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Our client was looking for a multisource data solution for a marina renovation and dredging project in a tributary to the Rehoboth Bay, in Sussex County, Delaware. Elevation data was required for the project area encompassing uplands, structure and water.
Problem
To provide detailed topographic data across a site comprised of uplands, tidal marsh and submerged/flooded areas. The site contained areas of marsh covered with several types of marsh grass, a mud flat with the consistency of chocolate mousse, and a substantial area of water in both a confined space within the marina and a general navigation channel, all of which was subject to a multiple foot tidal cycle.
AXIS GeoSpatial LLC was contracted to provide a comprehensive conventional land based topographic and bathymetric survey of a severely silted-in saltwater marina in White’s Creek, Rehoboth Bay, Delaware. The project area was comprised of three different types of terrain; dry land with a boat ramp, piers, bulkheading, gravel parking area, a few trees and two buildings on it, two areas of tidal flooded marsh covered with grass and a submerged area with varying amounts of extremely soft silt over a hard sand bottom. Complicating the bathymetric part of the project was extremely low water levels and large mats of algae which affected echo sounder readings. The bottom was so soft that wading was impossible, even though the water was only a foot deep at low tide. To accurately map the two bottoms, a dual frequency echo sounder was employed, yielding depth readings of both the soft silt and the hard bottom. The entire bathymetric work area was spot checked using GNSS technology on a specially constructed probe pole to locate the silt bottom and an additional GNSS mounted on a steel rod extension pushed through the silt until refusal was met. In many probe areas, the silt proved to be in in excess of 12 feet thick.
Solution
To fulfill the client’s needs, AXIS developed a technical approach that merged three different surveying technologies into a single, highly detailed and accurate data set that fulfilled the clients design and engineering requirements.
AXIS deployed a robotic surveying instrument for efficient, high speed data collection, using the automatic prism tracking system to record a grid and detail patterns of measurements across the site. Part of the survey detail included locations of piling, finger piers, bulkhead walls, buildings and security fencing, along with delineated marsh features.
In the areas too soft for foot travel, our robotic instrument was used in its laser scanning mode to scan the ground surface from the edge of the vegetation mat out onto the exposed mud flats at low tide. Once high tide returned, AXIS deployed a dual frequency echo sounder and dual frequency/dual constellation GNSS positioning system to collect water and sediment depths across the inundated area of the project. Water and sediment depths were confirmed, or bar checked, across the project area. In most of the project area, the sediment was more than 12 feet thick, which is shocking considering the maximum water depth in the work area was just over 6 feet at high tide.
All of the data was reduced and imported into AutoCAD Civil 3D 2016, where a field to finish coding system was implemented to efficiently convert the field data into planimetric drafting content and 3D surface feature lines. The laser scanner data was imported and the point cloud was used to produce a detailed mesh surface which was used to bridge the gap between the bathymetric data and the conventional survey data.
The final product was delivered to the client in conventional 2D and 3D AutoCAD formats. The 3D data was expanded into a fully rendered 3D model of the site, showing above and below water detail, improvement locations and vegetation information, referenced to water levels and tidal cycles, allowing accurate modeling of marsh fill and drain patterns and its effects on marsh habitat and wildlife.
Lessons Learned
While appearing to be a relatively straight forward, albeit somewhat equipment-intensive project, the biggest challenge proved to be fine tuning the echosounder equipment and then verifying performance of the water penetrating and sediment penetrating frequencies performance with conventional measurements. We did not expect to have sediment in excess of twelve feet thick in such a shallow, but highly used location.
Benefit
The final deliverable was a highly accurate and reliable topographic survey delineating site features and points of interest and verification of sediment conditions which had a direct impact on the design of shoreline protection, structural improvements to the marina, and planning for dredging operations to return the marina to a much higher functioning facility through improved navigation and better flushing of the water inside the marina berthing and launching areas.
William T. Derry, PLS is Director, Surveying Technologies at AXIS GeoSpatial LLC. He has 32 years of experience as a land surveyor and is licensed in Delaware, Maryland, North Carolina and Pennsylvania. For more information please visit www.axisgeospatial.com or contact Erica Insley at 410.822.1441
A 992Kb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE