.5%
3.9%
13.1%
5 -Year Starry
D/S 1-287
11.5%
13.8x1`0
45.5%
50.5%
D/S I-95
10.0%
12.0%
22.2%
26.6%
Ili -'Year Storm
DIS 1-287
6.2%
7.4%
27.8%
31.0%
D/S I-95
6.7%
8.0%
27.7%
30.7%
25 -Year Storm
D/S 1-287
2.3%
2.7%
12.6%
14.1%
DISI -95
2.4%
2.9%
12.3%
13.6%
50 -Year Storm
DIS 1-287
0.4%
0.5%
43%
5.4%
1.1/S 1-95
0.9%
1.0%
7.2°.fin
8.3%
100 -Year Storm
D/S 1-287
0.41%
0.5%
3.3%
3.9%
17/S I-95
0-6q,.
0.8%
4.5%
5.0%
Orifice Optirnization Alternatives
The existing Lowman Avenue Dam outlet consists of a concrete structure with a 20- foot long by
2 -foot high principal spillway along the top. Normal (low) stream flows pass beneath the
structure through a 15 -foot wide by 11.5 -foot high opening at the base of the dam that has its
flaw restricted on the upstream side by timber railroad ties, creating an opening of approximately
20.2 square feet (sf). As the Blind Brook's flaw increases and stream level goes above the top of
that opening the structure acts as an orifice. Flow rates that can pass through an orifice depend
upon two factors -- the size of the orifice opening and the head or water level above the opening.
Increasing the size of an orifice will result in a higher flow rate that can pass through. Likewise,
increasing the head above an orifice will also increase the exiting flow rate. The four
alternatives under this general category examined the effects of increasing the size of the opening
without modifying the storage volume behind the dam (i.e. existing conditions). The four orifice
opening sizes analyzed include:
Alt. 5. Orifice Area = 45.6 sf
Alt. 6. Orifice Area = 72.1 sf
Page 13 of 38`
Final Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
Alt. 7. Orifice Area = 105.6 sf
Alt. 8. Orifice Area = 139.1 sf
For any fixed opening size, as the incoming stream flaw increases and starts to exceed the flow
rate that can pass through, the water will build up behind the dam. As the water level increases it
creates a larger head on the opening, which will result in a larger flaw through the orifice, At the
Bowman Avenue Dam there are not only the dynamics of the size of the orifice opening versus
the water level behind the dam, but also the fact that at some point the water level will overtop
the dam at its weir, thus creating an additional flow area. The amount of storage volume in the
pond also impacts the way in which a particular size orifice opening will increase or decrease the
flow through the dam when compared to existing conditions.
The orifice optimization alternatives took into account each of these factors. As a result, for each
design storm frequency, the orifice size that would create the greatest reduction in flaw rate
varies. Table 5 contains the results of the analysis of the four orifice openings for each design
storm. The orifice opening size that creates the optimum flow rate reduction has been
highlighted.
Page 14 of 38 4 1I1 .11.s1:1.11 -.1N
Fxkling
2 -Year Storm
DVS Bw m n Dam
397
625
228
798
401
828
431
916
411
DVS 1-287
565
783
218
994
429
1014
449
1(X)4
43'
EVS I-95
681
ST)
192
1066
385
1070
389
1068
38
"_Year Storm
DPS Bwmii s Dam
1253
969
12(14.1
-53
1377
124
1565
312
DVS 1-297
14'73
1154
31 9
1458
-15
1659
1
1909
43
US 1-95
1413
1238
-175
15644
151
1760
34
192.8
51
10 -Year Storm
DVS Bmwmm Dam
2006
1593
-413
1397
-619
1632
-374
1822
-184
DVSI297
2006
1870
-136
1687
-31
1(J67
-39
2218
21
DVS 1-95
2006
1838
-168
1816
-190
2097
81
2319
313
25 -Year Storm
DPS Bowwman Dam
2800
2730
-70
2535
-209
2266
2388
-41
DV 1-287
3396
3279
-117
3010
-386
2676
2$88
-50
US 1-95
2194
2946
-
-48
2848
-
446
2773
-221
2975
-1
5Mear Storm
US Bommn Uam
3755
3722-33
3673
-82
3571
-185
3381
-37
DVS 1-287
4506
4455
-51
4370
-136
42;3
=273
4002
-504
DVS 1-95
4844
4730
-94
4616
-2.28
4101
43
3873
-971
l(l()�-Year Sturm
DS Bownen lam
432.2
4315
-
4261
-61
4210
-112
4078
2
DVS 1287
5162
5126
-36
504
-115
4971
-191
4809
35
1I-95
5621
55610
5443
-178
5341
-280
5136
-495
Page 14 of 38 4 1I1 .11.s1:1.11 -.1N
Final Project Report
Flood Mitigation Study
Bowman Avenue ,dam Site and Lower Pond
As can be seen in Table 5, this option produces more significant decreases in peals flow for the
25-, 50-, and 100 -yeas design frequency storms when compared to the pond resizing alternatives.
Implementation of the orifice optimization alternative can be accomplished by retrofitting the
Bowman Avenue Dam with an automated sluice gate. An automated sluice gate has the ability
to vary the opening size, thus providing the optimum orifice size for the flow rate in the stream.
The sluice gate would be automatically controlled based on water surface elevations measured at
a gauge mounted behind the Bowman Avenue Dam. The percent reductions for the optimum
orifice opening for the 5- through 100 -year storms are shown in Table 6.
Storm Event/Location Red.
5 -Year Storm
Orifice Opening (sf)
45.6
UIS Bowman Dam
22.7%
D/S 1-287
21.7%
DIS I-95
12.4%
10 -Year Storm.
Orifice Opening (sf)
72.1
D/S Bowman Dam
309%0
D/S 1-287
15,9%
D/S I-95
9.5%
5 -Year Storm
Orifice Opening (sf)
105.6
DIS Bowman Dam
19.1%
D/S 1-287
21.2%
DIS 1-95
7.4%
50 -Year Storm
Orifice Opening (sf)
139,1
DIS Bowman Dam
10.0%
DIS 1-287
11.2%
DIS 1-95
20.0%
100 -Year Storm
Orifice Opening (sf)
139.1
D/S Bowman Dam
5.6%
DIS I-287
6.8%
DIS 1.95
8.6%
The positive results for peak flow mitigation achieved by optimizing the orifice opening together
with the ability to provide an automatic means of accomplishing it warrants a more detailed
analysis of this alternative.
Raising the Bowman Avenue Dam Alternatives
The third alternative category considered was to raise the height of the Bowman Avenue Dam.
As part of the analysis the storage volume for the pond was also increased in the same manner as
Page 15 of 38 +.1I1 .1I -Sr 1',INt .
