FLOOD05425

• County: Delta
• Community: Delta
• Stream Name: Gunnison, Uncompahgre River
• Basin: Gunnison
• Title: City of Delta Confluence Park Project
• Date: 10/1/1989
• Prepared For: Delta
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

Additional subsurface information was obtained from
subsurface explorations for a proposed shopping center west
of Highway 50. The closest hole to the sewage lagoon is
approximately 2.,000 feet away. The logs indicate sandy
clay, clayey sand, and silty sand form the surficial
deposits to dep1:hs of 10 to 16 feet deep. Underlying
material consis1:s of gravel with sand, silt, and clay 4.5 to
13 feet thick. Claystone bedrock was encountered 15 to 24
feet below ground surface. Based on this subsurface data,
the average gravel -thickness was estimated :Eor the purpose
of making the snepage analysis.

Nine subsurface explorations were performed in and
around the abandoned sewage lagoon. Borings were, primarily
performed to explore the material above the gravel layer
which would function as a natural seepage blanket,.

The interior of the sewage lagoons consist.s of a thin
upper liner of clay (2 to 4 inches thick), possibly derived
from the Mancos Shale formation. The underlying material
from 1 to 15 fent consists of silt, silty sand, and lean
clay with variable amounts of silt and sand. The surficial
material surrounding the lagoons is generally similar except
for isolated arE!as of fill containing lime waste. The
underlying gravE!1 layer consists mostly of poorly graded
gravel. The upper gravels contain some clay and silt which
generally decrease with depth. Based on subsurface data,
this material varies in thickness from 4.5 to 21 feet.
Claystone of thE! Mancos Shale Formation underlies the
gravel.

Seepage control to reduce water loss can be
accomplished by installing complete or partial cutoffs, or
by using method!; which lengthen the seepage path. Control
measures must take into consideration any potential for high
exi t gradients 1:hat can produce heaving or boils. The
methods of seepage control most applicable to this type of
lake include complete cutoffs, liners, and lakeside and
lands ide blanke1:s.

a. Cutoff Systems - Cutoffs would have to be
extended to the shale bedrock or the efficiency of the
cutoff would be significantly reduced. Unless the cutoff is
continuous, flo,TS would flank or bypass the cutoff. The
only practicabln cutoffs are those that can be effectively
installed without dewatering. All cutoff systems have
limitations, CO!;t being the restrictive limitation common to
all. Given the low differential heads, it appears that the
cost of any effnctive cutoff would be disproportionately
high compared to the cost of water saved.

b. Geonembranes and Clay Liners - Both methods have
cost disadvantacres and construction constraints. Both
methods would bi, difficult to place underwa1:er, and above

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