Abstract
This study investigates the potential of a Rhizophora mangrove forest of moderate cross-shore thickness to attenuate wave heights using an idealized prototype-scale physical model constructed in a 104 m long wave flume. An 18 m long cross-shore transect of an idealized red mangrove forest based on the trunk-prop root system was constructed in the flume. Two cases with forest densities of 0.75 and 0.375 stems/m2 and a third baseline case with no mangroves were considered. LiDAR was used to quantify the projected area per unit height and to estimate the effective diameter of the system. The methodology was accurate to within 2% of the known stem diameters and 10% of the known prop root diameters. Random and regular wave conditions seaward, throughout, and inland of the forest were measured to determine wave height decay rates and drag coefficients for relative water depths ranging 0.36 to 1.44. Wave height decay rates ranged 0.008–0.021 m–1 for the high-density cases and 0.004–0.010 m–1 for the low-density cases and were found to be a function of water depth. Doubling the forest density increased the decay rate by a factor two, consistent with previous studies for other types of emergent vegetation. Drag coefficients ranged 0.4–3.8, and were found to be dependent on the Reynolds number. Uncertainty in the estimates of the drag coefficient due to the measured projected area and measured wave attenuation was quantified and found to have average combined standard deviations of 0.58 and 0.56 for random and regular waves, respectively. Two previous reduced-scale studies of wave attenuation by mangroves compared well with the present study when their Reynolds numbers were re-scaled by λ3/2 where λ is the prototype-to-model geometric scale ratio. Using the combined data sets, an equation is proposed to estimate the drag coefficient for a Rhizophora mangrove forest: CD = 0.6 + 3e04/ReDBH with an uncertainty of 0.69 over the range 5e03 < ReDBH < 1.9e05, where ReDBH is based on the tree diameter at breast height. These results may improve engineering guidance for the use of mangroves and other emergent vegetation in coastal wave attenuation.
Highlights
As coastal communities search for effective, resilient adaptation strategies to coastal hazards including sea level rise, erosion, and storm-driven wave and surge events, natural and naturebased systems have gained attention for their ecological, social, and engineering benefits
Results of this study indicate that even mangrove forests of moderate cross-shore width (∼10–50 m) can provide measurable reduction in wave heights, building on previous studies that have considered greater forest expanses (e.g., Maza et al, 2019)
(6) An equation is proposed to estimate the drag coefficient for a Rhizophora mangrove forest: CD = 0.6 + 3 × 104/ReDBH with an uncertainty of σ = 0.69 over the range 5e03 < ReDBH < 1.9e05, where ReDBH is based on the tree diameter at breast height
Summary
As coastal communities search for effective, resilient adaptation strategies to coastal hazards including sea level rise, erosion, and storm-driven wave and surge events, natural and naturebased systems have gained attention for their ecological, social, and engineering benefits. The present study shows a reduction in At at lower elevations, while the models of Chang et al (2019) and Maza et al (2019) consistently increase toward the base of the specimen This difference may be owing to root overlap in the analyzed stencils causing underestimation of the per-tree projected area per unit height. As noted earlier, Maza et al (2019) used the projected area as the characteristic length scale for Re. To provide a consistent basis for comparison, DBH was used as the characteristic length scale to rescale their reported values of Re. In the figure, drag coefficients from the present study for regular and random wave cases are shown as light red squares and dark red circles, respectively. This comparison reinforces the necessity of proper consideration of Reynolds-dependent CD values estimated from reduced-scale tests based on the vegetation archetype considered
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