Final Project Report
Flood Mitigation Study
Bowman Avenue Liam Site and Lower Pond
was considered under the pond resizing alternatives — to the 1925 configuration and to one that
maximizes the area, plus the option of with and without dredging (Alternative 1 through 4). The
four alternatives considered include:
Alt. 9. Raising the dam two feet and excavating the Upper fond to the 1925 configuration
without the dredging of any material in the pond itself, keeping the bottom elevation
at 39.0.
Alt. 10. Raising the dam two feet, excavating to the 1925 configuration, and dredging the
bottom of the pond area by 2 feet to elevation 37.0.
Alt. 11. Raising the dam two feet and maximizing the volume behind the pond by excavating
up to Bowman Avenue without the dredging of any material in the pond itself,
beeping the bottom elevation at 39.1.
Alt. 12. Raising the dam two feet, using the maximized the volume in Alternative 11, and
dredging of 2 feet of the pond area to elevation 37.0.
The flow rags resulting from these alternatives are shown in Table 7.
Existing
-SWrm E'vesall-ocatiom Ca)nd. Alt. 9 WE Alt. 10 - -- Diff. Alt. I I MIT. Alt. 12
Oift.
2 -Year Storm
D/S Bowman Dam 397
380
-17
37£
-21
345
-52
337
-60
D/S 1-287 565
547
-18
536
-29
513
-52
490
-75
D/S 1-95 681
651
-30
635
-46
646
-35
601
-80
5 -Year Storm
D/S Bowman Dane
1253
894
-359
873
-380
558
-695
527
-726
D/S 1-287
1473
1058
-415
1034
-439
750
-723
736
-737
DIS 1.95
1413
1090
-323
1069
-344
1096
-317
1032
-381
10 -Year Storm
D/S Bowman Ilam
1768
1508
-260
1486
-2821
1122
-646
1059
-709
DIS 1-287
2088
1769
-319
1743
-345
1312.
-776
1238
-550
DIS 1.95
2012
1702
-310
1676
-336
1302
-710
1239
-773
5 -Year Storm
DIS Bowman Dam
2800
2641
-159
2630
-170
2382
-418
2340
-460
DIS 1-287
3396
3150
-246
3135
-261
2801
-595
2751
-645
DIS 1-95
2994
2707
-197
2.782
-212
2560
-434
2516
-478
50 -Year Storm
DIS Bowman Daze
3755
30()95
-60
3692
-63
3554
-201
3533
-222
D/S 1-287
4506
4391
-115
4384
-122
4167
-339
4136
-370
D/S 1-95
48,14
4650
-194
4638
-206
4253
-591
4194
-650
100 -Year Storm
DIS Bowman Dann
4322
4282
-40
4281
-41
4187
-135
4171
-151
DIS 1-287
5162
5062
-100
5055
-107
4890
-272
4864
-298
D/S 1-95
5621
5467
-154
5458
-163
5239
-382
5207'
-414
Raising the top of the Bowman Avenue Dam by two feet, particularly when coupled with
maximizing the storage potential in the Upper Pond does result in sizeable reductions in peak
Page 16 of 38 [.11:15.11. -1 1 1 '-. 1',, .
Final Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
flow rates. However, this alternative does result in negative impacts to the stability of the dam
and upstream properties.
Stability Analysis
The stability of the dam was assessed according to criteria set forth by NYSDEC in the
publication "Uttr`delinesafcrr Design n of Danis." The functions of the NYSDEC's darty safety unit
include: safety inspection of darns; technical review of proposed dam construction or
modification; monitoring of remedial work for compliance with dam safety criteria; and
emergency preparedness. Although the Lowman Avenue Dam is not on the state's inventory,
rehabilitation and/or modification to the dam is considered a perinitted activity due to its size and
impoundment volume; fence subject to NYSDEC approval.
The dam was analyzed wader the 100 -year flood condition, with headwater elevation of 59.70'
and tailwater elevation at 42.64. This represents roughly a 2 -foot increase in elevation. The
darn was analyzed to determine its resistance to sliding and overturning.
A, dam's resistance to sliding is said to meet the guideline requirements if the factor of safety is
greater than or equal to 1.25 for 100 -year flood conditions. Resistance to overturning is
measured in terms of the eccentricity of the resultant force acting on the base. A dam's
resistance to overturning is said to meet the guideline requirements if the resultant force acts
within the middle third of the base for normal conditions and within the middle half for 100 -year
flood conditions..
In the absence of a detailed geotechnical investigation, uplift pressures (caused by seepage
beneath the spillway) were calculated using full hydrostatic head values. Uplift pressures were
included in the spillway calculations for both normal and 100 -year flood leading conditions.
The results of the analysis indicate a factor of safety for sliding equal to 1.01 which does not
meet the minimum guideline requirements of 1.25. With regard to overturning, the eccentricity
of the resultant force for the 100 -year flood was calculated to be 0.71 feet which is within the
minimum guideline requirements.
In summary, due to minimum sliding criteria, modification to the darn by raising the elevation of
its crest will requite extensive rehabilitation/reconstruction in order to satisfy the minimum dam
safety requirements.
Upstream Impacts
An analysis was performed to determine the effects on upstream properties and facilities should
the water surface elevation in the Upper pond was raised by two -feet. The results indicate that
the backwater effect would raise the 100 -year base flood elevation for a distance of
approximately % mile upstream of the dam. At the Bowman Avenue Bridge the water surface
elevation is raised by an additional 1.87 feet. The additional flooding on Bowman Avenue
would further impede emergency access thus creating a public safety issue. Additionally, the
increase in water surface elevation would result in additional flooding on private properties
within the''/2 mile influence.
Although the impacts in this area might be perceived as limited to additional flooding in parking
areas and other non-residential facilities, they still present significant issues based on the flood
Page 17 of38 W I - II " II. til I,l, , 1vt:.
Final Project Report
Floud Mitigation Study
,bowman Avenue Dam Site and Lower Pond
study status of the stream. Having been studied by detailed methods, the stream has a regulatory
floodplain, floodway, and base flood elevations. Any proposed increase to the flood elevation at
any point along the stream would require coordination through the Federal Emergency
Management Agency (FEMA) for conditional approval and for a follow-up physical revision to
the regulatory flood hazard information. This process includes a detailed review of increased
impacts as they relate to potential increased risk to public safety and private and public property.
For a conditional request to be considered, FEMA needs documented proof that the local
government and all impacted property owners have been made aware of the proposed increased
flood hazards and that they would accept the increases upon completion of the proposed project.
Typically, conditional request for proposed projects that might result in an increase between
4.00' and 1.00' are considered acceptable given that all technical data supporting the increase has
been certified by a Professional Engineer and reviewed by an independent party for full
co.rnpliame with regulatory requirements including the previously mentioned property owner
notification. For conditional request where the increase would be greater than 1.00', FEMA
takes into consideration a greater level of detail due to the higher degree of increased risk.
