San Diego Modified Rational Formula in InfoSWMM

Subject: San Diego Modified Rational Formula

San Diego Modified Rational Formula

The Rational method is a widely used technique for estimation of peak flows from urban and rural drainage basins (Maidment 1993; Mays 2001).

Stormwater modeling applications such as the design of detention basins require knowledge of total inflow volume obtained from runoff hydrographs. For these applications peak flow information alone may not be sufficient. The San Diego modified rational formula is a technique adopted by the San Diego County to generate runoff hydrograph by extending the traditional rational formula.

The San Diego County uses a 6-hour storm event for many stormwater design applications. Accordingly, for designs that are dependent on total storm volume a hydrograph has to be generated by creating a rainfall distribution consisting of blocks of rain, creating an incremental hydrograph for each block of rain using the modified rational formula, and adding the hydrographs from each block of rain where each rainfall block lasts for a duration of time of concentration. The rainfall blocks are distributed across the 6-hour duration using the “2/3, 1/3” distribution in which the peak rainfall block is centered at the 4-hour time within the 6-hour rainfall duration, and the additional blocks are distributed in a sequence alternating two blocks to the left and one block to the right of the 4-hour time. Please refer to the H2OMAP SWMM Users Guide section 4 for additional information.

Figure 1 How the SDMRH is selected in InfoSWMM

Figure 2 The Time of Concentration and Runoff Coefficient can be entered on the DB Subcatchment Table

Figure 3 The Time of Concentration and Runoff Coefficient can be entered in the Attribute Browser

Figure 4 The Peak Runoff is shown in the output file of InfoSWMM in the Subcatchment Summary Table

Figure 5 Graph of the Rainfall and Runoff for the SDMRH