continuum draft feis oct 2012 - appendix 4 stormwater management file_73

7/29/2019 Continuum Draft FEIS Oct 2012 - Appendix 4 Stormwater Management file_73

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THE CONTINUUM COMPANY

IRVINGTON, NEW YORK

STORMWATER

MANAGEMENT

REPORT

Prepared for:

The Continuum Company LLC590 Madison Avenue, 26th Floor

New York, NY 10022

Prepared by:

Divney Tung Schwalbe, LLPOne North Broadway, Suite 1407

White Plains, NY 10601

September 27, 2011Revised September 24, 2012


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IRVINGTON, NEW YORK

STORMWATER MANAGEMENT REPORT

TABLE OF CONTENTS

PAGENO.

EXECUTIVE SUMMARY

I. SITE PLANNING ..........................................................................................................I1

A. Site Location ............................................................................................................I1B. Existing Site Conditions ...........................................................................................I1C. Redevelopment Planning Strategies ..........................................................................I2D. Soil Types .................................................................................................................I5

II. WATER QUALITY TREATMENT VOLUME (WQv) ................................................II1

A. Redevelopment WQv Methodology .........................................................................II1B. Redevelopment WQv Calculations ...........................................................................II2

III. RUNOFF REDUCTION (RRv) ....................................................................................III1

A. Redevelopment Requirements for RRv ....................................................................III1

B. Redevelopment RRv Calculations ............................................................................III1

IV. ADJUSTED WATER QUALITY TREATMENT VOLUME (WQv) ...........................IV1

A. Standard Stormwater Management Practices (SMP) ................................................IV1B. Alternative Practices Accepted by the NYSDEC .......................................................IV1

V. WATER QUANTITY CONTROL ................................................................................V1

VI. EROSION & SEDIMENT CONTROL .........................................................................VI1A. Temporary Erosion & Sediment Control Measures ..................................................VI1B. Maintenance of Stormwater Management Measures ................................................VI2


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IRVINGTON, NEW YORK

STORMWATER MANAGEMENT REPORT

LIST OF TABLES

TABLE PAGENo. NO.

1 Existing Drainage Conditions .............................................................................. II32 Redeveloped Drainage Conditions ....................................................................... II43 Stormwater Quality Management Measures ......................................................... II54 Stormwater Quantity Design Flow Summary ....................................................... V3

5 Stormwater Management Basin Design Summary ................................................ V46 Stormwater Management Inspections & Maintenance of Permanent Structures ... VI2

LIST OF FIGURES

FIGURE PAGENo. NO.

SWM1 Site Location Map ................................................................................................ I3SWM2 Existing Drainage Conditions Map ...................................................................... I4SWM3 Proposed Drainage Conditions Map..................................................................... I6


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Executive Summary

This Stormwater Management Report has been prepared to address changes to the stormwater

characteristics at the 4.63 acre 30 South Broadway site that would result from its redevelopment

into The Continuum Company facility project in the Village of Irvington, New York. Stormwater

management requirements of the proposed redevelopment, including the anticipated site

disturbance of approximately 3.5 acres, will be evaluated and quantified in terms of stormwater

runoff and pollutants associated with onsite stormwater discharge.

The stormwater management design of the redeveloped facility adheres to a five step process

outlined by the New York State Department of Environmental Conservations (NYSDEC) New

York State Stormwater Management Design Manual (NYSSMDM), dated August 2010. The

following report is a summary of the strategies employed to redevelop the existing developed site

while meeting the stormwater regulations and requirements included in the New York State

Department of Environmental Conservation (NYSDEC) SPDES General Permit GP010001 and

the Village of Irvington Town Code, Chapter 183, Stormwater Management requirements.

Through the use of proposed site planning methods, planned green infrastructure techniques,

standard and alternative stormwater management practices (SMPs) at the redeveloped site, the

proposed stormwater peak rates of stormwater runoff are estimated to be at or below existing

conditions for each 24hour storm event up to and including the 100year storm event. In addition,

through the use of stormwater runoff reduction measures where feasible, the increases in total

runoff volume from the entire redeveloped site will be mitigated.


