R2001-120 AUTHORIZING AGREEMENT - Laserfiche WebLink

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r � Sedimentation Surve of Lake Decatur B ' y s asin 6 � � Macon County, Illinois Introduction The Illinois State Water Survey (ISWS) conducted a sedimentation survey of Basin 6 (northern basin) of Lake Decatur during the Summer of 2000. The survey was undertaken to provide information on the storage and sedimentation conditions of the lake. Lake Decatur is owned and operated by the City of Decatur. The city withdraws water from the lake as the sole raw water source for direct distribution of finished water to the community. Sedimentation detracts from the use of any water supply lake by reducing depth and volume. As a result, there is a reduction of reserve water supply capacity and possible burying of intake structures. Sedimentation of a reservoir is a natural process that human activities in the watershed can either accelerate or slow. In general, sedimentation of a lake is presumed to be accelerated unintentionally as a secondary impact of other developments within the watershed. For example, construction and agricuItural activities in a lake watershed generaliy aze presumed to increase sediment delivery to the lake due to increased exposure of soil material to erosive forces. Reductions of the sedimentation rate in a lake due to human impacts almost always are the result of programs intentionally designed to reduce soil and streambank erosion, and they are often the result of implementing lake remediation programs. These programs might include, but are not limited to, the implementation of watershed erosion control practices, streambank and lakeshore stabilization, stream energy dissipaters, and lake dredging. Sedimentation of a reservoir is the final stage of a three-step sediment transport process. The three steps aze watershed erosion by sheet, rill, gully, and/or streambank erosion; sediment transport in a defined stream system; and sediment deposition, in which stream energy is reduced such that the sediment no longer can be transported in suspension or as bed load. Sediment deposition can occur throughout the stream system. Lake sedimentation occurs when sediment-laden water in a stream enters the reduced flow velocity regime of a lake. As water velocity is reduced, suspended sediment is deposited in patterns related to the size and fall velocity of each particle. During this process, soil particles are partially sorted by size along the longitudinal axis of the lake. Larger, heavier sand and coarse silt particles are deposited in the upper end of the lake; finer silts and clay particles tend to be carried further into the lake. Several empirical methods have been developed for estimating sedimentation rates in Illinois (ISWS, 1967; Upper hiississippi River Basin Commission, 1970; Singh and Durgunoglu, 1990). These methods use regionalized relationships between watershed size and lake sedimentation rates. As estimates, they serve well within limits. A more precise measure of the sedimentation rate is provided by conducting a sedimentation survey of the reservoir. The sedimentation survey