In the case of raising the elevation of this dam, for example, consideration would be given to the
decreased level of service on bowman Avenue, potential for significant property damage to cars
parked in areas of inundation, and an evaluation of alternatives. In this case, there are feasible
alternatives to provide a decrease in flooding downstream of the darn without resulting in
increases upstream of the darn. Finally, the review process for a conditional request to increase
flood hazards, especially when a flood control device is involved, can be lengthy, ranging from a
3 month period to in excess of a full year of ongoing coordination. The time that would likely
lapse in the process to implement solutions to flooding problems might create an added burden
should flood conditions persist while no action if being taken.
Based on these issues, this alternative is removed from further consideration.
Combined Upper Pond Resizing and Orifice Optimization Alternatives
These alternatives combine Alternatives 3 and 4 that increase the volume of storage behind the
Bowman Avenue Dam to the maximum with Alternatives 5 through 8 that vary the size of the
orifice (opening) at the dam. A total of eight alternative configurations were considered:
Alt. 13. Maximizing the volume behind the pond, no dredging of the pond (bottom elevation
at 39.00) and an orifice area = 45.6 sf.
Alt. 14. Maximizing the volume behind the pond, no dredging of the pend (bottom elevation.
at 39.00) and an orifice area = 72.1 sf
Alt. 15. Maximizing the volume behind the pend, no dredging of the pond (bottom elevation
at 39.00) and an orifice area = 105.6 sf
Alt. 16. Maximizing the volume behind the pond, no dredging of the pond (bottom elevation
at 39.40) and an orifice area = 139.1 sf
Alt. 17. Maximizing the volume behind the pond, dredge the pond two feet (bottom elevation
at 3 7.00) and an ori f ice area = 45.6 sf.
Alt. 18. Maximizing the volume behind the pond, dredge the pond two feet (bottom elevation
at 37.00) and an orifice area = 72.1 sf
Page 18 of 38 Il I A ' ll. 1»1 1 114, N,
Final Project Report
Flood Mitigation Study
_ Bowman Avenue Dam Site and Lower Pond
Alt. 19. Maximizing the volume behind the pond, dredge the pond two feet (bottom elevation
at 37.00) and an orifice area = 105.6 sf
Alt. 20. Maximizing the volume behind the pond, dredge the pond two feet (bottom elevation
at 37.00) and an orifice area = 139.1 sf
Fables 9 and 10 contain the results of the analysis of Alternatives 13 through 20. As discussed in
the section on optimizing the orifice opening, the orifice size that would create the greatest
reduction in flow rate varies with the design storm frequency and the orifice opening size that
creates the optimum flow rate reduction has been highlighted.
oth the resizing of the upper Pond and the optimization of its outlet produce flow reductions that
warrant more detailed analysis. As part of that analysis the combination of the alternatives will
be included. The optimum percent reduction of flows for these alternatives is as follows:
Page 19 of 38
2 -'Year Storm
DI$ Bowman Dam
397
550 153
731
334
710
313
713
316
DIS 1-2117
565
690 125
902
337
865
300
869
304
DIS 1--95
681
782 101
964
283
932
251
935
254
5 -Year Storm
DRS Bowman Dam
1253
778
-475
1083
-170
1257
4
1418
165
D/S I-287
1473
1000
-473
1317
-156
1515
42
1693
220
DIS 1-95
1413
1128
-285
1421
8
1597
184
1711
298
10 -Year Storm
VIS Bowman Dam
1768
983
-785
1250
-518
1486
-282
1719
-4()
DIS 1-287
2088
1172
-916
1524
-564
1789
-299
2080
-8
DISI -95
2012
1309
-703
1649
-363
1900
-112
2154
142
5 -Year Storm
DIS Bowman Dam
2800
2314
486
1925
-875
1922
-878
2220
-581)
DIS 1-287
3396
2719
-677
2261
-1135
2317
-1079
2674
-722
DISI -95
2994
2570
-418
2332
-662
2484
-514
2777
-217
50 -Year Storm
DIS Bowman Dam
3755
3535
-220
3360
-395
3163
-592
2824
-931
DIS1-287
4506
4156
-350
3965
-541
3716
-790
3337
-1169
DISI -95
4844
4235
-609
3564
-1280
3502
-1342
3424
-1420
100 -Year Storm
DIS Bowman Dam
4322
4190
-132
4035
-287
3897
-425
3659
-663
DIS 1-287
5162
4890-272
4716
-446
4564
-599
4317
-845
DISI -95
5621
5239
-382
5020
-601
4832
-789
4513
-1108
Page 19 of 38
Final Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
MWE
2 -Year Storm
Orifice Opening (sf)
45.6
45.6
q457M690q293
37.9%
38.7%
DI5 Bowman Dam
397
536
135
717 320
854
10-Yeor Storm
DISI -287
565
669
104
879 314
1106
541 839 274
1-95
681
749
68
935 254
1148
467 902 221
5 -Year Storm
OriGee Opening (sf)
72.1
72.1
DIS Bowman nam
31.3%
32A%
DIS L-287
DIS Bowman Ilam
1253
768
-485
1071
-182
1244
-9
1392
139
D/S 1-287
1473
981
-492
1300
173
1496
23
1658
185
DISI -95
1413
1106
-307
1401
-12
1574
161
1673
260
10 -Year Storm
16.7%
1r)1S 1-95
19.7010
20.1010
018 Bowman Data
1768
957
-811
1240
-528
1475
-293
1709
-59
DIS 1-287
2088
1140
-948
1509
-579
1772
-316
2064
-24
DIS 1-95
2012
1129
-883
1631
-381
1880
-132
2132
12
25 -Year Storm
DIS Bowman Dam
2800
2268
-532
1894
-906
1913
-887
2212
-588
DIS 1-287
3396
2665
-731
2225
-1171
2305
-1091
2663
-733
DIS 1-95
2994
2539
-455
2299
-695
2470
-524
2763
-231
50 -Year Storm
DIS Bowman Dam
3755
3518
-237
3342
413
3144
-611
2815
-940
DIS 1-287
4506
4131
-375
3940
-566
3693
-813
3326-1180
D/SI-95
4844
4186
-658
3540
-1304
3481
-1363
3411
-1433
100 -Year Storm
D/S Bowman Dam
4322
4177
-145
4020
-302
3883
-439
-676
D/51-287
5162
4869
-293
4698
-464
4349
-613
J64
D/S 1-95
5621
52.13
-40$
4998
-623
4814
-807
-113
5 -Year Stam,
Orifice Opening (sf)
45.6
45.6
DIS Bowman Dam
37.9%
38.7%
DIS I-287
32.1%
33.4%
DIS 1-95
20.2%
21.7%v
10-Yeor Storm
Orifice Opening (sf)
45.6
45.6
D/S Bowman Dam
44.4%
45.9%a
D/S I-287
43.9%
45.4%
D/S I-95
34.9%
43.9%
25 -Year Storm
OriGee Opening (sf)
72.1
72.1
DIS Bowman nam
31.3%
32A%
DIS L-287
33.4%
34.5%
DIS I-95
22.1%
23.2%
50 -Year Storm
Orifice Opening (sf)
139.1
139.1
D/S Bowman Dam
24.8%
25.0%
D/S 1-287
25.9%
26.2%
D/S 1-95
29.3%
29.6%
100 -Year Storm
Orifice Opening (sf)
139.1
134.1
1]/S Bowman Dam
15.3%
15.60 -
D/S 1-2$7
16.4%
16.7%
1r)1S 1-95
19.7010
20.1010
Page 20 of 38r-,k(LCIAS. ItSLLLti. INC.
