WMS:FHWA Sheet Flow Equation
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Sheet flow generally occurs for the first 300 feet at the headwater of streams. The following equation is used to describe sheet flow:
where:
- = travel time for open channel flow segments.
- = emprical coefficient equal to 0.933 for English units and 6.943 for Metric.
- = rainfall intensity (in/hr).
- = 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.
- = length of the overland flow segment (ft).
- = ground slope of the flow segment (ft/ft).
Surface Description | |
---|---|
Smooth asphalt | 0.011 |
Smooth concrete | 0.012 |
Ordinary concrete lining | 0.013 |
Good wood | 0.014 |
Brick with cement mortar | 0.014 |
Vitrified clay | 0.015 |
Cast iron | 0.015 |
Corrugated metal pipe | 0.024 |
Cement rubble surface | 0.024 |
Fallow (no residue) | 0.05 |
Cultivated soils | |
Residue cover £ 20% | 0.06 |
Residue cover > 20% | 0.17 |
Range (natural) | 0.13 |
Grass | |
Short prairie grass | 0.15 |
Dense grasses | 0.24 |
Bermuda grass | 0.41 |
Woods | |
Light underbrush | 0.40 |
Dense underbrush | 0.80 |
The rainfall intensity is actually a function of the travel time for the flow segment. In order to iteratively solve for the travel time you must 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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