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I1

I. Site Planning

A. Site LocationThe project site is located in the Village of Irvington, New York at 30 South Broadway.

The site is bounded to the north by a residential coop complex, to the west by the Old

Croton Aqueduct, to the south by two residential homes and the Barney Brook which runs

parallel to Station Road and to the east by South Broadway. Via underground storm

drainage pipe systems and overland flow, the site ultimately discharges to the Hudson River

via the Barney Brook, part of the Lower Hudson River Basins Watershed. Refer to the Site

Location Map, Figure SWM1, for an aerial view of the site and surrounding areas.

B. Existing Site Conditions:The existing 4.63 acre site is currently developed and occupied by the Foundation for

Economic Education. The property consists of a threestory main building with adjacent

accessory buildings, sheds, parking and drives. Outside the limits of these impervious areas

and associated lawns, the terrain is partially wooded and sloped. No water quality measures

exist as part of the currently developed site. Per the Federal Emergency Management

Agency, the project site is not located within a 100 or 500year flood zone, but ultimately

discharges to the Barney Brook which has been mapped to be in these flood zones.1

The 4.63 acre site is part of a larger overall study watershed totaling 10.28 acres, including

neighboring parcels north to Main Street and south to the Barney Brook. An Existing

Drainage Conditions Map, Figure SWM2, has been prepared to identify the natural

resource areas and existing drainage patterns and discharge points (DP) of the study

watershed. The site and surrounding drainage areas in the study watershed are divided into

four separate subwatersheds, with two of the four subwatersheds encompassing almost 90%

of the study watersheds. Nearly half of the site discharges via overland flow to the Barney

Brook along the southern boundary of the site (DP 3), with the remainder of the main

1Federal Emergency Management Agency Flood Insurance Rate Map Number 36119C0261F, effective date September28, 2007.


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I2

portion of the developed portion property discharging to the east to an existing storm

drainage system along South Broadway (DP 1). Less than 20% of the site discharges via

sheet flow to the same existing storm drainage system along South Broadway just upstream

of the existing culvert discharging into the Barney Brook (DP 2). The final subwatershed

encompasses approximately half of the existing parking area and discharges via both existing

underground storm drainage system and overland flow to the Old Croton Aqueduct (DP 4)

and ultimately discharges into the Barney Brook.

The entire existing study watershed, approximately 10.28 acres in size, includes

approximately 3.62 acres of impervious area, an estimated 35% of the study area. Of the

impervious area within the study watershed, an estimated 1.06 acres of impervious area is

located on the project site, an estimated 23% of the property.

C. Redevelopment Planning Strategies:The main goal of the redevelopment of the site is to minimize the additional disturbance of

undeveloped portion of the property in order to preserve the remaining natural features of

the developed site, while maintaining the general drainage patterns of the existing site. The

proposed action will be a combination of both redevelopment and new construction, as

defined by the NYSDEC New York State Stormwater Management Design Manual

(NYSSWMDM), August 2010. The portions of the site considered redevelopment are areas

of the site that are currently disturbed and will be reconstructed as either pervious or

impervious surfaces. The estimated 1.3 acre additional impervious area proposed will be

treated and designed as new construction as defined by the NYSSWMDM.

By substantially retaining the main building, driveway and dropoff circle and then by siting

the remainder of the proposed facility across the former footprint of the existing buildings,

parking areas and lawn areas, the northern and eastern portions of the site were generally

maintained, and the impacts to the remaining natural features are minimized and increases

to the total impervious controlled. By milling and repaving these existing areas, grading,

earthwork cut and site disturbance will be minimized. In certain locations where the main


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I5

house, driveway and dropoff circle are to generally remain, there is inadequate space for

treating stormwater runoff. The western and southern portions of the site will be

substantially developed, while maintaining the existing buffer where feasible. Green

infrastructure techniques, standard stormwater management practices and alternative

measures will improve the stormwater quality conditions of the runoff from the site from

current conditions.

