FLOOD10390

• County: Boulder
• Community: Boulder
• Stream Name: Fourmile Canyon
• Basin: South Platte
• Title: Fourmile Canyon Creek - Major Drainageway Planning - Phase A Report - Alternatives Analysis
• Date: 6/1/2000
• Prepared For: UDFCD, Boulder
• Prepared By: Love & Associates
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

developed hydrologic models for the 10-, 50- and lOa-year storm events. As a part of t
project, the SOO-year storm was estimated using flood frequency analysis based on the 10-
50- and lOa-year storm events.

HYDROLOGIC ANALYSIS

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The Original Report identified a 400-cfs spill flow from Fourmile Canyon Creek to
Wonderland Creek at Broadway and additional spill flows at 19th and 26th Streets. However,
the Original Report did not reduce the flows in Fourmile Canyon Creek to account for these
spill flows.

Model

Corrected H

3.3.2

GENERAL

The hydrologic model previously prepared for Fourmile Canyon Creek, as documented in the Major
Drainageway Planning. Boulder and Adjacent County Drainageways. Phase B report, dated May of
1987, (hereafter referred to as the Original Report) was utilized as the hydrological basis of this
study. The input parameters of the Original Report were checked for reasonableness and corrections
were made, where appropriate. The Original Report assumed, incorrectly, that all floodwaters
remained in the Fourmile Canyon Creek floodplain. The hydrology model for this study was
modified as a part if this project to account for flows that spill from Fourmile Canyon Creek and flow
into the Wonderland Creek drainage

3.1

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The original hydrologic model was re-created by entering the input parameters from the
Original Report into the latest versions of the CUHP hydrologic catchment modeling and the
UDSWM hydrologic routing models. "Corrected" CUHPF9S and UDSWM9S computer
models were re-created for the 2-, 5-, 10-, 50- and lOa-year storm events

The CUHP input parameters were checked for reasonableness and corrections were made,
where appropriate. Based on an analysis of aerial photography, dated October 1998, it was
determined that imperviousness of 12% for Sub-catchment 905 was too low. The models
were updated with a weighted imperviousness of33%

The hydrologic analysis was completed for the entire Fourmile Canyon Creek watershed. The
watershed boundaries are shown in Figures 3-1a and 3-1b. The total tributary area of the Fourmile
Canyon Creek watershed is approximately 10.2 square miles. The area tributary to the study reach is
approximately 9.24 square miles. Fourmile Canyon Creek discharges to Boulder Creek below State
Highway 119.

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The UDSWM input parameters were checked for reasonableness and corrections were made,
where appropriate. Based on the City of Boulder's current base mapping, flown May 1993, it
was determined the slope of 17% for the conveyance element between Sub-catchments 903
and 904, was in error. The models were corrected with a slope of 1.7%. Channel roughness
coefficients were changed from 0.045 to 0.056 and overbank roughness coefficients were
changed from 0.06 to 0.075 to approximate the routing of the original HEC-l model.

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Fourmile Canyon Creek is unique in that a portion of the existing channel is perched within an
alluvial floodplain. The perched portion of the channel occurs from the mouth of the canyon
downstream to approximately 19th Street. In this reach, once the channel capacity is exceeded, flood
flows overtop the south bank and flow in a southeasterly direction towards Wonderland Creek. The
result of these spills is flows in Fourmile Canyon Creek downstream of the spill locations are
reduced, while the flows in Wonderland Creek are significantly increased.

Analysis

Supporting figures and documentation are presented in Appendix A, ''Hydrologic

SPILL FLOWS

3.2

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ic Predictions

Existing development within the study area is predominately residential development with
some commercial development near Broadway. The study area is predominately built-out.
Development is unlikely to occur within large parcels of open space that have been designated
near the mouth of the canyon and in the vicinity of 47th Street and State Highway 119. With
the exception of minor in-fill and redevelopment, future land use will remain very similar to
existing land use patterns

Future Land Use H

3.3.3

With one exception, the spills do not return to Fourmile Canyon Creek. The one exception occurs in
the vicinity of the Crestview Elementary School. Spills, which leave Fourmile Canyon Creek
between Broadway and 19th Street, flow in a southeasterly direction towards the school. The school
acts as a barrier and a further split in the floodwaters causes a portion of the spills to flow north
around the school and re-enter Fourmile Canyon Creek. The remaining portion of the spills move
southward around the school towards Wonderland Creek.

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Imperviousness for future development conditions was estimated by analysis of the City of
Boulder North Boulder Sub-Community Plan and Boulder County future zoning maps. The
estimated future imperviousness values were input into the CUHPF9S model to determine the
peak flows under future land use conditions

HYDROLOGIC ANALYSIS

Model

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3.3.1

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the corrected

Report and

the Original

Table 3-1 summarizes the lOa-year flood peaks from
and future land use hydrologic models.

Love & Associates, Inc. obtained the technical notebook containing the hydrologic models
prepared by MSM/Greiner in 1983 for the Original Report. The Original Report's hydrology
was modeled using CUHP-B and routing was modeled using HEC-1 The original report

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3-1

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