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Using the Rational Method in HEC-HMS

Generally, drainage basins less than 200 acres are modeled by the Rational Method, however the Rational Method does not consider the time-duration of a storm, only focusing on a single peak point in time. The rational method relies on a particular drainage area (A), the Manning’s C-factor (c) and the intensity (I) of a storm event based on a particular Time of Concentration (Tc). Another method used to model storms is the Unit Hydrograph method with distributes a storm of a given duration (typically 24 hours) over time to mimic actual rain distribution. The downfall of the Unit Hydrograph method is that it typically overestimates the flows in the basin, especially with smaller basins, such as the case in less than 200 acres. The standard dimensionless Unit Hydrograph can be transformed into a discharge versus time hydrograph for any watershed, given drainage area, time of concentration, and rainfall intensity.

This session will explore a case study where a proposed development project with a basin of less than 200 acres was modeled as a Unit Hydrograph and that model modified to mimic the Rational Method flows. The results of this model were routed through a set of detention ponds to make the proposed project compliant with Texas Water Code 11.086.



Travis Attanasio PE, CFM, M.ASCE
Senior Capital Engineer
City of Burleson


Call: 512-472-8905

Email: office@texasce.org

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