GENERAL56300

• Permit No: C1981037
• IBM Index Class Name: General Documents
• Doc Date: 3/30/1999
• Doc Name: MEMO INSPECTION REPORT OF EROSION AT GEC
• From: DMG
• To: DAN HERNANDEZ
• Permit Index Doc Type: GENERAL CORRESPONDENCE
• Media Type: D
• Archive: No

• III IIIIIIIII
999
~~'A~' ~, IIIIII

~:.;a~ I

COLORADO
DIVISION OF MINERALS AND GEQL(~~'
Depanmem of Natural Resources
1313 Sherman St., Room 215
Denver, Colorado 00203 DIVISION O F
Phone [phone] MINERAL S
FAx: [phone] $c
GEOLOGY
interoffice RECLAMATION
MINING•SAFETY
MEMORANDUM
Bill Owens
Governor
to: Dan Hernandez Greg E. Walther
ExecNive Director
IU tUrU: °/' DaVld Berry Michael B. Long
i+ Division Director i
~-~/~` i
from: 13"y~or~G. Walker
subject: Inspection Report of Erosion at GEC
date: March 30, 1999
Larry Routten, Jim Stark, and 1 made an inspection of erosional features at GEC on March ] 6.
]999. This report has been coordinated with Harry Ranney, Jim McArdle, and John Nelson.
In summary, additional work is required to arrest erosion a[ GEC.
Existing Conditions:
The silt-sandy gravel soils are cohesionless and highly erodible. In a section of the channel at
1% grade, the channel was 8 feet wide and 1 foot deep (vertical embankment to a debris line).
Thus at least one flow since installation may have approximated 50 cfs (SEDCAD regression
analysis) (Enclosure 1). The results at Enclosure 1 indicate a scour situation (limiting velocity of
1.5, analysis velocity of 6.36 fpsj. The large number of cobbles and small boulders that have
been transported in the channel validates this on the site. However, such t7ows must be
infrequent as grasses are starting to stabilize the channel at (in a very few areas) grades less than
1%. Vegetation in the bottom of the channel appears to be inhibited as much by being buried
under sediment as much as it is by being eroded.
Pictures 1 and 2 are of the upper reaches of the main drainage. The steeper grades of the
drainage channel from the tree line to the floor are around 20% (inclinometer readingsj. This
reach is in a state of accelerated erosion. The grades of the adjacent slopes are aroL113d I ~%, and
are experiencing rapid erosion (gullies). Armoring of the channel by residual rocks in the soils is
not likely in these areas. There is too much sand and silt that washes away from rocks. The
rocks then tumble on down the drainage way due to gravity and the forces of the water.
Similar extensive rill and gullies are forming the full length of the drainage way (slopes from 10
to ] 5 % (Enclosure 4). Concentrations of runoff result in severe erosional features (Enclosure ~).