Turning the Tide
As the 2008 hurricane season draws to a close, it leaves behind a stark reminder that seashores and coasts are constantly changing due to storms and other natural phenomena. Observations and maps from high-altitude sensors provide comparisons of large areas over relatively long time intervals. What’s missing is the ability to gather and analyze information for small areas over shorter time periods, which would allow developers and environmental planners to guide efforts to control or mitigate changes to coastal areas.
Dr. Ivano Aiello, a geology professor at Moss Landing Marine Laboratories (MLML) in California, is working to change this situation. Located along Monterey Bay about 75 miles (120 kilometers) south of San Francisco, MLML is a dedicated marine research and educational unit of the California State University system. Aiello and his team are investigating coastal change along the shores of the Monterey Bay National Marine Sanctuary, the largest protected marine area in the United States, as well as in the Elkhorn Slough National Estuarine Research Reserve (California’s second-largest estuary).
The quantity and detail of data needed for these projects called for a new technical solution. To conduct the measurements and analyses, Aiello and his team decided to use spatial imaging technology--specifically the Trimble VX Spatial Station and Trimble RealWorks Survey software. “We looked at other scanners, but they were too bulky and heavy and did not provide the flexibility we need,” Aiello says. “I needed something that one person could easily carry--even in the marshes.”
Using the Trimble system, Aiello and his team are developing field and office strategies to collect high-resolution geospatial information and quickly process and interpret the data. According to Aiello, it’s the first time researchers have been able to obtain data for high-resolution, time-based analyses of the effects of individual storms.
Scanning at Moss Landing BeachFor the work at Monterey Bay, Aiello selected a portion of Moss Landing Beach located in the central part of the bay at the head of Monterey Canyon, the world’s largest undersea canyon. At this location, the Trimble VX could scan a section of the beach about 1,000 feet (300 meters) long and 100 feet (30 meters) wide.
A permanent marker was used as the instrument point and georeferenced with differential GPS. Backsight points set in a seawall provide a fixed basis for observations. Aiello is careful to maintain and preserve his control markers, and he achieves consistent measurements as a result. (The Trimble instrument is capable of collecting up to 15 points per second with 0.01-foot [3-millimeter] accuracy.)
The team defined three individual sectors visible from the instrument. Each measurement session scanned all three sectors to collect 3D points and photographic images. The measurement sessions were repeated at approximately one-month intervals starting in January 2008.
The speed and flexibility of the Trimble VX system also allowed Aiello’s team to scan the beach quickly on short notice. They could react to storms and other events that could--and did--erode or add to the beach. In one location, sand accumulated to cause changes in the surface by approximately 1.6 feet (50 centimeters) in just a few days. Instead of only tracking cumulative changes over several months, the team was able to document changes of just a few centimeters that occurred in the hours it took for the storm to pass. Not only could they say that the coastline changed at Moss Landing Beach, but they could also say where it changed and by how much.
Knowing how the erosion/accretion process works in short time cycles helps researchers better understand the long-term record. “It’s analogous to weather versus climate change,” Aiello says. “Understanding the short-term variability lets us know how [this variability] affects the long-term trends. We can make decisions on short-term actions with a much better understanding of how they will affect the coastal areas over the long term.”
Studying Elkhorn SloughElkhorn Slough, one of the largest and most varied wetland ecosystems in California, has experienced significant changes from both human and natural causes. In 1947, the U.S. Army Corps of Engineers completed work to remove sand dunes and modify the channel to create Moss Landing Harbor. In addition to this work, numerous dikes, levees and other structures have been constructed. As a result, tides and flows in the slough have changed substantially. Today most of the slough’s delicate marsh ecosystems are eroding rapidly.
While performing various studies of the slough, Aiello’s team has used the Trimble VX system to collect data from different locations to help accomplish several project goals. In Yampah Marsh, the team surveyed a sediment fence well upstream from the estuary’s outlet into Monterey Bay. Sediment fences are used to delay erosion by holding sediments in place and controlling the flux of water during tides. Scanning and imaging the site, Aiello’s team developed contour maps, surface models and 3D images. The work will show how well the different types of fences function and how they affect tidal flow rates and sedimentation.
