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HydroCAD Stormwater Modeling - Since 1986

Overland Flow

Why can't I route a hydrograph through a subcatchment?

A subcatchment is used to generate a runoff hydrograph for a given area of land.  But it does not include the information required to route an existing inflow hydrograph.  In cases where a separate upstream flow must also be routed across the subcatchment, a separate reach routing is often used in conjunction with the subcatchment.

Note: Runoff entering a swale along it's edge is commonly modeled as a channel flow segment within the Tc path.  Details here.

Modeling Overland Flow

If the "reach" flow is particularly shallow, such as general overland flow, this can be modeled with a corresponding reach geometry, such as a wide bottom and/or very low side slopes.

In addition, a higher Manning's value may be used to account for the low flow depth.  This commonly occurs when a "level spreader" is used to re-create very shallow flow.  The "Sheet Flow Roughness" coefficients originally designed for Tc calculations may be appropriate in these situations:

Surface Description n
Short Grass .150
Dense Grass .240
Bermuda Grass .410
Range .130
Woods w/light underbrush .400
Woods w/dense underbrush .800

Since these values are an order-of-magnitude higher than a standard Manning's value for the same surface, it is essential to check the resulting flow depth to ensure that sheet flow is actually occurring.  If there is significant flow depth (over 0.1 feet), a standard Manning's value should be used as listed in Appendix C of the HydroCAD Reference Manual.

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