Nature based solutions for coastal defense: Wave attenuation and economic analysis of marsh-fronted seawalls

Abstract

A seawall fronted by salt marshes is a hybrid, nature-based solution for rural and urban coastal protection. A one-dimensional wave attenuation model was developed using first principles to capture four mechanisms that impact wave evolution: Wave breaking, vegetation drag, shoaling, and bed friction. In particular, the vegetation drag was modeled using the stem and leaf morphology and material properties of specific marsh species. The model was validated with field wave height data. A benefit-cost analysis framework was used to present an economic argument for hybrid infrastructures. The additional wave attenuation from vegetation drag results in cost savings from lower seawall required for the same protection level, in addition to reduced scouring erosion, and the additional ecosystem services, such as habitat, water quality improvement, and carbon sequestration. Both the one-dimensional wave model and benefit-cost analysis framework were applied to an urban marsh-fronted seawall case study at Juniper Cove, Salem, Massachusetts. The presence of vegetation was found to reduce significantly the occurrence of wave breaking, which would be beneficial for sediment accretion to maintain a healthy marsh habitat and a less turbulent aquatic habitat. From the case study, narrow vegetation widths of 20 to 40 m can provide essential wave attenuation that would justify the marsh restoration in front of the existing seawall.