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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15

Storm Water Management Model (SWMM), as modifi ed by the Urban Dra i nage and
Flood Control District in Denver, Colorado. The purpose of the hydrologic
analys i s is to predi ct the fl oodi ng magnitude for the 5-, 10-, 50, and
100-year events. Within the central, northeast, and northwest basins, the SCS
methodo logy was used. Thi s model is well-sui ted to agri cultura 1 and rangel and
area, and was specifically developed using rainfall and soil type as the major
variables in defining the hydrologic characteristics of rural basins. Within
the Planning Area, the SWMM model was applied. The SWMM methodology has been
specifically developed to assist in the hydrologic assessment of urban runoff,
which is characterized by flow in streets and storm sewer systems. Table 4
shows selected basin data for the Northwest and Northeast Basins.
Table 5 presents the basin data used for the hydrologic modeling within
the Planning Area, including input for the undeveloped and developed basin
conditions. The sub-areas correspond to those shown on the map in Figure 4.
The SWMt~ model can also account for such factors as infiltration, surface
retention of rainfall and basin slope. The values used in the SWMM model of
Ordway for these parameters are listed below.
Surface Retenti on, Impervi ous Area: 0.1 inches
Surface Retention, Pervious Area: 0.2 inches
Maximum Infiltration Rate: 3 inches/hour
Final Infiltration Rate: .5 inches/hour
Infiltration Decay Factor: .0018
Typical Urban Basin Slope: 2.0 percent

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Results
The peak flows generated in the hydrologic analysis are listed in Table
6. Peak flows in the Northeast and Northwest Basins were determined using the
SCS TR-20 computer model, as discussed above. The peak flows presented have
been calculated at Design Points 1, 2, 7, and 8, as shown on Figure 3. Table
7 summarizes the peak flows estimated at various locations witnin the Planning
Area. The key design points have been previously shown on Figure 4. The com-
puter input and output corresponding to the discharges presented in Tables 6
and 7 are included in a technical addendum to this report.