Protective capacity of mangroves during tropical storms: a case study from “Wilma” and “Gamma” in Belize
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E. F. Granek & B. I. Ruttenberg


Globally threatened mangrove forest habitat is often considered an important buffer protecting coastlines from wave and storm impacts and coastal erosion. However, there is little empirical data quantifying the protective effects of mangroves during storms, primarily because of the difficulty of predicting where and when a storm will intersect the shoreline, to facilitate data collection before and after storm events. In 2005, opportunistic results from an ongoing study quantifying differences between intact and cleared mangrove areas on Turneffe Atoll, Belize, provided such pre- and post-storm data from tropical storms ‘Wilma’ (later a Category 5 Hurricane) and ‘Gamma’. We compared differences in equipment retention rates of 3 types of experimental devices previously installed in adjacent intact and cleared mangrove areas. Retention rates were greater in intact mangrove areas, empirically demonstrating the protective capacity of mangroves during moderate magnitude storm events. The results support the assumption that removal of mangroves diminishes coastal protection not only during catastrophic storm events such as hurricanes or tsunamis, but also during less energetic but more frequent events, such as tropical storms. This highlights the importance of improved coastal zone management, as storm events may increase in frequency and intensity with changing climate, and coastal mangrove forest habitats continue to decline in size and number. 

Main Results and Conclusions:
  • The study used comparative techniques between areas of mangrove coverage and areas that had been cleared of mangrove forest: “…7 paired study sites were arrayed along a 30 km stretch of coastline of Turneffe Atoll, Belize (Fig. 1). Each site consisted of one area of intact and one area of cleared mangroves”(102).
  • The study used three types of cages to study wave attenuation: herbivore exclusion cages, sediment Traps, and shades 102-103).
  • Results from the study are as follows: “The percentage of herbivore exclusion cages and cage controls lost during the storms was more than 6- fold higher in the cleared areas compared to the intact mangrove areas (paired t-test; t = 7.78, df = 6, p = 0.0002) (Fig. 3). Sediment traps were also lost at a significantly higher rate in the cleared areas than the intact areas (paired t-test; t = 4.60, df = 6, p = 0.0037) (Fig. 3)”(103). 
  • Based on their findings, the hypothesis that mangroves help attenuate waves from tropical storms was supported: “Our equipment survived significantly better in areas with intact mangroves (Fig. 3), demonstrating that mangroves provide substantial coastal protection from tropical storms relative to areas that have been cleared”(103-104).
  • In conclusion: “Results from this study provide the first quantitative evidence that mangroves can provide coastal protection from these less intense but more frequent storms, in addition to the protection they may provide from major events such as tsunamis (Dahdouh-Guebas et al. 2005, Danielson et al. 2005). Our equipment survived these storms at significantly higher rates behind a single layer of live mangrove root structure compared to similar areas with no remaining live root structure”(104).



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