Seabee Divers complete inspection of Gathright Dam for US Army Corps of Engineers
April 29, 2014
By BUC (SCW/DV) Brian Strantz
Underwater Construction Team 1
COVINGTON, Va. - Located 19 miles from the town of Covington in western Virginia, Gathright Dam provides flood and water quality control for the Jackson and James Rivers. The completion of the Gathright Dam in 1981, created Lake Moomaw, which now provides 43.5 miles of recreational shoreline. Management of the area and oversight of the dam operations and maintenance is the responsibility of the U.S. Army Corps of Engineers (USACE). The USACE recently requested inspection services from U.S. Navy Underwater Construction Team 1 (UCT-ONE), based on their requirement to have a documented underwater inspection of Gathright Dam overseen by a registered Civil Engineer. Typically contracted to an engineering firm, the inspections were the first working venture between UCT-1 and the USACE.
UCT-ONE Construction Dive Detachment Bravo was tasked with the inspections and began training for the mission in early March. Lt. Dave Hallam, the UCT-ONE Executive Officer and registered Civil Engineer, augmented Detachment Bravo and served as the on-site engineer. SCUBA was chosen as the most effective means of completing the inspections and would provide complete mobility throughout the project. The timeframe of the inspection would require the detachment to make dives in cold water and the elevation of the dam (1,500 feet above sea level) necessitated the use of altitude dive charts. To compensate for the water temperature of 40 degrees and air temperatures at 29 degrees, the detachment utilized dry-suits and Poseidon cold-water regulators. To relay the details of the inspection to the surface, the divers used full-face masks with through-water communications. Divers also wore a helmet-mounted camera and light system that transmitted video to a digital recorder located in the support boat to provide documentation of the inspection.
Dive inspections were required at two different locations. The first of the locations was where the floodwaters empty from the south side of the dam and flow into the Jackson River. Known as a stilling basin, this area is a concrete lined outlet tunnel that provides an area to calm the turbulent water before it flows downstream. The stilling basin is approximately 320 feet long and 60 feet wide. The flow through the stilling basin is controlled by the USACE and can range from 100 cubic feet per second (cfs) to 10,000 cfs in emergency cases. To minimize the water flow through the basin and allow safe diving operations, the flow was regulated to approximately 145 cfs. Depth of the basin varies with the flow rate, but a maximum depth of 35 feet was achieved during the inspection of the basin.
The second location was a bridge access pile located in Lake Moomaw. The bridge access pile supports the connecting bridge that allows access between the intake station and the dam. Constructed of two parallel concrete columns connected by a concrete diaphragm, the structure was 21 feet wide and 8 feet deep. Maximum depths at the base of the pile were 45 feet, but dropped off dramatically towards the intake tower, the deepest point of the lake at 150 feet.
Following initial set-up of the Transportable Recompression Chamber System (TRCS) and dive side, the detachment conducted hydrographic inspections of the basin. Utilizing a bathymetric and side-scan sonar system, the detachment was able to provide an underwater image of the basin floor to ensure that bottom contours were verified and divers would not become entangled in unforeseen submerged objects. After confirmation of a clear dive site, diving operations were started on the stilling basin side of the dam. The detachment used two inflatable dive boats with one containing divers and the second one holding the video survey equipment and communications system. Outflow from the basin was reduced to a manageable level and the dive teams were launched from the boat. Following numerous dives, the detachment completed a 100 percent visual inspection of the stilling basin and removed any debris that was found in the basin. Structural issues identified by the dive teams were relayed to the engineer on the small boat and visually documented with the underwater video system.
The inspection of the access bridge pile was conducted in the same method as the stilling basin. Using dive pairs, the detachment conducted a complete visual inspection of the pile. In order to ensure the condition of the pile, divers removed a build-up of algae in 18-inch strips at the mud line, mid-depth and at the surface. All identified issues with the pile structure were photographed and noted in the inspection report.
The inspection project provided a real-world training experience that focused on skills not frequently utilized at UCT-ONE. Cold water and altitude diving required the detachment to conduct concentrated pre-inspection training and ensure that the required gear was available and ready for use. Coordination with USACE District Diving Coordinators ensured that all diving operations were in accordance with the OSHA Safety Manual and EM-385-1-1 Army Safety and Health Requirements Manual prior to any diving in an area overseen by the USACE. The successful execution of the inspection was a direct result of continued coordination between the USACE and UCT-ONE and will undoubtedly lead to a continued working relationship in the future.
UCT-ONE provides responsive inshore and ocean underwater construction, inspection, repair and maintenance to ocean facilities for Navy, Marine Corps and joint forces engaged in military operations.