In addition to careful site planning strategies, stormwater management measures have been

proposed to ensure that the developed conditions of the site generally maintain the pre

construction hydrology of the site, including the implementation of underground detention

pipes, porous pavement, rain gardens and stormwater planters. Refer to the Proposed

Drainage Conditions Map, Figure SWM3 for locations of these measures. These measures

significantly improve the overall water quality of the stormwater runoff from the site by

providing diversion, infiltration and an increased travel time allowing for sediment removal

and uptake into the plant regime.

D. Soil Types:The soil classifications and their limits of the study watershed were provided from mapping

compiled by the U.S. Department of Agriculture Soil Service Soil Survey of Putnam and

Westchester Counties, New York which has classified the soil types as follows:

Soil Symbol Hydrologic Soil Group Description

ChE B Charlton LoamSuB B Sutton LoamUhB B Urban Land Charlton Complex

The project site is estimated to be Urban Land Charlton Complex, hydrologic soil group

(HSG) B, referred to as very deep, well drained, and gently sloping Charlton soil2. Geotech

findings indicate that the site is overlain by natural glacial deposits. The limit of disturbance

area for the proposed redevelopment of the site is in this area of urban land and HSG B.

2Soil Survey of Putnam and Westchester Counties, New York USDA Soil Conservation Service, pages 6667.


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II1

II. Water Quality Treatment Volume (WQv)

As a project containing portions of redevelopment and new construction, the required water

quality volume (WQv) is estimated as a percentage of WQv from the existing, previously

untreated, onsite area plus 100% of the WQv of the increased impervious area. Table No.1,

Existing Drainage Conditions, illustrates a breakdown of the impervious and pervious areas

within each subwatershed discharging to each of the design points. Table No. 2, Redeveloped

Drainage Conditions, tabulates the proposed conditions of the redeveloped site for The

Continuum Company facility. The WQv will be calculated based on the original 1.06 acres of

previously untreated impervious area on the current developed site plus the WQv from the 1.3

acres of newly created impervious area.

A. Redevelopment WQv MethodologyPer Chapter 9,Redevelopment Projects, of the New York State Department of Environmental

Conservation (NYSDEC) New York State Stormwater Management Design Manual

(NYSSMDM), August 2010, required WQv is calculated using one or more of four separate

options. The first option proposes the reduction of impervious cover from existing

conditions to redeveloped conditions by 25% or more. In order to meet The Continuum

Company facility program, provide circulation, adequate parking and emergency access

while generally preserving the existing buffer, it was not feasible to reduce the impervious

cover of the site.

The second option for redevelopment WQv calculation proposes that a minimum of 25% of

the WQv from the existing disturbed impervious area is captured and treated by the

implementation of a NYSDEC standard practice or green infrastructure technique. The 1.3

acres of newly created impervious area would require 100% of its WQv be captured and

treated. The proposed redeveloped site will meet the NYSDEC water quality treatment

objective using this option.


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II2

The third option allows for the use of alternative practices, sized to meet a minimum of 75%

of the water quality volume from the disturbed, impervious area. It is anticipated the

stormwater runoff from the existing main house to remain, portion of the dropoff circle

and entry driveway will be treated by using a hydrodynamic separator, an approved

alternative practice by the NYSDEC.

The fourth option proposes a combination of the first three options, allowing for the use of

standard and alternative practices. It is anticipated that this option will need be employed.

B. Redevelopment WQv CalculationsRefer to Table No. 3, Stormwater Quality Management Measures, for a detailed breakdown

of the proposed stormwater management measures to be provided to address stormwater

quality management on the project site. The calculations are based on the existing total

impervious area on site of 1.06 acres plus the addition of 1.29 acres of impervious area,

totaling 2.35 acres under redeveloped conditions, a mixture of redevelopment and new

construction.


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TABLENO.

1

CONTINUUM

IRVINGTON,NEW

YORK

EXISTINGDRAIN

AGECONDITIONS

(3)

(4)

CN

Tc

Roof

Pavement

IMP.TOTAL

(HRS)

A

0.9

5

1.7

1

2.6

6

2.1

2

4.7

8

82

0.2

3

1

B

0.0

0

0.3

4

0.3

4

0.8

5

1.1

8

68

0.0

7

2

C

0.2

6

0.2

8

0.5

4

3.5

7

4.1

1

65

0.1

8

3

D

0.0

0

0.0

9

0.0

9

0.1

3

0.2

2

76

0.0

6

4

TOTALAREA

3.6

2

6.6

6

10.28

1.

