FLOOD04855

• County: Boulder
• Community: Boulder
• Stream Name: South Boulder Creek
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 7/1/1986
• Prepared For: Boulder County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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S.H. 93 Split Flow

- FUTIJRE flOODS -

Another area where a special floodplain analysis was completed included the area
between U.S. Highway 36 and State Highway 93. A split flow condition exists
along South Boulder Creek over the 2.2 mile reach which encompasses these road
crossings. At both of these roadway crossings the existing bridges are high in
relation to the western road embankment leading up to the bridge structure.
Existing elevations along the western roadways leading up to each bridge are up
to 10 feet below the road elevations at the bridges. As a result, a split flow
condition occurs at both road crossings. The bridge structures and a portion of
the roadway adjacent to each bridge are not inundated during the 100-year flood.
As a result, there exist two separate, fairly distinct flow pathS between State
Highway 93 and U.S. Highway 36.

Flood Frequency and Discharge

Determi nation of the 100-year flood event is based on analysis of recorded
historical preCipitation and other valid data. Statistically, the 100-year flood
has a one-percent chance of being equalled or exceeded in any given year. Floods
of this magnitude or greater have occurred on South Boulder Creek in the past and
will occur in the future.

Flow that splits from the South Boulder Creek main channel at State Highway 93 is
conveyed in a separate channel to U.S. Highway 36. Hydraulic analyses indicates
that approximately 3850 cfs crosses the roadway at State Highway 93 and is
conveyed west of the main channel whereas 2100 cfs passes through the bridge and
is conveyed in the South Boulder Creek main channel. These split flow portions
of the 100-year flood are conveyed to a point approximately 3000 feet upstream of
U.S. Highway 36. At this point the capacity in the west overbank area decreases
as the split flows (left overbank and main stream channel) converge. An area of
shallow flooding connects the two split flow areas with flow returning from the
west overbank channel into the main channel of South Boulder Creek. The capacity
in the overbank area decreases to the point where approximately 1660 cfs of the
total 6160 cfs (lOO-year) will overtop the highway near its intersection with
State Highway 57. The remaining 4500 cfs passes through the bridge opening.

The 100-year flood is identified by the Urban Drainage and Flood Control
District, the Colorado Water Conservation Board and the Federal Emergency
Management Agency as the flood magnitude used for the regulation of flDodplains
and for the design of flood control improvements. In Colorado, the 100-year
floodplain represents an area of state interest as defined in HoB. 1041. Due to
widespread utilization of the 100-year floDd as a basis for floodplain
regulation, it is commonly referred to as the base flood, regulatory flood,
and/or intermediate regional flood.

Discharge-probability profiles shown in Figure 2 graphically present the relative
magnitudes of the 10-, 50-, 100-, and 500-year flood events. This discharge
information serves as an aid for regulating future floodplain development and
also for planning and engineering of channel improvements and flood protection
measures.

Flooded Areas

A separate flood prOfile for the split flow reach between U.S. Highway 36 and
State Highway 93 is plotted on Sheets 30 and 31. This overbank split flow reach
is herea fter referenced as the State Hi ghway 93 spl it flow. Limits of the Sta te
Highway 93 split flow floodplain are presented on Sheets 9 through 15.

Flood Hazard Area Delineation maps showing the limits of the 100-year flOOdplain
are contai ned in the Appendix of this report (Sheets 1 to 21). Sheets 22 to 31
present water surface prOfiles corresponding to the 10-, 50-, 100-, and 500-year
flood events. Tabulations of discharges and computed water surface elevations
for the 10-, 50-, and 500-year events are presented in Table 3. Table 4,
entitled "lOO-Year Floodplain and Floodway Reference Data" presents infDrmation
pertinent to regulation of the 100-year floodplain. Table 4 also includes
information related to flood discharges, channel thalweg elevations, and other
descriptive data.

The split flow returns to the main channel just downstream of U.S. Highway 36
near South Boulder Road.

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