FLOOD00155

• County: Boulder
• Community: Boulder County
• Stream Name: Upper Boulder Creek, Fourmile Creek
• Title: Floodplain Information Report
• Date: 12/1/1981
• Prepared For: Boulder County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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regulation as set forth in the state statutes. The State of Colorado
considers the 100-year frequency flood as the flood event to be used in
desi gni ng and protecti ng structures and dwell i ngs for human occupati on.
Therefore, all floodplain regulations are based upon the 100-year flood.
Al so, the area which woul d be inundated by the 100-year flood may be
delineated as an area of state interest as set forth in H.B. 1041 -- the
state's land use bill.
500- Year Event - The 500-year flood event is useful in maki ng the
public aware that floods larger than the 100-year flood can and do
occur. Because an individual is living outside the 100-year flood
boundary does not mean that he is completely safe from flooding. The
500-year flood event can also be used for regulating high risk
developments within the floodplain such as nuclear power plants, or the
storage or manufacture of toxic or explosive materials.
Table 4 lists the discharges and water surface elevations for the
10-, 50-, 100-, and 500-year flood events at selected reference points.

Also upon official approval of this report by the Colorado Water
Conservation Board, the area outlined by the 100-year flood boundary will
be desi gnated a f100dpl ai n area and may be regul ated accordi ngly by the
local officials.

Flooded Areas
The 100-year frequency flood has been selected by the State of
Colorado as the flood event to be used for floodplain delineation and
regulation. Thus, the Flooded Area plates show only the boundary of the
100-year floodplain. Since the base map for the Flooded Area plates is a
two-foot contour map, the outlines of the other events - 10-, 50, and
500-year can be readily plotted uSing the information in Table 4.
The area delineated on the Flooded Area plates as the 100-year
floodplain meets the requirements of H.B. 1041 as an area of state
interest.

Flood Elevations
Fl ood crest el evati ons for the 10-, 50-, 100-, and 500-year floods
may be found in three separate 1 ocati ons in thi s report. The Peak
Di scharge and Water Surface El evati on Data Tabl e (Tabl e 4) 1 i sts these
elevations at selected reference points. The Flooded Area plates give
the pl an vi ew of the flooded area on a contour base map and the hi gh
water elevations for the 100-year flood can be interpolated from this.
Pl ates 3 through 18 cover Upper Boul der Creek and Pl ates 19 through 30
cover Fourmile Creek. The Flood Profile plates show the streambed
elevation and the high water elevations for all four frequency floods.
Pl ates 31 through 37 cover Upper Boul der Creek and Pl ates 38 through 42
cover Fourmile Creek.
The Flood Profiles may be used in areas where controversy arises over
the 100-year flood boundary on the Flooded Area plates. Since the Flood
Profile plates given the elevations and distance or stationing from a
known point, the high water elevations can be surveyed on the ground to
alleviate any discrepancies on the base map. Survey control markers
exist in the field in the vicinities of Boulder Creek and Fourmile Creek
and their associated floodplain areas. A summary of these control
markers and their locations is given in Table 5.

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