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Hydrological Theory

Calculating Runoff

SCS Triangular Response

 

From the MIDUSS Version 2 Reference Manual - Chapter 7
(c) Copyright Alan A. Smith Inc.

 

Figure 7-19 - Representation of the triangular response function.

A very common and popular technique proposed by the Soil Conservation Service uses a triangular response function as shown in the upper part of Figure 7-19 above.  The time to peak tp of the triangular IUH and the time base tb are given by equations [7-42] and [7-43] respectively.

The ‘Triangular SCS’ option in MIDUSS  represents a modification of this method in that the value of tc is obtained by [7-41] and is assumed to vary in a nonlinear fashion in much the same way as for the rectangular response function.  For each time step the effective rainfall intensity is known and the triangular response function for the corresponding time of concentration tc is discretized and multiplied by the effective rainfall.  The resulting contributions to the overland flow hydrograph are lagged and accumulated.  The computation is made more efficient by representing the triangular response as the difference between two right‑ angled triangles as indicated in the lower part of Figure 7-19.

It will be found that with both the “Rectangular’ and ‘Triangular SCS’ methods the time of concentration of the impervious fraction is much shorter than that for the pervious area due to smaller values of Manning's 'n' and higher effective rainfall intensities.  In cases where the contribution from each fraction is of the same order of magnitude the total runoff hydrograph exhibits a double peak because of this feature.

 

 

 


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