During Hurricane Harvey in 2017, the world was focused on torrential, unprecedented rainfall and flooding that occurred in Houston, Texas. While no city could withstand four feet of rain in a few days, it was clear to all that Houston has a flooding problem. Bayous and creeks, man-made channels and flood control reservoirs simply couldn’t keep up with the stalled hurricane’s endless downpours on this coastal plain that is flat as a pancake. Land here drains slowly even on a good day. Less well known about Hurricane Harvey’s impacts was that there was a mild storm surge that just made draining the rain-soaked land even more difficult.
Led by Rice University engineering professor Dr. Phil Bedient, the Severe Storm Prediction, Education and Evacuation from Disasters (SSPEED) Center takes a collaborative and multi-university approach to studying and helping solve the Houston-Galveston region’s severe storm issues. In addition to studying inland flooding and developing a flood alert system for one bayou, the SSPEED Center has also led research involving a number of faculty members and researchers on storm surge in Galveston Bay. Storm surge is not like either bayou flooding or overland flow, both of which are caused by rain. Storm surge is caused by tropical storms/hurricanes pushing sea water onshore. Hurricane winds push ocean water onto the adjacent land to such an extent that the land temporarily becomes part of the sea/bay.
In 2008, Hurricane Ike, a Category 2 storm, came ashore between the Bolivar Peninsula and Galveston Island. As a result of the storm’s track, the highest surge of 16-18 feet was focused on Bolivar, where almost all structures were demolished. Galveston Bay and the region’s petrochemical industrial complex, including the Houston Ship Channel, experienced some storm surge damage but were spared the worst.
Galveston Bay and the Houston Ship Channel are home to one of the largest petrochemical complexes in the world, where tremendous amounts of gasoline, plastics, and military-grade jet fuel are refined and produced by vast petrochemical plants at low elevations vulnerable to storm surge. The Houston Ship Channel is one of the world’s busiest shipping lanes and is lined with above ground storage tanks. NASA is located on the western edge of Galveston Bay, as are many towns. All of this infrastructure is vulnerable to storm surge. Consequently, the regional, state and even national economy are also vulnerable to storm surge impacts in Galveston Bay.
The SSPEED Center’s goal is to understand storm surge better, evaluate its risks, and develop solutions. Since 2009, the SSPEED Center has been studying a regional storm surge protection system called Houston-Galveston Area Protection System. Part of this research effort has been extensive surge modeling executed on High Performance Computers (HPC).
Working with Dr. Clint Dawson from the University of Texas at Austin, the SSPEED Center modeled a variety of hurricane sizes and paths and identified a reasonable worst-case storm for the Houston-Galveston region. Dawson used the hydrodynamic model ADvanced CIRCulation (ADCIRC), a 2D- and 3D-code along with the Simulating Waves Nearshore (SWAN) model to simulate hurricane storm surge. Dawson ran ADCIRC+SWAN on the Texas Advanced Computing Center’s “Stampede” supercomputer.
ADCIRC+SWAN modeling results for a reasonable worst-case storm making landfall at the west end of Galveston Island. Source: Analysis by Dr. Clint Dawson for the SSPEED Center. Image prepared by Christina Walsh for the SSPEED Center (Click image to enlarge).
Initially, the SSPEED team studied historical storms to establish baseline storm surges. A reasonable worst case hurricane track was identified as making landfall on the west end of Galveston Island. With this track, the highest storm surge would be directed at Galveston Bay and the Houston Ship Channel. The results of Dawson’s ADCIRC+SWAN modeling were startling, showing a storm surge of 23-24 feet on the west side of the bay and almost 25 feet in the ship channel due to the funneling effect of the bay. A 25-foot surge would cause significant damage to the petrochemical complex, the ship channel, and western Galveston Bay population centers.
As a result of this research, the SSPEED Center developed a storm surge mitigation strategy called the Galveston Bay Park/Mid-Bay Alternative and then modeled it with ADCIRC+SWAN to see how effective it would be. The Mid-Bay alternative consists of an in-bay berm system that generally runs along the ship channel, gate structures for small craft, a large navigation gate for ship traffic, and levees and elevated roads. Dawson’s ADCIRC+SWAN supercomputer modeling revealed that western Galveston Bay and the ship channel would be much more protected from storm surge with the Mid-Bay Alternative in place, than without it.
ADCIRC+SWAN modeling results showing effectiveness of Galveston Bay Park/Mid-Bay Alternative mitigation proposal by SSPEED Center to protect the area from storm surge. Source: Analysis by Dr. Clint Dawson for the SSPEED Center. (Click image to enlarge)
To complicate matters, storm surge and inland flooding can happen at the same time during a hurricane. This occurs when inland (freshwater) flooding, traveling downriver to the coast, meets storm surge (saltwater) being pushed upriver by a hurricane. When these events happen at the same time, it’s called compound flooding. Compound flooding has two components that amplify each other and make the flooding worse: freshwater flooding piles up on the land and can’t drain into the ocean because it’s blocked by salt water storm surge coming inland. Low-lying coastal regions like the Houston-Galveston area are particularly vulnerable to compound flooding because the land is very flat and drains slowly under normal conditions. Hurricane Harvey’s mild three-foot storm surge made it harder than normal for the region’s bayous to drain the tremendous amounts of water that fell inland. But fortunately, the storm surge did not cause devastating impacts like Hurricane Ike’s storm surge on Bolivar Peninsula in 2008. Very little research has occurred on compound flooding in the US and little is known about how it happens in the Houston-Galveston area.
The SSPEED Center’s storm surge modeling is helping advance knowledge of our region’s complex hydrology. This type of complicated research can only occur on supercomputers. Development of a regional HPC-based flood alert system would provide a foundation for complicated HPC research about coastal storm surge and compound flooding in the Houston-Galveston area. Improving our regional flood resilience starts with increasing our regional understanding of hydrology—how individual watersheds work, how inland flooding occurs, how storm surge behaves at the coast, and how compound flooding puts coastal residents at increased risk. Armed with a better understanding of the science, we can be better prepared for floods and hurricanes. Increasing our knowledge of regional hydrologic systems with supercomputing is vital to improving Houston’s flood resilience.
Read Part 1 of this story here.
Author: WENDY PETRUZZI