Mapping Bluffs and Beaches to Quantify Sediment Supply

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This project will help ESRP begin to understand how much sediment input is needed to sustain coastal processes and maintain nearshore ecosystem functions within individual drift cells. Baseline boat-based lidar surveys collected as part of this project will establish current physical shoreline conditions. Geomorphology and habitat metrics can be used to strategically prioritize restoration projects and assess restoration effectiveness.

Sediment supplied from eroding bluffs is important for sustaining Puget Sound beaches and the nearshore ecosystem. Under present shoreline development trends and rising sea levels, sediment supply is among the most at-risk service, while intact beaches and the connectivity of nearshore habitats are among the most difficult to restore. Yet sediment supply rates remain unknown and their effects on beach structure and ecosystem services are only conceptually understood.

This project involved the collection of high-resolution bluff and beach topographic data that can be used to quantify physical aspects of the coastal landscape, including morphology, shoreline armoring, and habitat. The project demonstrates the usefulness of boat-based lidar technology to map drift cells efficiently, with sub-decimeter accuracy and detail. The horizontal-look angle of the mobile laser scanner provides an advantageous view of the coastline, particularly for bluff-backed beaches common to the Salish Sea.

High-resolution digital elevation models and point clouds can be used to detect physical changes in morphology and habitat as needed for monitoring restoration project effectiveness, to understand how much sediment input is needed to sustain beach functions, and to strategically prioritize drift-cell restoration. Morphological and ecological metrics can be extracted from the data, such as bluff toe elevation, back-beach width, overhanging riparian vegetation, large woody debris, beach wrack, and shoreline armor dimensions.

While this data collection effort can only provide one snapshot in time, repeat surveys of the same sites in the future can help us answer key questions about the spatial and temporal variability of beach structure, bluff supply rates, and downdrift effects of sediment input or barriers to sediment transport. As more data is collected, we hope to (1) gain a better understanding of beach response due to altered sediment supply, (2) determine bluff sediment delivery rates, (3) evaluate effectiveness of beach restoration measures, and (4) characterize key linkages between physical and ecological processes.

Data Collection[edit]

In 2013, 2015, & 2016, the Washington State Department of Ecology Coastal Monitoring & Analysis Program (CMAP) collected boat-based lidar and GPS topography data at 16 sites around the Salish Sea. Sites from 8 of the 12 Salish Sea coastal counties in Washington were surveyed. The sites include a variety of shoretypes, including feeder bluffs, vegetated bluffs, low-lying spits, and modified shorelines. Each site took 1-5 days to survey, depending on the shoreline length, which varied between 1-36 km. A total of 220 km of shoreline, comprising over two dozen drift cells, were surveyed in 31 days.

CMAP uses a combination of boat-based lidar and GPS topography data collected on foot to map beaches and bluffs around the Salish Sea. CMAP collects lidar data using a laser scanner mounted to the top of our boat, the R/V George Davidson. From this position, the laser scanner can scan bluffs and beaches as we navigate along the shoreline. The data collected by the laser scanner is much denser than could be collected by GPS on foot, with dozens of points per square meter.

The position and heading of the vessel are derived using two Global Navigation Satellite System (GNSS) antennas mounted to either side of the cabin top, while the motion of the vessel is measured using a series of gyroscopes and accelerometers inside an inertial measurement unit, or IMU, mounted next to the laser scanner. Together, the IMU and GNSS antennas make up the Position and Orientation System (POS MV) onboard the vessel and allow us to correct for the boat’s movement due to waves and tides in real time. Data from the POS MV are integrated with data from the laser scanner to provide an accurate, georeferenced, 3D point cloud of all the features in the coastal landscape.

While boat-based lidar data provides high-resolution coverage of the coastal landscape, there may be gaps in the data created by shadows from large objects such as boulders, large woody debris, or beach ridges. GPS topography data collected on foot can fill these gaps as well as provide accurate ground-truthing of the lidar data. Together, these data are used to produce high-resolution, 0.5-m digital elevation models of the beaches and bluffs at each survey site.

