October 16, 2014
Hurricane Storm Surge and Storm Tide
by Hinman Team
The United States has been experiencing increased frequency of high intensity hurricanes over the last decade, as predicted in climate change models. Six of the top ten most expensive natural disasters in US history have been tropical storms – two of which have occurred in the last 10 years. Hurricane Katrina tops the all natural hazards list with $148.8 Billion in estimated costs and 1,833 dead1. Superstorm Sandy, the third most expensive, had estimated costs of $65.7 Billion and 159 dead1. The increasing loss associated with natural hazards has led leaders to questions our national and local hazard preparedness. The National Resource Council has called improving resilience a “National Imperative”.
Tropical storms and hurricanes are complex phenomenon. Most early hurricane research was focused on wind speed, and the rating system for hurricane strength (i.e. the Saffir-Simpson Scale) is based upon the system’s wind speed as a measure of storm intensity3.
The Saffir-Simpson Scale has served the community well, to a point. Given the evidence from the current Most Expensive Natural Hazards List, there is more to describing hurricane intensity and the risk associated with individual storms than the wind speed alone. Engineers and scientists model these storms using three-dimensional computational fluid dynamics that take into account variables including: velocity, temperature, salinity, density, pressures, Coriolis force, gravity, viscosity, and diffusion2.
Insurance company data has estimated that 60% of the damage from storm hazards is associated with surge effects 5. Surge is the water pushed ashore due to wind force and pressure gradient from the storm. This estimate illustrates that more than half of the damages from Hurricane hazard are water-based and not wind-based. Current research is showing that overall hurricane size, measured as the radius to peak sustained winds, forward speed of the storm system, and continental slope approaching landfall are better metrics to model surge height prediction. A Storm Tide is a combination of the normal coastal tides and the storm surge that creates a water level much higher than the surge alone. Storm tides can raise the mean water level by 15 feet or more, and some models have exhibited a maximum surge tide height at 38.5 feet above mean seal level6. In the case of Katrina, total surge effects were observed at 27.8 feet above mean sea level6.
The aftermath of Katrina and Sandy clearly show the risk storm surge presents to coastal communities. The coastal flooding in these storms dominated the damage conditions in both cases. Even though Katrina was downgraded to a Category 3 storm before landfall in New Orleans, it caused “catastrophic damage” consistent with a much greater Saffir-Simpson Category storm. The surge associated with Katrina was much larger than typical storms with greater intensities because the storm was much larger in radius, had slower forward system motion, and approached landfall along a track of gentle continental slope.
The United States has some of the most advanced codes and construction processes in the world, however, natural hazards continue to cause record breaking monetary losses. At Hinman, we are developing services to aid emergency managers and planners on the federal, state, and local levels in assessing the risks and developing mitigation solutions to reduce the losses associated with these high intensity storm events, advancing the discussion concerning natural hazards and resilience.
I have heard on numerous occasions that, “we cannot engineer our way out of natural hazards.” While we cannot make these hazards “disappear,” I firmly believe that thoughtful engineering can successfully manage the effects of natural hazards, reduce losses, and meet the challenge of improving resilience nationwide.
1 Knutson, Thomas R., John L. Mcbride, Johnny Chan, Kerry Emanuel, Greg Holland, Chris Landsea, Isaac Held, James P. Kossin, A. K. Srivastava, and Masato Sugi. "Tropical Cyclones and Climate Change." Nature Geoscience 3.3 (2010): 157-63.
2 Unger, David J. "The 10 Most Expensive US Natural Disasters." The Christian Science Monitor. The Christian Science Monitor, 27 June 2013. Web. 15 Sept. 2014.
3 Chen, C., R. C. Beardsley, G. Cowles, J. Qi, Z. Lai, G. Gao, D. Stuebe, Q. Xu, P. Xue, J. Ge, R. Ji, S. Hu, R. Tian, H. Huang, L. Wu, and H. Lin. "An Unstructured Grid, Finite-Volume Coastal Ocean Model." (n.d.): n. pag. Sea Grant College Program. Massachusetts Institute of Technology, 28 Mar. 2013. Web. 12 May 2014. <http://seagrant.mit.edu/publications/MITSG_12-25.pdf>.
4 Irish, Jennifer L., Donald T. Resio, and Jay J. Ratcliff. "The Influence of Storm Size on Hurricane Surge." Journal of Physical Oceanography 38.9 (2008): 2003-013.
5 The National Hurricane Center. "2014 Tropical Cyclone Advisory Archive." 2014 Tropical Cyclone Advisory Archive. National Oceanic and Atmospheric Administration, 24 June 2014. Web. 24 June 2014. <http://www.nhc.noaa.gov/archive/2014/>.
6 Infantino, Laurie, and Marjorie L. Segale. "Lessons Learned from Hurricane Sandy." Insurance Community University. Insurance Community University, n.d. Web. 15 Sept. 2014.
7 Masters, Jeffrey. "U.S. Storm Surge Records." U.S. Storm Surge Records. NOAA, n.d. Web. 15 Sept. 2014.