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12 SEDIMENT CONTROL „u T' . A MATTER OF MATHEMATICS For the Washington Suburban Sanitary Commission (WSSC), an undetected miscalculation was hiding ' good news. WSSC owns and operates =a the Triadelphia and Rocky Gorge Reservoirs as water supplies for the suburban Washington D.C., area. As part of a water quality protection program,both reservoirs have been surveyed once every 10 years since their construction in 1942 to determine sediment accumulation and loss of reservoir volume. Surveys conducted in 1974 and 1984 indicated an increase in the rate of sedimentation in the reservoirs that would cause the reser- voirs to lose 50 percent of their original capacity within the next 30 years. _� y Faced with the potential for major dredging and construction costs, WSSC asked EA to analyze the sedimentation data to identify areas that were repre- unusual conditions would affect the management of the reservoirs. One sentative of sedimentation in the entire calculations, sediment study findings, major recommendation of the study was reservoir and that could be monitored or the validity of the data. to increase the frequency with which on a more frequent basis. The researchers found that WSSC measures the sedimentation, to When EA scientists examined and miscalculation in the 1964 reservoir ensure that such changes come to the analyzed the data, they discovered studies caused significant exaggeration attention of the watershed managers as significant and unexplainable of the sedimentation rate in later soon as possible. discrepancies and contradictions from surveys. Because they are in the same It now appears that the reservoirs survey to survey. They performed a watershed and subject to the same will not lose 50 percent of their complete recalculation by writing a climatic conditions, sedimentation rates capacity until sometime in the twenty- computer program that would calculate for Rocky Gorge are expected to be second century, and WSSC now has original reservoir capacity and sediment similar to the ones for Triadelphia. more than a century to plan dredging volume of the Triadelphia Reservoir, Sedimentation rates in reservoirs programs for the reservoirs. Of course, for which a more complete set of data vary over time with changes in land use such miscalculations could just have was available,against the actual sedi- and population, severe weather events, easily led to the opposite conclusions, ment survey data taken in 1950, 1964, and other factors. The quality of the but it is safe to say that this is one 1974, and 1984. Scientists reviewed water used by the communities served, instance where nobody minds--the the climatological conditions of the and the maintenance of the water correction will save WSSC time and watershed over the period of the supply, ecosystems, and recreational money. calculation to determine whether facilities are all dependent on the Contact: Jeff Elseroad 301-771-4950