Site Selection[edit]

The surveyed drift cells were selected based on a rigorous and systematic geospatial analysis of bluff-backed beaches. Many factors were assessed, including:

The potential of the bluffs to supply sediment to intact shorelines (i.e., feeder bluffs located on beaches with little shoreline armoring).
The presence of nearshore resources, such as an abundance of habitat for forage fish, eelgrass, herring, shellfish, and geoduck.
Proximity to existing or planned beach restoration projects funded by ESRP or ongoing work by local Marine Resource Committees.
The potential for future shoreline armoring or habitat loss based on population growth scenarios.

Scoring for each of the criteria was standardized on a numerical scale in order to assign a value for each criteria to every drift cell in the Salish Sea. The scores were tallied, and drift cells were ranked based on their total score. The highest ranking drift cells were put into three categories, or “tiers”: Tier 1, Tier 2, and Tier 3. For this project, all 17 Tier 1 sites were surveyed plus about 8 of the Tier 2 or 3 sites, depending on their proximity to other planned surveys. Adjacent sites were combined to create a total of 16 geographically distinct survey sites. Survey sites have feeder bluffs with relatively greater sediment supply, associated forage fish and shellfish resources, and past or planned restoration project work. Sites are distributed across Puget Sound, with a greater number of sites in the north due to the prevalence of exposed feeder bluffs. Because of the selection criteria, the surveyed drift cells are top candidates for implementing drift cell-scale protection and restoration strategies.

Applications[edit]

The high point density achieved with boat-based lidar allows for decimeter-level, detailed mapping of shoreline characteristics at a drift-cell scale. Features within the digital elevation models and lidar point clouds can be extracted for a more comprehensive inventory, classification, and assessment of site conditions and variability. Furthermore, features can be correlated to characterize how the shoreline landscape may be affecting nearshore ecosystem services.

Boat-based lidar data can be used for a variety of geomorphological and ecological applications, including:

Determining bluff erosion rates; this high-resolution bluff topography data provides a baseline dataset for determining bluff erosion rates once repeat surveys are conducted. Erosion rates are needed by coastal managers and property owners to develop appropriate setbacks and to better quantify the amount of sediment supplied by the largest contributor of sediment to Puget Sound beaches.
Quantifying sediment budgets within a drift cell; accurate mapping of where, how much, and how fast sediment moves within a drift cell, as well as the identification of erosion hot spots or sediment deficits, are important for understanding how to strategically restore beaches in Puget Sound.
Restoration effectiveness monitoring; not only does boat-based lidar provide high-resolution elevation data on beaches and bluffs that can be used to track sediment movement over time, it also provides detailed 3D mapping of physical habitat features, such as large woody debris, beach wrack, overhanging vegetation, and shoreline armoring, and change in those features over time.
Developing shoreline inventories of morphological and ecological features; quantitative metrics such as bluff height, bluff slope, bluff toe elevation, beach slope, beach width, back-beach width, shoreline armor elevation, width and height of overhanging vegetation, area or volume of large woody debris can be extracted from the boat-based lidar point cloud in order to make comparisons within and across drift cells. This information is useful as indicators of beach quality and function, and it can help provide feedback about whether beach restoration and regional recovery efforts are working.
Shoreline armoring inventory and assessment; boat-based lidar provides 3D mapping of shoreline armor attributes such as type, length, elevation, height, slope, and condition within the context of beach morphology. Because of this, it can be used to accurately assess encroachment, beach impacts, and downdrift effects such as changes in sediment supply or localized scour due to end effects.

Notes[edit]

This effort is positioned to provide monitoring services to shoreline restoration projects to evaluate effects of sediment supply manipulation. For example, see our report on the results of monitoring the Edgewater Beach shoreline armoring removal project.
Project work is led by George Kaminsky, PhD, PE, at the Washington State Department of Ecology.
PRISM contract for 2013-15 work.
The final project report is available at Weiner et al 2018 and contains extensive well illustrated examples of the capabilities of boat-based LIDAR.
Three Posters are available from the Salish Sea 2018 Conference:

Proposal for Future Funding[edit]

This project extends the current project of the Washington Department of Ecology, utilizing boat-based LiDAR to survey Puget Sound bluffs and beaches. Detailed mapping of beach structure and change will support quantification of sediment input at the drift cell-scale and evaluation of how a change in supply translates to a change in beach structure. This project performs repeat surveys of “Tier 1” drift cells for better understanding of beach change, and baseline surveys of “Tier 2” sites in order to expand the population of surveyed drift cells to obtain more complete geographic coverage and representation of Puget Sound.