FLOOD06828

• Designation Number: 177
• County: Jefferson
• Community: Unincorporated Jefferson County
• Stream Name: Weaver Creek
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Weaver Creek
• Date: 5/1/1981
• Designation Date: 9/1/1981
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Denver Water Department, stores municipal and irrigation
water. Embankment and outlet improvements are being
considered which would allow storage of the probable maximum
flood. Other offstream reservoirs include the two Bergen
Reservoirs located just east of the Hogback Ridge and
Kingfisher Lake located in the upper basin. The offstream
reservoirs contain sufficient flood storage above their

material is not considered stable, and it could seriously
affect the hydraulic efficiency of the culvert under the
highway during a flood. The filling was in the very early
stages at the time of the aerial photography for the
topographic mapping, and the flood hazard area shown in this
report is considered to represent the condition before
filling.

normal maximum water
tributary to them. A
throughout the basin.

levels to store the 100-year
number of smaller reservoirs are

floods
located

Flood Hydrology

There are 15 major roadway and canal crossings of the
drainageway involving culvert structures of various sizes.
All, except the Youngield Avenue bridge, are inadequate for
passage of the 100-year flood, and their embankments would be
overtopped. The blockage potential for each crossing was
considered in the backwater computations for this study.

Hydrographs were derived for the 10-, 50- and lOO-year flood,
using synthetic unit hydrograph techniques and assuming
future basin development. Descripiions of the unit hydrograph
techniques are included with the backup calculations on file
with the Urban Drainage and Flood Control District. Future
development was estimated, assuming low density (less than
two units per gross acre) for those areas not already
developed, in accordance with Jefferson County's current land
use plan. Existing development was determined from base maps
of the area obtained from Jefferson County. The effect of
on-site detention that might be included with future
development was neglected. The basin was divided into the 11
subbasins and 7 design points shown on the Basin Map. All of
subbasin number 1 and part of 2 was considered undevelopable,
because of excessive grade.

Three irrigation ditches cross the basin, including the
Bergen Ditch in the upper basin, and the Harriman and Warrior
Canals in the lower basin. Historically, the Harriman and
Warrior Canal s have intercepted Weaver Creek. A culvert has
been installed under the Warrior Canal. The irrigation
function of the Harriman Canal has been abandoned with
construction of a large pipeline which extends from u.s.
Highway 285 to Harriman Lake. A drainageway has been cut
through the canal embankment to convey the 100-year flood.
For this study it was assumed that the irrigation ditches
would not convey flood water into or out of the basin during
the flood.

A significant amount of fill was placed in the flood plain of
Weaver Creek, just upstream of the culvert under U.s. Highway
285 after the original mapping was completed. The fill

The 10-, 50- and 100- year flood hydrographs were generated
for subbasins 1 through 9. Flood volumes only were
conservatively estimated for subbasins 10 and 11. Analysis of
existing records and field inspection, showed that the
reservoirs ci 11 subbasins 5, 6, 10 and 11 could store the
100-year flood volumes tributary to them above their normal
maxim~c water levels, without significant discharge into
Weaver Creek.

~ Leonard Rice Consulting Water Engineers. Inc.

~

Leonard Rice Consulting Water Engineers. Inc.