WMS:FHWA Sheet Flow Equation
Sheet flow generally occurs for the first 300 feet at the headwater of streams. The following equation is used to describe sheet flow:
- Tt = travel time for open channel flow segments.
- K = emprical coefficient equal to 0.933 for English units and 6.943 for Metric.
- i = rainfall intensity (in/hr).
- n = Mannings roughness coefficient for overland flow. Suggested values are given in Table 3-2 of the FHWA HEC 22 manual and : arerepeated in the table below.
- L = length of the overland flow segment (ft).
- S = ground slope of the flow segment (ft/ft).
|Ordinary concrete lining||0.013|
|Brick with cement mortar||0.014|
|Corrugated metal pipe||0.024|
|Cement rubble surface||0.024|
|Fallow (no residue)||0.05|
|Residue cover £ 20%||0.06|
|Residue cover > 20%||0.17|
|Short prairie grass||0.15|
The rainfall intensity is actually a function of the travel time for the flow segment. In order to iteratively solve for the travel time, define an IDF curve (function) to be used in conjunction with the equation. IDF curves are defined in WMS using Hydro 35, NOAA Atlas 2, or user defined rainfall intensities for specific durations. The IDF Curves dialog used as part of the rational method is used to set up equations relating i to Tt.
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