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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typical of a more regional ized storm patterns, and cause floods typically
within large basins, 10 square miles and up.

Rainfall Distribution
Not all rainfall, when viewed frrnn the standpoint of total depth, produce
floods. However, when cons i deri ng the hydrology of a gi ven bas i n, and to a
greater extent of a regi on, certai n storm patterns have been quantifi ed for
use in hydrologic modeling. For this study, a thunderstorm pattern has been
assumed to be of the type of storm which would cause significant flooding.
For the type of area and hydrologic model used in preparing this study, a Type
II Rainfall distribution, as classified by the Soil Conservation Service, was
used within the northeast and northwest basins. Within the planning area, a
2-hour storm pattern developed specifically for modeling a high-intensity
rainfall event occurring over an urban area was assumed. Table 3 presents
this pattern for the 10- and 100-year storm.
Local experience and accounts of past flood events generally justify the
use of a short-duration, high-intensity storm pattern. Additionally, of cri-
tical concern is the adequacy of the proposed drainage system improvements,
and a more realistically designed system will be achieved by the use of a
thunderstorm model.

Basin Descriptions
Figure 4 presents the boundaries of the major drainage basins north of
the Town of Ordway.
The northeast basin covers approximately 1.5 square miles, at the design
poi nt shown on Fi gu re 4. The basi n trends to the south, and has an average
stream slope of 0.8 percent, and surficial soils of clays. The Ordway
Reservoir lies within the upper portion of the basin. The reservoir and tri-
butary area were assumed as not contributing to downstream runoff. The
drainage basin is drained by an open-grass and earthlined channel, which flows
along the east right-of-way of Colorado State Highway 71. This channel even-
tually drains into Lake Meredith, south of the study area.
The northwest basin drains north to south and covers an area of approxi-
mately 7.9 square miles of tributary to the design point shown on Figure 4.
This basin is both rangeland and irrigated cropland, with surficial soils of