Areabasedonwatershedevaluation,

includingareasupstreamofprojectsite.

Areaw/inPL(ac)=

4.63

Imperviousareaw

/inPL(ac)=

1.06

2.

TotalAreausedfo

rTR55Modeling;WaterQuality

evaluationtobebasedonareawithinlimitofdisturbance

3.

CN=CurveNumb

er

4.

Tc=TimeofConc

entration,

InTR55ModelingminimumTc=0.0

83hrsused

DESIGN

POINT

#

A

REA(AC)(1)

IMPERVIOUS

WATERSHED/

SUBBASINID

TOTAL

AREA(2)

PERVIOUS


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TABLENO.

2

CONTINUUM

IRVINGTON,NEWYORK

REDEVELOPED

DRAINAGECONDITIONS

(3)

(4)

CN

Tc

Roof

Pavement

IMP.TOTAL

(HRS)

AA1

0.9

6

1.2

3

2.1

9

0.9

9

3.1

8

86

0.2

1

1

AA2

0.0

0

0.1

3

0.1

3

0.0

1

0.1

4

96

0.0

8

1

AAEx

0.7

7

1.2

2

1.9

9

1.6

2

3.6

1

81

0.2

3

1

BBEx

0.0

0

0.3

3

0.3

3

0.3

7

0.7

0

75

0.1

4

2

CCEx

0.0

8

0.1

8

0.2

6

2.3

4

2.6

0

62

0.1

6

3

DD

0.0

0

0.0

0

0.0

0

0.0

4

0.0

5

64

0.1

3

4

TOTALARE

4.9

1

5.3

7

10.28

1.

Areabasedonw

atershedevaluation,

includingareasupstreamofprojectsite.

Impervio

usareaw/inPL(ac)=

2.35

acres

2.

TotalAreaused

forTR55Modeling;WaterQualityevaluationtobebasedonareawithinlimitofdisturbance

3.

CN=CurveNum

ber

4.

Tc=TimeofConcentration,

InTR55ModelingminimumTc=0.0

83hrsused

5.

A2%increasein

imperviousareainSubbasinsAA1andAA2areincludedtoaddress

modestdesignchangesintheSPAprocess

WATERSHED/

SUBBASINID

DESIGN

POINT

#

AREA(AC)(1)

IMPERVIOUS(5)

TOTAL

AREA(2)

PERVIOUS


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TABLE NO. 3

CONTINUUM

IRVINGTON, NEW YORK

STORMWATER QUALITY MANAGEMENT MEASURES

ONSITE SUMMARY

I3 Rv4

S7

IMP IMP TOTAL Standard Redevel Standard Adj RRv Provided Required Provided1

(ac) (ac) (ac) (%) (cft) (cft) (cft) (cft) (cft) (cft) (cft)

1.06 2.35 4.63 51 0.51 0.40 11,063 7,174 10,525 4,163 5,207 1,966 2,022

REDEVELOPMENT SELECTION PRACTICES NOTES

S.A. WQv/RRv Add'l WQv 1.Stormwater Quality based on redevelopment and new construction.

(sf) (cf) (cf) 2.Design per New York State Stormwater Management Design Manual, 08/10.

Green Infrastructure Measure: 3. I=Impervious Cover (%)

Green Roof 2,350 242 4. Rv = 0.05+0.009(I), Minimum Rv=0.2

Rain Garden 2,275 1,365 5. P=90% Rainfall Event Number

Stormwater Planter 2,945 2,618 6. P (in) = 1.3 (See Figure 4.1, NYSSMDM, August 2010)

Porous Pavement 1,165 130 7. S=Hydrologic Soil Group (HSG) Specific Reduction Factor

Stabilized Grass Emg Access 6,450 853 8. WQv=[(P)(Rv)(A)]/12; Redevelopment WQv: 25% Ex WQv (using NYSDE

Alternative SWM Measures: Standard Measures or GI of Ex Imp Area (using NYSDEC Standard or GI RRv

Hydrodynamic Separator 19,645 2,022 Measures) + 100% WQv of Increased Impervious Area.

