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S T <br /> ATE OF COLORADO <br /> DIVISION OF MINERALS AND GEOLPFy <br /> Department of Natural Resources <br /> 1313 Sherman St., Room 215 <br /> Denver,Colorado 80203 D I V 1 5 1 0 N O F <br /> Phone:(303)866-3567 MINERALS <br /> FAX:(303)832-8106 & <br /> GEOLOGY <br /> RECLAMATION <br /> interoffice MIN ING•SAFETY <br /> MEMORANDUM <br /> Bill Owens <br /> Governor <br /> to: Dan Hernandez Greg E.Walther <br /> Executive Director <br /> in turn: David Berry Michael S.Long <br /> J, Division Director <br /> from: gyro rf G. Walker <br /> subject: Inspection Report of Erosion at GEC <br /> date: March 30, 1999 <br /> Larry Routten, Jim Stark, and I made an inspection of erosional features at GEC on March 16, <br /> 1999. This report has been coordinated with Harry Ranney, Jim McArdle, and John Nelson. <br /> In summary, additional work is required to arrest erosion at GEC. <br /> Existing Conditions: <br /> The silt-sandy gravel soils are cohesionless and highly erodible. In a section of the channel at <br /> 1% grade, the channel was 8 feet wide and 1 foot deep (vertical embankment to a debris line). <br /> Thus at least one flow since installation may have approximated 50 cfs (SEDCAD regression <br /> analysis) (Enclosure 1). The results at Enclosure 1 indicate a scour situation (limiting velocity of <br /> 1.5, analysis velocity of 6.36 fps). The large number of cobbles and small boulders that have <br /> been transported in the channel validates this on the site. However, such flows must be <br /> infrequent as grasses are starting to stabilize the channel at (in a very few areas) grades less than <br /> 1%. Vegetation in the bottom of the channel appears to be inhibited as much by being buried <br /> under sediment as much as it is by being eroded. <br /> Pictures 1 and 2 are of the upper reaches of the main drainage. The steeper grades of the <br /> drainage channel from the tree line to the floor are around 20% (inclinometer readings). This <br /> reach is in a state of accelerated erosion. The grades of the adjacent slopes are around 15%, and <br /> are experiencing rapid erosion (gullies). Armoring of the channel by residual rocks in the soils is <br /> not likely in these areas. There is too much sand and silt that washes away from rocks. The <br /> rocks then tumble on down the drainage way due to gravity and the forces of the water. <br /> Similar extensive rill and gullies are forming the full length of the drainage way (slopes from 10 <br /> to 15 % (Enclosure 4). Concentrations of runoff result in severe erosional features (Enclosure 5). <br />