PROJ00060

• Contract/PO #: C153721
• Contractor Name: Lyons, Town of
• Contract Type: Loan
• Water District: 5
• County: Boulder
• Bill Number: SB 96-153
• Loan Projects - Doc Type: Feasibility Study

Description of the Proiect and Statement of Need

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3.2 Diversion Structure and Intake: Assessment of Feasibility

The diversion of raw water is done about one-half mile upstream of the WTP on North
St. Vrain Creek, Diversion, regulation of inflow to the water treatment plant,
presedimentation and sediment sluicing all take place at this site. Refer to Figure 1 for the
diversion location. Inspection of the site and the facilities in March of 1995 determined that a
major rehabilitation or replacement of these facilities is required. Through time, the channel
flow regime has established a pool upstream of the diversion dam. The diversion structure is a
rockfill dam with a deteriorating concrete cap. Over time most of the concrete cap has
deteriorated and eroded. Water is lost through cracks. During periods of extreme low flow,
sheet plastic of other material must be spread to capture water for the intake gate.
Additionally, sediment appears to build up adjacent to and down-ditch from the intake gate,
requiring periodic maintenance. Large sediments are dropped just after entering this gate. The
inlet slide gate is bent, making the diversion flow rate difficult to set. A concrete, rectangular
presedimentation channel separates smaller fines from the flow. A side gate is available to
sluice this channel and purge accumulated fmes. Raw water next passes through a debris
screen to enter the raw water pipeline. This intake system appeared to operate at the time of
the field visit, but will operate poorly during adverse conditions. It requires significant
improvement to assure reliable operations. The entire facility is subject to periodic flooding,
sediment intake and winter icing conditions.

Discussions with the former Public Works Director, Mr. Herb McConnell, have fixed the
scope of the rehabilitation of this site. Subsequent requirements of the CWCB call for
including flow measurement and fish passage. The existing diversion dam is to be demolished
and rebuilt as a new structure. The existing presedimentation basin will be cleaned and
substantially modified for more efficient operations. All new structures must be designed to
maintain sufficiently high velocities to transport stream sediments downstream through the
diversion dam. The headworks will require a trash rack and the design must exclude bedload
sediments from entering the presedimentation basin and grit channel. Another feature of the
diversion dam is that it will include a flow measuring device in the stream channel. This
device is included at the suggestion of the CWCB, due to discussion at the CWCB meeting of
November 8, 1995. The diversion dam is considered non-jurisdictional under the regulations
of the Colorado Division of Water Resources, State Engineer's Office, dam safety.

RED, Inc. conducted a hydrologic analysis of this portion of North St. Vrain Creek in 1991
for the Apple Valley Road Bridge Project. It has been determined that much of the data
generated in that project used in flood analysis of the site. Allowances must also be made for:

demolition of the existing structures, bypass flows, minimum instream flows,
transmigration of fish, temporary intake to the raw water transmission line during
construction, sluicing of captured sediments, passing of flood waters and wintertime
icing conditions.