Find ,project Report
Flood Mitigation Study
Bowman Avenue Dant Site and Lower Fond
Modifications to the tower Pond Alternatives
The final category of alternatives involves modifications to the Lower Pond. The modifications
to the Lower Pond that were considered contained two components. The first was the removal of
the 1 -acre "peninsula" adjacent to Bowman. Avenue at the northwest side of the pond so as to
create additional storage and the second was modifications to the outlet from the pond.
The outlet modifications involved three scenarios. The first resulted from an evaluation of the
streatribed profile between the pond and 1-287 that was field surveyed by Sells. There is an
apparent "bump" in the profile approximately 300 feet from 1-287 where an 80 foot section of
the streambed rises up about 1.5 feet. The first outlet modification involves the removal of that
material. The second and third outlet modifications involve the creation of a spillway section for
the Lower fond, one of which is 75 feet and the other 120 feet, together with the downstream
strewnbed change and the removal of the peninsula.
Alt. 21. Removal of Lower Pored peninsula.
Alt. 22. Removal of Lower Pond peninsula and lowering the downstream overflow section to
elevation 27.5.
Alt, 23. Removal of Lower Pond peninsula, lowering the downstream overflow section to
elevation 27.5, and providing a 75 foot spillway at elevation 33.0.
Alt, 24. Removal of Lower Pond peninsula, lowering the downstream overflow section to
elevation 27.5, and providing a 120 foot spillway at elevation 33.0.
The peak flow rates are shown in Table 11.
Removal of the peninsula adjacent to Bowman Avenue at the northern side of the Lower Pond
and modifications to the overflow section have negligible effects in reducing the peals flow rates.
Based on these results these alternatives were not carried forward for more detailed analysis.
It should be noted that in order for the Lower Pond to function as a flood control measure, the
pond would need to exist in a pre -drained condition. This can be accomplished via gravity -based
and mechanical -based means. The study of these alternatives is beyond the scope of this project.
Page 21 of 38(,IIA,,: MI_ w41.1M. IN(,
Final Project Report
Flood Mitigation Study
Bowman Avenue Dant Site and Lower Pond
PREFERRED ALTERNATIVES
Based on the results of the Alternatives Analysis, three preferred alternatives were developed and
further analyzed to determine water surface elevations. Selection of the preferred alternatives
was based on several factors: cost, anticipated level of mitigation, and potential impacts on
upstream neighborhoods. The three alternatives are:
Alternative A: Optimizing the orifice opening at the dam
Alternative E: Optimizing the orifice opening and maximizing the Upper Pond
Alternative C. Optimizing the orifice opening, maximizing the Upper Pond area and
dredging 2 feet of sediment material (bottom elevation 37.0)
Methodology
Blind Brook and East Branch Blind Brook were studied by detailed hydrologic and hydraulic
methods for FEMA's preliminary FIS for Westchester County. Backup data was made available
to Sells through Michael Baker, Jr., Inc. The area studied in this report on Blind Brook is from I -
Page 22 of 38 CHAS. 11, 1'i-
2 -Year Storm
coad.
All. 21 Diff.
DIS Bowman Dam
397
397 0
397 0
30 0 397 0
DIS 1.287
558
$57 -I
555 -3
553 555 3
Dis 1-95
681
678 -3
668 -13
671 -10 687 6
5 -Year Storm
DIS Bowman Dam
1253
1253
0
1253
0
1253
0
1253
0
DIS 1-287
1467
1464
-3
1463
-4
1434
•33
1459
-8
DIS 1.95
1413
1412
•1
1413
0
1403
-10
1412
-1
10 -Year Sturm
DIS Bowman Dam
1768
1768
0
1768
0
1748
0
1768
0
DIS 1-297
2079
2077
-2
2077
-2
2032
-47
2068
-11
DIS 1-95
2012
2010
-2
7010
-2
1989
-23
2008
4
5 -Year Storm
DIS Bowman Dam
2800
2800
0
2800
Il
2800
0
2800
0
DIS 1-287
3384
3390
6
3378
-6
3335
•49
3347
.37
DIS 1-93
2994
2987
-7
2995
1
2987
-7
2993
-I
50 -Year Storm
DIS Bowman Dam
3755
3755
0
3755
0
3755
01
3755
0
D1S 1-287
4490
4459
-1
4503
13
4487
-3
4472
-18
DIS 1-95
4844
4857
13
4855
Il
4819
-25
4792
-52
100 -Year Storm
DIS Bowman Damn
4322
4322
4322
0
4322
0
4322
0
DIS 1-257
5 14LI
5100
44
5206
62
5192
48
517.5
31
DIS 1.95
5621
5554
(i7
5749
128
5708
87
5673
52
PREFERRED ALTERNATIVES
Based on the results of the Alternatives Analysis, three preferred alternatives were developed and
further analyzed to determine water surface elevations. Selection of the preferred alternatives
was based on several factors: cost, anticipated level of mitigation, and potential impacts on
upstream neighborhoods. The three alternatives are:
Alternative A: Optimizing the orifice opening at the dam
Alternative E: Optimizing the orifice opening and maximizing the Upper Pond
Alternative C. Optimizing the orifice opening, maximizing the Upper Pond area and
dredging 2 feet of sediment material (bottom elevation 37.0)
Methodology
Blind Brook and East Branch Blind Brook were studied by detailed hydrologic and hydraulic
methods for FEMA's preliminary FIS for Westchester County. Backup data was made available
to Sells through Michael Baker, Jr., Inc. The area studied in this report on Blind Brook is from I -
Page 22 of 38 CHAS. 11, 1'i-
Final Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
95 (south) to Interstate I-287 (north). For this study of this reach of Blind Brook, base data from
the FIS model was used as presented with the exception of flow rates. For our analysis we used
the discharges developed in our August 2007 Hydrologic Deport (see Appendix A). All other
data including crass sections, distances between crass sections, Manning's n values, bridge
geometry, ineffective flow areas, etc. was applied as represented in the FEMA study. The model
created was used as the baseline model for this report.
