MAXIMUM DISCHARGE ESTIMATES FOR TERLINGUA CREEK AT THE HIGHWAY 170 BRIDGE, TERLINGUA, TEXAS

Terlingua creek is a major tributary to the Rio Grande in the Big Bend region of Texas. It has a drainage basin area of 2,851 km² and enters the Rio Grande immediately below the mouth of Santa Elena canyon in Big Bend National Park (BBNP). The International Boundary and Water Commission (IBWC) maintains a stream gage near Terlingua Abaja below the Highway 170 bridge over the creek. This gage has been in operation since 1932, and has recorded a maximum discharge of 487 cms (17,191 cfs) in 1937 and 458 cms (16,167 cfs) in 2004. We initiated this project preliminary to initiation of a larger project looking at watershed-discharge relationships at several locations along the Creek. Our data collection consisted of establishing three cross sections that extend above the elevation of the bridge immediately upstream of the bridge using total station and RTK GPS survey equipment. We also collected stream channel elevations 100 meters above and below the bridge in order to assess the local stream bed gradient. The cross sections are located at the bridge, and at 12 and 32 meters upstream from the bridge. After merging the total station and RTK GPS data, the cross section data were extracted. We produced our maximum discharge estimates using a simple Manning equation style of analysis. Our wetted perimeter and cross sectional areas were determined mathematically from the cross sections. The slope of the energy grade line for various stages could only be estimated from the local stream gradient, and a value of 0.003 was chosen. A composite Manning’s roughness coefficient (0.065) was estimated by observation of channel conditions and comparison to published coefficients. Maximum discharge estimates for the two cross sections nearest the bridge are 655 and 700 cms (23,000 and 24,570 cfs).