Assessing Drivers of Alongshore Variation in Historical Coastal Dune Evolution: A Field and Model-Based Approach

Abstract

The protective services provided by coastal foredunes vary spatially due to alongshore differences in dune morphology and hydrodynamic forcing. This study aims to quantify the drivers of alongshore variability of dune evolution across spatial (40-km) and temporal (multi-decadal) scales. The alongshore variability in dune evolution is extracted from long-term beach and dune monitoring surveys on Long Beach Peninsula, Washington, USA, and analyzed to explore the drivers of dune volume change. To quantify the relative influence of geomorphic, geologic, and environmental forcings on dune growth, field analyses are complemented by numerical model sensitivity tests. The sensitivity tests are executed using a locally calibrated, reduced-complexity dune evolution model on three dune profiles representative of accretional, stable, and erosive conditions. Shoreline change rate best explains the observed variability in dune volume change at multi-decadal scales because it influences sediment availability to the dunes. Beach slope is identified as a secondary driver on dune volume change because it influences wave runup and therefore dune erosion during periods when dunes are in the collision regime, as well as affecting the fetch length of wind driven sediment transport. Grain size influences dunes where accretive processes dominate over erosive processes. Model sensitivity tests suggest dunes may become taller, steeper, and eventually undergo significant erosion under increased water levels and associated collision hours. Modeled dune evolution hindcasts indicate the importance of including shoreline change rate and the appropriate grain size into long-term dune evolution modeling, which can inform risk assessments and management planning for coastal communities.