FLOOD09312

• Designation Number: 370
• County: Crowley
• Community: Ordway
• Stream Name: Local Drainage
• Basin: Arkansas
• Title: Floodplain Study and Master Drainage Plan - Ordway, Colorado
• Date: 6/1/1987
• Designation Date: 7/1/1994
• Floodplain - Doc Type: Floodplain Report/Masterplan

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clays, and loamy clays. The slope ranges from 1.3 percent in the upper half
of the basin, to 0.5 percent in the lower half of the basin. The basin is
drained by a network of several small trenches apparently designed to lower
the water table. During a flood event, County Road 17 would become the main
conveyance of floodwaters. Flows from this area would generally move
overland, slowly draining under the railroad tracks via small channels to Bob
Creek, and eventually to Lake Meredith. The Colorado Canal, an irrigation
structure, traverses the northwest basin, and has the abil ity to alter tile
runoff patterns of the natural basin. For the hydrologic analysis carried out
in this master plan, the Colorado Canal was assumed to be full at the time of
a fl ood event.
Between these two major basins is a smaller basin directly tributary to
the Town of Ordway, which has been termed the Central Basin. The area of this
basin up to 11th Street is approximately 0.2 square miles. Below this basin
the Town itself is drained at three locations along the railroad grade,
denoted as design poi nts 5, 6, and 10 on Fi gure 4. Th i s basi n is compri sed
of approximately one-half open or agricultural area, and one-half urban areas,
predominately single-family residential in nature. Areas within the corporate
limits are currently SUbject to development pressure, and therefore both unde-
veloped and developed conditions were modeled as part of the hydrologic analy-
s i s.

Numerous small surface drains have been excavated in the basin apparently
to lower the water table, but which can also serve to impede or carry flood
waters. These facil ities were assumed full at the time of a heavy rainfall
event, and therefore providing little or no flood conveyance. The exception
to this is the Town of Ordway Drainage Ditch, which has been dedicated for the
use of conveying storm flows, and therefore used in the hydrologic and
hydraulic analyses. Figure 5 shows the Planning Area broken down into the
sub-basins used in the SWMM hydrologic model.

Hydrologic Methodology
A hydrologic analysis was performed to determine the peak rate of flow,
volume of runoff, and flood routing tendencies. The hydrologic analysis uti-
1 ized both the U.S. Department of Agriculture, Soil Conservation Service,
TR-20 Hydrol ogi c Computer Model, and the Envi ronmental Protecti on Agency's