“Hurricane” Hal Needham is a storm surge scientist who specializes in data-driven storm surge analysis. He is the founder and president of Marine Weather and Climate.
Hurricane Matthew made landfall as a Category 4 storm with sustained winds of 145 mph along the Tiburon Peninsula in south Haiti Tuesday morning. Matthew is forecast to emerge into open water around the western end of the Gulf of Gonave and make landfall or a very close approach to the southeastern tip of Cuba, in Guantanamo Province, before threatening the Bahamas on Wednesday.
Fortunately, from a storm surge perspective, the areas impacted in both Haiti and Cuba are not susceptible to large storm surges. Deep bathymetry, or offshore water depth, in these areas reduces storm surge potential, because displaced water can redistribute itself where the bathymetry is deep. The water depth just 5 miles (8 km) offshore in these areas of Haiti and Cuba exceeds 3,000 feet (914 m) deep in many areas.
Potential Impacts in Haiti
Bathymetry is shallower in Port-au-Prince Bay, and storm surges could threaten areas like Cite Soleil, a densely-populated area within the Port-au-Prince metropolitan area, if not for the orientation of the bay. This region would require intense, sustained northwest winds to generate a large storm surge near Port-au-Prince, but a hurricane could only do this if it was centered inland, due to the counter-clockwise rotation around hurricanes in the Northern Hemisphere.
Even wrap-around winds on the backside of Matthew should not create a dangerous storm surge near Port-au-Prince, and any coastal flooding impacts in Haiti will be overshadowed by severe impacts from heavy rainfall runoff and mudslides.
The National Hurricane Center predicted storm surge levels of 7 to 10 feet along the south coast of Haiti, and this should occur within a narrow area near the eastern eyewall at landfall. Although this is a relatively low storm surge level for a Category 4 hurricane, we should expect extraordinary waves in this region.
Areas with deep bathymetry observe high waves because the energy of incoming waves is not broken until very close to shore. The difference between storm surge and waves is that a wave is a rise in water that may only last several seconds, whereas a storm surge is a dome of water that can last anywhere from 30 minutes to several days.
Deep bathymetry offshore usually translates to high mountains on land, which is the case in both Haiti and eastern Cuba. The orographic influence of these mountains enhances the lifting of air, increasing rainfall potential and exacerbating the impacts of heavy rain, as steep slopes experience rapid runoff and mudslides. As with most hurricanes affecting this region, Matthew’s greatest impact on both Haiti and Cuba will be flash flooding and mudslides.
In 1963, Hurricane Flora impacted this region, causing more than 7,000 deaths, mostly in Cuba and Haiti. Flash flooding caused majority of these deaths.
Potential Impacts in Cuba
The National Hurricane Center predicts storm surge levels could reach 7 to 11 feet along the southern coast of Cuba, east of Cabo Cruz. This region is similar to Haiti in regards to deep offshore water depths and mountainous terrain on land.
Cuba has experienced catastrophic storm surges before, but not in this region of the country. A hurricane in 1932 generated a 21.3-foot (6.5m) storm surge that killed 2,870 people at Santa Cruz del Sur in Camaguey province.
A 25-mile (40 km) wide shelf in this region exacerbates storm surge by providing a pool of shallow water that can be easily displaced. However, in southeastern Cuba, where Matthew’s impact will be greatest, hurricanes inflict most damage from heavy rainfall runoff and mudslides.
Potential Impacts in the Bahamas
The Bahamas are more vulnerable to storm surge than Cuba and Haiti, and storm surge will likely be the major hazard that Matthew inflicts upon this country. The National Hurricane Center predicts a storm surge that will reach 10 to 15 feet (3.1 – 4.6m).
Many areas in this archipelago contain broad reefs that provide large pools of shallow water for hurricanes to displace and inflict storm surge damage. The impacts of these surges are often severe, sea water can overwash small islands, completely inundating them with salt. Such surges often destroy fresh water and food supplies, as saline soils can take years to lose high salt content.
Of particular concern is the possibility that Matthew will track far enough east to produce a large storm surge on Crooked Island, Acklins Island and Long Cay. Hurricane Joaquin generated a devastating 15-foot (4.57m) storm surge in this area just last October, taking advantage of a shallow reef that is open to the southwest. A second large storm surge in two years would have grave impacts for this region.
Whether Matthew produces a comparable storm surge on these three islands remains to be seen, but one thing looks likely: Matthew will inflict catastrophic storm surge damage in the Bahamas, as it is forecast to slowly move through the Bahamas as a major hurricane.
This originally appeared on Hurricane Hal’s Storm Surge Blog.