ASSESSING THE EFFECTS OF CHANGE IN IMPERVIOUS AREAS ON FLOODING IN TEXAS

Changes in land-use, such as conversion of forest land into cropland, or cropland and grassland into urban areas, increase surface runoff and consequently flood magnitudes and frequencies mainly due to increase of impervious surfaces. The goal of this study is to evaluate the effects of change in impervious surfaces on flood frequency at selected major watersheds (San Antonio, Colorado, Brazos, San Jacinto and Trinity) in Texas. These watersheds had significant changes in their impervious surfaces compared to other watersheds across the state. The National Land Cover Database (NLCD) was used to identify the change in impervious areas across these watersheds during the past decade. Precipitation and streamflow data used in this study were obtained from the National Climate Data Center (NCDC) and USGS, respectively. These data were used to generate rainfall-runoff coefficients for the major watersheds in Texas. The rate of changes in rainfall-runoff coefficients and impervious areas across Texas were evaluated. Results of the analysis showed that the highest rate of change in impervious area was at San Jacinto watershed between 2006 and 2011; as such 1.5% (105 km²) area of this watershed was changed into 50-100% impervious surface between 2006-2011 whereas the change at Trinity and Brazos watersheds were 0.49 and 0.13% respectively. Although the effects of change in impervious areas on flooding was not consistent across the state, the majority of watersheds showed an increasing trend of flooding with respect to increased impervious areas during the study period; such as, excluding 2011 drought year, 6% increase of rain between 2007 and 2010 produced 63% more runoff at Brazos river watershed.