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Storm surge reduction by wetlands

By: Dr. Jeff Masters, 10:31 AM GMT on May 08, 2009

It has long been argued that coastal wetlands provide critical protection against incoming hurricane storm surges, and that restoration of lost wetlands should be a key component of any strategy to protect vulnerable regions such as New Orleans. But exactly how much protection do wetlands afford? This has been a contentious issue, and there is no one number that works to define the value of wetlands in protecting against storm surge. In fact, in some cases, wetlands do not reduce the storm surge at all.

The traditional rule of thumb: each 2.7 miles of marsh knocks down the storm surge by 1 foot
Historically, many people have used the rule of thumb that each 2.7 miles of marsh knocks down the storm surge by 1 foot (1 meter reduction per 14.5 km of marsh). This estimate is based on a US Army Corps of Engineers report from 1963 (USACE, 1963), which examined the inland penetration of the storm surge from seven storms in southern Louisiana between 1909 and 1957. However, the data from this study varied by about a factor of three--attenuation rates as high as 1 foot per 1.3 miles of marsh were seen in one storm, and as low as 1 foot per 3.8 miles of marsh in another.

Thus, the simple rule of thumb of a 1 foot storm surge reduction per 2.7 miles of marsh is not a very good one to use in most situations. The inland penetration of the storm surge is an extremely complicated function of storm track, speed, duration, size, and associated waves; the regional topography, geometry of the shore, presence of barrier islands, and slope of the ocean bottom; plus the type and thickness of vegetation, and presence or absence of levees. Wetlands will always act to slow down the inland penetration of a storm surge, so the surge will not be able to advance very far inland before the winds die down if a region is exposed to strong winds for a short period of time. One example of this was in western Louisiana during Hurricane Rita of 2005. As the hurricane approached western Louisiana at 11 - 14 mph, the coast was initially subjected to offshore winds that blew water away from land. In the final few hours before landfall, the counter-clockwise circulation of air around the hurricane brought on-shore winds and a storm surge of up to 15 feet to the western Louisiana coast. However, this portion of the coast was only subject to on-shore winds for a few hours, and the surge was reduced by the wetlands by 1 foot per 2.1 - 3.6 miles of inland penetration, according to an ADCIRC storm surge model simulation by Resio and Westerink (2008).

Wetlands don't always decrease the storm surge
However, the situation is very different for slow moving storms, or for portions of the coast subjected to strong winds for many hours. If a marshland is subject to strong winds for long enough, the wetlands will completely flood, and there will be no reduction of storm surge at all--and an increase in storm surge is even possible, according to the mathematical equations governing the surge (Resio and Westerink, 2008). This has occurred in Louisiana during a number of storms--Hurricanes Rita, Katrina, Gustav, Ike, and Hurricane Betsy of 1965, along the eastern side of the protruding delta of the Mississippi River (Figure 1). Resio and Westerlink (2008) found that during Hurricane Rita of 2005, strong winds blew along the east side of the Mississippi for almost a full day, completely flooding the 25 miles of wetlands fronting the Mississippi River levee at English Turn. In fact, the model results show that the surge probably increased in height, by 1 foot per 8.7 miles of inland penetration in the Hurricane Rita simulation, since the day-long period of strong winds allowed the surge to pile up against the levee. Thus, while the wetlands were able to slow down the speed with which the surge reached the levee, the wetlands had no impact on the surge height in that location. A similar effect was seen during Hurricane Carla in 1961, a ferocious Category 4 hurricane that brought the highest storm surge ever observed to the Texas coast--a massive 22.7 feet at Port Lavaca. Carla moved so slowly--just 8 mph--that the surge had plenty of time to inundate marshes, and along one inland bluff fronted by wetlands, the surge was higher than at the coast.

Figure 1. For portions of the coast subjected to strong winds for a long period of time, wetlands do little to prevent high storm surges. This NOAA SLOSH model simulation of Category 3 Hurricane Betsy as it tracked west of New Orleans shows the highest storm surge occurred (pink colors) in a region where the surge had passed over 25 miles of wetlands. The Mississippi River levee at English Turn dammed up the storm surge.

In a 2008 conference presentation, Dr. Pat Fitzpatick used a SLOSH storm surge model to show that wetlands reduced the inland penetration of Katrina's storm surge near the hurricane's inital landfall over the Mississippi River "bird's foot" by 1 foot per 1.5 miles of wetlands traversed. The effect varied with the depth of the surge--an eight foot high surge was knocked down about 13% by wetlands, while a one foot high surge was reduced 59%. However, he found that where people lived along the river's levees (Venice), the surge piled up, and the marshes did not decrease the surge at all.

How much will future wetland loss increase storm surge?
Over the next century along the Gulf Coast, sea levels will continue to rise, coastal land will continue to subside, and human impacts due to shipping and the oil and gas industry will continue to cause erosion that will reduce wetland acreage. According to Harold Wanless of the University of Miami, global sea level is presently rising at 3 mm per year, and the land along the Louisiana Mississippi River delta is subsiding at 4-11 mm per year. Relative sea level is, thus, presently rising at 70-140 cm (2.3 - 4.6 feet) per century. Global warming may increase this sea relative sea level rise even further. Wetlands are being lost at an average rate of more than 23 square miles (60 square km) per year, with higher pulses during hurricanes. Water is about one meter deep across recently lost marshes and up to two meters deep in earlier lost marshes. This is of concern not only for the potential loss of hurricane storm surge protection, but because wetlands serve to increase fisheries production, filter pollutants out of water, and provide wave protection.

