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

ground water placement would have a limited effect
considering that t:he most pervious zone ig at or below
ground water.

c. LakesidE' and Landside Blankets - This method of
controlling seepaqe losses utilizes the existing impervious
surface material. Where there is suffici~nt area for the
blankets, this met:hod is the most cost effective.
Accordingly, use of existing blankets is considered a
practicable solution.

The validity of a seepage analysis is limited to the
accuracy in estimating the representative ,coefficients of
permeability, the permeable and impermeable layer
thicknesses, and t:he differential head. Assumptions for
lakeside and lands ide blankets are (1) now through the
impervious blanket: is vertical, (2) flow through the
pervious foundation (gravel layer) is horizontal, and (3)
flow through the blanket and pervious foundation are laminar
and steady state. It is noted that the problem is three
dimensional; such a rigorous analysis is not warranted for
the level of data and low differential head.

a. Permea,bility Estimates. Deposits of naturally
occurring soils are variable, and any detejrmination of
permeability can only be estimated in ternls of order of
magnitude. The coefficient of permeabilitiy (k) of the clay
liner, sandy silt, sandy clay, and silty ~andy overlying the
gravel layer was determined from laboratory falling head
tests. A permeability rate of 2.5 X 10-3 feet per day was
estimated for material overlying the gravEH. The gravel
permeability was estimated using various ~ethods that
correlate permeability with material gradcttion and partiCle
size. A permeability of 100 feet per day was estimated for
gravel.

b. Soil Layer Thickness. The thickness of the
sandy silt to sandy clay blanket and the pervious gravel
foundation was conservatively adopted from site explorations
and from explorations performed north of the lagoons. The
gravel layer is bcunded by bedrock at the [bottom which was
encountered at a relatively uniform or cOl1stant depth. The
maximum thickness of gravel encountered wcts 21 feet. For
the purpose of the analysis, a depth of 20 feet was selected
based on the nearby logs. The blanket thickness varied from
1 foot to 15.5 feet with 50 percent of the explorations
indicating a thickness between 3 to 8.5 feet. On this
basis, a thickness of 1 foot was conservatively used for
both the lakeside and landside blankets.

c. Effective Blanket Lengths. The effective
blanket length is dependent on the permeabilities of the
blanket and pervious foundation and corresponding layer
thicknesses. In that portion of the lake :where there is no
planned excavation, blanket lengths exceeq 200 feet. The
lands ide blanket requires isolated low areas along the north

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