Climate change, coastal hazards, and coastal dune habitat restoration in the Pacific Northwest

Background/Question/Methods

Hurricane Sandy wrought destruction on mid-Atlantic coastal communities, but also served as a reminder that coastal dunes provide coastal protection against storm-related flooding and erosion. The U.S. Pacific Northwest (PNW) also faces severe coastal hazards with one of the strongest wave climates on Earth. However, PNW dunes present a social-ecological conundrum: invasive beach grasses (Ammophila spp) create tall foredunes that provide coastal protection, but simultaneously harm native flora and fauna. In response, multiple U.S. agencies are mechanically removing Ammophila from PNW coastal dunes. Given Ammophila’s role in building foredunes, and given the potential for sea level rise (SLR) and storm intensification in the PNW, we ask: (1) How does Ammophila removal affect foredune shape? (2) How does Ammophila removal alter exposure to coastal flooding and erosion during extreme storm events? Finally, (3) How will projected SLR and changes in storminess alter the impacts of Ammophila removal on coastal vulnerability? To investigate these issues, in 2012, we conducted foredune vegetation and topographic surveys at Ammophila removal locations and nearby reference sites. We then employed total water level models to estimate wave runup, flooding, and erosion during current and projected extreme storm scenarios, and the Bruun rule to estimate erosion from SLR.

Results/Conclusions

Ammophila removal reduced foredune toe and crest elevations by an average 1.2m and 1.6m, respectively, with minor impacts on beach slope. As a result, the lowered foredunes from Ammophila removal were more susceptible to flooding and erosion. Under the current wave climate, 55% of Ammophila removal locations may experience wave overwash as compared to 22% of reference sites during a present-day one-hundred year storm event. If storm intensification trends continue, a 2050 extreme storm event may result in flooding at 80% and 60% of Ammophila removal and nearby reference sites, respectively, with removal sites experiencing more severe flooding. Similarly, Ammophila removal locations would experience greater erosion than non-removal sites under both current and projected wave scenarios. Because PNW dune habitat restoration areas (i.e., Ammophila removal areas) are typically 100-300m in width, increased erosion from SLR and storm intensification may significantly reduce the size and conservation value of habitat restoration areas. Further, if fixed structures such as coastal development preclude inland expansion of restoration areas, then SLR or storms associated flooding and erosion could both undercut conservation and place coastal communities at increased risk. This case study demonstrates the importance of considering indirect effects of management actions in controlling invasive species.