SWMM5 - Stormwater Management Model
SWMM 5, Watersheds, Water Quality,Hydrology, Hydraulics - Watersheds
One of the advantages of use of the curve number (CN) method for loss estimates (Section 3.3) is the highly structured method for CN estimates provided by the NRCS (SCS, 1972,1988; McCuen, 1998, Bedient and Huber, 2002 and virtually every hydrology text). Such estimates are embedded in engineering practice through Table 3-18, in which CN values are given as function of land use and soil Hydrologic Soil Group (A through D). Hydrologic Soil Group has the meaning indicated in Table 3-17, ranging from well-drained (type A) to poorly drained (type D). Every soil in the United States has an A-D classification, or sometimes a dual classification, such as B/D, meaning drained (artificially) and undrained (natural) condition.
Table 3-17. Hydrologic soil group meanings (SCS, 1972, Chapter 7).
|
Group |
Meaning |
|
A |
Low runoff potential. Soils having high infiltration rates even when thoroughly wetted and consisting chiefly of deep, well to excessively drained sands or gravels. |
|
B |
Soils having moderate infiltration rates when thoroughly wetted and consisting chiefly of moderately deep to deep, moderately well to well-drained soils with moderately fine to moderately coarse textures. E.g., shallow loess, sandy loam. |
|
C |
Soils having slow infiltration rates when thoroughly wetted and consisting chiefly of soils with a layer that impedes downward movement of water, or soils with moderately fine to fine textures. E.g., clay loams, shallow sandy loam. |
|
D |
High runoff potential. Soils having very slow infiltration rates when thoroughly wetted and consisting chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a clay-pan or clay layer at or near the surface, and shallow soils over nearly impervious material. |
Hydrologic Soil Group is provided on the Soil Survey Interpretation sheets discussed in Section 3.8.8.2. For instance, the Conestoga Silt Loam of Figure 3-37a is in hydrologic soil group B. A more concise listing is provided in the Soil Survey for each county, published by the NRCS or the local Soil and Water Conservation District.
Hence, parameter estimation for SWMM (and any use of curve numbers) is straightforward and relatively unambiguous. Curve numbers may also be used to back-calculate estimates of initial moisture deficit, as discussed earlier in conjunction with Equation 3-135, since the CN and storage, S, are related through
where it is essential to remember that units of S are in inches. And early discussion of Horton infiltration parameters has also been related to Hydrologic Soil Group.
Table 3-18. Runoff curve numbers for selected agricultural, suburban, and urban land use (SCS, 1986). (Antecedent moisture condition II; Ia = 0.2S)
|
Hydrologic Soil Group |
|||||
|
Land Use Description |
A |
B |
C |
D |
|
|
Cultivated land1 |
|
|
|
|
|
|
Without conservation treatment |
72 |
81 |
88 |
91 |
|
|
With conservation treatment |
62 |
71 |
78 |
81 |
|
|
Pasture or range land |
|
|
|
|
|
|
Poor condition |
68 |
79 |
86 |
89 |
|
|
Good condition |
39 |
61 |
74 |
80 |
|
|
Meadow |
|
|
|
|
|
|
Good condition |
30 |
58 |
71 |
78 |
|
|
Wood or forest land |
|
|
|
|
|
|
Thin stand, poor cover, no mulch |
45 |
66 |
77 |
83 |
|
|
Good cover2 |
25 |
55 |
70 |
77 |
|
|
Open spaces, lawns, parks, golf courses, cemeteries, etc. |
|
|
|
|
|
|
Good condition: grass cover on 75% or more of the area |
39 |
61 |
74 |
80 |
|
|
Fair condition: grass cover on 50 – 75% of the area |
49 |
69 |
79 |
84 |
|
|
Commercial and business areas (85% impervious) |
89 |
92 |
94 |
95 |
|
|
Industrial districts (72% impervious) |
81 |
88 |
91 |
93 |
|
|
Residential3 |
|
|
|
|
|
|
Average lot size |
Average % impervious4 |
|
|
|
|
|
1/8 ac or less |
65 |
77 |
85 |
90 |
92 |
|
1/4 ac |
38 |
61 |
75 |
83 |
87 |
|
1/3 ac |
30 |
57 |
72 |
81 |
86 |
|
1/2 ac |
25 |
54 |
70 |
80 |
85 |
|
1 ac |
20 |
51 |
68 |
79 |
84 |
|
Paved parking lots, roofs, driveways, etc.5 |
98 |
98 |
98 |
98 |
|
|
Streets and roads |
|
|
|
|
|
|
Paved with curbs and storm sewers5 |
98 |
98 |
98 |
98 |
|
|
Gravel |
76 |
85 |
89 |
91 |
|
|
Dirt |
72 |
82 |
87 |
89 |
|
1. For a more detailed description of agricultural land use curve numbers, refer to the SCS (1972) National Engineering Handbook, Section 4, “Hydrology.”
2. Good cover is protected from grazing and litter and brush cover soil.
3. Curve numbers are computed assuming that the runoff from the house and driveway is directed toward the street with a minimum of roof water directed to lawns where additional infiltration could occur.
4. The remaining pervious areas (lawn) are considered to be in good pasture condition for these curve numbers.
5. In some warmer climates of the country a curve number of 95 may be used.
Last updated by Robert E Dickinson Apr. 21, 2008.
© 2010 Created by Robert E Dickinson.