Port Gamble SíKlallam Tribe Coastal Analysis: Shoreline Change and Extreme Water Levels

The Port Gamble SíKlallam Tribe (PGST) relies on coastal resources for recreation, cultural enrichment, spiritual enhancement, and food. Shoreline change and extreme water levels associated with climate change will impact the future of the PGST. As part of their effort to create a coastal management plan, PGST contracted with our project team to assess geologic aspects of coastal risk and provide recommendations for future monitoring. There is limited information on coastal geomorphology, sediment transportation, and accurate water levels for the PGST coast (Ladd et al., 2016; McCollum et al., 2016). This report addresses shoreline changes, extreme water levels, and coastal hazards associated with climate change along the PGST coastline. I designed a sediment transport monitoring system and conducted water level measurement and analyses. For the first Phase of this project, I assessed historical coastal bluff and shoreline changes using aerial photographs, historical maps and photographs, shoreline topographic surveys, LiDAR analysis, and time-lapse photography. I established shoreline transects for future monitoring of the PGST shoreline and collected baseline data. Using historical T-sheet survey data with modern LiDAR, I found the PGST bluff erosion rates to be less than 3.7 ± 2.8 in/yr over a 162-yr period from 1856 to 2018, with the highest rate along the Tribal Center bluff. Overall, the beach face appears to be relatively stable with little evidence of change from our GNSS beach transect surveys. To evaluate extreme water levels, I collected water level data along the PGST coast and compared our local water level measurements to long-term water level records at Port Townsend and Seattle. Water level data at PGST suggest that using longer water level records from Seattle and Port Townsend would reliably predict flood magnitude and frequency at PGST. Our data show bluffs currently undergo frequent interaction with sea water. During our study, time-lapse photography showed small (< 1 ft) waves with limited wave run-up. However, while not entirely common along the PGST coastline, the combination of larger storm events with high tides may cause flooding of Point Julia and increase bluff erosion rates. Lastly, I assessed the response of coastal flooding to climate change along the PGST coastline. Extreme water levels will flood most of Point Julia under different climate change scenarios. We created a series of inundation maps at Point Julia based on recent sea level rise projections for the area (Miller et al., 2018). Climate change and sea level rise will impact the coastline and how the tribe interacts with it. The development of detailed sediment budgets and shoreline change models requires long-term, high-resolution datasets. While our data provide a baseline, continued study and additional data are recommended to make informed coastal management decisions. I recommend performing frequent (2-3 years, seasonally, or event-aligned) repeat surveys of the established shoreline transects. I recommend yearly surveying for transects in areas of highest erosion (i.e. near the Tribal Center). Flooding and bluff erosion may be mitigated by projects which support large woody debris and increased sediment on beaches. Coastal inundation maps may be helpful for planning and management strategies, considering the time frame and likelihood of each scenario