9. Runoff Reduction Vol, RRv=[(P)(Rv*)(Ai)]/12; per NYSSMDM Chapter 9,

meeting RRv is not required for redevelopment projects, but encouraged.

1. Green Roof:

depth 0.25 SWM Surf Area Stor Vol WQv

porosity 0.2 Measure (sf) (cf) (cf)

depth 0.17 Mem Care 1,400 163 144

porosity 0.4 Equivalent Depth: 0.1 Main Bldg 950 111 98

2,350 242

2. Rain Garden: Af=WQv x (df) / [k x (hf + df)(tf)]

Egr Soil: k=(ft/d) 0.5 SWM Surf Area Stor Vol WQv quiv Imp

depth 2.5 Ponding Depth: 0.5 Measure (sf) (cf) (cf) Area (sf)

porosity 0.2 Filter Time: 1.0 RG1 925 1,172 555 5,393

Gravel: depth 0.67 RG2 750 950 450 4,372

porosity 0.4 Equivalent Depth: 1.3 RG3 600 760 360 3,498

2,275 1,365 13,263

3. Stormwater Planter: Af=WQv x (df) / [k x (hf + df)(tf)]

Egr Soil: k=(ft/d) 4 SWM Surf Area Stor Vol WQv quiv Roof

depth 1.5 Ponding Depth: 0.5 Measure (sf) (cf) (cf) Area (sf)

porosity 0.2 Filter Time: 0.2 Planter1 815 978 724 7,039

Gravel: depth 1.00 Planter2 635 762 564 5,484

porosity 0.4 Equivalent Depth: 1.2 Planter3 645 774 573 5,571

Planter4 850 1,020 756 7,341

2,945 2,618 25,4364. Porous Pavement:

Reservoir: depth 0.67 SWM Surf Area Stor Vol WQv Avail Vol Add'l Imp Add'l

porosity 0.4 Equivalent Depth: 0.3 Measure (sf) (cf) (cf) (cf) Area (sf) WQv (cf)

Circle Pkg 590 157 61 97 762 10

Gar Walk 575 153 59 94 0 0

1,165 120

ONSITE

CALCs

Adjusted WqV

After RRvEX AREA

PROPOSED

AREA

Runoff Reduction Volume

RRv9WQv

5,207

STORMWATER

MANAGEMENT MEASURE

TOTAL 2,022

TOTAL

SITE

725.04 Stormwater Analysis GP010001.xlsx 8/17/2012 Divney Tung Schwalbe, LLP


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III1

III. Runoff Reduction (RRv)

A. Redevelopment Requirements for RRvChapter 9,Redevelopment Projects, of the New York State Department of Environmental

Conservation (NYSDEC) New York State Stormwater Management Design Manual

(NYSSMDM), August 2010, explains that though encouraged, meeting the Runoff

Reduction Volume (RRv) sizing criteria is not required for redevelopment projects.1

Because the redevelopment of the project site includes both redevelopment and new

construction, the potential for incorporating green infrastructure techniques to the entire site

was evaluated, and it was determined that the site could employ these measures.

B. Redevelopment RRv CalculationsThe application of green infrastructure techniques is prohibited in certain locations within

the proposed limit of disturbance by the existing steep slopes of the site and by the proposed

preservation to the maximum extent practicable of existing buffers surrounding the existing

developed areas. With these site constraints as justifications for calculating minimum RRv,

it is estimated that the minimum volume reduction using green infrastructure techniques

with RRv capacity is approximately 4,200 cubic feet for the entire study watershed area.

Due to the hydrologic soil group (HSG) B and the site limitations, the NYSDEC considers

a minimum of 40% of the RRv to be an acceptable threshold. Though the goal would be to

meet 100% of the RRv, an estimated 5,200 cubic feet of RRv is provided, which is greater

than the minimum required by NYSDEC and equal to approximately 50% of the 100%

RRv. The following is a summary of the breakdown of the onsite green infrastructure

techniques proposed.

