Real-Time Location Methods for Underwater Repair of the Ludington Pumped Storage Plant Reservoir Foundation

The Ludington Pumped Storage Plant is an 1872 mega-watt facility located on the eastern shore of Lake Michigan. The 842 acre upper reservoir is lined on the bottom with a glacial clay till that ranges in thickness from three to five feet. As early as 1975, trenchlike features developed in and through the clay liner, ranging in size from four to ten feet in width, three to forty feet in depth and hundreds of feet in length. A program was initiated in l989 to investigate the causes of the trench features and design an effective repair. The investigation, engineering design, and remedial construction methods for this project were presented at Dam Safety '94. Since de-watering the reservoir would be logistically difficult, have a negative impact on plant operations and result in further damage to the clay liner, a repair method was developed that could be implemented underwater. The necessity for underwater repair over such a large area required an accurate map of the trench features and precise, real-time positioning of the vessels and divers implementing the repair. The positioning methods had to be effective in the planned water depth range of 25 to 100 feet. This paper presents the technical methods for mapping the trench features, positioning vessels, and locating divers to inspect and fill the trench features from 1992 through 1997. Current positioning methods include local area differential GPS (LDGPS) for vessel positioning and ultra-short baseline (USBL) tracking systems for locating divers. Accuracy for these systems has improved to plus or minus one to two feet for the LDGPS and plus or minus three to five feet for the USBL system relative to the vessel position. The continued improvements in positioning have allowed the trench feature maps to be refined annually using side-scan sonar data, fathometer records, and diver tracking data. For each dive inspection and remedial operation, the diver's USBL location is integrated with the dive vessel's LDGPS position and displayed in real time with the digital trench feature map on the onboard navigation computer. This system has been proven to be an effective method for guiding the divers to the trench features and for documenting the locations of the dive inspections and remedial activities. 12 pp., 5 references.