FLOOD03283

• Designation Number: 349
• County: Adams
• Community: Unincorporated Adams County
• Stream Name: Beebe Draw and Left Bank Tributaries
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Beebe Draw and Left Bank Tributaries
• Date: 7/1/1993
• Designation Date: 1/1/1994
• Floodplain - Doc Type: Floodplain Report/Masterplan

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V. SUMMARY OF HYDRAULIC ANALYSIS
I-lydraulics
Hydraulic analyses of the study area were completed to determine the water surface
elevations for the 10-, 50- and 1oo-year storm events. The water surface elevations were
compiled using the U.S. Army Corps of Engineers (COE) HEC-2 Water Surface Profiles
computer program with digitized and map interpolated cross sections. The digitized sections
were taken from the I" = 100' scale. two-foot contour topographic mapping, and supplemented
with cross sections taken from the topographic maps, as needed. Some culvert cross section data
were provided by the UDFCD. and other information was obtained by field measurements.
Channel and overbank roughness coefficients were estimated by field inspection and
photograph comparisons along the drainageways. Mannings 'n' values for channels varied
between 0.035 and 0.042 and the overbank roughness varied between 0.04 and 0.06. Irrigation
ditches were assumed to be flowing full and would be overtopped by the full flood flow. In
reality, flood flows may be diverted and overtop the canal banks at different locations when the
limited capacity of the canal is exceeded.
Starting water surface elevations for Beebe Draw were set based on critical depth. The
starting point for Beebe Draw was the E. l68th Avenue embankment which acts as a weir.
Beebe Draw is crossed by four roadways in the study area. The capacities of the road crossings
were calculated by hand and subtracted from the total flow. Irrigation control structures are also
located along Beebe Draw to direct flow from the Beebe Seep Canal to the irrigation storage
facilities adjacent to the Canal. The irrigation facilities are lower than the canal banks.
Starting elevations along Meadowlark Run, Magpie Run and Redwing Creek were either
set at critical depth or estimated by the slope - area method. Culvert capacities were calculated
by hand and subtracted from the total flow at the appropriate cross section.

East 160th Avenue is overtopped by the 100-year flood and, again, the roadway
embankment, flat terrain and the presence of an irrigation storage facility combine to create a
very wide floodplain. The computer model utilizes encroachments into the cross section to
reflect the ineffective portions of the flow area. An irrigation diversion structure upstream of
East 160th Avenue acts as a drop structure and raises the stream bed by about four feet. The
stream makes a 90 degree bend at East l52nd Avenue. Upstream of Piccadilly Road, Beebe
Draw narrows as it approaches Barr Lake. In this area the channel size greatly increases and has
nearly 1oo-year capacity.

b. Redwing Creek
Redwing Creek concentrates at 1-76. The 120th Avenue Interchange is currently under
construction. The re-aligned 1-76 blocks the drainageway and will force the flows to the south.
The floodplain is defined assuming that this diversion would not occur. The Burlington
Northern Railroad will also divert flood flows. The railroad embankment would channel some
of the flood flow to the northeast near East 152nd Avenue where the flood flows would cross the
railroad and continue to Beebe Draw. The Speer Canal and Burlington Canal will be
overtopped. The drainageway is almost totally undefined downstream of the irrigation canals
and flood flows would follow the wide, shallow swale towards Beebe Draw.

Floodplain Descriptions
The Flood Hazard Area Delineation maps and profiles are given in the Appendix.
Floodplain descriptions for the drainageways follow:
.l... Beebe Draw
The Beebe Draw 1oo-year floodplain is, in general, very wide and relatively shallow.
The channel and valley slope are about 0.2 %. The floodplain width exceeds 4,000 feet in some
locations. East 168th Avenue acts as a dam across the floodplain. The floodplain width results
from backwater conditions upstream of E. 168th Avenue, the presence of Bowles Reservoir No.
2 and the flat terrain. The floodplain east of the channel extends beyond the extent of the
mapping at Calhoun Reservoir. The floodplain west of the channel extends to Interstate 76. A
portion of the Burlington Northern Railroad is overtopped.

.3.. Meadowlark Run
Meadowlark Run also concentrates at 1-76. Meadowlark Run crosses under the west 1-76
frontage road in two 60" culverts. Flows exceeding the culvert capacity are blocked by the road
and will be forced to the north to Magpie Run. The downstream floodplain is defmed assuming
that this diversion would not occur. The Burlington Northern Railroad will also divert flood
flows. The railroad embankment would channel some of the flood flow to the northeast near
East 152nd Avenue where the flood path would cross the railroad and continue to Beebe Draw.
The Speer Canal and Burlington Canal will also be overtopped. The drainageway is almost
totally undefined downstream of the irrigation canals and flood flows would follow the wide,
shallow swale towards Beebe Draw.
4. Ma~ie Run
Magpie Run also concentrates at 1-76 where three 42 inch diameter culverts carry low
flows under the road. 1-76 will be overtopped by large floods. The Burlington Northern
Railroad will divert flood waters. The railroad embankment would chaunel some of the flow to
the northeast. The Speer Canal and Burlington Canal will be overtopped and also potentially
divert flood waters. The drainageway is almost totally undefined downstream of the irrigation
canals and the flood path would follow the wide. shallow swale towards Beebe Draw.

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