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

SECTION 4

4.2

FLOOD HAZARD AREAS

CURRENT PROBLEMlISSUE IDENTIFICATION

The effective Flood Insurance Study (FIS) and Flood Insurance Rate Maps (FIRMs) for
Georgetown are dated June 5, 1989. The FIRM community number is 080035 and a single map
panel covers the entire incorporated area (Panel No. 0001 C). FEMA mapping of the 100-year
floodplain boundaries is based on a "detailed analysis" of two flooding sources: Clear Creek and
South Clear Creek. The effective FEMA floodplain boundaries have been transferred to the most
current topographic and planimetric mapping available for the town which is dated June 1991.
Figures 4-1, 4-2 and 4-3 present the extent of lOO-year flooding. Additionally, several areas of
historical flooding which are currently unmapped were identified by citizens of the Town. These
general areas are denoted in Figures 4-1- 4-3.

4.1 FLOOD HISTORY

Based on the current FEMA Flood Insurance Rate Map (FIRM) for Georgetown and June 1991
aerial mapping, there are approximately 88 residential and business structures (excluding sheds
and small outbuildings) located within the boundaries of the 100-year floodplain for Clear Creek
and South Clear Creek. In recent years, runoff from the spring snowmelt has repeatedly caused
flooding in Georgetown. The most significant recent flood events occurred in 1965, 1983, 1984
and most recently in 1995. Overflows during these events have typically been most severe along
South Clear Creek and the resulting flooding was generally confined to those areas south and
east of the confluence of South Clear Creek and the main Clear Creek channel. In the most recent
flood of 1995, streets were sandbagged to contain and direct South Clear Creek channel
overflows to Clear Creek. Reportedly, these artificial "channels" conveyed water for more than a
month. Local news accounts reported the rescue of one fireman and a volunteer worker from the
swollen river. Surface inundation was accompanied by flooding of many residential basements
due to flood-induced high groundwater levels. The damage and control and cleanup costs
resulting from this event were substantial, as was the danger to human life.

4.3 FLOOD HAZARD DATA

For the purposes of floodplain delineation and regulation, FEMA generally limits its attention to
those drainage courses having a drainage area of I sq mi or more at the point of interest. Within
the Town of Georgetown there are two FEMA designated flooding sources:

. Clear Creek
. South Clear Creek

Existing hydrology (flows) and hydraulics (capacities) have been derived from the February,
1987 report by the Colorado Water Conservation Board (CWCB) entitled Flood Hazard
Identification Report. Clear Creek and South Clear Creek, Georgetown Colorado. Table 4-1
provides a summary of existing flows, limiting capacities and existing protection levels for three
study reaches:

Peak flood events generated by these two sources have generally been the result of snowmelt or a
combination of rainfall on snowmelt. Snowmelt generated flooding events can be anticipated to a
much greater degree and provide longer warning times than rainfall induced events. Snowpack
depths at the end of the winter season give the first indication of the magnitude of spring
snowmelt flows. Impending spells of unusually warm weather in late spring can generally be
anticipated up to a week in advance. Localized spring and summer thunderstorms are far less
predictable, but only result in heavy runoff when combined with a lingering snowpack. For most
past flood events in Georgetown, warning times have been sufficient to take necessary
precautions.

. Lower Clear Creek - Clear Creek from Georgetown Lake upstream to the confluence point
with South Clear Creek
· Upper Clear Creek - Clear Creek from the confluence point with South Clear Creek upstream
to the southern corporate limit of Georgetown
. South Clear Creek

Typical depths of flooding in Georgetown during the lOO-year event are expected to vary from I
ft or less to 2 ft. Georgetown is a mountain community and the topography of the town is
relatively steep. The average slope of South Clear Creek within the corporate limits is
approximately 0.022 ftlft (2.2 percent). The average slope of Clear Creek within the corporate
limits is approximately 0.023 ftlft (2.3 percent). Given the relatively steep topography of the
Town, typical flow velocities in floodplain areas tend to be high, varying from 3 to 8 fps.

Table 4-2 presents recent years in which flooding has occurred in Georgetown and approximate
recurrence intervals based on peak flows measured at the USGS streamgage at Lawson (USGS
Gage No. 06716500). Note from the table that flooding has repeatedly occurred for events
having a recurrence interval of less than ten years. Thus, recent historical events confirm the
limiting creek capacities and computed protection levels presented in Table 4-1. Based on this
information it can be concluded that without implementation of a Flood Hazard Mitigation Plan,
Georgetown should expect to experience significant flooding an average of once every to years.

4.4 PROBLEM ASSESSMENT

The major factor influencing the occurrence of flooding, the amount of resulting damage and
possible flood mitigation alternatives in Georgetown is the proximity of homes and businesses to
the two creeks. A majority of the property adjacent to Clear Creek and its southern tributary is
privately owned. On South Clear Creek, many homes are located within yards of the creek bank.

4-1

4-2