Result difference when using runoff coefficient loss method

Product(s): SewerGEMS, CivilStorm
Version(s): 08.11.XX.XX, 10.XX.XX.XX
Area: Modeling

Problem

Different results are seen when comparing multiple subareas in a single catchment Vs individual catchments for subareas with “runoff coefficient loss method”.

Background

When using “runoff coefficient” Loss method for a catchment and comparing results for multiple subareas in a single catchment Vs separate catchment areas, results are not same. Whereas if other loss method like “initial loss and constant loss rate” or the “initial loss and constant fraction” are used then flow from catchments in both cases is same.

Solution

Let’s understand this by considering below example of multiple subareas – CM1 and an individual catchment for each subarea CM-21, CM-22 & CM-23.

Catchment CM1 is consisting of 3 pervious subareas: 

For this case,

  1. It consolidates all the pervious areas into a single catchment, and calculates the runoff hydrograph off of that.
  2. It consolidates all the impervious areas into a single catchment, and calculates the runoff hydrograph
  3. The combined two hydrographs are the resultant runoff hydrograph for the catchment.

Weighted runoff coefficient is considered for subareas case, 

It consolidates the subareas as follows:

  1. It sums the area: 2.5 + 3.5 + 4.0 = 10 acres
  2. It computes the weighted runoff coeff: 0.25 * 0.8 + 0.35 * 0.85 + 0.4 * 0.9 = 0.8575
  3. All the Tc values are summed up: 10 + 5.553 + 10 = 25.553  min

Whereas CM-21, CM-22 & CM-23 are having following properties separately which are same as subareas of catchment CM-1 with loss method-Runoff coefficient. 

Catchment

Area (ac)

Runoff Coeff

Tc (min)

CM-21

2.5

0.8

10.0

CM-22

3.5

0.85

5.553

CM-23

4.0

0.9

10.0

After computing the results, flow through conduits CO-1 and CO-2 are not same despite same properties of the catchments as seen below. 

Whereas if other loss method like “initial loss and constant loss rate” or the “initial loss and constant fraction” are used then flow from catchments in both cases is same.

Reason for this is, when you are using runoff coefficient loss method, calculations for individual catchment for each subarea will be different because of the weighting of the runoff coefficient and the effect of the greater time of concentration. so catchment runoff from individual catchments (subareas) will be higher compared to the single catchment with subareas. 

See Also

Assumptions behind Subarea Routing with the EPA-SWMM Runoff Method 

Reporting weighted Runoff Coefficient for catchments that use the Multiple Subareas option 

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