Wave attenuation of coastal mangroves at a near-prototype scale

Abstract

A physical model study investigating the dissipation of wave energy by a 1:2.1 scale North American red mangrove forest was performed in a large-scale flume. The objectives were to measure the amount of wave attenuation afforded by mangroves, identify key hydrodynamic parameters influencing wave attenuation, and provide methodologies for application. Seventy-two hydrodynamic conditions, comprising irregular and regular waves, were tested. The analysis related the dissipation to three formulations that can provide estimates of wave attenuation for flood risk management projects considering mangroves: damping coefficient β, drag coefficient C𝐷, and Manning’s roughness coefficient 𝑛. The attenuation of the incident wave height through the 15.12 m long, 1:2.1 scale mangrove forest was exponential in form and varied from 13%–77%. Water depth and incident wave height strongly influenced the amount of wave attenuation. Accounting for differences in water depth using the sub-merged volume fraction resulted in a common fit of the damping coefficient as a function of relative wave height and wave steepness. The drag coefficient demonstrated a stronger relationship with the Keulegan–Carpenter number than the Reynolds number. The linear relationship between relative depth and Manning’s 𝑛 was stronger than that between Manning’s 𝑛 and either relative wave height or wave steepness.

Mangroves as Coastal Protection for Restoring Low-Energy Waterfront Property

“…Paddle In general, the wave reduction factors ranged from 0.60-0.80 for both the live mangroves and the dowels with the flap paddle. These reduction factors are similar to those found in the literature [18]; however, direct comparison with previous studies is limited due to the differences in variable ranges in experimental setups. Differences in the wave reduction factor between the live mangroves and dowels ranged between 0.7-5.7%.…”

Small-Scale Experiments’ Ability To Augment Large Lab Testing For Designing Nature-Based And Hybrid Solutions For Coastal Flood Hazard Mitigation

“…The highest density modelled by Bryant et al (2022) was 1.42 trees per meter squared at prototype scale. This translated to 90.88 trees per meter squared at 1:8 scale, as density scales with the square of the scaling factor.…”

“…In order to create a model mangrove specimen, the mangroves constructed at 1:2 scale by the U.S. Army Corps of Engineers (Bryant et al, 2022), which were used in the overtopping experiments of Libby et al (2024), were redesigned to 1:8 scale for construction and installation in the Coastal Basin at the USNA's Hydromechanics Laboratory. The nominal dimensions of mangrove trunk diameters, height, root diameters, and highest root height are listed in Table 1.…”

“…The roots are formed from a more flexible material, cut to specific lengths in order to facilitate a parabolic spreading to the experimental seafloor (Ohira et al, 2013). Libby et al (2024) and Bryant et al (2022) utilized PVC pipe and PEX tubing as the trunk and roots, respectively. These specimens used for the 1:8 scale experiments were constructed from 0.015m (3/5-inch) brass rod and 6-gauge (diameter 0.004 m) galvanized steel wire.…”

“…The experiments considered configurations including mangroves and a seawall, without other protection features, to isolate the effects of the mangrove forest on overtopping of an otherwise undefended seawall. The density for tested for experiments was reduced from the high-density forest reported by Bryant et al (2022)…”