Using Mangroves to Mitigate Hurricane Damage to the Southern US Coast

Authors:

Jodie Berezin, Natural Resource Conservation

Samantha Gray, Pre-Veterinary

James Woodward, Building & Construction Technology

 

In August of 2005, hurricane Katrina made landfall in New Orleans, Louisiana. Sitting directly on the Gulf Coast, New Orleans is no stranger to tropical storms. Katrina, however, hit the city like no other. The levees, which were the city’s primary defense for keeping storm surge out, failed. Wind and water devastated the once vibrant streets and shops, leaving the same streets that hold the famous Mardi Gras celebrations each year completely underwater. By the time the storm passed, the ocean had claimed eighty percent of the city, submerging people’s homes beneath ten feet of flooding (Plyer, 2016). Katrina left nearly a thousand bodies in New Orleans’s streets, with drowning accounting for 40% of the death toll. About half of these victims were elderly citizens taken by surprise, who failed to prepare themselves for the terrifying waters rushing over them (Brunkard, J., Namulanda, G., & Ratard, R., 2008, p. 2-3). This tragic scene plays itself over and over as large hurricanes continue to ravage the Gulf Coast, and something must be done.

The Gulf Coast of the United States regularly sees hurricanes that result in severe damages. Four of the top five cities in the U.S. most vulnerable to hurricanes span the southern coast (Freedman, 2012), including New Orleans and Houston, which have both been devastated by hurricanes in recent years. While hurricanes already present significant danger, the related literature agrees that hurricane intensity, and the associated damages, will only increase with the rise of climate change. The global phenomenon has already seen an annual average rise in sea surface temperature (SST) of 0.13℉ between 1901 and 2015 (Environmental Protection Agency, 2016). Even small temperature increases have significant impacts on climate, as the difference between a 1.5℉ rise and a 2℉ rise can mean heat waves that last 33% as long and rain storms that are 33% as intense (Silberg, 2016, para. 2).

These rising SSTs will intensify hurricanes, feeding the storms more heat energy the way candy might feed the hyperactivity of a child. The heat energy being released into the upper atmosphere is what lowers a storm’s pressure, and in turn creates its spiraling winds (NOVA, 2009). These winds push waters to shore and create unusual rise in sea level during tropical storms, a phenomenon called storm surge. Storm surges can also push farther and farther inland as general sea level rises, as they have a higher starting point. Sea levels already rose by an average of 2.6 inches between 1993 and 2014, due to thermal expansion (water expanding as temperatures rise) and melting of glaciers and ice sheets (National Oceanic and Atmospheric Administration, 2017, para. 1).

Rising SSTs working in concert with general rising sea levels can increase storm surge, and thus coastal flooding, to catastrophic levels (Mousavi, M., Edge, B., Irish, J., Frey, A., & Olivera, F., 2010). On average, hurricane flood elevations (the elevation to which floodwater will rise) along the Gulf of Mexico will increase by 0.3m-0.8m by 2080, while more powerful hurricanes (category 4 & 5) will increase 0.3m-1.8m by the same year (Mousavi et al., 2010, p. 592-593).

Coinciding with this intensification is the expectation of increased damages. Already, Hurricane Katrina cost the United States $160 billion in damages, accounting for inflation to 2017 dollars. Twelve years later, in 2017, Hurricane Harvey swept through Houston, Texas and cost $125 billion. Irma (2017) and Ike (2008) hit the southern US as well and cost the US $50 billion and $34.8 billion, respectively (NOAA, 2017, p. 3). These already-staggering costs are only expected to see an upward trend. In 2015, hurricanes accounted for 0.16% of the US GDP, which is $28 billion in damages (Dinan, 2017, p. 195). However, by 2075, hurricane damages will have increased to 0.22% of the US GDP, which is $151 billion in projected 2075 US GDP ($38 Billion in 2015 US GDP) (Dinan, 2017, p. 195).  As we face the threat of hurricanes destroying our infrastructure and threatening our lives, finding ways to mitigate these damages is imperative.

Hurricane damage may seem like an issue isolated to coastal communities, but it affects all taxpayers in the United States. The National Flood Insurance Program (NFIP) is a key effort by the federal government to remediate the financial burden on flood victims. To pay back hurricane damages, the NFIP borrows funds directly from the US Department of the Treasury. Since the Treasury is in turn funded by taxpayers, this means taxpayers across America are footing the bill for these damages. However, the government fears the program will not produce enough revenue to pay off what it owes to the Treasury; covering neither claims from the 2005 and 2012 hurricanes, nor potential future claims (U.S. Government Accountability Office, 2017). The Federal Emergency Management Agency (FEMA) owed the Treasury $23 billion for the NFIP as of March 2016, increased from $20 billion debt in November 2012. It made only a $1 billion principal repayment in December 2014, which was its first since 2010 (U.S. GAO, 2017). As hurricane damages continue and increase, the NFIP will only require more funds that it cannot pay back, costing all American taxpayers greatly.

