Attachment 8 - Conceptual Storm Water Management Plan

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

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

---

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

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

---

Area Listing (all nodes)

---

Summary for Subcatchment 1S: Pre-Development

Runoff = 0.00429 m³/s @ 12.41 hrs, Volume = 0.026 MI, 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

Hydrograph for Subcatchment 1S: Pre-Development

A chart showing flow (m³/s) over time (hours), peaking at 0.00429 m³/s at approximately 12.41 hours.

• Type II 24-hr 10-yr Pre-Development Rainfall = 84 mm
• Runoff Area = 0.1597 ha
• Runoff Volume = 0.026 Ml
• Runoff Depth = 17 mm
• Tc = 37.7 min
• CN = 65