Recent studies in conjunction with Lahontan’s development of a Total Maximum Daily Load (TMDL) for Lake Tahoe have determined that the greatest impact on the Lake’s clarity is due to the suspension of fine sediment (less than 16 microns). Furthermore, it has been found that the greatest source of this sediment is from urban upland, or basically developed areas within the Basin.
The development and urbanization of the Basin altered natural drainage paths and runoff patterns and decreased the opportunity for precipitation to infiltrate into the ground. It also resulted in SEZs being filled to accommodate development, thus further decreasing the opportunity to reduce sediment and nutrients through natural functions before runoff reaches the Lake. The roads, structures, and related infrastructure, like parking lots, associated with urban areas cause increased runoff volumes and rates that concentrate flows and erosive forces. These flows generate and carry with them fine sediment and nutrients, often directly into streams and to the Lake.
The Conservancy has been a leader in addressing this problem and started the Soil Erosion Control Program in 1985 to address the problems associated with the water quality from urban areas. This program looks to address these problems through grants to local governmental agencies (Placer County, El Dorado County, City of South Lake Tahoe) and local public utility districts. These agencies implement projects that address the water quality of runoff from their streets, facilities, and urbanized areas.
Projects are implemented utilizing the Storm Water Quality Improvement Committee’s (SWQIC) Project Delivery Process. This involves establishing a Project Development Team, made up of regulators, funding agencies, implementing agencies, and others, to assist in identifying problems and alternatives. Projects also use the Preferred Design Approach in the development of alternatives.
The Preferred Design Approach looks to improve water quality in urban areas by first addressing the sources of sediment and trying to stabilize these sources by utilizing methods like vegetation, rock slope protection, or other methods. Next, hydrologic design is employed in an effort to disperse rather than concentrate flows and focuses on trying to achieve infiltration of flows rather than just conveyance of flows. Often times, this is accomplished by directing water to and enhancing SEZs. Lastly, flows that cannot be infiltrated must be treated to reduce their fine sediment loads. This may be accomplished through the use of devices like media filter vaults or more natural methods like infiltration basins and constructed wetlands.
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