Analyzing the Effect of Urbanization on Surface Runoff for Intense Rainfall Storm Events: Case Study of Olmos Creek in San Antonio, Texas
High intensity rainfall storms can have devastating impacts on watersheds. Rainfall events can produce flooding which is typically magnified by urbanization and land use change. This thesis investigates how urbanization affects the generation of surface runoff in terms of peak flow and runoff volume for high intensity rainfall events. The study was conducted for the Olmos Creek basin in San Antonio, Texas. A physically-based distributed hydrological model Gridded Surface Subsurface Hydrological Analysis (GSSHA) was built, calibrated (NSE = 0.83; RSR = 0.42; PEP = -5 %; PEV = 8 %; PBIAS = -8 %), and validated based on two historical events of 25th May 2014 – 29th May 2014 (calibration period) and 28th October 2015 – 31st October 2015 (validation period). In order to analyze the effect of urbanization on surface runoff for intense storms. Six scenarios of land use land cover change were considered, consisting of the National Land Cover Dataset (NLCD) 1992 to 2011 and two hypothetical scenarios (undeveloped; and a fully developed watershed). Two type II 24-hour SCS design storms, one with a 5-year return period (5.30 inches or 134.62 mm), a 100-year return period (11.9 inches or 302.26 mm), and a hypothetical Hurricane Harvey (56.1 inches or 1423.8 mm) were simulated for each scenario. Results of the simulations showed that the evolution of land use land cover change, from 1992 to 2011, increased both peak discharge and runoff volume. In the case of 5-year storm, even though the peaks and volume were much lower compared to 100-year and Harvey, the overall relative increase was greater. A 100-year storm and Harvey will result in a greater peak and volume but showed an overall smoother relative increase over time as urbanization evolves. The results suggest that, the intensification of population density will relatively increase the magnitude of storm events with shorter return period. That indicates that while flood control systems still need to be designed to address low frequency high intensity storm events, more recurrent flood events also need to be analyzed by floodplain and urban planners, in order to mitigate the adverse impacts of urbanization on water resources.