Mapping Bluffs and Beaches to Quantify Sediment Supply 2021
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This project conducted by the Washington State Department of Ecology Coastal Monitoring & Analysis Program (CMAP) will help ESRP aim to understand how much sediment input is needed to sustain coastal processes and maintain nearshore ecosystem functions within a drift cell. Baseline boat-based lidar data collected between 2013 and 2016 will be compared to boat-based lidar collected as a part of this project in 2021 to quantify beach and bluff on a drift cell scale. Quantifying and characterizing morphologic change on beaches and bluffs can help to identify target conditions for assessing restoration effectiveness.
Sediment supplied from eroding bluffs is important for sustaining Puget Sound beaches and their ecosystem functions, however sediment supply rates are not well understood. As efforts to reconnect coastal bluffs to beaches by removing hard shoreline armoring become more common, it is essential to gain an understanding of typical sediment dynamics on unarmored shorelines as a target for restoration.
This project draws from existing high-resolution bluff and beach topographic data collected in 2015 and 2016 to direct efforts for collecting repeat surveys of beaches and bluffs in 2021. The high-resolution topographic data is collected using boat-based lidar technology. The horizontal look angle of the vessel-mounted laser scanner provides an advantageous view of the coastal landscape because it captures more detail on vertical faces like bluffs or shoreline armor than traditional airborne lidar systems.
The topographic data can be used to quantify physical aspects of shoreline morphology including beach width, beach slope, backshore width, bluff toe elevation, and bluff height. Comparing beach metrics between surveys can offer insight into the rate and volume that sediment is supplied to the beach by feeder bluffs. Review of beach and bluff change spatially within a drift cell can help us understand alongshore sediment dynamics.
Data Collection[edit]
In 2021, CMAP conducted lidar surveys at 17 sites around the Salish Sea, totaling 136.8 kilometers of coastline. Of these sites, 9 were repeat surveys and 8 were new sites.
CMAP primarily uses boat-based lidar supplemented with GPS topography data collected on foot to comprehensively map beach and bluff topography. A laser scanner mounted to the top of our research vessel, the R/V George Davidson, scans the nearshore environment. Returns from the laser scanner collectively create a lidar point cloud, which is stripped of vegetation and man-made structures to produce a high-resolution bare-earth digital elevation model (DEM).
Site Selection[edit]
The surveyed drift cells in 2021 were selected based on quantitative ranking of all drift cells within the Washington Salish Sea followed by a qualitative review for the final selection. The factors considered include:
- Ratio of mapped feeder bluff length to feeder bluff exceptional length
- Total length of feeder bluff exceptional
- Whether the site was previously surveyed during 2015 and 2016
- Proximity to existing or planned beach restoration efforts
- Visual inspection of bluff erosion likeliness based on oblique photos
- Feasibility and logistics of mobilizing for surveys
Repeat survey locations were ranked highest in the quantitative ranking, however some new sites were selected to increase the geographic and geomorphic distribution of baseline datasets, and to capture baseline surveys of planned restoration efforts.
Interim Update[edit]
The study conducted by the Washington State Department of Ecology Coastal Monitoring & Analysis Program (CMAP) aims to understand the changes of Puget Sound beaches and bluffs. Results from the study will gain insight into how much sediment input bluffs contribute to beaches to sustain coastal processes and maintain nearshore ecosystem functions within a drift cell. Baseline boat-based lidar data collected between 2013 and 2016 will be compared to boat-based lidar collected in 2021 to quantify beach and bluff changes on a drift cell scale. Quantifying and characterizing morphologic change on beaches and bluffs can help identify target conditions for assessing restoration effectiveness and prioritizing restoration projects.
Just over 15% of Washington’s Salish Sea shoreline is comprised of feeder bluffs, i.e., bluffs that act as a sediment source for adjacent beaches and their littoral cell. Feeder bluffs are part of an important cycle that supplies sediment to beaches to maintain a healthy habitat for many species functions, such as forage fish spawning. Forage fish are consumed by salmon, which are a keystone species in the Salish Sea, and culturally important to local tribes. Since salmon’s diet is comprised of forage fish, suitable spawning grounds are crucial for their survival.