Countless methods to reduce hurricane impacts exist, but mangrove forests may offer an especially valuable remedy to southern coastal states. Mangroves are tropical trees and shrubs that rest on the edge of a body of water, often where rivers meet oceans or seas. They have the ability to grow in both fresh and salt water areas, making them resistant to changing conditions and can survive where other trees can not. Mangrove forests have historically spread across the southern coast of the United States in the Gulf Coast area, from Florida to Texas (United States Environmental Protection Agency, 2016). Healthy mangrove forests have the ability to lessen the impact of flooding caused by hurricanes and tropical storms (Madren, 2012), but have seen alarming losses in recent decades. Globally, mangrove forests have seen area losses of about 35% (Valiela, Bowen, & York, 2001) to 50% (Feller et al., 2012) since original recordings in the early 1980s. Their annual loss rate is about 2.1%, caused by both natural forces such as hurricanes and associated winds, and anthropogenic forces such as deforestation and conversion to farmland (Valiela et al., 2001). If mangroves are properly restored and maintained by human effort, they can withstand and protect against the natural forces that threaten them along with coastal communities. To reduce damages from future hurricanes, local governments in the southern coastal US should invest in the restoration of declining mangrove forests.

Across the literature, there is a general consensus that mangrove forests reduce the height of storm surges and subsequent flooding. The drag of the water moving through the dense roots and stems of a mangrove forest is strongly related to wave reduction (Krauss et al., 2009). Increasing the area of mangrove forests can lead to more drag on incoming waves and storm surges, thus reducing their effects. Mangrove forests can reduce storm surges by 26-76% (Blankespoor et al., 2017; Sheng and Zou, 2017; Zhang et al., 2012). In addition, peak water level height decreased by 4.2 to 9.4 cm on average across multiple mangrove forest patches (Krauss et al., 2009, p. 145). Wave height was reduced by about 20% in mature forests in Vietnam (Mazda, Y., Magi, M., Kogo, M., & Hong, P.N, 1997). One study also suggests that without the mangroves on Florida’s coast, the storm surge of hurricane Wilma would have extended up to 70% further inland (Zhang et al., 2012).

Another option for mitigating damage caused by hurricanes is building a levee; a compact, earthen, hill-shaped flood protection barrier (FEMA 511). The effectiveness of levees depends on the specific site, it suggested that they are used in conjunction with a non-structural flood control method (Yen, 1995). While levees are a good short term solution, mangroves offer long term protection to coastal communities. Levees are a set height when they are built and will never change but mangroves, through conservation, have the potential to expand their range and mature, and to grow thicker, more tangled roots and stems, which is ideal for reducing storm surge (Mazda et al., 2009). Mangroves also provide valuable ecosystem services to the local community, in addition to storm surge reduction, that other types of barriers cannot. Mangrove trees purify water; cycle nutrients; prevent shoreline loss and soil erosion; provide high quality fisheries habitat; offer recreational and educational opportunities; and sequester carbon (Millenium Ecosystem Assessment, 2005). Carbon sequestration is the long term storage of carbon in plants, soils, geological formations and the ocean (Selin, 2011), and through the planting of mangroves, more carbon dioxide can be taken out of the air and reduce its impact on the climate (Environmental Protection Agency). Conserving mangroves for the purpose of carbon sequestration is economically viable for developing nations in Latin America, including Mexico (Siikamaki, Sanchirico, & Jardin, 2012; Vázquez-González et al., 2017) and could be viable in the United States as well.

Planting mangroves for the purpose of mitigating coastal damage is a long term investment that is time consuming and potentially costly. It can take around a year to complete a small scale (less than one hectare, or 2.47 acres) restoration project (Bayraktarov et al., 2016). On the bright side, even smaller, denser patches of mangroves have the ability to significantly reduce the impacts of faster storms (Zhang et al., 2012), which we can expect to see more of as the frequency of higher category storms increases (Zhao & Held, 2010). On top of this, the long term benefits make it worth the wait.

As far as cost goes, it could take anywhere from less than US$100 per hectare to US$216,000 per hectare, depending on whether more or less expensive methods are used (Lewis III, 2001; Primavera & Esteban, 2008, p. 53). Of all types of natural restoration efforts, however, mangroves are the least expensive and covered the largest areas of all coastal ecosystems (Bayraktarov et al., 2016). Mangroves acting as a fishery resource have a general potential market value of US$750-16,750 per hectare of forest (Rӧnnbӓck, 1999, p. 47), while the Gulf of Mexico, specifically, has mangrove fisheries valued at around US$37,500 per hectare (Aburto-Oropeza et al., 2008, p. 10457). If cheaper, or even middle-ground replanting methods are used, the cost could potentially be more than covered by fishery revenue alone. As restored mangrove forests promote local industries, mitigate climate change, and – most importantly – lessen otherwise devastating hurricane damages, they are sure to repay themselves and then some.

