Managed coastal dunes and the role of slope in storm impact and recovery

Abstract

Coastal dunes protect landward infrastructure from flooding and wave impact during storms. Managed coastal dune systems are becoming the new ‘normal’ along developed coastlines where natural dune growth is often inhibited. Understanding and optimizing managed dune systems is critical for maintaining dune health and resilience. Previous research evaluating dune management strategies and morphology broadly evaluated morphologic impact at varying spatiotemporal scales, but more work is necessary to translate these results to future managed dune systems. This paper investigates the role of the seaward stoss slope for dune evolution and response for two managed systems in South Carolina, U.S.: a nourished dune with a stoss slope of 12° (Site A) and a managed upper backshore with a stoss slope of ∼2° (Site B). Field-based topography and vegetation coverage data are presented for one year, including Hurricane Ian's near-direct impact. Results are subdivided into temporal periods (TPs) to present managed pre-storm, storm-response, unmanaged recovery, managed recovery, and full-study duration comparisons between both sites. Findings support that the variance in stoss slope between sites resulted in differential response throughout the entire study period, attributing to initial dune development rates and recovery potential. Statistically significant (p < 0.01) hot spots of erosion and accretion throughout each TP present site-specific patterns, coupled with contrasting post-storm volumetric changes: −10.5 m³/day for Site A and +0.73 m³/day for Site B. Vegetation growth from initial planting suggest correlations exist (p < 0.05) between cross-shore placement (e.g., slope) and vegetation growth and retention. Results demonstrate post-storm recovery is best supported in managed dune systems when roughness elements are reinstalled, warranting further investigation on recovery potential for managed dune design. Overall findings from this study suggest trade-offs exist in the benefits derived (e.g., rate, protective capacity) from managed coastal dune system construction metrics that should be carefully considered for future dune management, especially in regions where storms are frequently experienced.