Many people desire to live atop the coastal bluffs of the Salish Sea and install hard shoreline armoring as an attempt to protect their properties from erosional processes, the same processes that feed our beaches. Hard armoring includes structures such as seawalls, bulkheads, and boulder riprap. It is estimated that over a third of shoreline in the Puget Sound has hard armoring (Trimbach, 2019). Consequences of hard armoring include less sediment input from bluffs, increased beach erosion, and degradation of nearshore habitat.
In this update, four surveyed sites are compared: Cattle Point (4 km), West Whidbey Island (20 km), Edgewater Beach (3 km), and Maury Island (9 km). These sites have varying geographic positions, wave energy interaction, exposed bluffs, armoring, and nearby development. Each were surveyed in 2015 and 2021 using a variety of data collection methods to ensure accurate, high-resolution data. Data types collected include boat-based lidar, real-time kinematic GPS, and digital grainsize analysis.
- Puget Sound Bluffs and Beaches
All four surveyed sites predominantly glacial sediments deposited by the Cordilleral ice sheet that covered the Salish Sea area ~12,000-14,000 years ago (Booth et al., 2003). The bluffs are primarily composed of mixed sediment glacial outwash deposits and primarily contain grain sizes ranging from sand to gravel.
The surveyed section of West Whidbey Island was the longest extent of drift cell that we monitored. The adjacent waterbody is Admiralty Inlet, which has currents up to 6 knots. Whidbey Island is also a heavily populated area, with significant shoreline development and armoring. The surveyed portion of Whidbey featured 5 km of shoreline armoring. Two Whidbey Island beaches, Hastie Lake to the north and Ebey’s Landing to the south, were analyzed to see how armoring effects these beaches. Both sites showed a relatively narrow beach, less large woody debris and vegetation, and lower toe elevation in areas with armoring. Ebey’s Landing also had a low amount of vegetation in unarmored areas too, in part due to a higher volume of pavement near the State Park and National Historical Reserve. This reach also features a barrier beach and lagoon protecting the landward bluffs.
Cattle Point is located on the southeast end of San Juan Island, along Haro Strait. The drift cell is ~4 km long, and has no hard armoring, which makes it a unique site for comparison. The area falls within the San Juan Island National Historical Park, and is a tourist destination for its history, beaches, scenery, and wildlife viewing. The shoreline along the western half of the drift cell is backed by gently sloping prairie land and is characterized as an accretion shoreform for its wide backshore area (Avery, 2004; MacLennan et al., 2013). The eastern half is dominated by bluff-backed beaches with 1.4 km characterized as “exceptional feeder bluffs” where active erosion supplies sediment to the nearshore (MacLennan et al., 2013). On the beach, we observe erosional and depositional trends. Localized erosion has occurred on the foreshore of beaches down-drift side of the bedrock. Accretion is detected on the up-drift side of where bedrock jets out to the strait. On the foreshore of the main beach, we detected erosion fronting the vertical and west half of the sloping bluffs. Accretion was detected on the foreshore of the east end of the main beach.
Maury Island is located in the southern portion of the Salish Sea and is relatively sheltered by nearby land. The survey extent was 9 km, and there was 4 km of hard shoreline armoring. Like other sites, Maury saw higher amounts of vegetation and large woody debris in unarmored areas and lower toe elevation and narrow beach along shoreline armoring.
Edgewater Beach includes a restoration site where 241 m of armoring was removed in 2016. Lidar surveys were conducted in 2015, 2017, 2019, and 2021. Preliminary results show that armoring removal led to an initial influx of sediment to the beach from adjacent bluffs, bluffs steepening, increase in mean beach elevations, and beach widening. The armor removal also allowed for a back beach to develop where LWD can accumulate and provide habitat and shade for organisms.
In conclusion, we found across all sites that armoring results in a loss of beach habitat, lower bluff toe elevation, and narrower beach. Armoring removal can lead to the steepening of bluffs, widening of beaches, and influx of sediment. The Salish Sea offers a wide range of nearshore habitat and beaches provide vital ecosystem functions.
