Lower South Platte River Study

• Contractor Name: Colorado Water Conservation Board
• Loan Projects - Doc Type: Feasibility Study

Lower South Platte River Study
Colorado Water Conservation Board
December 2001
' Johnson Lake West could be supplied by pumping from the South Reservoir Ditch
by wells, or by pumping from the river. Johnson Lake West would create 3,500
acre-feet of storage with a 60-foot-high dam. Cost would be $12,620,000, or$3,610
per acre-foot of storage capacity at the Johnson Lake West site. Road and building
relocations would not be required. Some wetland areas are shown on the USFWS
wetland inventory maps.
The Johnson Lake Enlargement would involve an 80-foot-high, 2,600-foot-long
' embankment creating 5,400 acre-feet of storage. This storage project would be filled
by pumping from a reactivated Carlson Ditch or by pumping from wells or the river.
Several ranch buildings and a dwelling would be impacted. Wetlands are prominent
' around the existing reservoir and below the existing small dam. Cost would be
$21,500,000, or$3,980 per acre-foot of storage capacity at the Johnson Lake
1 Enlargement.
4.1.2 Preliminary Cost Estimates for Storage Development
' Preliminary cost estimates for the potential storage sites were developed by GEI based on the
following general criteria and assumption:
'
Ring-Dike Storage Sites (Ovid, Sedgwick, Gerk and Troelstrup Reservoirs)
• Typical outlet works cross-section shown on Figure 4.3
• Typical embankment cross-section shown on Figure 4.4
' • Small overflow spillway to handle precipitation on the reservoir during the probable
maximum flood (PMF)
' The basic layout for the ring-dike storage sites envisions a 3:1 downstream slope and 6:1
upstream slope. The flat upstream slope is provided to avoid the need for upstream slope
' protection(riprap or soil cement) for resistance to wave erosion. Primary technical concerns
at these sites relate to constructing a reservoir that does not experience excessive seepage
during periods of low flows in the river or excessive groundwater inflow during high river
' stages. Shallow clay layers near the ground surface will be relied on at the Ovid and
Sedgwick sites to help retain water. The embankment would be zoned with a clay core and
upstream and downstream shells of sands and gravels excavated from the ground surface
' above the clay layer. Other technical concerns with the proposed dam layout include: drying
and cracking of the clay liner during reservoir drawdown, which could lead to preferential
' seepage paths through the dam; and the possible existence of sand and gravel "stringers"
interbedded with clay, which could result in high seepage rates; and the availability of
adequate materials to construct the dams for the higher storage volume reservoirs.
GEI Consultants, Inc. 4-19 01002 Dec 2001 Lower South Platte River Study