Closer to the ocean, the team is monitoring an intertidal creek along the mud banks of the Old Salinas River. The information provided by the Trimble system showed areas of erosion and accretion separated by less than 33 feet (10 meters), which would have been difficult to detect through other means. Data from the scanning system was also the key to another important discovery. In addition to the 3D positions used for the surface models, the Trimble VX Spatial Station provides information on the intensity of the return signal from the electronic distance measurement (EDM). Aiello uses the return-signal intensity as an indicator of the reflectivity of the material he is scanning. While examining the point cloud of a mudflat on the Old Salinas River, Aiello noted a different reflectivity in one area. Because a change in reflectivity can indicate a change in materials, it drew Aiello’s attention. He collected coordinates of the locations from the software and loaded them into the Trimble CU Controller.
Using the Trimble VX’s stakeout functions and robotic surveying, Aiello’s team can repeatedly collect sediment samples from a specific spot. Since storms and erosion change the appearances of a site, the team relies on the spatial imaging technology to obtain the samples from the correct locations. The Geological Oceanography Lab at Moss Landing analyzes the samples to determine their physical properties and to understand how the composition of the sediments affects their reflectivity under wet and dry conditions.
Knowing the physical properties improves models for predicting and understanding potential changes in Elkhorn’s wetlands. Properties such as water content and soil saturation affect the survival of the various species of plants and wildlife, and these properties can change in as little as 3 feet (1 meter) from one sample to the next. So for Aiello’s research, it’s critical that samples be taken in the same place over the various time intervals. When using the more conventional hand-held GPS receivers for this work, the team could collect samples with accuracy of only a few feet. Using the stakeout function in the Trimble VX, Aiello is confident that the team will gather repeated samples with positional accuracy of around 0.03 foot (1 centimeter).
New Uses for Spatial ImagingAiello notes that the Trimble instrument’s speed and accuracy, lightweight portability and high-resolution imaging are beneficial to his research. Additionally, Aiello says that the image-capture capability of the Trimble spatial imaging system is an essential component of MLML studies. He can “drape” the photo images over the terrain models to create 3D photographic views, which allows the results of the geospatial analyses to be seen in the context of the actual location where the changes are occurring. As a result, it’s much easier for people to visualize what is happening. “They can easily see the characteristics, speed and magnitude of changes.” Aiello says. “Without the images, it would be much more difficult to share and explain the information.”
Aiello’s team uses the tools in Trimble RealWorks Survey to analyze and quantify the changes at each site, and they share their results by exporting data from RealWorks Survey into ArcGIS and other systems. The information is used by local, state and federal agencies for land and resource management, transportation and environmental protection.
The ability to collect geomorphologic data frequently and with high precision is a major advance for Aiello’s work. It is providing information that has not been available with airborne or other sensors. As a result, Aiello has become a staunch supporter of spatial imaging technology and sees it playing an increasing role in scientific and research projects. “The benefits go in many directions,” he says. “We are safeguarding the wetlands. We are preserving the beaches, which, in turn, protects the inland developments and assets. We’re also helping to improve restoration after wildfires, hurricanes or other disasters. We will have better answers to the questions of what’s happening and what we should do about it. It’s a big step forward in managing and protecting our natural assets.”
Sidebar: Geomorphology and SurveyorsGeomorphology, the study of the physical features of the Earth’s surface and their relation to its geological structures, is a discipline that relies on surveying and mapping technology to obtain the measurements needed to assess those structures. A good example is the survey of India in 1856, which produced the discovery and first measurements of Mt. Everest as the planet’s highest peak. More recently, the Plate Boundary Observatory (PBO) project in the western United States is using more than 800 survey-grade GPS receivers to observe plate tectonics. And airborne or satellite LiDAR and photos of geomorphologic changes--commonly seen as illustrations in the scientific and popular press--are important tools for geologists.
While many geomorphologic changes take place over very long periods of time, significant coastal erosion often occurs in a matter of hours. The tools used to record large-scale, slow-moving changes are not well suited for detailed studies of coastal erosion. The ground-based spatial imaging technology used in surveying is emerging as a versatile tool for this type of work.