Updating rainfall zones and intensities in Nebraska for improved design of non-bridge sized drainage structures: Phase I

NDOT utilizes the Rational Method to calculate peak design discharge flows for small drainage areas (less than 640 acres) when sizing culverts and storm sewers. This method requires information about land use, watershed travel time, and precipitation data for the location of interest. NDOT’s current implementation of the Rational Method simplifies the precipitation data into three rainfall zones with intensity-duration-frequency (IDF) curves determined from UNL research completed in 1988. That research developed the three rainfall zones based on the National Weather System’s Technical Paper 40 (1961) by using spatial analysis of the general pattern of rainfall isohyets shown in TP-40. Climate scientists typically define a climate normal in terms of 30-year averages, meaning that the 60 years old precipitation data being utilized by NDOT to date is likely not representative of the current climatic conditions in Nebraska. The proposed research project will review the past research and summarize the development of the current and historic rainfall zone maps and IDF curves. Also, using the additional 60 years of rainfall data, a wider net of weather stations, and advancements in instrumentation, the research would entail reviewing the accuracy of the current rainfall zones that NDOT is using. The proposed research will look at the currently compiled precipitation data as well as new weather stations within Nebraska. If it is determined that the current rainfall zone maps and IDF curves are outdated, the spatial analysis of the current data will be used to develop new rainfall zone maps and the intensity-duration-frequency curves for NDOT to use in the Rational Method