Attachment 7 - Conceptual Stormwater Management Plan – Eltham Road

RE: Conceptual Stormwater Management Plan – Eltham Road

Introduction

The purpose of this report is to provide an overview of the stormwater management design for the proposed development permit application at the address noted above.

Background

The development consists of two existing residences, along with paved driveways and concrete patios. For the purposes of this report, we have used pre-development flow to the ‘natural’ state of the area.

The proposed development will result in approximately 85% of the property area to being impervious. It is proposed to construct a stormwater management system large enough to detain and release stormwater at the pre-development flows.

Design Considerations

Stormwater storage calculations are based on the Hydrograph SCS Method. See attached HydroCAD report for input parameters and specifications. Flow rates for pre-development and post-development are based off of the rational method input values below:

Pre-Development:

• C = 0.35 (forested area)
• I = 28mm/hr (10-year, 10-minute storm)
• Area = 0.1597 hectares

Post Development:

• C₁ = 0.9 (impervious area)
• C₂ = 0.35 (landscaped areas)
• I = 28mm/hr (10-year, 10-minute storm)
• Area₁ = 0.1385 hectares (impervious area)
• Area₂ = 0.0212 hectares (landscaped area)

See attached Rational Method Calculations.

Pre-Development Flows: See attached HydroCAD calculations

• 10 year: Q = 0.00429 cu.m/s (4.29 L/s)

Post Development Flows: See attached HydroCAD analysis

• 10 year: Q = 0.01043 cu.m/s (10.43 L/s)
• Release Rate: Q = 0.00399 cu.m/s (3.99 L/s)
• Required Orifice Size = 38 mm
• Required Storage Volume = 53 cu.m

Summary

It is proposed to incorporate detention into the building with flow-control structures limiting flows to pre-development rates.

Yours truly, JE Anderson & Associates

Colton Killip, EIT Reviewed by: Ross Tuck, P. Eng 2022-04-29

Enclosures: 20220425 - 33862 – Stormwater Management Calculations – Rational Method 20220425 - 33862 – HydroCAD Report

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Stormwater Management Calculations (Rational Method)

STORAGE VOLUMES & ORIFICE SIZING FROM HYDROCAD SOFTWARE. HYDROCAD USES SCS METHOD. CONFIRMATION OF PRE & POST FLOWS BY USING RATIONAL METHOD.

PRE-DEVELOPMENT FLOW C = 0.35 (FORESTED AREA) I = 28 mm/hr (10-YR, 10 min) A = 0.1597 ha

$Q_{PRE} = \frac{CIA}{360} = \frac{0.35 \times 28 \times 0.1597}{360} = 0.0043 \text{ m}^3\text{/s} = 4.3 \text{ L/s}$

POST-DEVELOPMENT FLOW C₁ = 0.9 (IMPERVIOUS AREA) I = 28 mm/hr (10-YR, 10-min) A₁ = 0.1385 ha C₂ = 0.35 (GARDENS / PERVIOUS AREA) A₂ = 0.0212 ha

$Q_{POST} = \frac{C_1IA_1}{360} + \frac{C_2IA_2}{360} = \frac{0.9 \times 28 \times 0.1385}{360} + \frac{0.35 \times 28 \times 0.0212}{360} = 0.0103 \text{ m}^3\text{/s} = 10.3 \text{ L/s}$

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HydroCAD Pre-Development Report

33862 - Pre Printed 2022-04-25

Area Listing (all nodes)

Summary for Subcatchment 1S: Pre-Development

Runoff = 0.00429 m³/s @ 12.41 hrs, Volume= 0.026 Ml, Depth= 17 mm Runoff by SCS TR-20 method, UH=Gamma-500, Weighted-CN, Time Span= 0.00-36.00 hrs, dt= 0.05 hrs Type II 24-hr 10-yr Pre-Development Rainfall=84 mm

[Graph: Hydrograph for Subcatchment 1S: Pre-Development showing runoff peak of 0.00429 m³/s at approximately 12.4 hours]

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HydroCAD Post-Development Report

33862 - Post Printed 2022-04-25

Area Listing (all nodes)

Summary for Subcatchment 1S: Post-Development

Runoff = 0.01043 m³/s @ 12.32 hrs, Volume= 0.107 Ml, Depth= 67 mm Runoff by SCS TR-20 method, UH=Gamma-150, Weighted-CN, Time Span= 0.00-36.00 hrs, dt= 0.05 hrs Type II 24-hr 10-yr Post-Development Rainfall=84 mm

[Graph: Hydrograph for Subcatchment 1S: Post-Development showing runoff peak of 0.01043 m³/s at approximately 12.3 hours]

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Summary for Pond 2P: SWM System

Inflow Area = 0.1597 ha, 86.73% Impervious, Inflow Depth = 67 mm for 10-yr Post-Development event Inflow = 0.01043 m³/s @ 12.32 hrs, Volume = 0.107 Ml Outflow = 0.00399 m³/s @ 13.95 hrs, Volume = 0.075 Ml, Atten = 62%, Lag = 98.0 min Primary = 0.00399 m³/s @ 13.95 hrs, Volume = 0.075 Ml

Routing by Stor-Ind method, Time Span = 0.00-36.00 hrs, dt = 0.05 hrs Peak Elev = 1.649 m @ 13.95 hrs Surf. Area = 32.0 m² Storage = 52.8 m³

Plug-Flow detention time = 233.5 min calculated for 0.075 Ml (70% of inflow) Center-of-Mass det. time = 133.9 min (976.6 - 842.8)

Primary OutFlow Max = 0.00399 m³/s @ 13.95 hrs HW = 1.649 m (Free Discharge) 1 = Orifice/Grate (Orifice Controls 0.00399 m³/s @ 3.52 m/s)

[Graph: Hydrograph for Pond 2P: SWM System showing Inflow peak of 0.01043 m³/s and Primary outflow peak of 0.00399 m³/s]