The role of mangroves in attenuating storm surges

Study Number:

33

Author:

K. Zhang, H. Liu, Y. Li, H. Xu, J. Shen, J. Rhome & T. J. Smith III

Abstract:

Field observations and numerical simulations indicate that the 6-to-30-km-wide mangrove forest along the Gulf Coast of South Florida effectively attenuated storm surges from a Category 3 hurricane, Wilma, and protected the inland wetland by reducing an inundation area of 1800 km2 and restricting surge inundation inside the mangrove zone. The surge amplitude decreases at a rate of 40-50 cm/km across the mangrove forest and at a rate of 20 cm/km across the areas with a mixture of mangrove islands with open water. In contrast, the amplitudes of storm surges at the front of the mangrove zone increase by about 10-30% because of the “blockage” of mangroves to surge water, which can cause greater impacts on structures at the front of mangroves than the case without mangroves. The mangrove forest can also protect the wetlands behind the mangrove zone against surge inundation from a Category 5 hurricane with a fast forward speed of 11.2 m/s (25 mph). However, the forest cannot fully attenuate storm surges from a Category 5 hurricane with a slow forward speed of 2.2 m/s (5 mph) and reduced surges can still affect the wetlands behind the mangrove zone. The effects of widths of mangrove zones on reducing surge amplitudes are nonlinear with large reduction rates (15-30%) for initial width increments and small rates (<5%) for subsequent width increments.

Main Results and Conclusions:

• Study site description: “The mangrove forest in South Florida, which is the largest in the United States, is mainly distributed along the southwest coast of Florida next to the Gulf of Mexico and the south coast adjacent to Florida Bay, covering a coastline of 200 km and an area of 2800 km2 (Fig. 1). The width of the mangrove zone varies from 6 to 30 km along the coast…”(12).
• The effect of mangrove width on surge attenuation is as follows: “The results show that surge amplitudes, inundation extents, and surge decay patterns change remarkably as the width of the mangrove zone varies (Fig. 5).
  • “With a 1-km mangrove zone, surge amplitudes at the front of the mangrove zone increase by 3-18% along four profiles (Fig. 4), compared to surge amplitudes there computed without mangroves. Surge amplitudes at the back of the mangrove zone are initially reduced drastically by 16-30%, compared to surge amplitudes at the front of the mangrove zone, and then exhibit a gradual decay pattern. With a 3-km mangrove zone, surge amplitudes increase and decrease by about 9-32% and 46-57%, respectively, at the front and back of the mangrove zone.
  • “With a 5-km mangrove zone, surge amplitudes at the back of the mangrove zone are reduced by 54-71%. Surge amplitudes at the front of the mangrove zone increase by 12-34%, changing little in comparison to surge amplitudes with a 3-km mangrove zone.
  • “With a 7-km mangrove zone, surge amplitudes at the back of the mangrove zone decrease by more than 72-86%, largely reducing the impact of storm surges to the ecosystems behind the mangrove zone.
  • It appears that the threshold width of the mangrove zone for significant attenuation of the storm surge from Hurricane Wilma is about 7-8 km. This indicates that a mangrove zone with a width of several kilometers is needed to attenuate most of the storm surge from hurricanes like Wilma”(17).

• Despite the ability of mangroves to successfully attenuate waves, there are still storm surges that mangroves cannot safely attenuate, indicating the importance of preserving as much mangrove coverage as possible to help combat even the largest of storms: “Although the mangrove forest reduces surge amplitudes by 26-76%, surges can still impact the areas behind the mangrove zone because the wind has sufficient time to push the ocean water through the mangrove zone”(19).
• In conclusion, the thicker the mangrove forest, the better and more successful that forest will be at attenuating storm surges:
  • “Kilometers of mangrove forests are needed to reduce surge heights to a less damaging level for a Category 3 hurricane like Wilma. For slowly moving Category 4 and 5 hurricanes, even a 15-30 km mangrove zone is not wide enough to completely attenuate storm surges…The different requirements for widths of vegetation zones must be considered when bioshields are designed to defend the coast against impacts from wind waves and storm surges…
  • “Since large tsunamis have much higher amplitudes than storm surge waves, widths of mangrove zones required to reduce inundation from large tsunami events are probably comparable to those for slowly moving, intense hurricanes, e.g., on the order of tens of kilometers…
  • “The mangrove forests with widths of 6-30 km along the Gulf Coast of South Florida attenuated storm surges from Hurricane Wilma (Category 3) by reducing both the amplitude and extent of overland flooding, protecting the freshwater marsh behind the mangrove zone from surge inundation. Numerical simulations show that the inundation area by Wilma would extend more than 70% further inland without the mangrove zone, causing severe inundation of the wetlands behind the mangrove zone. The amplitude of storm surges at the front of the mangrove zone increase by 10-30% because of the “blockage” of mangroves to surge water, thus, structures in front of mangroves suffer more impacts than the case without mangroves. (Coastal Development). The decay rates of surge amplitudes are about 20-50 cm/km across mangroves…Without the mangrove zone, surge amplitudes would decrease gradually landward in almost a linear fashion with rates of 6-10 cm/km”(22).