SMP Sizing in Redevelopment Projects

• Scenarios are countless

• Tools– Simple method– CN back calculation– Peak Discharge Rate

• Chapter 10 Criteria

• List of Verified Practices

Site Description

• Total area = 4 acres

• Disturbed are = 1 acre

• Impervious area = .6 acre

• Precipitation = 1”

• Storm Type 2

• Justification

• Sizing and SMPs selection– 4 scenarios

Justification

Bethlehem, NY

• Site information– approximately 29.2% Imp– florist, green houses, parking, planting beds

• Proposed development – 54% impervious – condominium complex, 9 buildings and 4 covered garages

• Constraints– relocating a sanitary sewer– maintaining setbacks

• Proposed practice– CDS units– Underground storage

25% Impervious Cover Reduction

Scenario 1•Reconstruction of existing impervious area not exceeding footprint.•Selected Practice: Reduction of Impervious area by 25%•Plan appropriately•Document footprint reduction•Ensure adequate infiltration

•Soil amendment•Decompaction

Redevelopment Project with an Existing Detention Pond

Keep the detention pondAddress WQ treatment

WQv Calculation -1

WQv Calculation –2100% Impervious

Reduce upstream drainage and control an area that is all impervious.

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WQv Calculation –3Rv Constant

25% Standard practice

Scenario 2•Reconstruction of existing impervious area not exceeding footprint.•Selected Practice: Infiltration Trench•Calc. WQv x 25%•Locate sub-catchment & diversion structure• Check feasibility & limitations•Document maintenance plan, etc.

Site DescriptionEddy Greenhouses

• Nursing facility in Cohoes• Area encompass 22 acres• Northwestern portion of the property undeveloped

(wetlands)• Remainder is buildings and parking areas• Existing impervious area 8.29 acres• Proposed construction 16 Greenhouses

– New construction 7.8 acres– Reconstruction 6.17 acres

Redevelopment Project with an Existing Detention Pond

May modify the pond to meet pond design standards

Redevelopment Project with Increased Footprint

Proposed PlanEddy Greenhouses

• Reduce impervious area by 25%• Design a P-1 Pond• Maintain existing water quantity controls in pond• Provide treatment for all the new disturbance• Provide treatment for a portion of the existing

imperviousness• Allow sheet flow for ½ of the site that did not have

detention in existing condition• Did not restore soil property• Increase HSG for new pervious area

75% Alternative practice

Scenario 3•Reconstruction of existing impervious area not exceeding footprint.•Selected Practice: Hydrodynamic System•Back calc. CN

•If only imp. Connected, CN = 98•Define flow rate•Locate sub-catchment, flow splitter•Look up the corresponding model from table 1•Check all the requirements for suitability of the practice, feasibility, limitations (head, soil, separation distance, etc.)•Document calculation method, manufacturer’s recommendation, maintenance plan, etc.

Redevelopment Project with Increased Footprint

Inputs Parameter Redevelop. UnitsNew Construction

Site Acreage A 2 acres 0.5

Impervious Area IA 1.5 acres 0.5

Impervious Cover% I 75 % 100

Runoff Coefficient Rv=.05+.009*I 0.73 constant 0.95

Min. Rv Rv 0.2 constant 0.2

90% Rainfall P 1 inches 1

Water Quality Volume WQv = [PRvA]/12 0.1216667 Acre-Feet 0.0395833

    5299.8 ft^3 1724.25

Alternative Practice, sized to 75% of WQv, define priority areas that generate a runoff volume equivalent to the target WQv. Rv constant. 0.75 3974.85 Total (ft^3) 5699.1

EquationsWQv = [PRvA]/12Rv=.05+.009*I

I = imperviousness

P= precipitation (inch)

A= area

CN Back-CalculationPeak Discharge Rate

http://www.dec.ny.gov/chemical/29089.html

Hydrodynamic SystemsVortechs System

http://www.dec.ny.gov/chemical/29089.html

Hydrodynamic Systems

WQv VSM + VDL + (DP x ARG)

 VSM = ARG x DSM x nSM

 VDL (optional) = ARG x DDL x nDL

 where:VSM = volume of the soil media [cubic feet]

VDL = volume of the drainage layer [cubic feet]

ARG = rain garden surface area [square feet]

DSM= depth of the soil media, typically 1.0 to 1.5 feet [feet]

DDL = depth of the drainage layer, typically .05 to 1.0 feet [feet]

DP = depth of ponding above surface, maximum 0.5 feet [feet]

nSM = porosity of the soil media (≥20%)

nDL = porosity of the drainage layer (≥40%)

WQv= Water Quality Volume [cubic feet], as defined in Chapter 4 of the New York Stormwater Management Design Manual

Alternative PracticesRain Garden

Redevelopment Project with Increased Footprint