Hydraulic Model Calibration on the Coastal Plain: A Case Study in Cameron County, Texas

Development of a representative hydraulic model in flat coastal regions is often problematic. This can be complicated further if the study area is extensively crossed by elevated irrigation canals, roadways or railroad tracks that tend to impede or alter stormwater conveyance. In addition standard hydrologic assumptions, unit hydrograph shapes and hydraulic modeling techniques are often not applicable. This analysis will consider various hydrologic and hydraulic calibration parameters. Hydrologic calibration parameters considered will focus on time of concentration, antecedent moisture condition and peak rate factor. Hydraulic parameters will focus on Manning's "n" value and storage routing. A concern for floodplain managers in this region is that improving the efficiency of the drainage system would reduce storage volumes, thereby increasing peak discharge and causing additional flooding downstream of improvements. The presentation looks at this problem as encountered in the development of a Master Flood Protection Plan for Cameron County Drainage District #5. Drainage District #5 encompasses most of the City of Harlingen, and the Towns of Primera, Combes and Palm Valley. The District drains to the Arroyo Colorado, part of the Rio Grande floodway system and the Gulf Intracoastal Waterway.