Modeling variable dune evolution in Cape Lookout National Seashore, North Carolina, USA

Abstract:

Coastal dunes are dynamic features which evolve constantly, eroding in response to storm events and recovering during relatively calm periods via aeolian and wave driven sediment transport. Predicting the intensity, rate, and style of these morphologic changes is complicated by many factors including the pre-existing geomorphology, tidal range, wind and wave climate, sediment and vegetation characteristics, and sediment supply. Recent efforts have coupled dune evolution models with nearshore hydro-morphodynamic models to coevolve the beach-dune system holistically. These state-of-the-art models provide platforms to enhance our knowledge of the interdependent physical and ecological processes controlling coastal dune evolution and, ultimately, develop better estimates of present and future coastal hazards.

In this research we quantify annual beach and dune changes from 2016-2019 along Cape Lookout National Seashore, NC with in-situ measurements and explore the drivers of observed alongshore varying rates and styles (i.e., vertical and lateral accretion) of dune recovery using the process-based, beach-dune evolution model Windsurf. In-situ measurements including cross-shore topographic profiles using Real Time Kinematic (RTK) GPS techniques, ecological quadrat sampling measuring dune grass species and density, and sediment grab samples were collected at 77 transects in the fall of 2016-2019. Field measurements, which capture snapshots of post-Hurricane Florence (2018) and Hurricane Dorian (2019) morphology, show spatially and temporally varying beach-dune response and recovery. These geomorphological and ecological observations, along with environmental conditions (e.g., total water levels and wind data) compiled from hindcast and observed data, are incorporated into Windsurf. Windsurf is used to explore the relative importance of environmental (e.g., wave and wind energy, grain size distribution, pre-existing morphology) and ecological factors (e.g., beach grass density, minimum vegetation elevation) at cross-shore profiles that experienced different rates and styles of dune evolution. Additionally, the sensitivity of specific model parameters (e.g., skewness and asymmetry parameters, vegetation friction coefficient) on model results is explored.