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'" December 8, 1972 '. '~" ji. l \'_;.llv 1,-"-,'-t, , ) ~ I........"~ : ') p J, I' Mr. John Rold, Director Colorado Geological Survey 1845 Sherman Street, Room 254 Denver, Colorado 80203 Dear Si 1': This is a review of the conclusions drawn from the meeting held in your office on November 27, 1972. In attendance were representatives from Marble Ski Area, Inc.; Charles Robinson & Assoc.; Chen & Assoc.; KKBNA; and, yourself. Basi- cally, we were in agreement with the nature of the problem and the manner in which the problem could be solved. However, at your request, we have attempt- ed to roughly outline the design criteria used in obtaining a solution to Slate Creek. The major problem in designing hydraulic structures, or channels, for a mud type flow, is the determination of the density and water content of the fluid debris. As reported in the \~rightwood Study, "Density and water content are bound to va ry wi de ly" . Although the maximum soli d content of fl ow debri s on the Wrightwood cone indicated a water content of roughly 15-20 per cent, "dens iti es of around 2.0 (Viool ey) and water contents of 30-60 per cent (Ri ck- mers) are reported in other fl O\vs". We, therefore, assumed a \Vater content of 40% for the Slate Creek mud flow, supported by the following factors: (1) The Slate Creek mud flows are caused chiefly by high intensity summer rainstorms. The vJright~/ood mud flow, on the other hand, was caused primarily by rapi d me lti ng of deep wi nter snow. (2) As reported by Dr. Robinson (attached letter), the material that forms the mud flows of Sl" te Creek "i s be 1 i eved to be deri ved from eros i on of the channel". Conversely, the debris for flowage of the Wrightwood Mud Flow was caused primarily from the rapid weathering of bedrock within the San Andreas rift zone. We attempted to verify our assumption by determining the water content of a mud flow that occurred during the summer of 1972. Material was deposited up- stream of a temporary fill constructed for Crystalline Drive. A 24" culvert had been installed to carry the normal flow of Slate Creek, with the knowledge that it would not carry any type of mud flows. The culvert plugged and most of the material was deposited above the road. The total volume measured was ap- proximately 2150 c.y. From frequency and duration studies, it has been estab- lished that there exists a 75% chance that the 2-year frequency flood will occur dl;,'ing a 2-year period of observation. Furthermore, there is also a 75% proba- bility the 5-year flood will occur during a 6-year period of observation. With " ,-, .......1\ ::." ~J .' .~.. ::.;,...' : ~ ~ ..-" t:~' I