The software used for the FIS for developing water surface protides for Blind Brook and East
Branch Blind Brook is the ACOE's EIEC -RAS software. HCC -RAS is an improved windows
version of the DCS based HEC -2. The program is designed to perform one-dimensional
hydraulic calculations of natural and manmade channels. Water surface profiles are computed
using an iterative procedure called the standard step method. The water surface elevations are
calculated from section to section by solving the Energy equation. The bridge modeling
approach chosen in the FIS is the "momentum" for law flows and "pressure and/or weir" for
high flaws.
To take into account the peak discharge reduction at Bowman Dam site for each alternative, the
computed discharges were adjusted by an inflow/outflow ratio developed by the WinTR-20
software. The boundary condition (starting point of the backflow analysis) for each alternative
was determined from a rating curve (included in Appendix C) developed from the existing FIS
HEC -RAS water surface elevations at a section located approximately 850 feet downstream of
Interstate 1-95. The water surface elevations for the existing and three alternatives of
improvements at Bowman Avenue Dam site were computed and presented in the following
sections. Copies of the HEC -RAS outputs are also included in the Appendix C.
Table 12 provides a comparison of the discharge rates and water surface elevations arrived at by
the FIS and those presented in this report. Since Sells' August 2007 Hydrological Report
determined that the discharge rates in this reach of Blind Brook are greater than those used by
FEMA for the existing conditions our results are larger. For the alternative analyses differences
in the results stem from not only the variation in discharge rates but also from the manner in
which the outlet system at the Bowman Avenue Dam was modeled. In Sells' analysis we took
into account the ability of varying the orifice opening at the base of the dam as well as the other
outflow features such as the weir and dam site overflow. In the FEMA model the dam was
modeled as an "in-line structure". Input for this feature is a single weir and a single orifice, a
much simpler configuration than the existing conditions that Sells modeled. The differences in
the manner in which the actual dam, pond, overflow, and outlet work depending upon the water
surface elevation leads to the variations between our values and FEMA's. In some instances our
flow rates in the Alternatives will be greater than FEMA.'s and in others less.
Page 23 of 38
Final Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Fond
Oplitnix Orifice Opening
FIS Sells FIS sells
10 -Year Storm
1.95 (UIS) 1,521 22.93 1,982 24.59 1,521 22.93 1,789 23.79
Highland Rd, (UIS) 1,521 24.15 1,982 25.88 1,521 24.15 1,789 25.24
Purchase St. (UIS) 1,434 27.35 1,663 2833 1,434 27.35 1,344 27.73
1-287 DIS 1,374 32.32 1,663 32.73 1374 32,32 1,344 32.27
50 -Year Storm
1.95 (UIS)
2,497
26.55
3,078
3056 2,497
26.55
2,461
26.41
Highland Rd. (UIS)
2,497
27.49
3,478
31.01 2,497
27,49
2,461
27.39
Purchase St. (U1S)
2,353
30.12
2,767
3191 2,353
30.12
2,458
30.18
1-287 DIS
2,255
33.45
2,767
34.11 2,255
33.45
2,458
33.66
100 -Year Sto rm
1-95 (UIS)
2,984
30.33 3,583
32.17
1,984 30.33
3,274 31.12
Highland Rd. (UIS)
2,984
30.78 3,583
32.60
2,984 30.78
3,274 31.57
Purchase St. (UIS
2,812
31.71 3,346
33.44
2,812 31.71
3,117 32.55
1-287 i
2 694
34.01 3 346
34.97
2 694 34,01
3 l 17 34.54
Opt.Orifice
1
Opt. Orifice Openir"'
FIS
1181. Vol. willh Button]
Sells
FIS
t
Sells
Discharge
10 -Year Storm
1.95 (UIS)
1,521
22.93 1,289
22.12
1,521 22.93
1,1 12 21,48
Highland Rd.WU
1,521
24.15 1,289
23.04
1,521 24.15
1,112 21.72
Purchase St. (UIS)
1,434
27.35 933
26.45
1,434 2735
908 26.14
1-287 91S
1,374
12.321 933
31.47
1,374 32.32
908 31.41
50 -Year Storm
1-95 (VIS)
2,497
26.55
2,176
25.32
2,497
26.55
2,167
25.29
HiBhl and Rd. (UIS)
2,497
27.49
2,176
26.51
2,497
27.49
2,167
25.41
Purchase St. (UIS)
2,353
30.12
2,049
2900
2,353
30.12
2,042
2898
1-287 (DN
2,255
33.45
2,049
33.20
2,255
33.45
2,042
33.19
1IlMear Storm
1-95 (UIS)
2,984
30,33
2,877
30.07
2,984
30.33
2,861
30.04
Highland Rd. (1115)
2,984
30.78
2,877
30.52
2,984
30.78
2,861
30,08
Purchase St. (UIS)
2,812
31.71
2,798
31.54
2,812
31.71
2,787
31.51
1.287 MI5
2,694
3491
7.798
34.08
2,694
1 34,01
2,787
34,06
The upstream impact of each flood control improvement alternative was also determined for
areas upstream of the Bowman. Avenue Darn and on East Branch Blind Brook. The water
surface calculations were performed using; the FIS HEC -RAS model with Sells, discharges and
boundary conditions. The calculations were extended only to Long Ledge Court since most of
Page 24 of 3 8 (11 AS.
Final Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
the flooding experienced on Bast Branch Blind Brook is occurring south of Avon Circle located
approximately 500 feet south of Long Ledge Court.
The discharges used for East Branch Blind Brook were New York regression discharges for
unregulated streams as presented in the July 2007 Hydrologic Report. The starting water surface
elevations for each run were interpolated from rating curves developed at the Lower Pond just
downstream of the East Branch Blind Brook. The rating curves were derived from 1VinTR-20
computed water surface elevations at the Lower Pond for each alternative.
The rating curves at the Lower Pend for each alternative are included in Appendix E. The water
surface elevations for the existing and the impact of the three alternatives of improvements at
Bowman Avenue Dam Site are presentee! below.
For each of the alternatives the water surface elevation for existing and improved conditions was
computed at the downstream face of the 1-287 culvert and upstream face of the I-95 culvert, the
extents of the Indian Village neighborhood, as well as two intermediate locations (Purchase
Street and Highland Road).
Analysis Results
Alternative A: Optimizing the Orifice Opening at the Dam
As previously described in the Alternatives Analysis section of this report, this alternative
consists of retrofitting the Bowman Avenue Dam with an automated sluice gate. An automated
sluice gate has the ability to vary the opening size, thus providing the optimum orifice size for
the flow rate in the stream. The sluice gate would be automatically controlled based on water
surface elevations measured at a gauge mounted behind the Bowman Avenue Ilam. The results
of this alternative are provided in Table 13.
