Experimental study of wave dynamics in coastal wetlands

Abstract

This paper presents laboratory experiments of wave-driven hydrodynamics in a three-dimensional laboratory model of constructed coastal wetlands. The simulated wetland plants were placed on the tops of conically-shaped mounds, such that the laboratory model was dynamically similar to marsh mounds constructed in Dalehite Cove in Galveston Bay, Texas. Three marsh mounds were placed in the three-dimensional wave basin of the Haynes Coastal Engineering Laboratory at Texas A&M University, with the center of the central wetland mound located in the center of the tank along a plane of symmetry in the alongshore direction. The experiments included two water depths, corresponding to emergent and submerged vegetation, and four wave conditions, typical of wind-driven waves and ocean swell. The wave conditions were designed so that the waves would break on the offshore slope of the wetland mounds, sending a strong swash current through the vegetated patches. Three different spacings between the wetland mounds were tested. To understand the effects of vegetation, all experiments were repeated with and without simulated plants. Measurements were made throughout the nearshore region surrounding the wetland mounds using a dense array of acoustic Doppler velocimeters and capacitance wave gauges. These data were analyzed to quantify the significant wave height, phase average wave field and phase lags, wave energy dissipation over the vegetated patches, mean surface water levels, and the near-bottom current field. The significant wave height and energy dissipation results demonstrated that the bathymetry is the dominant mechanism for wave attenuation for this design. The presence of plants primarily increases the rate of wave attenuation through the vegetation and causes a blockage effect on flow through the vegetation. The nearshore circulation is most evident in the water level and velocity data. In the narrowest mound spacing, flow is obstructed in the channel between mounds by the mound slope and forced over the wetlands. The close mound spacing also retains water in the nearshore, resulting in a large setup and lower flows through the channel. As the spacing increases, flow is less obstructed in the channel. This allows for more refraction of waves off the mounds and deflection of flow around the plant patches, yielding higher recirculating flow through the channel between mounds. An optimal balance of unobstructed flow in the channel, wave dissipation over the mounds, and modest setup in the nearshore results when the edge-to-edge plant spacing is equal to the mound base diameter.