HydroCAD Stormwater Modeling - Since 1986

Swamp Modeling

When modeling a swamp or wetland, the conveyance effects (how upstream flows are routed through the area) must be considered, as well as the direct runoff from the wetland area.

Conveyance Effects

Instead of the pond and swamp adjustment factor used in TR-55, HydroCAD provides full reach or pond routing capabilities to account for these effects.  The exact approach will depend on the situation:

bulletFor ponding with a well-defined outlet control, use a standard pond routing.  If one of the standard HydroCAD outlet devices (such as a weir) is not appropriate, a special outlet can be used to provide a custom stage-discharge curve.  This data can be obtained from field observations of the pond level vs. flow rate.
bulletFor flow through swamps or wetlands without a specific outlet control, use a standard reach routing.  The reach might be a vee or trapezoidal shape, typically with low side-slopes (high Z-value).  The key is to use a suitably high Manning's value (up to 0.15) to represent the extreme "roughness" of flow through weeds, brush, or woods.  For specific Manning's values, see the Appendix C of the HydroCAD Owner's Manual.

Runoff Effects

In order to allow for rainfall on the surface of the wetland, this area must also appear in the model as part of a subcatchment, or perhaps as a separate subcatchment.

bulletIf the wetland is saturated or inundated with water, a high Curve Number of 98-100 can be used to represent the negligible infiltration capability.
bulletA lower CN value may be appropriate if the wetland is not saturated, and still has some absorption capability.


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