A case study done in the Bhitarkanika Conservation Area in India found the ecosystem services provided by the mangrove forest to be more numerous than having no mangrove forest (Badola & Hussain, 2005). They compared mangrove protected villages with villages not sheltered by mangroves (in response to a hurricane) using variables such as: damage to houses, livestock or fisheries; crop yield; losses incurred from storms per household; and villager attitudes toward mangroves. The villages protected by mangroves had overall, lower levels of adverse effects, such as damages to houses and higher levels of positive factors, such as crop yield (Badola & Hussain, 2005).  Losses incurred per household were also lower in mangrove protected villages (US $33.31) than unprotected villages (US $153.74) (Badola & Hussain, 2005, p. 89). In addition, 89% of local villagers ranked hurricane mitigation and flood control as the most valuable ecosystem services (Badola & Hussain, 2005, p. 89). The villagers are aware of the mangroves and appreciated the ecosystem services they provide because they protect their livelihoods from increased damages (Badola & Hussain, 2005).

One of the largest barriers to replanting and expanding mangrove forests is deciding what land will be used. First, scientists need to determine where the best places are for planting and expanding mangrove patches and who that land belongs to. In the U.S., 28% is owned by the federal government and a majority of land in the south is privately owned (Vincent, Hanson, & Argueta, 2017, p. 6). For example, in Louisiana, most of the coast is privately owned (Masson, 2017). For land that is state or government owned, converting unused or abandoned land could be beneficial. Private land, however, will require talking to land owners and making decisions at a community level. Going over people’s heads about their property is ill-advised; people are likely to cooperate when involved, as multiple case studies show.

Community based natural resource management (CBNRM) is a way to involve a community in its own restoration projects. CBNRM promotes conservation of natural resources, and directly governs and makes decisions about the resources within a community (USAID, 2016). This entails encouraging scientists to volunteer their time to gather people in a community willing to go out, plant trees and any other necessary steps to promote mangrove growth. Public education and promotion of CBNRM can bring a community together to learn about nature while protecting their homes and businesses, and contributing to mitigating carbon emissions. Ideally, this will lead to an appreciation of nature and give locals a sense of ownership of the forests, making residents more likely to take care of the saplings, thus, increasing the survival rate of newly planted trees. Such a program saw success in the Philippines. While centrally planned mangrove rehabilitation (institutional) had limited success, community driven initiatives were far more effective despite the smaller budget (Primavera & Esteban, 2008). Throughout Southeast Asia and Southern Thailand, this has also been proven to be true (Datta, Chattopadhyay, & Guhu, 2012; Sudtongkong & Webb, 2008).

While these case studies are from different parts of the world, the United States can apply similar measures. Pelican Bay, Florida utilized a form of CBNRM, following the die-off 63 acres of mangrove forest (Conservancy of Southwest Florida, 2018). With the help of the Conservancy of Southwest Florida and Collier County, the locals ultimately restored the entirety of that patch of forest (Conservancy of Southwest Florida, 2018). A project partner, the Florida Gulf Coast University (FGCU), successfully obtained permission from landowners to carry out the restoration, and held meetings with the City of Naples and its residents to plan and organize it (USF Water Institute, 2013, p. 4-5). FGCU staff and students, consultants, fundraisers, and stakeholders all got involved to turn this project into a successful reality (USF Water Institute, 2013, p. 4-5). More southern US cities can follow Pelican Bay’s lead by implementing CBNRM programs on a region-wide scale.

Mangroves are not the be all and end all to stopping damages from intensified hurricanes, but throughout the literature we covered, they are proven to lessen the blow. The land surrounding the Gulf of Mexico is frequently hit with powerful hurricanes and is home to several large cities that are highly vulnerable to damages from flooding and wind (Freedman, 2012). There are, however, many mangrove forests throughout the coasts of this area that can be restored and grown. If nothing is done to help the growth of mangrove forests, they will be at risk of deteriorating due to the effects of climate change (Blankespoor et al., 2017), and we will continue to lose their natural protection and other benefits. Human effort to maintain mangroves is the key to fighting the effects of the very force that threatens them. If we can raise awareness of these valuable mangroves slipping away from us and save them, the southern US coast will be prepared for the future hurricanes that threaten to unleash their full fury.

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Evan

2 Comments

  1. Hi Jodie, Sam and James. As can be seen in other countries, especially in the tropics, mangroves are natural inhibitors to storm surges. They are long-term measure compared to levees and are self-propagating in the sense that once you plant them they grow and increase in a few years. Cost effective too of you really think about it.

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