As can be seen, the reduction in water surface elevation, measured in feet, is particularly notable
during the 50 -year storm event. The 4.15 -foot reduction in water surface elevation upstream of
1-95 is attributed to the fact that the flow is passing through the I-95 bridge with little backwater
effect. During the 100 -year event, the stream flow does not pass the structure thus creating
backwater. See Figures 6 through 8 for water surface elevation of 10-, 5-, and 100 -year design
storms.
Slight modification to the upstream dam face would be required to accommodate the sluice gate.
A detailed inspection and analysis including darn cores would be required during subsequent
design phases. Additionally, upstream channel work and clearing and grubbing would be
required. The budgetary cost for this alternative is $1 - 2 million.
Fuge 25 of 38 ;11 t , I I.'sF:
Final Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
Table 13. Alternative
Optim lziog Orifice Opening
Water Surface Elevation
Exisfiiig Proposed
0111d.
A -
Orifice
2 -Year Storm
S
20.2
DIS 1-287
31.07 31.07 0.00
Purchase Street
25.65 25.65 0.00
Highland Road
21.41 21.43 0.02
UIS 1-95
20.77 20.80 0.03
5 -Year Storni
45.6
DIS 1-287
32.15
31.62
-0.53
Purchase Street
27,20
26,61
459
Highland Road
24.19
23.35
-0.84
UIS 1-95
1 2295
22.36
459
10 -Year Storm
72.1
D/S 1.287
32.73
32.27
-0.46
Purchase Street
2$.33
27.73
-0.60
Highland Road
25.88
25.24
-0.64
U/S 1-95
1 24.59
23.84
-0.70
25 -Year Storm
105.6
D/S 1-287
33.44
32.87
-0.57
Purchase Street
30.06
29.21
-0.$5
Highland Road
27.79
27.20
-0.58
U/S 1-95
1 26.93
26.19
-0.74
50 -Year Storm
- 139.1
D/S 1-287
34.11
33.66
-0.45
Purchase Street
31.91
30.19
-1.73
Highland Road
31.01
27.39
-3.62
U/S 1-95
30,56
26.41
-4.15
100 -Year Storm
139.1
DIS 1-287
34.97
34.54
-0.43
Purchase Street
33.44
32.55
-0.89
highland Road
32.60
31.57
-1.03
U/S 1-95
32.17
31.12
-1.05
Alternative B: Optimizing the Orifice Opening and Maximizing the Upper Pond
This alternative includes the work described in Alternative A above in conjunction with
maximizing the area of the Upper Pond. Maximizing the pond size will include removal of in-
situ soils along the northern side of the pond, removal of previously dumped material and rock
excavation (see Figure 4). The results of this alternative are provided in Table 14 below. As can
be seen, the water surface elevations are further reduced. See Figures G through 8 for water
surface elevation of 10-, 5-, and 100 -year design storms.
Page 26 of38 CHAS 11. 1N(.:.
Final Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
The budgetary construction cost for this alternative is $10 - S15 million. This includes the
removal of approximately 160,000 CY of material. For estimating purposes, it is assumed that
approximately 50% of this material is rock. Soil borings would he required during subsequent
design phases to accurately determine the extent of rock removal.
Table 14: Alternative 11 - Optimizing Orifice and
Maximizing
Upper Pond
Sm -face Elevatinil
011kil.
Difference
Water
Size Colid,
2 -Year Storm
20.2
D/S 1-287
31.07
30.90
-0.17
Purchase Street
25.65
25.52
-0.13
Highland Road
21.41
21.29
-0.12
U/S 1-95
20.77
20.66
-0.11
5 -Year Storm
45.6
D/S 1-287
32.15
31.29
-0.86
Purchase Street
27.20
26.27
-0.93
Highland Road
24.19
22.72
-1.47
U/S 1-95
22.95
21.89
-1.06
10 -Year Storm
45.6
D/S I-287
32.73
31.47
-1.25
Purchase Street
28.33
26.45
-1.88
Highland Road
25.88
23.04
-2.84
UIS 1-95
74.59
22, 12
-2.17
25 -Year Storm
72.1
D/S 1-287
33.44
32.51
-0.93
Purchase Street
30.06
28.28
-1.78
Highland Road
27.78
26.01
-1.77
UIS 1-95
26.93
24.73
-2.20
50 -Year Stormy
139.1
DIS 1-287
34.11
33.20
-0.91
Purchase Street
31.91
2.9.00
-2..91
Highland Road
31.01
26.51
-4.50
UIS 1-95
30.56
25.32
-5.24
100 -Year Storm
139.1
D/S [-287
34.97
34.08
-U9
Purehese Street
33.44
31.54
-1.90
Highland Road
32.60
30.52
-2.08
U/S 1-95
32. L7
30.07
-2.11)
Page 27 of 38 IN' -
Final Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Fond
Alternative C; Optimizing the Orifice Opening, Maximizing the Upper Pond and
Dredging 2 -feet of Sediment Material from Upper Pond
This alternative includes the work described in Alternative A and B above in conjunction with
dredging up to 2 feet of sediment accumulated in the Upper Pond (see Figure S). As previously
stated, the sediment is likely contaminated with typical roadway pollutants, such as lead, oil,
copper, zinc, iron and chromium. Soil sampling and testing would be rewired during
subsequent design phases. The results of this alternative are provided in Table 1-5 below. As
compared to Alternate B, this alternative only provides benefit during the lower intensity storm
events (2 -year or less). During more intense storms, this alternative provides virtually the same
water surface elevations as compared to Alternative B. See Figures 6 through 8 for water surface
elevation of 10-, S-, and 100 -year design storms. The budgetary construction cost for this
alternative is $18 - $22 million, This includes the removal of approximately 30,000 CY of
contaminated material.
Page 28 of 38
Final Project Report
Flood Mitigation Study
Bowman Avenue ferry Site and Lower Pond
2 -Year Storm
20.2
D/S 1-287
31.07
30.79
-0.28
Purchase Street
25.65
25.32
-0.33
Highland Road
21.41
20.93
-0.48
UIS 1-95
20.77
20.32
-0.45
5 -Year Storm
45.6
D/S 1-287
32.15
31.25
-0.90
Purchase Street
27.20
26.22
-0.98
Highland Road
24.19
22.70
-1.49
U/S 1-95
22.95
21.79
-1.16
10 -Year Storm
45.6
D/S 1-287
32.73
31.41
-1.32
Purchase Street
28.33
26.14
-2.19
Highland Road
25.88
22.62
-3.26
WS 1-95
24.59
21.48
-3.11
25 -Year Storm
72.1
DIS 1-287
33.44
32.47
-0.97
Purchase Street
30.06
28.20
-1.86
Highland Road
27.78
25.92
-1.86
UIS 1-95
26.93
24.62
-2.31
50 -Year Storm
1:39.1
D/S 1-287
34,11
33.19
-0.92
Purchase Street
31,91
28.98
-2.93
Highland Road
31.01
26.48
-4.53
U/S 1-95
30.56
25.29
-5.27
100 -Year Storm
1.39.1
D/S 1-287
34.97
34.06
-0.91
Purchase Street
33.44
31.51
-1.93
Highland Road
32.60
30.48 1
-2.12
U/S 1-95
32.17
30.04
-2.13
Page 29 of 38
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CITY vFRYEFLOOD MITIGATION MEASURES
FIGURE NO.
