### Table of Contents

# Basin Efficiency

## Design

Basins must be designed to meet applicable ordinance performance standards, such as peak rate control and sediment reduction. Basin design should also take into account the physical characteristics of the site and be designed in accordance with Dane County and WDNR technical standards.

If the basin is located in the watershed of a cold-water stream, the design must provide Thermal Mitigation. Possible ways to prevent stream warming from the basin include eliminating permanent pool storage, providing extended draw down by use of tile drainage, or installing stone cribs at the outlet.

Basins must have the volume of storage necessary to settle the particle size necessary to meet either the soil loss standard during the construction phase or the TSS reduction standard for post-development. Stormwater models (HydroCAD and WinSLAMM) are now frequently used to determine peak rates, infiltration and sediment reductions, but the process described below can help to determine the approximate size of a stormwater facility.

Please note that MSE4 rainfall distributions must be used to model peak flow rates in Dane County.

## Volume of Storage Required

- From TR-55 find the pre-construction and post construction peak flow for the 1-year, 24-hour storm event.
- Calculate the ratio of peak inflow to peak outflow. The inflow rate will be the post construction condition and the outflow rate will be the pre-development conditions.
- With the peak ratio, use the chart on the next page to find the ratio of storage volume to runoff volume.
- The volume of storage necessary can now be calculated by multiplying the ratio from the table by the post-development runoff.

**Approximate detention basin routing for rainfall types I, IA, II, and III**

## Calculating Trapping Efficiency

The sediment reduction is selected from the particle size versus expected efficiency for a Plano silt loam during construction. The expected efficiency is given by the USLE in the column named “Percent reduction required to meet Ordinance.” With this expected efficiency, use the chart below to obtain the particle size required to be retained in the pond. The particle size has a settling velocity that may be calculated using Stokes Law.

**Expected trapping efficiency of a Plano silt loam**

Convert the storage volume from the 1-year, 24-hour storm event into cubic feet. This volume of storage is then divided by the time required to settle the particle obtained by Stokes Law.

Q_{maximum} (cfs) = V_{Storage} (ft3) / Time (sec)

Q_{maximum} is the rate at which the basin must be released in order to obtain the expected efficiency. The time required to trap the necessary particle can be found on the table below.

**Settling velocities for spherical particles based on Stokes Law**