Ecosystem-based solutions to dune and beach erosion

Abstract

Sandy beach and dune ecosystems make up two-thirds of the world's ice-free coastline and constitute an important part of the global land-sea interface; however, rising sea levels, changing disturbance regimes, and expanding coastal development is expediting their disappearance. Today, coastal managers lean towards the implementation of natural ecosystems for coastal protection because these methods are less expensive, more sustainable, and more resilient compared to engineered structures. We present investigative results on the protective abilities of Sargassum spp - a widely-distributed pelagic macroalgae, commonly found as beach wrack and heretofore referred to as ‘sargassum’. We tested increasing volumes of sargassum in a flume and found that it reduced erosion-causing processes in the offshore and swash zone. Moreover, sargassum directly reduced erosion when placed atop an embryonic dune and exposed to waves. As such, our study provides information for beach managers on the physical benefits of sargassum to beach-dune ecosystems. We also document the effects of wind and run-up on dune vegetation. We tested common dune plants in a wind tunnel and wave flume and measured the effects with a custom-built force sensor. We found that wind caused relatively constant forces over time whereas run-up produced more varied forces. Additionally, plants were affected differently by wind and run-up depending on their morphology. These results have important implications for the persistence of coastal vegetation under a changing disturbance regime and give insight to a possible evolutionary trajectory of dune plants based on their cross-shore location. Finally, we demonstrate a complex role for vegetation in the process of storm-induced dune erosion. We performed a near prototype-scale laboratory experiment on a vegetated dune and a bare dune and found that the cumulative erosion rate and total erosional volume of the vegetated dune was greater than that of the bare dune. Additionally, our results showed that vegetation induced scarp formation more quickly and caused a deeper scarp. Vegetation initiated the formation of relatively steep microslopes, which then fed back with wave scouring processes to induce still steeper slopes, cascading into more erosion over time.