Mapping & Surveying: Monitoring the Active Replenishment of Subsiding Habitat Project (MARSH) - Elevation Survey
To evaluate the success of the spray dredging experiment, very detailed surveys of the marsh surface were required before and after the dredging operation. Since anticipated changes would be very small, a technique that could measure very slight elevation changes over time was required. To accomplish this a GPS survey using dual frequency, carrier phase measurements and Real Time Kinematic (RTK) positioning was employed. Carrier phase measurements provide accuracy at the centimeter level. RTK surveys use a radio link between the base station receiver and the roving receiver so corrections are made as the points are logged in the field. In this project a base station receiver was established near the marsh sites. The rover receiver, or the unit on the marshes, occupied multiple locations on the marsh surface where the dredge sprayed material would be deposited. Approximately 80 points were measured at each experimental site. The resulting coordinates were used to generate the marsh surface topography. The experimental sites were resurveyed after the dredge spray activity to produce contoured before and after maps. Spray dredging using fine silt and mud from the marsh creeks is not a particular effective method of increasing marsh surface elevation. Based on calculations of material accumulation the amount of material successfully placed on the marsh surface was very modest, given the amount of material originally excavated. The material available in the marsh creeks is so fine-grained, that it does not settle out on the marsh surface quickly enough to accumulate. The experimental sites will be resurveyed each year to determine the stability and longevity of the material that did settle on the marsh surface.