San Diego Modified Rational Formula in InfoSWMM and InfoSWMM SA

The Rational method is a widely used technique for estimation of peak flows from urban and rural drainage basins (Maidment 1993; Mays 2001). Mathematically, the Rational formula is expressed as:

where Q = peak runoff rate (flow unit)

C = runoff coefficient (unitless)

I = rainfall intensity (intensity unit)

A = watershed area (area unit)

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. The actual rainfall amount for each rainfall block is calculated as:

where PN = actual rainfall amount for each rainfall block (inch or mm)

P6-hr = six-hour storm depth (inch or mm)

N = an integer representing position of a given block number of rainfall. N is 1 for the peak rainfall block, and is assigned according to the “2/3, 1/3” distribution rule for other rainfall blocks.

Tc = time of concentration (minutes)

Once rainfall distribution is created, triangular hydrograph is generated for each rainfall block. Peak flow for the triangular hydrograph is computed according to the rational formula. Finally, the overall hydrograph for the 6-hr storm event is determined by adding all the triangular hydrographs from each block of rain. The final hydrograph has its peak at 4 hours plus ½ of the Tc. The total volume under the hydrograph is the product of runoff coefficient, the six-hour precipitation depth, and area of the subcatchment.

Stormwater Runoff and the Rational Method from Innovyze H2OCalc for Reference

For storm sewer loading, the focus shifts to hydrologic analysis of excess precipitation and associated runoff. Common techniques for analysis include the rational method and unit hydrograph methods, as well as the use of more advanced hydrologic models.

For small drainage areas, peak runoff is commonly estimated by the rational method. This method is based on the principle that the maximum rate of runoff from a drainage basin occurs when all parts of the watershed contribute to flow and that rainfall is distributed uniformly over the catchment area. Since it neglects temporal and spatial variability in rainfall, and ignores flow routing in the watershed, collection system, and any storage facilities, the rational method should be used with caution only for applications where the assumptions of rational method are valid.

Rational Method from Innovyze H2OCalc for Reference

The rational formula is expressed as

where Qp = peak runoff rate (m3/s, ft3/s)

C = dimensionless runoff coefficient (see Table 3-9)

I = average rainfall intensity (mm/hr, in/hr) for a duration of the time of concentration (tc)

A = drainage area (km2, acres)

K = conversion constant (0.28 in SI, 1 in English)

The time of concentration tc used in the rational method is the time associated with the peak runoff from the watershed to the point of interest. Runoff from a watershed usually reaches a peak at the time when the entire watershed is contributing; in this case, the time of concentration is the time for a drop of water to flow from the remotest point in the watershed to the point of interest. Time of concentration, tc (min), for the basin area can be computed using one of the formulas listed in Table 3-10.

Table: Runoff Coefficients for 2 to 10 Year Return Periods

Source: Nicklow et al. (2006)

Table 3-10: Formulas for Computing Time of Concentration

Method	Formula
Kirpich (1940)	L = length of channel (ft) S = average watershed slope (ft/ft)
California Culverts Practice (1942)	L = length of the longest channel (mi) H = elevation difference between divide and outlet (ft)
Izzard (1946)	i = rainfall intensity (in/h) c = Retardance coefficient	Retardance factor, c, ranges from 0.007 for smooth pavement to 0.012 for concrete and to 0.06 for dense turf; product i times L should be < 500
Federal Aviation Administration (1970)	C = rational method runoff coefficient (see Table 3.9)
Kinematic wave	n = Manning’s roughness coefficient
SCS lag equation	CN = SCS runoff curve number (see Table 3.11)
SCS average velocity charts	V = average velocity (ft/s)
Yen and Chow (1983)	KY = Coefficient N = Overland texture factor (see Table 3.13)	KY ranges from 1.5 for light rain (i<0.8) to 1.1 for moderate rain (0.8<i<1.2), and to 0.7 for heavy rain (i>1.2)

Source: Nicklow et al. (2004)

Table 3-11: Runoff Curve Numbers for Urban Land Uses

Adapted from SCS (1985)

Table 3-12: Description of NRCS Soil Classifications

Adapted from SCS (1985)

Table 3-13: Overland Texture Factor N

Adapted from Yen and Chow (1983)