BEsT MANAGEMENT PRACTICEs HANDbooK - Tahoe BMP

infiltrate into dune sands because of their porosity (i.e., large connectedinterstitial void spaces resulting in limited detention or contact time). LakeTahoe dunes are comprised primarily of quartz sands derived from theweathering of granite. The chemically inert nature of quartz limits its ability toeffectively filter and remove dissolved nutrients from infiltrating stormwater.The proximity of Lake Tahoe’s dune lands to the littoral beach and nearshorewaters creates a concern that exfiltrating hyporheic groundwater contaminatedby shorezone development could rapidly reach the lake without beingadequately treated or filtered. Projects and activities planned for ToleranceDistrict 1 areas continue to be controlled because of this sensitivity todisturbance, susceptibility to erosion, and limited capacity to treat surface andgroundwater before it interfaces with the open waters of the nearshore.Shorezone Tolerance Districts 2, 4, 6, and 7 represent shorelines made ofvolcanic, alluvial, or morainic materials. These Tolerance Districts are lesssensitive than Tolerance District 1, but are still subject to potentially high ratesof erosion. The Tolerance Districts are separated solely on the basis of slopemeasurements, with Tolerance District 2 consisting of the steepest andTolerance District 7 containing the gentlest slopes. This ranking does notdifferentiate between the relative stability of the different materials.Significant differences can be observed in Shorezone Tolerance Districts of thesame classification simply because of their exposure to differing frequency andmagnitude of wave action. The presence of deep water near and foreshoreincreases wave action on the backshore. Under these conditions, shorezonescomposed of the same geological material will show a different landformcharacter. However, similar backshore substrate will respond to the forcesacting on them from the lake in similar ways. 18 For example, the backshore ofGlenbrook Bay is composed of younger lakebed deposits that are easily eroded.From Slaughterhouse Creek in the north and proceeding south, the shorezoneconditions change predominantly because of exposure to wave action. Theyounger lakebeds exposed at the north exhibit a steep eroding escarpment,while wide beaches form further south.Tolerance Districts 3, 5 and 8 delineate shorelines that consist of armoredgranite with slopes ranging from over 30 percent for District 3 to less than 10percent for District 8. Armored granite shorelines are the most stable and leasterodible of all types at Lake Tahoe. Although the erodibility of the soilsoverlying the granitic bedrock increases with greater slope, the stability of thebedrock remains the same.18TRPA, 1998, Shorezone EISCHAPTER 8: Shorezone Protective Structures and BMPsTRPA BMP HandbookPage 8-14 2012