FLOOD01787

• County: Boulder
• Stream Name: South Boulder Creek
• Title: South Boulder Creek/ Flatirons
• Date: 12/5/1997
• Prepared For: Boulder
• Prepared By: Consultants
• Floodplain - Doc Type: Community File

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FIELD WORK

We measured many of the culvert and bridge entrances including the Highway 36
and 93 bridges over South Boulder Creek, the Highway 36 barrier, the Lower
Flatiron's levee, the Viele culverts, five culverts along the Anderson Ditch, the Dry
Creek spill structure at the Anderson Ditch, the Baseline and Foothills culverts, two
Crossover Channel culverts, and three locations along Dry Creek Ditch No.2 in the
vicinity of 55th Street, and selected features of the Wellman.

COMPARISONS WITH MWE MODELING OF AREAS TRIBUTARY TO 55TH AND
WELLMAN DITCH

As part of planning efforts for improvements to 55th Street, McLaughlin Water
Engineers (MWE) prepared a CUHP/SWMM hydrology model of the local area.
MWE notes that results were within 20 percent of prior WRC SWMM modeling.
Earlier WRC modeling had no simulation of the flow splits which existed. MWE
simulated the spills at Foothills and Baseline, noting that the flow would leave the
basin. Also they simulated a minor split at 55th and Baseline, which we have not
explored. MWE noted a flow of about 800 cfs arriving at the storages at Foothills
and Baseline, 230 cfs leaving the basin there, with 1320 cfs arriving at the
Wellman at 55th. They recommend a design flow of 1820 cfs for the 55th
crossing of the Wellman Ditch, some 500 cfs greater than their model to account
for trans basin water that could come into the Wellman at Bear Canyon Creek.

While our initial modeling had similar main element flows, subbasins were noted to
discharge at significantly lower unit discharges (cfs/acre). Numerous parameters
were checked and refined, but the major cause was a significantly different rainfall
being used. We used a rainfall similar to MWE's in one version of the model in
which we disconnected the South Boulder spill to test the response of the local
drainage areas. Unit discharge values were then found comparable and similar.

LIMITATIONS

In addition to leaving the Corps' model above Node 44 as described above, the
following assumptions were used to facilitate the modeling effort.

Because of the fact that the model is an initial investigation, the runoff from the
individual basins was simplified. Rather than using separate catchments for streets,
direct flow from lots, and indirect flow from lots, a single catchment representing
all of catchments as an average was used for each subbasin. Imperviousness,
overland slope, and catchment width attempted to represent these averages. The
model does not account for backwater effects, associated storage, or floodplain
storage.

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