The sizing of detention basins can be done using a reservoir routing method such as the Level-Pool Routing Procedure, which computes storage routing by solving the continuity equation and the storage function.
The continuity equation or the equation of conservation of mass simply expresses the condition that the rate of inflow less the rate of outflow at any instance in time is equal to the rate of change in storage in the basin as follows:
The above equation may be expressed in finite difference form as follows:
The above equation can be rearranged such that all known variables are placed on the left side of the equation and all unknown variables on the right as follows:
|I||: The instantaneous inflow rate of discharge to the basin (m3/s)|
|Q||: The instantaneous outflow rate of discharge from the basin (m3/s)|
|S||: The volume of temporary storage in the basin (m3)|
|j, j+1||: Time steps j and j+1, respectively|
|Δt||: The time interval defining the finite difference approximation of the continuity equation.|
It is evident from the above equation that a second equation is necessary to solve the two unknown variables of Qj+1 and Sj+1. This second equation is referred to as the storage function, which expresses the relationship between the storage in the basin and the discharge from the basin in the form of Q = f(S).
The storage function represents the combined effect of:
- The discharge characteristics or the “rating curve” as represented by Q=f(H)
- The topography of the site i.e., the geometric properties as represented by the storage curve or H versus S data of the storage facility, expressed as H= f(S).
By combining Q=f(H) and H= f(S), the storage-discharge relationship for the basin or the storage function can be derived as Q = f(S).
A level pool routing can be carried out using either one of the following approaches:
- a spreadsheet like MS Excel (see Work Example 1.1)
- a simple computer program (MSMAware Software)
Figure 1.1 Reservoir Routing Through Detention Basin
Figure 1.2 Reservoir Routing