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Section 4: Hydrology Study Requirements

The design engineer must coordinate with the district office at the beginning of a hydrology study to establish study requirements. Typically, TxDOT requires the designer to:

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  • Select an appropriate frequency for the event that will be the basis for design or evaluation, considering the risk of capacity exceedance. The event selected may range from one with a 50% AEP (2-year event) to one with a 1% AEP (100-year event). ( Design Flood and Check Flood Standards provides guidance on selection of the risk-based flood event.)
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  • Choose an appropriate hydrologic analysis method, following guidance given herein, and use that method to compute the flow for the selected frequency.
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  • Explain and justify all assumptions. This includes explaining and justifying choice of the analysis method and choice of parameters and other inputs used with the methods.
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  • Verify and support results. This may be accomplished, in part, by demonstrating that the method used for design flow computation also can reproduce observed streamflow. Or it may be accomplished by comparing results of the selected method with those of valid alternative methods or other studies for nearby locations to establish confidence in results. (Driveway culverts and storm drains may be exempted from this requirement.)
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  • Provide, in reports and on plan sheets, information to enable a reviewer to understand and to reproduce the results. When using computer software for the computations, this will require the designer to identify the program used and version, to display all relevant input values, and to specify options and methods used in the software.
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  • Calculate flows for flood frequencies larger than and smaller than the selected design level, repeating the tasks described above. This will provide the designer with information for testing the resiliency and robustness of designs.
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