Physical model investigation of mid-scale mangrove effects on flow hydrodynamics and pressures and loads in the built environment

Abstract

Large (km-scale) mangrove forests can provide protection to shorelines and near-coast structures during extreme coastal flood events, including tsunamis and tropical cyclones. However, little is known about the effects of mangroves with a modest cross-shore thickness (~10–50 m), on flow hydrodynamics and resulting inland pressures and forces on near-coast structures. We constructed a 1:16 geometric-scale physical model of a Rhizophora mangle (red mangrove) fringe with modest cross-shore thickness to measure the effects of a mangrove forest's cross-shore thickness on wave attenuation and subsequent load reduction on near-coast structures, idealized during experiments with an array of cubes. Three configurations, one baseline with zero mangroves and two with mangrove cross-shore thicknesses corresponding to prototype-scale forest widths of 8.2 m and 19.0 m, were considered in front of an array of idealized slab-on-grade residential buildings. Transient wave conditions with varying incident parameters (wave amplitude, wave representative time scale, water level/mangrove emergence, and presence of a background current) were considered. Water surface elevations, water velocities, cross-shore forces, and pressures measured near and against the building array indicate that mangroves affected inland flow hydrodynamics and forces. The presence of mangroves was associated with elevated water levels and reduced peak velocities between the mangroves and inland structures. Increasing the mangrove cross-shore thickness reduced the cross-shore force on a structure by 11%–65% compared to the baseline case without mangroves. The force reduction by the mangrove configurations varied with incident wave representative time scale; waves with longer representative time scales required larger cross-shore thicknesses to provide similar force reductions to those observed for shorter waves. Further investigation into a wider range of mangrove cross-shore thicknesses, trunk densities, and wave conditions is needed to inform engineering performance of natural and nature-based features for resilient coastal design.