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
filter blanket and toe drain system and was constructed to address concerns of
' seepage at the downstream toe and slope stability.
Seepage discharge from the embankment drain has continued and is periodically
monitored by the owner. The discharge is reported to be free from sedimentation.
' Dam 4 (Dike)
Dam 4 (Dike) forms the eastern embankment of the reservoir. The dam is
' approximately 4,000 feet in length with a crest width of approximately 20 feet, and
varies in height between 10 to 16 feet. The upstream and downstream slopes of the
embankment are somewhat irregular. The upstream slope is approximately 3:1. The
' downstream slope varies between 2.5 to 3:1.
' Extensive seepage had been observed below the downstream toe of the embankment
for a number of years. A weir was constructed at the main seepage drainage
channels to monitor the seepage flow.
' To alleviate concerns about the potential piping of foundation materials and to
prevent rise of phreatic surface within the embankment, a toe drain was constructed
' to a minimum depth of 4 feet into the bedrock in 1991. Water seepage has not been
observed at the ground surface below the downstream slope of the dam since the toe
' drain was installed.
Main Dam 4 and Outlet
As originally constructed, the main dam was somewhat of an irregular shape with
the crest width varying between 25 to 30 feet and upstream slope of the embankment
' approximately 3:1 and downstream slope between 1.5 to 2:1. The original outlet
works consisted of three concrete conduits, 48 inches wide and 52 inches high. The
' steel gates controlling water discharge leaked excessively. Studies performed by
Wheeler& Kumar indicated excessive seepage through the steel gates and
deterioration of the concrete conduits due to chemical attack over a period of years.
' There was also concern about the seepage occurring at interface of the bottom of the
head wall and top of outlet conduits, and around the wingwalls. Due to the concerns
of the long-term integrity of the outlet works, it was recommended that the outlet be
' replaced.
A new outlet works and main dam were constructed downstream from the original
' location. The new embankment has a 25-foot-wide crest with upstream slope of
2.8:1 and downstream slope of 2.5:1. A chimney filter was placed to control the
' internal phreatic surface and an earthfill cutoff was constructed 5 feet into bedrock.
GEI Consultants, Inc. 4-8 01002 Dec 2001 Lower South Platte River Study