Impacts of Updated Design Rainfall Values on Louisiana Infrastructure

Abstract

The Louisiana Department of Transportation and Development (LADOTD) has a Hydraulics Manual which sets forth drainage design standards and codes for projects. The Manual contains maximum annual 24-hour rainfall maps and Intensity-Duration-Frequency (IDF) curves for the return periods of 2, 5, 10, 25, 50, and 100 years. The Manual divides Louisiana into three regions and specifies that all of the cities within a given region should use the same estimated rainfall intensities and rainfall depths. NOAA Atlas 14, the Precipitation-Frequency Atlas of United States, contains the latest rainfall data and statistics for much of the United States and provides the ability to gather rainfall intensities and depths on a city-wide scale. Using Atlas 14, intensities and depths for the return periods of 2, 5, 10, 25, 50, and 100 years were gathered at the local (rather than the regional) level enabling comparisons of individual cities’ rainfall intensities and depths to those published in the Manual. Six cities were chosen in three different LADOTD designated regions: Baton Rouge, New Orleans, Lafayette, and Lake Charles (Region 1), Natchitoches (Region 2), and Shreveport (Region 3). Comparisons with NOAA Atlas 14 show that the methods employed in the Manual underestimate the design rainfall intensities and depths in most of the cities for every return period. Intensities and depths in Baton Rouge presented the least underestimation, while New Orleans and Lake Charles presented the largest. Further comparisons were done using the upper bound limits of the 90% confidence interval from NOAA Atlas 14 versus the same LADOTD estimates. With this approach, the underestimates increased significantly in all cities. The second part of this thesis analyzed the impact of design storm intensities on two types of water related projects – open roadside channels and culverts. Results have shown that the differences in design parameters follow the pattern of underestimating correspondingly to the resulting differences in rainfall intensities. For the design of channels, in all cities, area of flow, depth, and width parameters increased slightly and the actual design using LADOTD values, with addition of Free Board, would be able to carry the peak discharges derived using NOAA Atlas 14mean intensities. However, using upper bound estimates, parameters increased significantly and the actual design using LADOTD values would not be able to carry the peak discharges, derived using NOAA Atlas 14, in all cities. Allowable Differential Head (ADH) and Allowable Headwater (AHW) are the two primary design parameters