The proposed redevelopment of the site will provide for the implementation of green roofs,

rain gardens (incorporating elements of bioretention practices), stormwater planters and

porous pavement as its green infrastructure techniques to address stormwater volume

1New York State Department of Environmental Conservation New York State Stormwater Management Design Manual,August 2010, page 94. RRv requirements pertain to new construction portions of the redevelopment project.


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III2

reduction. The green infrastructure techniques are placed to disconnect impervious areas,

increase flow paths and minimize stormwater runoff.

Though NYSDEC does not require that redevelopment portions of projects meet RRv

requirements, the portion of the project that would qualify as redevelopment, approximately

1.06 acres of impervious area, when combined with the new construction portion,

approximately an additional 1.3 acres of impervious area will meet or exceed the calculated

minimum RRv values if all impervious area were to be considered new construction.

The water quality volume (WQv) requirements, estimated to be approximately 7,175 cubic

feet, have been substantially met with the use of these green infrastructure techniques with

RRv capacity provided. Two hydrodynamic separators are proposed to treat the stormwater

runoff from the roof of the existing house and partial dropoff area and from the existing

entry driveway. These alternative measures, when combined with the green infrastructure

techniques employed on the rest of the site, will meet the WQv requirements for the project.


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IV1

IV. Adjusted Water Quality Volume (WQv)

A. Standard Stormwater Management Practices (SMPs)The New York State Department of Environmental Conservation (NYSDEC) New York

State Stormwater Management Design Manual (NYSSMDM), August 2010, outlines the

standard stormwater management practices (SMPs) that can be evaluated for use as

stormwater quality measures. The acceptable practices area divided into five broad groups

as follows:

I. Stormwater PondsII. Stormwater WetlandsIII. Infiltration PracticesIV. Filtering PracticesV. Open Channel Practices

Due to the existing surrounding slopes, the topography of the site and the proposed

preservation of existing buffers to the maximum extent practicable, ponds, wetlands,

infiltration and open channel practices are not acceptable SMPs for the project site.

B. Alternative Practices Accepted by the NYSDECAs previously discussed, on redevelopment projects the NYSDEC allows for the use ofaccepted verified manufactured technologies alternative practices, sized to meet a minimum

of 75% of the water quality volume from the disturbed, impervious area. An acceptable

alternative practice is a hydrodynamic separator, a preferred choice when there is the lack of

space for a standard SMP or green infrastructure technique. Due to the proposed use of

green infrastructure techniques such as porous pavement and rain gardens, over 70% of the

WQv requirements have been met, but additional practices are required to meet minimum

water quality volume (WQv) requirements, therefore a hydrodynamic separator is proposed

for The Continuum Company facility project at the main house/dropoff circle and at the

entry drive.


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V1

V. Water Quantity Control

Through the use of the suggested site planning methods and planned green infrastructure

techniques at the proposed redeveloped site, stormwater runoff reduction results mitigate

proposed peak rates of runoff from the site. The proposed measures allow for stormwater

runoff storage and infiltration for smaller storm events with overflow control structures to

discharge larger storm events in a controlled manner at rates equal to or less than existing

conditions. In Subwatersheds DP 2, DP 3 and DP 4, with the modified subwatershed

boundaries and reduction in impervious surfaces, it is estimated that the peak rates of runoff

from the proposed developed site are equal to or less than existing conditions.

In order to meet the program requirements of the onsite facility, the proposed buildings were

generally located along the open space areas and within the vicinity of the existing buildings.

The reconfigured subwatershed discharging to DP 1 was increased by approximately 2.15 acres

across the entire study watershed. To provide peak rates of runoff from this DP for each 24

hour storm event up to and including the 100year storm event to be at or below existing

conditions, underground detention pipes1 are proposed along the eastern edge of the loading

dock and emergency access road.