Wamsley et al. (2007) performed a surge surge simulation using the ADCIRC model of what would happen if the wetlands were allowed to continue to deteriorate with no restorative efforts over the next 50 years. Their results suggested that 50 years from now, storm surge heights would increase by 10-15% along Louisiana coastal areas to the east of New Orleans. These results held for both a severe Katrina-like hurricane, and a more modest hurricane (both making landfall at about 12 mph). However, the authors cautioned that "the impact of landscape features on surge propagation is a relatively new application for surge models and research is required". To underscore this lack of understanding, a White Paper put together by 25 coastal scientists and engineers held in July, 2007 found that adequate storm surge data do not exist for calibrating and verifying the models used to predict the impact of wetlands (or other features) on storm surge. Wamsley et al. are working on a field program in southern Louisiana to reduce these uncertainties. They intend to measure water level and wave attenuation across a wetland between Lake Borgne and the Mississippi River Gulf Outlet channel, using four non-directional water level/ wave gauges, an anemometer, and a periodic characterization of the wetland, including elevation, plant type, plant density, and plant height. The data collected will be analyzed to determine the surge and wave attenuation based on the vegetation type, density, and height.

Some expert opinions: Do wetlands reduce storm surges?
At an October 2008 meeting of the Geological Society of America, Dr. Robert Young of Western Carolina University stated that while he agreed with proposed ambitious efforts to restore wetlands in Louisiana, the potential storm protection benefits were "unknowable, but are most likely to be minimal".

Dr. Stephen Baig, who retired in 2008 as the head of the National Hurricane Center's storm surge unit, commented to me: Marshes are functionally useless as storm surge dissipators--disregarding for the moment their acknowledged utility for ocean breeding stock and other necessary and/or desirable functions. Once a marsh has more than a few feet of water overlying it the frictional effect of the grass is erased. The mythical "2.7 feet of surge reduction per inland mile of marsh" is just that, a myth. Also, it's unfortunate that the sand islands that front the shoreline are called "barrier" islands. They are certainly not barriers to storm surge. They get over-topped or breached with regularity. They are functionally useless as surge protection.

Dr. Joannes Westerink, who was the originator of the ADCIRC storm surge model, had this to say: "I think it depends on geography, storm direction, speed and size. For storms that track to the west of the Mississippi River with their sustained easterly winds impacting the eastern side of the Louisiana Mississippi River region, the marshes are essentially irrelevant. For the east-west coastline in western LA, there does appear to be some attenuation of surge elevations for many storms.

Outspoken hurricane scientist Dr. Ivor van Heerden, who as I reported, will be removed from his position at the Louisiana State Hurricane Center in 2010, had this to say in his 2006 book, The Storm: Wetlands can protect us from storm surge. Along with barrier islands, they are the best, most natural, least expensive buffer available...Joe Suhayda has already published computer studies showing that the 9.3-foot surge at Cocodrie, LA during Hurricane Andrew in 1992 would have been a foot higher without the barrier islands. Joe has calculated that a completely healthy marsh system could cut the storm surge in New Orleans by half.

Figure 2. Hurricane Ike of 2008 pushed a massive storm surge far inland over Texas and Louisiana, as the brown areas along the coast in this NASA Terra satellite image show.

The take home message from all this is that the degree of protection wetlands provide from storm surges is extremely complicated and is largely unknown. A simple rule of thumb that "X" miles of marshland will knock down the surge by "Y" number of feet is not going to be valid for most situations. Storm surge models do have equations to estimate the attenuation of the storm surge by wetlands, but these equations have not been validated using real world data. Thus, any model estimates of storm surge reduction by wetlands must be considered suspect. Wetlands will slow down the progress of a storm surge, and so will be most effective for for weaker and faster-moving storms. But if the wind blows strongly enough for long enough, it doesn't matter how many miles of wetlands you have, the storm surge will come. Thus, efforts to restore wetlands for the primary purpose of reducing hurricane surge will be ineffective in many cases, particularly for large, slow-moving hurricanes.

Resio, D.T., and J.J. Westerink, 2008, "Modeling the physics of storm surges", Physics Today, September 2008, pp. 33-38.

Corps of Engineers, US Army Engineer District, New Orleans, Interim Survey Report, Morgan City, Louisiana and Vicinity, serial no. 63, US Army Engineer District, New Orleans, LA (November 1963).

Fitzpatrick, P., 2008, "The impact of Louisiana's levees and wetlands on Katrina's storm surge", 28th Conference on Hurricanes and Tropical Meteorology, American Meteorological Society, May 2008.

Wamsley et al., 2007, "Influence of Wetland Degradation on Surge", Proc. 10th International Workshop on Wave Hindcasting and Forecasting and Coastal Hazard Symposium.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.