FLOOD01589

• County: Clear Creek
• Community: Georgetown
• Stream Name: Clear Creek, South Clear Creek
• Title: Flood Hazard Identification Report
• Date: 2/1/1987
• Prepared For: Georgetown
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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The bridge capacities were analyzed by a procedure in which bridges
were modelled using the HEG-2 computer program and computer calculations
were checked by hand calculations. Ten percent blockage of bridge
openings due to debris was assumed. The following table summarizes these
findings:

Stream Bridge
Clear Creek
22nd St.
foot bridge
15th St.
foot bridge
11th St.
9th St. foot br.
7th St.
6th St.
foot bridge
S. Clear Creek
Rose St.
9th St.
Taos St.
8th St.
Main St.

Finding

sufficient capacity
sufficient capacity
low overbank; water
low overbank; water
low overbank; water
would wash out
Barely sufficient capacity; insufficient
capacity if the low chord is touched. *
Barely sufficient capacity; insufficient
capacity if the low chord is touched. *
sufficient capacity

bypasses
bypasses
bypasses

bridge *
bridge *
bridge *

insufficient capacity
insufficient capacity
insufficient capacity
Barely sufficient capacity; insufficient
capacity if the low chord is touched. *
sufficient capacity

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* When the water surface of a river touches the low chord of a
bridge, the hydraulic capacity of the bridge decreases suddenly and
significantly. Because of high channel velocities, the water surface of
the flood water is not stable and the bridge's hydraulic capacity may be
reached suddenly if the low chord is touched. It is not possible to
predict at which of these bridges this may occur.

Flooded Areas

The areas covered by the lOO-year flood are shown on Plates 2
through 7. The plates should be used carefully since the scales of the
plates change from plate to plate. In addition, the elevations shown on
the plates are in map datum. Further information with regards to flood
elevations at various locations is given in the following sections of
this report.

It has been determined that there is extensive overbank flooding in
the town of Georgetown. In general, these flooding depths are shallow.
These shallow depths will allow "islands" to be free from flooding, but
be surrounded by floodwaters. Some areas will experience sheet flow,
i.e. shallow flooding areas characterized by unclear flow patterns.
Because of the high channel velocities, water surface elevations are
highly unstable. These unstable elevations in conjunction with possible
debris and unclear flow patterns make it impossible to predict every area
that will get wet by floodwaters. Thus not all sheet flow areas have
been identified.

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