FLOOD02248

• County: Clear Creek
• Community: Georgetown
• Stream Name: Clear Creek
• Basin: South Platte
• Title: Flood Hazard Mitigation Report
• Date: 9/1/1998
• Prepared For: Georgetown
• Prepared By: Montgomery Watson
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

· The footbridge at 9th Street will be closed or removed; it is anticipated to wash out during the
100:year storm.

6.2.1 Clear Creek

Interim emergenc:( services measures for South Clear Creek will consist of the same kind of
flood fighting measures that have been employed by the Town in past years offfooding. These
measures are summarized below.

The preferred alternative for Clear Creek consists of a combination of dredging of the channel
invert and constructing floodwalls and levees. The typical depth of dredging in Clear Creek will
be 1-2 ft. Downstream of the II th Street Bridge, the proposed depth of dredging will be a
relatively constant 2 ft. Upstream of the bridge, dredging will be limited to a relatively constant
depth of I ft. Dredging will reach a maximum depth of 2.5 ft at a location approximately 600 feet
upstream of the 15th Street Bridge. Proposed dredged channel invert profiles for Clear Creek and
South Clear Creek are presented in Figures 6-1, 6-2, and 6-3.

· The right channel bank will be partially breached at Griffith Street to divert excess flows into
the street right-of-way. Diverted flows will be confined with sandbags. Flows wilL be
conveyed down Griffith Street to the ball field at Park Street, and finally through the train
station parking lot to be discharged to Clear Creek.
. In the flood of 1995 (approximately a IO-year event), diversions to Griffith Street were not
adequate to eliminate overflows along downstream reaches of South Clear Creek. Based on
this, it is anticipated that even with the temporary diversion operating, much of South Clear
Creek will still have insufficient capacity for the 100-year flood and will have to be
sandbagged. Specifically, these reaches include the following:
Right and left channel banks from the confluence point with Clear Creek upstream to
approximately 9th Street .
Left channel bank in the vicinity of Taos Street
Right channel bank from 9th Street upstream to approximately Main Street
· Again, based upon experience obtained during the 1995 flood, it is anticipated that even with
the temporary diversion in place some bridges will still have insufficient capacity to pass the
100-year flood. Bridges which may require closure for safety reasons include the following:
Pedestrian bridge downstream of Rose Street
Rose Street bridge
9th Street bridge
Taos Street bridge
. The Town will continue to coordinate closely with Public Service Company during flood
conditions to maximize the flood mitigation potential of the upstream hydroelectric storage
facilities, within their required operational ranges.

In order to provide fulllOO-year protection for the reach of Clear Creek between the 11th and 15th
Street bridges, the proposed dredging depths will have to be combined with a system of
floodwalls and levees approximately 2 ft high. Figures 6-4 - 6-6 illustrate the location and extent
of the proposed floodwalls and levees. Starting at the II th Street Bridge, a 2-ft high levee would
be constructed for a distance of 350 ft downstream. From the end point of the levee and
extending downstream for approximately another 900 ft, the protection system would be
continued utilizing a floodwall. The proximity of existing structures to the creek bank precludes
the use of a levee along this reach. A conceptual level cost estimate for dredging and
floodwallllevee construction on Clear Creek is provided in Table 6-1.

The Town of Georgetown is in the process of developing a recreation and trail plan for the area
between Georgetown Lake and II th Street. If a trail is planned to parallel Clear Creek between
the Lake and 15th Street, there will be opportunities to combine the trail with a flood control
levee. This could lead to development of a multi-use project with land use and funding benefits
for the Town. Such a plan could reduce or eliminate the need for dredging in portions of Lower
Clear Creek, and could have a lower cost than the dredging alternative. The Town will integrate
this flood mitigation concept into the trail plan as it evolves.

6.2.2 South Clear Creek

In the above discussion, sandbags are identified as the means for controlling overbank flows.
Georgetown and Clear Creek County have recently experimented with water-filled tubular dams
as alternates to sand bags. They were found to be effective and quick to put in place. The County
currently has two 100-ft lengths of inflatable dam available, which is obviously not enough to
handle even minor Georgetown flooding. The Town will support the County in acquiring
additional inflatable dams which could be used at locations throughout the County, depending on
which area~ may be threatened with flooding in a particular year.

Two separate proposed alternatives have been developed for South Clear Creek. Both
alternatives provide lOO-year protection and are discussed in the following sections.

6.2 STRUCTURAL FLOOD HAZARD MITIGATION

Alternative A ( 100- Year Dredging). Alternative A for South Clear Creek consists of dredging the
channel invert to a depth sufficient to provide lOO-year capacity. Hydraulic analysis of channel
and bridge cross sections indicates that the average depth of dredging required to provide 100-
year capacity is approximately 1.5 ft. The maximum required dredging depth would be 3 ft in the
vicinity of the Rose Street Bridge. A proposed new dredged invert profile for South Clear Creek
is presented in Figure 6-3. A cost estimate for the dredging and disposal of excavated material
from South Clear Creek is presented in Table 6-1.

This section describes the structural components of the 100-year flood hazard mitigation plan
adopted by the Town of Georgetown.

Lowering the South Clear Creek invert is expected to uncover foundations of some of the
existing channel bank cribbing. In addition, some sections of cribbing require maintenance or
replacement. A full inventory of type, condition and foundation depth of existing cribbing has
not been prepared at this time. As a result, the plan assumes that cribbing will have to be
replaced wherever dredging depths exceed 1.5 ft. Based on this, 500 ft of channel length will

6-2

6-3