Table 4, Stormwater Quantity Design Flow Summary, outlines the existing flow rates at each

DP for the 1year through the 100year 24hour storm events. Also included are the

anticipated design flow rates for the improved redeveloped site and a delta indicating that at

each DP for each storm event the proposed peak rate of runoff is at or below existing

conditions.

Where applicable, channel protection volume (CPv), Overbank Flood (Qp) and Extreme

Storm (Qf) considerations have been provided, consistent with the New York State

Department of Environmental Conservation (NYSDEC) New York State Stormwater

Management Design Manual (NYSSMDM), August 2010 thereby meeting the Village of

1Full detention is not recommended in the subwatershed due to the delayed timing of the release of each storm event intothe downstream offsite storm drainage system that connects to the Barney Brook which has historic evidence of causing

flooding conditions. A preferable option would be to release the runoff from the site before the confluence of upstream

drainage areas to this section of the Barney Brook.


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V2

Irvington stormwater management requirements. CPv, estimated to be 0.25 acrefeet, is

accommodated in the underground detention pipes which for the 1year storm event has a

storage volume of 0.29 acrefeet, in excess of the CPv. Qp addresses the control of the peak

discharge of the 10year storm event to at or below predevelopment rates. For design point

DP1, the existing 10year storm event peak rate of flow is estimated to be 12.8 cubic feet per

second (cfs) and under redeveloped conditions is approximated to be 10.7 cfs; thereby meeting

the Qp requirements. Qf addresses the control of the peak discharge of the 100year storm

event to at or below predevelopment rates. For design point DP1, the existing 100year storm

event peak rate of flow is estimated to be 22.1 cubic feet per second (cfs) and under

redeveloped conditions is approximated to be 21.8 cfs; thereby meeting the Qf requirements.

The detention pipes safely pass the 100year storm event.

Refer to Table 5, Stormwater Management Basin Summary, for a hydrologic analysis of the

detention pipes modeled as part of the overall study watershed analysis.


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CONTINUUM

IRVINGTON,

NEW

YORK

STORMWATERQ

UANTITYDESIGNFLOWSUM

MARY

S

WFlow

RunoffVolumeDetention

Time

SWFlow

SWFlow

SWFlow

SWFlow

SWFlow

SWFlow

(CFS)

(CFT)

(HRS)

(CFS)

(CFS)

(CFS)

(CFS)

(CFS)

(CFS)

1

Existing

5.06

21,214

7.45

11.01

12.83

16.51

20.21

22.06

Redeveloped

3.89

28,377

5.73

8.49

10.66

15.51

19.54

21.79

Delta

1.17

7,164

1.72

2.52

2.17

1.00

0.67

0.27

2

Existing

0.56

2,265

1.06

1.87

2.31

3.24

4.21

4.71

Redeveloped

0.54

2,134

0.88

1.41

1.68

2.25

2.84

3.13

Delta

0.02

131

0.18

0.46

0.63

0.99

1.37

1.58

3

Existing

1.13

6,229

2.47

4.79

6.07

8.85

11.80

13.34

Redeveloped

0.46

3,049

1.20

2.58

3.35

5.05

6.88

7.84

Delta

0.67

3,180

1.27

2.21

2.72

3.80

4.92

5.50

4

Existing

0.20

697

0.31

0.49

0.58

0.77

0.96

1.06

Redeveloped

0.01

44

0.03

0.06

0.07

0.11

0.15

0.17

Delta

0.19

653

0.28

0.43

0.51

0.66

0.81

0.89

(1)WestchesterCoun

tyRainfall,NYSDECAmendmentN

Y1,November7,1990,page214.5

(2)Qp(cfs)=Control

thepeakdischargefromthe10years

tormto10yearpredevelopmentrates.

(3)Qf(cfs)=Controlthepeakdischargefromthe100yearstormto100yearpredevelopmentrates.Safelypassthe100yearstormevent.

5.0

6.0

Westchester

CountyRainfall

(IN)(1)

7.0

7.5

4.5

56YEAR

10

6YEAR(2)

256YEAR

506YEAR

1006YEAR(3)

Detention

Pipes:

24.0

hrs

NotRequired

NotRequired

DESIGN

POINTNO.

