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
Dam 2 failed on M , th
' performed by Mr. George arch 11 Prince.1910. He conclInvestigation uded that of e the cause cause of of failure failure was was
washing out(piping) of fines with the bedrock upper stratification and development
of hydrostatic pressure of sufficient magnitude to lift the overlying mass of rock and
earth at the downstream toe of the dam. The report documents large blocks of
bedrock being carried by flood water for considerable distances downstream of the
' dam.
The breach of the dam was approximately 300 feet wide at the base and 400 feet at
' the top. Reconstruction of the breach included construction of two concrete cutoff
walls extending approximately 4 feet into bedrock.
' In 1978, a series of toe drain systems were recommended by Chen&Associates due
to excessive seepage and other concerns expressed by the State Engineer. A
' downstream toe drain system was installed in two sections,the western section being
approximately 750 feet long and the eastern section being 350 feet long.
A 25-foot-long,kidney-shaped boggy area was observed on the downstream slope of
Dam 2 in 1984. The 1985 remedial measures consisted of placing a 3-foot-thick
layer of filter material, a drain pipe and a 5-foot-thick earth berm over the seep area.
Due to downstream slope stability concerns, an earth berm consisting of a 15-foot-
wide shoulder and a toe drain system was constructed in 1988 along the entire length
' of the dam.
Seepage continues from Dam 2 and is periodically monitored at weirs and drainpipes
that collect the seepage water. Seepage or wet areas have not been observed on the
downstream slope or at the dam toe since the construction of the drainage facilities
and stabilizing berms.
Dam 3
Dam 3 is approximately 1,400 feet long and approximately 40 feet high at the
maximum section with a crest width of approximately 20 feet. The upstream face of
' the dam has a slope of 1.7:1 to approximately 20 feet below the crest. Below that
level, the upstream slope flattens out due to sediment accumulation at the toe. The
upstream slope is paved with concrete for slope protection and has a 3-foot-high
concrete parapet wall for wave runup protection.
' The downstream face of the dam is non-uniform. The slope near the crest is steep
for the upper 8 feet. The stabilizing berm constructed at the base of the downstream
toe in 1987 is 13 feet wide at the top and has a slope of 2.5:1. The berm has a sand
GEI Consultants, Inc. 4-7 01002 Dec 2001 Lower South Platte River Study