FLOOD08206

• County: Statewide
• Community: All
• Stream Name: All
• Basin: Statewide
• Title: Colorado State Hazard Mitigation Grant Program
• Date: 9/1/1997
• Prepared For: CWCB
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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PAWNEE CREEK OVERFLOW
B d di . . ALT. NO. 1
Proposed Improvementr: ase on SCUSSI0ns WIth the City, eWCB, and CDOT, A combination of
altematives I and 2 was selected as the recommended approach to improving the crossings at HWY 6.
Sheet 1 of 4 presents the proposed improvem~t for Pawnee Creek. The proposed improvements for
Pawnee Creek include two levees downstream of the UPRR nmging in height from 1 foot to 5.5 feet.
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These levees are shown on Map Sheet 1 and lA and are labeled Levee Number 1 North and Levee
Number 1 South. The north levee will extend east :from abom 300 feet downstream of the railroad to
the defined low flow channel of Pawnee Creek and then curve north and follow the channel to an
existing dirt road just west of the limits of the 1 DO-year floodplain of the South Platte River. This
levee will be approximately 1,400 feet in length. A 300 foot long floodwaIl will be required
immediately east of the railroad' bridge on the north side of the channel where right -of-way is limited
by an existing residence and livestock pen.

The south levee will extend east from proposed channel improvements downstream of the UPRR for
about 1,500 feet to a dirt road just east of the limits of the 100-year floodplain of the South Platte
River. The south levee will prohibit low flows from draining along their existing cow-se llrOWld a
4.400 feet long oxbow formed by the existing channel; therefore. a new low flow channel must be
constrUcted to convey stann water between stations 16+00 and 72+00. This channel will be
trapezoidal in shape utilizing a 30 foot bottom width and 3:1 sideslopes. The depth of the low flow
channel will nmge from 2 feel at the point where it lea'Yes the existing channel oxbow to about 9 feet
where it must daylight back into Pawnee Creek.

Channel improvements are also proposed from a point 100 feet upstream of HWY 6 to approximately
200 feet downstream of the UPRR. The hig!l'I\,ay bridges will need to be improved to accommodate
the increase in discharges due to the confinement of the split flows by the proposed upstream le'Yees.
As discussed earlier, the existing bridge for the ~orthboW1d traffic is a wooden structure and CDOT
plans to replace this bridge in the next 3 to 5 years. For the southbound lanes, 4 cells of20' x 8' RCB
would be insta1led just south of the existing bridge. The culverts could be pre-cast which would
minimize the time required to complete the installation, thus limiting the impacts to existing traffic.
Wing walls upstream and downstream. as well as riprap protection would be required. Conceptual
cross-sections of the channel and bridge improvements at HWY 6 and the UPRR are presented in
Figures 10 Ilnd 11. In order to conSll'uct the new box culverts south of the existing bridge, the existing
siphon along the Springdale Ditch which carries ir.ri.gation water WIder the railroad and highway may
need to be extended to the south. There is also an existing concrete ditch diversion strUcture located
just downstream of the UPRR which will have to be reDloved. This structure is presently abandoned
and preliminary discussions with the owners indicated that they are agreeable to its removable.

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