AH m
BOWMAN AVhNU ~; IMAM KtStKVUIK - MAAIMILLL) K-tShK v um A&LA - BOTTOM ELEVATION 37.0
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Final Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
Upstream Impacts
Although the goal of making improvements to the Upper Pond area is to alleviate flood
conditions downstream of 1-287, it is equally important to ensure that there are no negative
effects to upstream neighborhoods. Additional analyses were performed along a portion of Blind
Page 35 of 38
B
t
Proposed
2 -Year Storm
201
-
BowmanAvenueDam
11.75
51,75 0.00
51,15 482 •3,75
51,75 47.20 455
Bowman Avenue (UIS)
55.91
5591 0.00
5591 55.91 0,00
55,91 55.91 0,00
Westchester Ave. Culvert (UIS)
67.11
6711 0.00
67,11 67,11 0,00
6711 67,11 0,00
Deer Run Area
84.25
84.25 0.00
8411 84.25 U01
8415 8415 0.00
5 -Year Storm
45.6
Bowman Avenue Dam
53.90
53.85
•0.05
53.90
49.80
-4,10
53,90
49,20
-4.70
Bowman Avenue( 18)
58.06
58.06
0.00
58,96
58.06
0,00
58,06
58.06
0,00
Westchester Ave, Culvert (UIS'
68.30
68.30
0.00
68.30
68.19
-0.01
6830
6819
401
Deer Run Area
85,05
85,05
0.00
85,05
85,05
0.001
81,05
85,05
0,00
III -Year Sturm
71.1
BowmanAvoueDam
56.40
55.30
-1.10
56.40
51.40
-5.00
56,40
51.00
-1.40
Bowman Avenue (0)
5937
59.18
-0.19
59.37
59.2)
-0,14
59.37
59.23
•0.14
Westchester Ave. Culvert (UIS)
68,83
68,83'
0.00
68.83
6883
0,00
6883
68.83
0.00
Deer Run Area
85,49
85A9
0.00
85,491
85.49
0.001
854
85:49
0.00
25 -Year Storm
105.6
Bowman Aveoue Dam
58.95
57.80
•1.15
58.95
55,20
-3,75
5895
505
-4,00
Bowman Avenue (0)
62.03
61.24
479
62.03
60.P
-1.14
62,03
60.89
-1.14
Westchester Ave. Culvert (UIS)
70A5
70.45
0.00
70.45
10.45
0.00
70.45
70.45
0.00
Deer Run Area
8(x.01
86,01
0.00
86.01
86.01
0A0
86.01
8601
0.00
50 -Year Storm
139.1
Bowman Avenue Dam
59.10
58.45
-0.75
59.20
57.45
-1.75
59,20
5725
-195
Bowman Avemne(LYS)
62.92
62.50
•0.42
62.92
62.14
•0,78
62,92
6211
.0.81
Westchester Ave. Culvert {UIS)
70,90
7090
0,00
70.90
70.90
0,00
70.90
70.90
0.00
Deer Run Area
86.41
86.41
0.00
86A11
86,41
0.001
86.41
86,41
0.00
100 -Year Storm
139.1
Bowman Avenue Dam
59.60
5855
-1,05
590
57.90
-1,70
59,60
57.79
-1.81
Bowman Avenue (UIS)
63.54
63,66
0.12
6354
63.53
-0,01
63.54
63.53
•0.01
Westchester Ave, Culvert (DIS)
11.84
71.84
0.00
7184
71.84
0,00
71,84
71.84
0.00
Deer Run Arra
8ti,82
86.8?
0,00
86,82
8632
0,00
86,82
86,81
0,00
Page 35 of 38
Final Project Deport
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
Brook above the Upper Pond and East Branch Blind Brook. Upstream of the Upper Pond along
Blind Brook, water surface elevations were computed at Bowman Avenue, Westchester Avenue,
and the Deer Run (Brook Lane) area. Along East Branch Blind Brook, elevations were
computed at Bowman Avenue, Westchester Avenue, and Long Edge Drive. Tables 16 and 17
provide water surface elevations for Alternates A, B and C.
In all instances none of the alternatives will create any impacts on the upstream areas.
Page 36 of 38 1;11A4,, 11, IN' L.
Mini Elmlion 1)(39.11
wi Ill Doi loin F.Itutiona(All
2•Yeu Storm
202
Dilirtott
1.287
31.35 31,15
-0,20
31,35 3130 -0,05
3133 31.25 4,10
B owman Avenue (0)
33.68 33,68
0,00
33,68 33,66 0.00
33.68 33.68 03
Westchester Ave, Culvert (DIS)
39,94 34.94
0.00
39.44 39.44 0.00
39.94 3494 0.00
Long Ede Drive
1
62,07 62,071
0,00
61071 62.03 0,001
62.02 62,07 0,00
5 -Year Storm
43,6
1.281
3190
31.32
418
31.40
31,42
-04
31.90
31.40
-0,50
Bowman Aveuue(LIS)
3454
34,54
0,00
34,54
34,54
0.00
34.54
34.54
0,00
Westchester Ave. Culvert (DIS
40,49
40.44
0,90
40A9
40,44
02
40.49
40,44
0.00
Long Edge Drive Dl5
1 62,88
62.881
0,00
62,881
62.88
0,001
62.88
62,881
0,00
10 -Year Storm
72.1
1.287
32,35
32.05
•0,30
32,33
31,60
-0,75
3233
31,60
•0.85
BowmanAvenue(1115)
36,09
36,09
0,00
36.09
36,04
0.00
36.09
36.04
0,00
Westchester Ave. Culvert (H
40,67
4,067
0A0
40.67
40,67
0.00
40.67
40,67
0,00
Long Ede Drive DIS
63,25
63,251
0,00
63,251
63,25
0.001
63.25
63.251
0,00
25 -Peat Stornr
105.6
-
--
1.287
3235
32.05
.0,30
32,151
31.60
-0.71
3235
31,50
-0,85
Bowman Avenue (I1IS)
37,35
3735
0.00
37,33
37,35
0.00
3735
371
0.00
Westchester Ave. Culvert (D!S)
40,90
40.90
0,00
40,40
4090
0,00
490
40,40
0.00
Long E40 Drive (DN
1 63.65
63.651
0,00
63,65
63,65
0.001
63.65
63.651
0,00
50•Year Storm
139.1
-
1.287
32,75
32.35
-0.40
32,75
32,20
-0.55
32,75
32,13
•0.62
Bowman Avenue PS)
38,47
38.47
0,60
38,47
36,47
0,00
36,47
36,47
0.00
Westchester Ave, Culvert (D!S
42.09
4209
0.00
42,04
42,04
0.00
42.09
42.09
0.00
Long Ede Drive (181
1 64,40
64.901
0.00
64.901
6490
0.001
64.90
64.901
0.00
100 -Year Storm
134.1
1.287
33,10
32.40
.0.10
33,10
32.65
-0,45
33.10
3264
-0,46
Bowman Avenue (UIS)
40.06
40,06
0.00
40,06
40,06
0,00
40,06
40,06
0.00
Westchester Ave. Culvert (D!S
4131
41.31
0.00
41,31
4131
0,00
41,31
41.31
0,00
Lou:Ede Drive (DJS)
64.271
64.271
0,001
64,271
64.17
0.001
64.271
61.211
0.00
Page 36 of 38 1;11A4,, 11, IN' L.
