Data collection methods
Scientists in our Coastal Monitoring & Analysis Program (CMAP) use a number of techniques to monitor beaches and bluffs around the region. We measure the elevation and shape of the land's coastal topography to understand how quickly bluffs may erode or where beaches might be vulnerable to waves and flooding. We also map bathymetry — the depth and shape of the sea floor — to see how sand moves around offshore and learn about nearshore habitats. Changes in nearshore bathymetry influence the amount of wave energy that is available to impact the shoreline and cause beach, dune, and bluff erosion.
Collecting high-resolution data on Washington's coast
The ability to capture seamless coverage of bluffs, the beach face, and the adjacent sea floor is incredibly valuable. This helps us understand the nearshore zone, where most changes in morphology occur and where valuable ecosystem services exist.
It is difficult to collect data within this intertidal and shallow sub-tidal zone, so high-resolution data is unavailable in many areas of Washington's coastline. Our combination of various coastal zone mapping techniques overcomes such challenges.
Efficiently collecting data
To cover a large region efficiently, we often collect beach and nearshore profiles to capture a 2D cross-section of the coast. The beach portion can be walked by surveyors with GPS mounted on backpacks, while the nearshore portion is collected using personal watercraft (PWC) outfitted with a single beam sonar and GPS. The maneuverability of the PWC allows for data collection through the surf zone and in very shallow water (up to 0.5-m depth).
Cross-shore profiles are useful for providing information about a beach such as dune elevation, beach slope and width, presence of sand bars, etc., and when the same profile is collected multiple times, it helps make an assessment of how the beach is changing.
If a more detailed, high-resolution 3D model of the coastline is desired, we use equipment on our research vessel, the R/V George Davidson, to collect data that can be used to create a Digital Elevation Model (or DEM). Our boat is equipped with a laser scanner (lidar), which scans the vertical bluffs onshore, and multibeam echosounders (sonar), which measure the depth of the seafloor beneath the boat.
By performing lidar scans at low tide and multibeam surveys at high tide, CMAP achieves overlap between the data collection methods. Any gaps or shadows in the data are filled in by GPS surveys at low tide, bridging the gap between the laser and sonar data.
Beach profiles are normally surveyed at low tide to measure the elevation of the subaerial beach (the part of the beach uncovered by water) as far into the intertidal zone as possible. A GPS antenna attached to the backpack collects elevation data one point per second wherever the surveyor walks. Beach profiles are collected by walking along pre-defined transects starting in the dunes and walking across the shore to approximately waist deep in water. By walking the same profiles each season, we can measure how the beach face is changing over time.
In order to map how beach features differ and change alongshore, we attach a GPS antenna to an ATV and drive it back and forth along the beach. Data points are collected every 1-m, creating a 3D map of the beach surface. In one low tide, a surface map can be driven for a 4-km (2.5-mi) section of beach. These maps can be used for volume change analysis to determine how much sand is moving over time.