FLOOD06904 - Laserfiche WebLink

Home Browse Search

46 LAWN LAKE DAM AND CASCADE LAKE DAM FAILURES, COWRADO 100 l- SO Z w U '" w "- 60 ;:; "' w z 40 u: 20 0 0,001 0.01 0.1 PARTICLE SIZE, IN MILLIMETERS 10 '00 FIGURE 43.-A, A well-developed boulder berm along the right bank of the Roaring River on the large alluvial fan at river mile 4.73, showing the concentration of organic material in the front of the berm. View is upslope on the surface of Roaring River alluvial fan. B, Partical-size analysis of matrix from Roaring River alluvial fan boulder berm. that includes numerous large shattered tree trunks and other organic material as well as large boulders. Upslope, there is no organic material and the berm consists of very coarse boulders and a coarse, noncohesive matrix depos- ited directly on ground moraine with little or no erosion. The coarse texture of the matrix and the small clay con- tent are characteristic of such flows (Church and Desloges, 1984). , Some of the coarsest boulders are at or near the sur- face of the berms, and the top of the berm is well above adjacent high-water marks on the valley side (assuming a level water surface). The boulders have a pronounced im- bricated structure, with their long axes perpendicular to the flow direction and their intermediate axes dipping steeply at 300 to 90 ~ There are nine criteria that are char- acteristic of boulder berms formed by inertial gravity flows: 1. They form below rapidly expanding reaches. 2. They may not form on both sides of the channeL 3. They are short, but continuous; lengths range from 6 to more than 30 ft. 4. They form below reaches with large sources of debris, such as deep scour or landslides. 5. They are mostly grain-supported with little matrix. The matrix is coarse and usually contains less than 4 to 5 percent silt and clay. 6. Boulders have steep imbrication angles, commouly greater than 60 ~