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.
All of our data is collected using Global Positioning Systems (GPS). We use a local GPS base station setup on a known location to transmit real-time corrections for its position via radio transmission to other GPS receivers mounted to backpacks, an all-terrain vehicle (ATV), or our boat. This technique, called Real-time Kinematic (RTK) GPS, can achieve accurate position data within an area about the size of a golf ball (2-4 cm).
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.
Lidar (light detection and ranging) is a technology that uses beams of invisible light to scan objects and landscapes, measuring their distances, reflectivity, and 3D shapes. A laser scanner can be mounted to a number of platforms such as a tripod, plane, car, or boat, depending on the feature to be scanned. We collect lidar with a laser scanner mounted to the top of our boat, the R/V George Davidson. From this position, it 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.
Single beam sonar
At high tide, when the lower beach face is submerged, personal watercraft (PWC) are used to collect seafloor depths along the same transect walked by surveyors on the beach during low tide to continue the 2D profile offshore. The depths are measured using a single beam echosounder (sonar) that sends a ping of sound down to the seafloor and calculates a depth based on the length of time it took for the sound to return the sonar. The PWC can also be used in rivers or lakes that are too shallow for a larger boat to safely navigate.
A multibeam sonar is similar to a single beam sonar in that it emits pulses of sound through the water column that bounce off the seafloor and return to the sonar to measure the depth. Instead of having only one beam, though, our multibeam has 256 beams that fan out in a swath to map four times the water depth at once. This means that in four meters of water, we can map a width of 16 meters of seafloor in one pass. CMAP's research vessel, the R/V George Davidson, has a dual-mount, dual-head multibeam sonar system to deploy two multibeam sonars at once, one off each side of our boat. This helps us to more efficiently map very shallow water as close to the shoreline as possible (up to ~1 m depth). For more information about our vessel setup, read this article in xyHt magazine, titled "Dual-head Mapping".