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<br />9-07. The area to the west of the city would be a noncontributing <br />area with respect to interior drainage. The area contains numerous de- <br />pressions and is intersected by elevated irrigation ditches and roads <br />that inhibit outflow. The area in the southern section of the city <br />which is served by the drainage ditch to the Purgatoire River would <br />not pose an interior drainage problem. <br /> <br />9-08. Design aPite~a.- The interior drainage studies for Las <br />Animas were performed in accordance with EM 1110-2-1410, "Interior <br />Drainage of Leveed Urban Areas: Hydrology." The general class of <br />urban development in which ponding areas would be required was con- <br />sidered to be Class III under anticipated future conditions as defined <br />by the EM. The class of urban development was determined by a site <br />reconnaissance of the area. A 25-year average recurrence interval <br />rainfall was selected as the design storm. ponding capacities would <br />be provided to store the runoff resulting from a I-year rainfall or <br />to store the runoff resulting from a routing of the 25-year rainfall <br />excess coincident with a 5-year flood in the Arkansas River, whichever <br />is greater. <br /> <br />9-09. Hydrographs - smaZZ interior drainage areas.- Runoff for <br />all the subareas were computed by the rational method from the equation <br />Q = CIA <br /> <br />where: Q = peak runoff in c.f.s. from a given area, <br />C = runoff coefficient, <br />I = intensity of inches per hour for the time of <br />concentration, and <br />A = drainage area in acres. <br /> <br />A runoff coefficient, "C", was selected for each subarea. Items which <br />were given consideration in this selection include infiltration, area <br />slope, percent imperviousness, and results of studies in the area. The <br />selected values of "C" were 0.60 for business and densely built-up resi- <br />dential districts, 0.35 for pasture areas, and 0.25 for cultivated and <br />heavily vegetated areas. These values differ only s-lightly from those <br />used in the project document studies and are the result of a recent <br />field reconnaissance. The time of concentration for each area was de- <br />termined from figure 37, page 313 of "Engineering Hydraulics," edited <br />by Rouse. The intensity of rainfall in inches per hour for the selected <br />design frequency was determined from page 8 of U.S. Weather Bureau Tech- <br />nical Paper 25, "Rainfall Intensity Duration Frequency Curves," using <br />the Pueblo, Colorad~ curves. <br /> <br />9-10. As mentioned in paragraph 9-03, planning studies have not <br />progressed to the point of definitely locating the levee alignments. <br />Preliminary studies indicate that with a 5-year flow (12,500 c.f.s.) <br />in the Arkansas Rive~ all the volume from the interior drainage design <br />storm would need to be stored until the river stage would permit out- <br />flow. This is the most critical ponding condition and as the results <br />of studies for the general design memorandum become available some <br />downward revision of required ponding storage is possible. <br /> <br />30 <br />