TR-20 Segmental and Lag Time Approaches for Peak Flow: Comparison to Stream Gage Flow Frequency

Abstract Submission: Most roadway design sections of state Departments of Transportation predict peak flow for culvert design using NRCS TR-20 methodology. Even though the approach is more than 50 years old, there is no clear guideline for how large a single watershed drainage area is appropriate for use. We developed flow frequency estimates for 120 small-area stream gages in rural Nebraska and compared the calculated return period discharges with those from TR-20 using both the segmental and lag equation approaches for calculating the time of concentration. There appear to be no significant differences between peak discharges calculated using the two different methods. Overprediction of low return period discharges was common using both times of concentration. Preliminary results suggest that either method is most appropriate for drainage areas between about five and fifteen square miles, although statistical tests are continuing.

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