SURVEYING/GPS
Grains of Sand
GPS survey-grade technology provides critical data for
understanding beach erosion
By Megan Hamilton

A small team from the Center for Coastal Studies, Scripps Institution of Oceanography, spends a good part of their working day at the beach.
    Not to be confused with play, the monitoring of beach erosion in San Diego County, California, represents important work. Eroding beaches mean lost revenue from tourist dollars. From an aesthetic point-of-view, rocky beaches are not as attractive as sandy ones. Sandy beaches also provide a natural protection against the undermining action of waves on cliffs, providing protection to valuable property sites. A wide, sandy beach helps to prevent waves from reaching the infrastructures close to the beach.
    North County San Diego beaches lie within the Oceanside Littoral Cell, which extends approximately 81 kilometers (km) south from Dana Point to Point La Jolla. A littoral cell is closed, in that no appreciable quantity of sand enters or leaves around the boundaries defining the cell. Sources of sand contributing to the cell include river runoff, cliff erosion, and beach nourishment. Offshore canyons are the main sink for sand within the littoral cell. The damming of rivers and flood control measures has prevented a major source of sand from reaching the coastline. Without the input of sand from sediment-laden rivers into the cell and decreased beach nourishment, sand is gradually being lost from these beaches.
    One solution to the beach erosion problem is increased beach nourishment. During October 1997, approximately 144,000 cubic meters of sand was dumped in the near shore area of Del Mar beach. The sand was a by-product of the dredging of the San Diego Bay.
    Determining the fate of beach nourishment sand is one question being investigated through the beach profile data collected monthly using standard survey methods. The data also provides insight into the cyclical nature of sand transport processes.
    One of the monitored site extends from Solana Beach to Del Mar, and covers a stretch of coast approximately 4km long. Solana Beach is very narrow and is backed by steep coastal cliffs. Del Mar Beach is narrow, with houses built at the head of the beach where once sand dunes drifted. The possibility of water reaching these houses is real during high tide, stormy conditions.
    As a scientist doing field work for the San Diego County beach monitoring program, I have developed a greater appreciation for the dynamics of sandy beaches. Work at the beach is often augmented by discussions about the project with interested passers-by. On a typical day, as I set up the survey equipment, discussions with passers-by could include the following exchange.
    Passer-by: 'You're not going to build a McDonalds here are you?'
    Hamilton: 'No, we're not. We're actually surveying the beach. We have chosen a number of transect lines, situated 400 meters apart and oriented approximately perpendicular to the coast. Each month we return to the same transect and obtain elevations of the beach, extending from the top of the beach out to a depth of approximately 4m. Simultaneously, a boat equipped with an echosounder and GPS, completes the same transect out to a depth of 30m. The changes in beach width and sand volumes can be estimated from these beach profiles.'
    Passer-by: 'But how do you know where to return to each month?'
    Hamilton: 'At the beginning of each predetermined Range line, we established a benchmark. From this point, we set up our transect line, using a compass to determine its correct orientation.
   'The horizontal coordinates of the benchmark and the range line orientation are used to calculate the horizontal coordinates of each point surveyed along the range line. The vertical coordinate of the benchmark are used to adjust the elevation of each surveyed point to a common vertical datum.' Passer-by: 'Oh! Ahhh. Have a good one.'
    To obtain accurate horizontal coordinates and elevations of our benchmarks, the Sokkia GSR2300 RTK GPS was used. This system consists of a GPS base station and a GPS rover with a hand held Husky computer running TDS data collection software. This survey-grade system can obtain real time positions accurate to 1 centimeter in the horizontal, and 1.7cm in the vertical.
    On a recent field outing, Jerry Sain, systems support manager for Sokkia Measuring Systems, brought along the rented equipment while meeting with Doug Gibson, staff researcher, Tom Sestak, field worker, and me. Prior to meeting, I had given Sain a file of the known benchmark coordinates, for use as control points. Sain had pre-loaded these into the data collector. The control points were chosen so as to encompass the field area and are used to calibrate field data to the California State Plane Coordinate System, and to the National Geodetic Vertical Datum 29.
    Establishing a base station was the first priority of the day. The equipment was lugged to the top of the bluff, just north of the San Dieguito River mouth. This crystal clear day, views of the barreling surf and, even, Del Mar Racetrack, could be seen. The location provided excellent radio coverage to the necessary benchmarks.
    Occupying the control points took a couple of hours, as members shuttled from one point to the next. Throughout this process, the recorded residual errors were acceptable.
    The next step was to occupy our benchmarks in order to determine their coordinates. In choosing a benchmark location, I looked for locations that were accessible and permanent. Hence, benchmarks were often located on a seawall, or on the foundation of a house, when available. This allowed the process of establishing the position of the benchmarks to often be handled using the smaller, lighter carbon fiber Sokkia RoverRod, which comes complete with a GPS antenna, receiver, and radio. For the harder to get to benchmark locations, the system was converted to a backpack.
    On returning to the field to collect beach profile data, we discovered the inevitable had happened during one of the recent El Nino storms. The combination of high tide and large waves had undermined the foundations of a few houses built at the head of the beach. Our benchmarks were not in as permanent a location as we first thought. We also lost a benchmark during the construction of a seawall, which, incidentally, saved another group of houses from storm wave damage. In cases like these, we saw first-hand how the ravages of the ocean can act upon the beaches and infrastructure.
    A number of benchmarks required relocation, and the RTK GPS was needed again to provide their precise position and elevation. To our advantage, we knew exactly where we had previously set up our base station. Setting up the base station in the same location meant we did not have to reoccupy our control points, saving time. To ensure the setup was the same, we reoccupied one of our benchmarks that hadn't been destroyed. The system provided the same results as before (to within acceptable tolerances). The accuracy and repeatability of results was reassuring, and, within no time at all, we had the coordinates of our reestablished benchmarks.
    San Diego beaches are a valuable asset to the community. The beach profile data we collect will bring insight and understanding as to the fate of beach nourishment sand and the cyclical process of sand transport. Combined with historical data, long-term changes in beach width can be evaluated. The use of GPS survey grade equipment is providing greatly needed accuracy and consistency to the program.

About the Author:
Megan Hamilton is a staff research associate for the Center for Coastal Studies Scripps Institution of Oceanography.

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