# WMS:Detention Basin Calculator

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An important aspect of any hydrologic study is the development of on-site storage facilities. The effects of a detention basin on an inflow hydrograph can be analyzed and an output hydrograph created in WMS using the Detention Basin calculator. This same calculator is also used in to define detention basin parameter input for HEC-1, the Rational Method and other hydrologic models as part of an overall analysis for a planned development.

A level pool routing technique is used to determine the effects of storage-routing on an input hydrograph for given detention basin/reservoir parameters. Using the principle of conservation of mass, the change in reservoir storage, S, for a given time period, Δt, is equal to the average inflow, I, minus average outflow, O.

${\displaystyle {\frac {S_{2}-S_{1}}{\Delta _{t}}}={\frac {I_{1}+I_{2}}{2}}-{\frac {O_{1}+O_{2}}{2}}}$

The defined storage vs. discharge relationships are used to iteratively solve for the end of period storage and outflow.

The detention basin calculator requires three sets of input:

When computing an outflow hydrograph an initial storage is used to account for any volume of water that may be in the detention basin prior to the arrival of the inflow hydrograph. If depth or elevation is known, the elevation vs. volume storage capacity curve must be used to determine the initial storage. The units of the initial storage should be the same as the units defined for the storage-capacity relationship

The storage-capacity and elevation-discharge curves (no matter how they are defined) are plotted in the detention basin calculator. They can also be displayed in the hydrograph window by selecting the respective Plot to Hydrograph Window buttons. Each of the curves can be printed by selecting the Print button. The Plot Options button accesses the plot options dialog in order to allow for control in the overall appearance of the defined curves.

The detention basin calculator uses the Hydraulic Toolbox to perform the calculations.