The SSR is the total amount of storage required to ensure that the required PSD is not exceeded and the OSD facility does not overflow during the storage design storm ARI.
As stated earlier, the storage design storm for estimating the SSR is 10 year ARI.
In sizing the volume of the storage facility, the method assumes a triangular inflow hydrograph and an outflow hydrograph shape related to the type of storage adopted. These simplifications are acceptable providing the site catchment is small.
Typically, the critical storm duration that produces the largest required storage volume is different from the time of concentration used for peak flow estimation. Therefore, storage volumes must be determined for a range of storm durations to find the maximum storage required as shown in Figure 3.4.
Figure 3.4 : Typical Relationship of Storage Volume to Storm Duration
The Site Storage Requirement (SSR) for the site in m3 is calculated using the formula:
The factors c and d are different for above-ground and below-ground storages due to differences in storage geometry and outflow characteristics.
For above-ground storage:
For below-ground storage:
td= selected storm duration (min)
Qd= the peak post-development flow from the site for a storm duration equal to td (l/s)
Editor :- Dilah – 13 Feb 14