These are the estimates for both SWMM 5, InfoSWMM and ICM SWMM Hydrology, the low estimate is 0.2*SQRT(Area in Feet) and the High Estimate is 5*SQRT(Area in Feet), Figure 1. You can use higher or lower numbers to calibrate to monitored data but these are just guidelines using the InfoSWMM Subcatchment Manager Width Tool (Figure 2).
| Subcatchment Area (Acres) |
Low Estimate Width (Feet), W = 0.2*SQRT(Area) |
High Estimate Width (Feet), W =5*SQRT(Area) |
| 1 |
41.74 |
1,043.55 |
| 5 |
93.34 |
2,333.45 |
| 10 |
132.00 |
3,300.00 |
| 25 |
208.71 |
5,217.76 |
| 50 |
295.16 |
7,379.02 |
| 100 |
417.42 |
10,435.52 |
| 200 |
590.32 |
14,758.05 |
| 300 |
722.99 |
18,074.84 |
| 400 |
834.84 |
20,871.03 |
| 500 |
933.38 |
23,334.52 |
| 600 |
1022.47 |
25,561.69 |
| 700 |
1104.39 |
27,609.78 |
| 800 |
1180.64 |
29,516.10 |
| 900 |
1252.26 |
31,306.55 |
| 1,000 |
1320.00 |
33,000.00 |
| 5,000 |
2951.61 |
73,790.24 |
| 10,000 |
4174.21 |
104,355.16 |
| 50,000 |
9333.81 |
233,345.24 |
| 100,000 |
13200.00 |
330,000.00 |
Figure 1. High and Low Estimates of the ICM Subcatchment Dimension for SWMM Hydrology or the SWMM 5 Subcatchment Width.
Figure 2 InfoSWMM Subcatchment Manager Width Estimator.