26YEAR

3.5

16YEAR

2.8

NotRequired


22/73

TABLENO.5

CONTINUUM

IRVINGTON,

NEWYORK

STORMWATERMANAGEM

ENTBASINSUMMARY

HWE(1)

SV(2)

HWE(1)

SV(2)

HWE(1)

SV(2)

HWE(1)

SV(2)

HWE(1)

SV(2)

HWE(1)

SV(2)

HWE(1)

SV(2)

DetentionPipes:

Orifice/W

eir:

6'Diameter

(1)1.5"d@

140.0'

Bottom:140.00'

(2)8"[email protected]

142.83

0.29

143.0

0.33

143.56

0.39

143.89

0.43

144.67

0.53

145.65

0.62

145.86

0.63

Top:146.00'

6'[email protected]'

Length:975'

TOTAL

0.29

0.33

0.39

0.43

0.53

0.62

0.63

(1) HighWaterElevation(Feet)

(2) StorageVolume(AcreFeet)

(3) ChannelProtectionVolume(

AcreFeet),CPv:0.2

5 Forward Flow Only (UpStream to DnStream)

TW 140.000 146.000

Orifice-Circular O1 ---> TW 142.750 146.000

Weir-Rectangular W0 ---> TW 145.500 146.000

TW SETUP, DS Channel

S/N:

Bentley PondPack (10.01.04.00) 8:51 AM 8/16/2012

Bentley Systems, Inc.


72/73

File.... J:\725.04 Continuum Irvington\Engineering\PondPack\725.04 12-08-14 Redeveloped.pp

Name.... UG Detn Pipe OCS

Type.... Outlet Input Data Page 5.02

OUTLET STRUCTURE INPUT DATA

Structure ID = O0

Structure Type = Orifice-Circular

------------------------------------

# of Openings = 1

Invert Elev. = 140.00 ft

Diameter = .1250 ft

Orifice Coeff. = .600

Structure ID = O1

Structure Type = Orifice-Circular

------------------------------------

# of Openings = 2

Invert Elev. = 142.75 ft

Diameter = .6667 ft

Orifice Coeff. = .600

Structure ID = W0

Structure Type = Weir-Rectangular

------------------------------------

# of Openings = 1

Crest Elev. = 145.50 ft

Weir Length = 6.00 ft

Weir Coeff. = 3.200000

Weir TW effects (Use adjustment equation)

Structure ID = TW

Structure Type = TW SETUP, DS Channel

------------------------------------

FREE OUTFALL CONDITIONS SPECIFIED

CONVERGENCE TOLERANCES...

Maximum Iterations= 200

Min. TW tolerance = .01 ft

Max. TW tolerance = .01 ft

Min. HW tolerance = .01 ft

Max. HW tolerance = .01 ft

Min. Q tolerance = .00 cfs

Max. Q tolerance = .00 cfs

S/N:

Bentley PondPack (10.01.04.00) 8:51 AM 8/16/2012

Bentley Systems, Inc.


73/73

Storm... TypeIII 24hr Tag: 1

File.... J:\725.04 Continuum Irvington\Engineering\PondPack\725.04 12-08-14 Redeveloped.pp

Name.... UG DETENTION OUT Tag: 1 Event: 1 yr

Type.... Detention Time Page 6.01

DETENTION TIMES SUMMARY

HYG Dir = J:\725.04 Continuum Irvington\Engineering\PondPack\

Inflow HYG file = NONE STORED - UG DETENTION IN 1

Outflow HYG file = NONE STORED - UG DETENTION OUT 1

Pond Node Data = UG DETENTION

Pond Volume Data = UG DETENTION

Pond Outlet Data = UG Detn Pipe OCS

No Infiltration

APPROXIMATE DETENTION TIME

---------------------------------------

Qout+Infilt. Centroid = 37.8515 hrs

Inflow Centroid = 13.8823 hrs

Centroid to Centroid = 23.9692 hrs

---------------------------------------

continuum draft feis oct 2012 - appendix 4 stormwater management file_73

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