What is Hydrographic Surveying?
Hydrographic surveys produce maps of an underwater area. They reveal the depths, physical features and objects on the bottom. Onshore topography is a 3D model of the shape of the land, based on measurements of locations and elevations. Underwater, bathymetry is the equivalent of topography: the elevation and contours of the ocean bottom, seafloor, lakebed, river bottom, reservoir basin or any other body of water. Hydrographic surveys can also be used to locate objects or obstructions underwater, such as pipelines, sunken boats, old pilings and large rocks. Repeated hydrographic surveys can model natural or man-processes, such as sand waves movement, dredging activities, sediment transport and shoaling (sediment filling in a deep area and making it shallower). Like topographic or land surveys, hydrographic surveys use special equipment to measure water depths and create bathymetric maps to support marine and waterfront infrastructure development.
Specialized Hydrographic Survey Equipment
Both land and hydrographic surveying equipment use GPS and GNSS to determine equipment positions. Similar to how land surveyors may use laser scanners or lidar to remotely model an area, hydrographic surveyors use sonars to remotely measure water depths. Sonars, installed on a boat or ship, use sound waves to determine the distance to the bottom, while laser scanners and lidar use light to determine distance from the instrument to the ground. Sonar or echosounder systems can be either single beam or multibeam. A single beam system measures the depth of water in a very small area, or basically a single point, directly under the sonar. Multibeam systems take hundreds of measurements in a swath or fan shape on the bottom. As the vessel moves forward, a wide stripe (much wider than the vessel) data is collected. Single beams are often used in very shallow water or where complete bottom coverage is not necessary, such as widely spaced river cross sections, while multibeam systems are often used in deeper water or for projects that require complete coverage. Multibeam systems are paired with an inertial navigation system that use two GPS/GNSS antennas and a highly sensitive motion sensor to model the movement of the boat or ship in the water. The acquired multibeam and motion/positioning data is then processed to create bathymetric maps that show the features of the seafloor.
Applying a Multibeam Echosounder System on a Kuskokwim River Survey
R&M used a multibeam echosounder system this past fall to survey a section of the Kuskokwim River, near Bethel. This system is the most ideal system for the type of environment in Bethel as much of the area could be efficiently surveyed with multibeam, achieving a complete model of the bottom without any gaps or spaces in the data. The City of Bethel needed this area surveyed because it is a critical route for barge and larger vessel traffic along the City Dock and fuel dock. Most goods and supplies are shipped to Bethel via barge, making the harbor and waterway access essential. Hydrographic surveys help the City monitor water depths in these critical routes, ensuring the route remains accessible and identifying trouble areas for vessels to avoid. Areas important to local skiffs were also surveyed, including the small boat harbor and channel, Brown’s Slough and the area in front of Beach 1 on the Kuskokwim River.
The City of Bethel’s 28’ harbor skiff was mobilized with survey equipment and operated by R&M personnel. Multibeam data was acquired in most of the Kuskokwim River survey area. Single beam data was also acquired in the remaining very shallow (under 10’ deep) areas of the river, Brown’s Slough, the Small Boat Harbor and Small Boat Harbor Channel.
Total survey area was approximately 135 acres and completed in just a few days.
The survey was referenced to Mean Lower Low Water using a local tide station benchmark with published coordinates and elevation. Real Time Kinematic processing and real time data coverage were monitored during the survey while Post Processing Kinematic solutions were computed for final positioning and motion results.
Benefits and Successes of this System
The combination of using multibeam and single beam equipment allowed the R&M survey team to accurately and efficiently survey all areas of the Kuskokwim River critical to Bethel, ensuring safe access for barges and local skiffs to the City. The large swath of the multibeam echosounder system enabled the team to make fewer passes through the survey area, saving time while acquiring a higher quality of data. The more mobile and less costly single beam equipment enabled the survey team to effectively and safely survey the shallow areas of the river, while still acquiring accurate depth data.
Why Conduct a Hydrographic Survey?
Hydrographic surveys are an important foundational step for any in-water design or construction projects. It is important to know the exact site layout, features and water depths so that designs can be generated and developed accordingly. With this service now in-house, R&M’s clients can streamline their project management to a single consultant versus managing multiple consultants. This results in better communication, greater efficiency and reduced costs.
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