FLOOD00385

• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Title: South Boulder Creek Interim Hydrology Study
• Date: 7/1/2000
• Prepared For: Boulder County
• Prepared By: Taggart Engineering Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

1- 18

I
I
I
I
I
I
I
j
I
i
j
I
-I
a
I
I
j
I
I

Floodplain Hvdroloav Modelina below the Canvon throuah the Urban Area

While the model here gives reasonable results for the tributaries of the foothills and
floodplain areas, the actual main stream routing is deficient because of the inherent
limitations of UDSWMM, which doesn't simulate dynamic storage routing.

Review of the total discharges proceeding from Highway 93 downstream reveals
that there is essentially no reduction of peak flow until past Baseline. Also,
comparison of the hydrograph shapes at various points down the main stream and
the West Valley Overflow reveals that the spiky peaks are not being attenuated.
Floodplain dynamic storage routing is a well documented phenomena. We believe
different modeling techniques should be used such as either HEC-1 stream routing
based on HEC-2 model storage routing data, or a more advanced version of SWMM
with routing capability EXTRAN, or UNET. Most importantly, the peaks of the main
stream hydrographs are being spilled to the West Valley urban area. Any
attenuation of the peaks in the main stream results in direct reductions of spills to
the West Valley. Section V presents comparative hydrographs to illustrate these
points.

The 500-year flood event appears to be reasonably simulated, but probably could
be refined. For this and the other events, the model is subject to survey and
hydraulic analysis refinements of the numerous splits and subsequent floodplain
hydraulic modeling.

Reasons for Chanae from Previous Model

Basically the total flows are higher than the 1 977 Corps' model because of
increased and longer duration rainfall, increased area, and to a lesser degree,
because of more appropriate land surface and stream routing characteristics. The
flows would have been even higher, but Gross Reservoir was found to have larger
storage than that previously used by the Corps based on information from both the
Colorado State Engineer's Office and Denver Water.

GENERAL CONCLUSIONS

While we are satisfied with the results of the modeling effort and feel that the
report will meet regulatory requirements, allow reliable and safe delineation of
floodplains, hydraulic structure sizing, and form a base for initiating the master
planning effort; there are several conditions to be aware of:

1 . Gross Reservoir in reality provides significant flood storage below the crest of
its spillway, which is to the benefit of the downstream residents so long as
its operation is sustained. The model does not depict this storage below the
spillway crest, as directed by the sponsors and following CWCB and FEMA