Dave:
My understanding of the UK Soil Index System is that it is used for determining the soil nutrients and is classified by a soil test. In order to determine the runoff coefficient for the modified rational method, you need to know the hydrologic soil group classification which is a texture based system. I suggest you determine the runoff coefficient using the method described below.
The equation used to determine the peak runoff for the modified rational method is Qp = CiA where Qp is the peak runoff (cfs or cms), C is a runoff coefficient (dimensionless), i is the rainfall intensity (in/hr or mm/hr) and A is the drainage area (acres or hectares).
The runoff coefficient for the rational method and modified rational method is a function of the land use, rainfall return period, soil characteristics and land slope. The runoff coefficient is the proportion of rainfall that contributes to runoff.
The Natural Resources Conservation Service (NRCS) (formally the SCS), based here in the USA, groups soils into one of four types (A, B, C or D). The NRCS offers the following descriptions for the various soil types:
Group A soils.
Have a low runoff potential due to high infiltration rates. These soils consist primarily of deep, well-drained sands and gravels.
Group B soils.
Have a moderately low runoff potential due to moderate infiltration rates. These soils consist primarily of moderately deep to deep, moderately well- to well-drained soils with moderately fine to moderately coarse textures.
Group C soils.
Have a moderately high runoff potential due to slow infiltration rates. These soils consist primarily of soils in which a layer exists near the surface that impedes the downward movement of water or soils with moderately fine to fine texture.
Group D soils.
Have a high runoff potential due to very slow infiltration rates. These soils consist primarily of clays with high swelling potential, soils with permanently high water tables, soils with a clay pan or clay layer at or near the surface, and shallow soils over nearly impervious parent material.
TR-55 provides the following relationship between the soil group and soil texture.
Soil Group / Soil Texture
----------------------
A - Sand, loamy sand or sandy loam
B - Silt loam or loam
C - Sandy clay loam
D - Clay loam, silty clay loam, sandy clay, silty clay or clay.
Once you have determined the land use, rainfall return interval, soil group and land slope, you select the appropriate runoff coefficient using the runoff coefficient table in StormNET. You access the runoff coefficient table by double clicking the runoff coefficient box in the subbasins element manager. The number inside the parenthesis next to each soil group on the runoff coefficient table represents the land slope.
Please let me know if you still have questions.
Chris Maeder, M.S., P.E.
Senior Technical Engineer
chris.maeder@bossintl.com
Linear Mode
