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Leer 0 Colorado Geological Survey December 8, 1972 our observation over the last three years and based on the flows that occurred during that period and the probabilities listed above, we have bracketed the storm frequency at a 2-5 year occurrence. From the hydrologic analysis similar to that shown in Exhibit No.1, the minimum water content was approximately 80% assuming the storm has a 2-year frequency. This, of course, is a rough esti- mate as the basic data is crude, and the hydrologic rational is based on simpli- fied assumptions. Of additional concern was the gradient of Slate Creek. Again referring to Dr. Robinson's letter, it is believed that erosion has been "accelerated by the diversion of Slate Creek and the increase of gradient". The grade of Slate Creek before diversion was approximately 9 per cent. It has been documented by Yearke that a relocated channel should be designed so as to preserve the pre- existing hydraulic gradient. As a result of the diversion, the grade of Slate Creek is still 14% in many reaches; and the channel is continuing to seek its hydraulic gradient through erosion and scour. Ferrell (1959) recommends a stabilizing slope of 0.7 of the natural gradient of the area. Consequently, we have used an 8% grade as the stabilizing slope, which is also less than the pre- existing hydraulic gradient. Grade reduction will be accomplished by the installation of check dams. Silting will occur behind the dams, thus reducing the slope to 8 per cent. The spacing between drops then depends upon the existing grade; i.e., the spacing will be such than an 8% grade will exist from the toe of the above dam to the crest of the ne< dC\'i11stream dam, as shown in Exhibit No.1. Downstream toe and bank protec- tion will a1sooe necessary to minimize potential failure of the dam or of the adjacent banks. Single and twin prestressed tees will be utilized for the check dams. The single tees will be cut as shown in Exhibit No.1, to carry the 25-year frequency storm. Storms of greater intensity will spill onto the flood plain channel. The flood plain channel was designed to carry the estimated 100-year fluid de- bris, assuming that the main channel would be filled with mud and rock, as shown in Exhibit No.2. Again, the total flow was determined by assuming that the 100- year storm water discharge represents 40% of the total debris flowage. The grad- iel~t of the flood plain channel will be held at the natural ground grade along Slate Creek, which averages 13.5%. Assuming the total flow (3000 cfs) contains no mud and rock, then the average velocity is 27 fps and the depth of flow is only 2~-feet. However, we know from the Wrightwood Report that the velocity of a debris flowage is much less than of water due to greater bulk density. The average velocity reported was about 10 fps. On this basis, the depth of flow would then be 6.2 feet for a mud type flow. In addition to the aforementioned channel treatment, Slate Creek must also be channelized from a point above Crystalline Drive (located in Filing No.2), to a point south of the existing county road. In this area, flood waters spread out onto a relatively flat fan and deposit their sediment load. Therefore, we feel that the same criteria applies except that a channel, as shown in Exhibit No.3, ffiJSt be excavated to carry the total debris flow. The minimum freeboard will be held at 5 feet, especially at the crest of drop structures. Again, we assumed that the average velocity for mud flowage was 10 fps and calculated the depth of flow. KETCHUM . KONKEL . BARRETT . NICKEL . AUSTIN