Changes in streamflow following modifications to Sims Bayou waterway

Abstract

Houston Texas is one of the fastest growing cities in the nation with a large, developed land area. Due to the low elevation, proximity to the Gulf of Mexico, and large, developed urban area, Houston is susceptible to large scale flooding events impacting thousands of people. To manage the effects of increasing urbanization and subsequent flooding during rainfall events, the Harris County Flood Control District with the U.S. Army Corps of Engineers began constructing major waterway modifications to reduce out-of-bank flooding as part of the Sims Bayou Federal Flood Damage Reduction Project. The goal of the project was to implement flood reduction best management practice (BMP) projects including 19 miles of waterway enlargements and enhancements, 20 bridge modifications, the creation of 3 stormwater detention basins, and an oxbow restoration. Project construction began in 1994 and was completed in 2015. The aim of my study was to compare Sims Bayou streamflow during the timeframe before BMP implementation (Period 1; 1991-1995) and after the project BMP implementation (Period 2; 2016-2021). I investigated changes in impervious surface area using QGIS software with data obtained from the National Land Cover Database. Sims Bayou Watershed had an impervious surface area of 18.22% in 2001 and increased by 3.09% to 21.31% in 2019. Rainfall data from The Harris County Flood Control District were used with discharge data from the USGS NWIS to examine trends in precipitation, streamflow, and streamflow response to precipitation events. I analyzed data quantitatively using a two-tailed t-test to determine if there was a statistical difference between the Period 1 (1991-1995; pre-BMP implementation) and Period 2 (2016-2021; post-BMP implementation) timeframes for storm events. There was no statistically significant difference between the two periods in the magnitude of cumulative rainfall for individual rain events (p-value = 0.9) or the total storm event discharge for individual runoff events (p-value = 0.5). The median values for the ratio of runoff response to precipitation were greater in Period 2 (0.31) than in Period 1 (0.22) and the mean ratio of runoff response to precipitation was greater in Period 2 (0.62) than in Period 1 (0.31), but this difference was not statistically significant (p-value = 0.3) due to the large variance in runoff ratios for both periods. BMPs for reducing runoff were not able to offset the increase from impervious surface area during the project period. A more detailed analysis is necessary to isolate the effects of these competing drivers of flood response in Sims Bayou.