Effective Discharge Calculation Methods and Variability: Application to the Trinity and Brazos Rivers, Texas

Abstract:

Effective discharge calculations are based on continuous recordings of daily discharge, cross-sectional stream properties, and measured or calculated sediment transport rates. This study investigates how different flow frequency analysis and varying amounts of collected on-site cross sectional and sediment data impact the final calculated effective discharge. The analysis is carried out for six river stations on the lower Brazos River and four stations on the middle Trinity River in the state of Texas, USA. Data obtained at each site includes mean daily flow discharge, measured suspended sediment concentration over a range of flow conditions, bed sediment samples and cross sectional geometry. Bed load rating curves are developed based on bed sediment samples and the Einstein-Brown formula. The flow frequency analysis is carried out using (1) equal arithmetic intervals with bin numbers of 25, 50 and 100, and (2) the kernel density estimate method. To answer the question of what kind of on-site measured data is essential for calculating the effective discharge, 4 scenarios using different combinations of measured and unmeasured data are defined. At one end of the scenario spectrum, all measured data is used. At the other end, the only on-site data used is the bed material grain size distribution. Results show that if a smoothed sediment load histogram is used, then the effective discharge is not sensitive to the flow frequency analysis method or to the varying levels of on-site data used in the analysis. The outcome of this is that on the Trinity and Brazos rivers, effective discharge can be adequately calculated using only the bed material size information, USGS daily flow data, and the top width of the river obtained from Google Earth. Comparing the calculated effective discharge to other flow metrics showed that the effective discharge on the Trinity is significantly smaller than the bankfull discharge. Effective discharge on the Trinity ranges between 15000 to 21000 cfs with the return period of 1-1.2 years while the bankfull discharge varies between 25000 to 34000 cfs. On the Brazos the effective and bankfull discharges are closer. The analysis also showed that the half-load discharge is independent of the flow frequency analysis and ranges from 13000 to 24000 cfs on the Trinity, and from 21500 to 38000 cfs on the Brazos.