Efficacy of Dune Grading and Grass Planting for Controlling Excess Sand Supply in a us Pacific Northwest Coastal Community

Dune systems are an essential component in the protection of coastal communities from flooding and inundation. Typically, communities and coastal managers explore options that bolster dunes, by adding sand, constructing sand fences, or planting stabilizing grasses. In some cases, however, these systems experience an overabundance of sand, leading to recurring management problems (i.e., infrastructure burial) that require costly solutions. A balance must be found between encouraging dune growth and maintaining dune stability. Here we assess the ability of dune grading and grass planting to enhance coastal dune stability. This study aims to provide researchers and coastal community members with a better understanding of the effectiveness of nature-based dune management and inform future coastal decision-making practices.

The study area comprises 2 km of coastline adjacent to Pacific City, OR, a community that for decades has been dealing with the nuisance of an overabundance of sand that is burying homes, businesses, and roads. Monthly topography surveys began in August 2018 using Real Time Kinematic (RTK) GPS techniques and continue to be used to measure 29 cross-shore transects at 50 to 200 m alongshore resolution. In Spring 2019, approximately a third of the study area was graded and planted with two dune grass species. Post-planting ecological surveys are conducted to determine grass survivorship and the evolving density of the plants. These datasets, along with total water level (TWL) and wind time series are used to determine temporal variations in the relevant forcing and quantify the effectiveness of the management practices in mitigating the effects of high sand supply.

In conducting these surveys, we have seen spatial and temporal changes in the dune morphology as a response to natural and anthropogenic forcings. Seasonal variations in wave and wind forcings contribute to periods of accretion and erosion, while differing management practices result in spatially varying growth responses to these drivers. Comparisons between the response of the natural dunes and the recently managed ones are drawn to assess how the culmination of these different factors influence the dunes and if spatial variations in growth are a direct result of management practices or alongshore variances in natural processes.