Modeling and Forecasting Hurricane Floods over Coastal Urban Watersheds

Abstract

To best mitigate the damage from hurricane-induced extreme floods, it is essential to better understand the performances of existing flood risk management measures, as well as future climate change impacts on the floods from an event-based analysis perspective. In this dissertation, three modeling based studies focusing on the aforementioned aspects were carried out at Houston watersheds in the context of Hurricane Harvey flooding (2017).

1. The first study investigated how a set of factors influenced the inflows, peak pool elevations, and outflows of the two most important detention reservoirs in Houston, the Addicks and Barker Reservoirs. It was found that urbanization only led to slight increases in peak inflows, which insignificantly affected in peak pool elevations. Similarly, the antecedent rainfall contributed to the peak inflow and pool elevation slightly. In contrast, the cyclone track had the most significant impacts on the flooding magnitude, both in terms of the inflows and peak pool elevations.

2. The second study tested the skills of streamflow and floodplain inundation forecasts derived from Quantitative Precipitation Forecasts (QPF) of different durations. A flood modeling system was obtained through offline coupling of a hydrological model and a hydrodynamic model. It was then used to produce flood forecasts for three major flood events (including Hurricane Harvey) in the Brays Bayou watershed under a range of QPF durations (6‒72 h). The results show that inundation forecasts driven by single streamflow forecasts with longer duration QPF achieve higher skill for extreme flood events of relatively longer durations; and forecasts under 24- and 72-h QPF durations provide decent inundation forecasts around the onset of a flood event.

3. The last study aimed to investigate the future climate change impacts on hurricane rainfall, and more importantly, subsequent compound flooding at a coastal watershed. Using the Clear Creek watershed as a testbed, a modelling framework was designed by coupling a hydrological model, a hydrodynamic model, and a regional earth system model offline. Results show that significant increase in maximum rainfall would lead to considerable increase in maximum inundation extent, as well as increases in inundation depths and duration under the high-emissions scenario.