Hydrologic modeling practices used by TxDOT are outlined in the 2001 TxDOT Hydraulic Design Manual. Methods used by TxDOT primarily include unit hydrographs, the Rational Method, statistical analysis of stream flow data, and regional regression equations.

The NRCS (National Resource Conservation Service) dimensionless unit hydrograph along with the NRCS Runoff Curve Number Method (also known as the SCS curve number method) is the primary unit hydrograph technique used by TxDOT. The unit hydrograph, a method for estimating storm runoff, was first proposed by L. K. Sherman in 1932. The unit hydrograph is defined as the watershed response to a unit depth of excess rainfall uniformly distributed over the entire watershed and applied at a constant rate for a given period of time (Chow et al., 1988).

Unit hydrograph techniques consider the time distribution of rainfall, the initial rainfall losses, and an infiltration rate that decreases during the course of the storm. Variables include the drainage area, time of concentration, curve number (if the SCS curve number method is used), rainfall distribution, and total

design rainfall. Equations to watershed parameters such

calculate the time of concentration can also consider

as

watershed

length

and

slope.

Popular

unit

hydrograph application programs used by its TR55 variant) and HEC-1. TxDOT is HMS. (Stolpa, 2002)

TxDOT have included the TR20 (and now moving towards the use of HEC-

The Rational Method is very simple, and is best suited for small urban and rural watersheds. The statistical analysis of stream gauge data method is applied

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