How Important are Marsh Properties for Assessing Flood Mitigation Benefits?

Coastal ecosystems such as marshes provide protective benefits to communities affected by flood hazards. How much protective benefit marshes provide, however, depends on a variety of factors including storm intensity and duration, heterogeneities in vegetation morphology, ecosystem health, intertidal channels, and the presence of levees. The goal of this study is to identify which marsh properties govern protective benefits for a range of flood hazards from king tides to hurricanes. A quantitative understanding of how protective benefits depend on ecosystem properties and health could guide investments into the protection or restoration of marshlands. To address this problem we link numerical modeling with field data from Magothy Bay, north of the Chesapeake Bay mouth, in Virginia. Our field site experiences alongshore tidal currents but is not exposed to open ocean waves. During an intensive field campaign from June to October 2017, and March 2018, we measured both physical marsh vegetation parameters like stem diameter, height, and canopy density and hydrodynamic variables like in-situ pressure and flow velocity. A high-resolution GPS survey in April, 2017, mapped intertidal channels and other morphological features. A key advantage of this data set is the occurrence of multiple coastal hazards during the field campaign. The joint availability of vegetation and hydrodynamic data provides a unique opportunity for modeling. We adopt a multi-scale modeling approach to understand how the protective benefit of the marsh changed over time. To achieve this goal, we link two numerical models based on the 2D Shallow Water Equations. Within Magothy Bay, our model captures the local bathymetry and includes a drag parametrization that is based on the detailed vegetation parameters available to us. We compare this parametrization to the more common modified Manning's coefficient. At the regional scale, we use ADCIRC to constrain the inflow boundary conditions into our local domain for different flood intensities. Although the horizontal length scales of our field site are an order of magnitude lower than those typically needed to observe storm surge attenuation in other studies, they capture the spatial heterogeneities of interest to us, meaning that results could be generalizable to other marshes.