Comparison of Estimated Increase in Inundation Probability with Confidence Intervals for Tide Stations in Texas and Florida

Floods are one of the most common natural disasters that affect societies around the world. The combination of sea level rise and population growth in coastal regions makes it essential to continue improving flood management strategies. As sea levels increase, flood events will threaten more lives, and damage higher numbers of public and private properties. This problem will become even more severe due to population growth. For efficient planning it is essential to develop accurate flooding estimates that take into account both local effects such as vertical land motion and global effects such as estimated rates of sea level rise linked to climate change. The main objective of this research is to model and compare the impact of sea level rise on the frequency of inundation for two Gulf of Mexico stations: Pensacola, Florida and Galveston Pier 21, Texas. Both stations have long term sea level records but have substantially different sea level rise trends and bathymetry. Pensacola is a sea port on Pensacola Bay, which joins the Gulf of Mexico. This location is very vulnerable to hurricanes. Water levels records measured hourly are available starting in January 1924 till present. The Galveston Pier 21 station is located on a ship channel about 4 km away from the main Galveston Ship Channel and the mouth of Galveston Bay. The station's records are available starting in January 1904 till present and include water levels measured hourly. The impact of hurricanes on Galveston Island is well documented including 2008 hurricane Ike and the 1900 hurricane, the mostly deadly hurricane in US history. In our work, we construct models for the probability distribution of the annual maximum surge for each location. The General Extreme Value (GEV) distribution is recommended by FEMA for modeling floods, and has been used by previous researchers focused on flood risk estimation. Vertical land motion, global sea level rise, and tidal and atmospheric forcings are considered separately. Exceedance probabilities of past storms are compared after including the influence of past sea level rise. The extreme surge distributions are then combined with sea level rise projections to estimate future water level exceedance probabilities. The uncertainty associated with surges' climatic variability is investigated using a non-parametric bootstrap technique. Ensembles of surge models are computed based on the 108-year record of Galveston Pier 21, Texas and on the 88-year record of Pensacola, Florida. The extreme surge distributions are then combined with sea level rise projections to estimate future water level exceedance probabilities. Increases in inundation frequencies are computed based on two possible sea level rise scenarios, a conservative linear continuation of the past century's trend and a scenario based on the upper limit of the sea level range in the IPCC AR4 report, i.e. the A1FI scenario. A non-parametric bootstrap is used to estimate 90% and 95% confidence intervals around the estimated proportional change in water level exceedance probabilities for varying surges for these two sea level rise scenarios. The research shows that the projections of the probabilities of inundation differ significantly between the two sites. The relative increases in the estimated probabilities reach their maximum for different water levels: at 1.1 m for the Galveston Pier 21 station and at 0.7 m for the Pensacola station assuming a continuation of last century's rate of sea level rise. By the end of the century the frequency of exceeding water levels of 1.1 m is projected to increase at least by 4 times for station Galveston Pier 21, but only by 2.5 times for station Pensacola, Florida for 0.7 m water level for a conservative linear continuation of the past century's sea level rise. The differences in water level exceedance probabilities are discussed and associated with the statistical parameters of the distributions, local coastal settings and rates of sea level rise.