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<br />:.1 r. Don He i x e lljl tl n <br />:;QVelliber 2, 1972 <br />Page 2 <br /> <br />to the 1 per cent flood probability. The area adjacent to Carbonate <br />Creek and the area adjacent to the upper reaches--in the eroding <br />stretcn--of Slate Creek are in the 10 per cent flood probability <br />area, and the lower reaches of Slate Creek are in the 50 per cent <br />flood probability area. <br /> <br />Much of the area of the alluvial fan cou~d be protected from <br />flooding by the channelization of Carbonate and Slate Creeks. Basic <br />data for the design of a 100-year flood channel wil" in large part, <br />have to be estimated. It is recommended--as discussed during the <br />field examinationnthat a double channel-,be. designed. That is, a <br />channel be designed to handle the average spring runoff. In addition, <br />on. one side of the channel or on both sides a flood plain type channel <br />may be designed tnat will be adequate to handle the estimated 100- <br />year flood. The flood plain type channel will require periodic main- <br />tenance to assure that there is adequate volume within the channel <br />to manage the 100-year flood. Periodic flooding will, as the water <br />spreads out in the flood channel, build natural levies along the <br />margins.of the annual channel, and :;0 reduce the capacity of the <br />1 per cent probability flood channel. <br /> <br />The construction of check dams in the annual channel has been <br />suggested. These would soon silt up, but they would serve to decrease <br />the velocity of the streams, and so the erosion and the amount of <br />sedi~en: transported by the stream. This would be beneficial. <br /> <br />It vii 1 1 be difficult to control the flooding in the area at <br />the lower end of Slate Creek, which is presently subject to almost <br />annual mud flows. An abrupt change in gradient as Slate Creek issues <br />from a deep cilannel has created a subsidiary alluvial fan. The flood <br />waters issuing from the channel onto the relatively flat fan spread <br />out and deposit their load of sediment. To carry the flood waters <br />anj their sediment beyond the limit of the Marble road will require <br />that the 100-year flood channel maintain the same gradient in this <br />lower reach of Slate Creek as in the upper part. Geometrically this <br />may not be possible without considerable alteration of the gradient <br />in the upper reaches of Slate Creek. It is recommended, however, that <br />a channel at least adequate to handle the annual flow be constructed <br />through the mud flow area to below the road. To safely develop the <br />lower mud flow area of Slate Creek will require considerable engin- <br />eering study--more than has been part of the present investigation. <br /> <br />The general con~lusion reached as a result of the visit to the <br />Marble Ski Area on October 25, 1972, was that most of the alluvial <br />fan of Carbonate and Slate Creeks can be developed if channels ade- <br />quate to contain the estimated 1 per cent probability (lOO-year) <br />flood are constructed. The exception would be the lower end of Slate <br />Creek. Additional study will be required to determine if it is feasible <br /> <br />