Using historical aerial photography and softcopy photogrammetry for waste unit mapping in L Lake.

L Lake was developed as a cooling water reservoir for the L Reactor at the Savannah River Site. The construction of the lake, which began in the fall of 1984, altered the structure and function of Steel Creek. Completed in the fall of 1985, L Lake has a capacity of 31 million cubic meters and a normal pool of 58 meters. When L Reactor operations ceased in 1988, the water level in the lake still had to be maintained. Site managers are currently trying to determine the feasibility of draining or drawing down the lake in order to save tax dollars. In order to understand the full repercussions of such an undertaking, it was necessary to compile a comprehensive inventory of what the lake bottom looked like prior to filling. Aerial photographs, acquired nine days before the filling of the lake began, were scanned and used for softcopy photogrammetry processing. A one-meter digital elevation model was generated and a digital orthophoto mosaic was created as the base map for the project. Seven categories of features, including the large waste units used to contain the contaminated soil removed from the dam site, were screen digitized and used to generate accurate maps. Other map features include vegetation waste piles, where contaminated vegetation from the flood plain was contained, and ash piles, which are sites where vegetation debris was burned and then covered with clean soil. For all seven categories, the area of disturbance totaled just over 63 hectares. When the screen digitizing was completed, the elevation at the centroid of each disturbance was determined. When the information is used in the Savannah River Site Geographical Information System, it can be used to visualize the various L Lake draw-down scenarios suggested by site managers and hopefully, to support evaluations of the cost effectiveness for each proposed activity