Final Project Report
Flood Mitigation Study
Bowman Avenue loam Site and Lower Fond
CONCLUSION AND RECOMMENDATIONS � e
1. Orifice Optimization: It is our recommendation to move forward with detailed design
for the installation of the automated sluice gate as this option presents the most cost-
effective solution for mitigating downstream flooding. As previously stated, the
automated sluice gate has the ability to vary the outlet opening, thus providing the
optimum orifice size for the flow rate in the stream. The sluice gate would be
automatically controlled based on water surface elevations measured by an actuator and
level control at the dam. The sluice gate would have remote control abilities via a
SCALA system, however manual overrides will also be provided at the installation. The
budgetary construction cost for this alternative is estimated at $ 1 - $2 million. This
alternative will not result in upstream impacts.
2. Maximizing Storage at Upper Pond: Immediately conduct subsurface investigation at
the upper pond so as to determine location and condition of underlying bedrock.
Additionally, soil sampling and testing is necessary to determine level of contamination.
We believe this information is necessary to further evaluate the feasibility and cost-
etIectiveness of maximizing the storage capacity of the upper pond. In conjunction with
this, the City should evaluate means in which to provide maintenance access to the upper
pond.
3. Lower Pond Alternatives: Additional studies should be performed to investigate the
feasibility of modifying the Lower Pond so as to allow for it to function as a flood control
measure. These options include pre -draining via gravity -based and/or mechanical -based
means.
4. Revise FIS and FIRM Mapping: We feel the City should prepare a revised version of
the FIS and FIRM mapping incorporating the discharge values determined as part of this
study. We believe the discharge values developed as part of Sells' August 2007
Hydrologic Report are a more accurate representation of actual flood events based on
methodology, calibration, and historical information. In a community where there are so
many houses within and immediately adjacent to the floodplain, the difference in water
surface elevation could be the difference of dozens of houses being flooded or susceptible
to deeper Flooding and more damage.
5. Hydraulic Improvements at Avon Circle: The available FEMA HEC -RAS model was
used to assess possibilities of improving the flood conditions during various storm events
in two areas located in the Village of lye Brook: the Avon Circle area situated on the left
batik of East Branch Blind Brook between Westchester Avenue and Long Ledge Drive,
and the Brook Lane area located on the left bank of Blind Brook between Westchester
Avenue and Deer Run.
A few models were developed using lower tailwater depths that resulted from the
reduction of the peak discharges downstream of the Bowman Dam and from lowering the
overflow just upstream of I-275 from elevation 29.0 to elevation 27.5. Although the
Bowman Avenue Bridge on East Branch Blind Brook seems to be undersized, the bridge
backwater does not carry over to Westchester Avenue. Preliminary calculations suggest
Page 37 of 38 i 1115.11.tii t 1 �.I�r .
Final Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
that increasing the size of the existing Westchester Avenue culvert unit from 5 feet in
diameter to a 12 -foot by 6 -foot 'box culvert will lower the water surface elevations
between 0.8 and 4.0 feet during various storm events (Appendix F). In order to provide a
final sizing, the existing FEMA model should be supplemented with a topographical
survey and the model updated. Water surface profiles and cross sections are included in
Appendix F.
6. Evaluation of Brook Lane: The FEMA hydraulic model was also used in assessing
flood improvements in the Brook lane area. It seems that the flooding in this area is in
connection with a relatively wide flood plain in some sections. The condition of the
Bowman Avenue bridge on Blind brook seems similar to the one on East Branch :Blind
Brook; it is undersized but its backwater does not carry over to Westchester Avenue. In
order to lower the water surface elevations in the Brook Lane area, various bridge
opening sizes were analyzed; widening the bridge by as much as 10 feet and increasing
the bridge height be 2 feet, respectively. These changes in bridge openings resulted in
minimal water surface elevation reductions (Appendix F). To help determine the
influence the Westchester Avenue bridge has on the Brook Lane area, a model where:
there is no bridge at this location was developed. The hIEC-RAS output results showed
that during various storm events, the backwater created by the existing Westchester
Avenue bridge is lower than 6 inches. Therefore, it was concluded that the existing
bridge opening is basically adequate and the flooding in this area is connected with a
wider floodplain rather than an undersized structure. Water surface profiles and cross
sections are included in Appendix F.
Page 38 o°f38
Project Report
Flood Mitigation Study
Bowman Avenue Dam Site and Lower Pond
BIBLIOGRAPHY
1. Survey Report for Flood Control. Army Corps of Engineers (1968)
2. Flood Insurance Study. Federal Emergency !Management Agency (1979)
3. Watershed flan and Environmental Impact Statement Blind Brook Watershed. U.S.D.A.
— Soil Conservation Service (1979)
4. City of Rye Development Plan. City of Rye (198 5)
5. Long Island Sound Study — The Comprehensive Conservation and Management Plan.
(1994)
6. Controlling Non -point Source Pollution in Long Island Sound. Westchester Advisory
Committee 3 and Westchester County Department of Planning (1998)
7. Flood Mitigation Plan. City of Rye (200 1)
8. Technical Memorandum Evaluation of the Bowman Avenue Danz Site Blind Brook
Watershed City of Rye (2000)
9. Draft Hazard Mitigation Plan. City of Rye (2004)
10. Update on Preliminary Hydrologic Analysis, for the .Blind Brook TYatershed Management
Plan. Army Corps of Engineers (2007)
11. Summary Review of Existing Information for the !Blind Brook Watershed Management
Plan. Army Corps of Engineers (2007)
{� R.fl ti's. i�.�l.l i � 1'•.+.
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