Abstract

Most roadway design sections of state Departments of Transportation predict peak flow for culvert design using Natural Resources Conservation Service (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 may be while remaining appropriate for predicting peak discharge with this method. Our objective is to identify the drainage area where TR-20 peak flow predictions significantly deviate from flow frequency predictions. We aim to recommend to the Nebraska Department of Transportation (NDOT) a range of basin drainage areas appropriate for predicting peak flow using a TR-20 model. We developed flow frequency estimates for 130 small-area stream gage sites in rural Nebraska and compared the calculated return period discharges with those from TR-20 using both the segmental and lag equation approaches for estimating the time of concentration. Additionally, we compared regional regression estimates against flow frequency and TR-20 predictions. There appear to be no significant differences between peak discharges calculated using the TR-20 lag method and segmental method for estimating the time of concentration. Overprediction of low return period discharges were common using both times of concentration. Results suggest that TR-20 error becomes larger after 15 mi2, indicating a drainage area limitation of uncalibrated models in Nebraska. Despite having its lowest errors for watersheds between 1 mi2 and 15 mi2, using an uncalibrated TR-20 model is discouraged due to its likelihood to overpredict peak flows. TR-20 models meant for comparing the effects of land cover change on peak flow can be calibrated to regional regression equations to represent the pre-construction state, decreasing the likelihood of overprediction.

Degree

College and Department

Ira A. Fulton College of Engineering; Civil and Environmental Engineering

Rights

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