This option automatically runs STORM repeatedly to size a facility to achieve a selected number of overflows per year or a specified percent capture. This option can be used, for example, to determine the storage volume required to handle all flows up to a 5-year storm (0.2 overflows per year) at a treatment plant or to size a detention basin to achieve 95% capture of annual runoff.
The Simulation Parameters are the same as for the single STORM analysis with the addition of Target Value and the omission of secondary treatment rate. The user additionally specifies whether to hold storage, treatment, or drawdown time (storage: treatment ratio) constant. A Target Condition of Overflows per year, First flush failures per year, Percent capture, or Most efficient size must be specified.
The Most efficient size option directs the program to identify an optimal storage volume based on the point where the slope of the capture vs. normalized volume curve equals one. Normalized volume is the volume divided by the volume needed to achieve 99.99% capture. The program thus first solves for the 99.99% capture condition. It then compares results from multiple runs looking for the point where the slope is closest to one.
When using Most efficient size, set the initial storage:treatment ratio to the desired drawdown time. E.g. to optimize for a basin with 48-hour drawdown, set storage (MG) to twice the treatment rate (mgd), such as 4 MG storage and 2 mgd treatment.