GROUPE 3

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Example 3: Detention Pond Design

Plan

1. Problem Statement

2. System Representation

3. Model Setup

4. Model Results

5. Summary

6. Conclusion 2

Problem statement This example illustrates how to define, design, and evaluate a

detention pond using SWMM.

Detention pond for the post-development site3

System Representation • Defining the site after development (the designation of areas

representing the seven sub catchments). • Identifying the location of the pond and the network in which water

runoffs.

Study area map with storage unit SU14

System Representation

• The main elements used to design detention ponds in SWMM are:

Storage Units: described by a Storage Curve, an Evaporation Factor can be specified, and a Maximum Depth of storage must be defined.

Orifices: can be used to represent the opening along the side or bottom of the storage unit that serves as an outlet

Weirs: represent the opening at the top of the storage unit that serves as an overflow structure.

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Model SetupRunoff will be detained in

separate sections of the detention pond. Both sections will have the shape of a trapezoidal prism.

The upper prism will control the major storms (return periods: 10 years and 100-yr) while the lower prism will control the minor storms (WQCV and 2-yr). 6

Schematic Representation of a detention Pond

Model SetupConception Steps:

– Calculating the site’s average percent imperviousness.

– Determining the WQCV in watershed inches.– Identifying Pond dimensions.

7Geometry of the pond’s WQCV

Model Setup Conception Steps:

– Add a Storage Unit to the Model

• Size the WQCV Orifice “the entire WQCV is released within 40 hours” , so we

simulate that in 4 iterations. 8

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Results from SWMMFigure from QA

Comparison of Flow Through the Bottom Orifice of Example

EXTRAN3

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Figure from QA Results from SWMM

Comparison of Water Depth at the Inlet Node of the Orifice inExample EXTRAN3

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Figure from QA Results from SWMM

Flow Comparison for Link 1570 of Example EXTRAN3

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Figure from QA Results from SWMM

Flow Comparison for Link 1570 of Example EXTRAN3

Model SetupConception Steps:

– So after doing the 4

iterations we obtain

a curve which shows

the change in water

depth upstream of

the storage unit.

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WQCV drainage time for the 4 iterations

Model Setup

Outlet hydrographs for the 10-yr storm

Outlet hydrographs for the 100-yr storm

20Impact of urbanisation

Model Results

• The final SWMM model for the post-development site with the detention pond is shown in this picture.

21Details of the outlet structure

Summary

This table summarizes the characteristics of the different discharge elements included in the pond’s outlet.

Characteristics of the pond’s outlet structure 22

Conclusion

This example showed how SWMM could be used to design a

detention pond and its outlet structure to provide both a

water quality capture volume (WQCV) and peak runoff control.

The WQCV was designed to provide a 40 hour drawdown time

to satisfy water quality treatment requirements while the peak

runoff goal was to limit the maximum post-development

discharges for the 2-, 10- and 100-yr storms to their pre-

development values.

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