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!t fr f. ~ ~ .~t- ti. F Tile one exception is that the ponding area at station 290+00 IIould be required to store the entire inflO1J volur:le (57.9 acre-feet) of the 5-year rainfall because the outlet IIoulJ be blocked by the coincident 25-year flood (46,500 c.f.s.) on the Arkansas River. A storage area 600 feet ,.;ide by 1,300 feet long IIith a maximum usable depth of 3.8 feet would be excavated to-provide this storage. The outlet size was adjusted so that ponding from the 25-year storm lJOuld not exceed the storage capacity. ~ '- 7-16. Infil tration values selected to determine the outflm-l hydrographs for the subareas varied fror:l 0.70 inch per hour for cultivated land to 0.50 incil per hour for urban areas. The I-hour J 25-year recurrence interval rain- fall (2.30 inches) was used to determine the volume of outflolI from each subarea. Studies of substantial rainfall coincident \vith flood stages in t'ne Arkansas River indicate about 45 percent chance .of occurrence. Triangular- sllaped storm hydro graphs Here cons tructed for the subareas. The storm llydrographs were routed using the Successive-Average-Lag-Hethod (Tatum) to key points. Routed flmls were combined ,-lith side drainage flmJs at appropriate points to obtain design discharges for the collection system conveying interior drainage runoff to the levee outlets. In the routing of flm,s to levee out- lets, consideration was given to the effects of existing structures on peak fImvs. 7-17. Area AI' plate 29, would be separated from area A2 by a section of relocated, elevated irrigation ditch. The point of concentration for area Al ,""auld be at station 84+75. The point of concetration for area AZ \'lould be at the outlet at station 107+50. Runoff from area AI' was routed to and combined with runoff from area A2 at station 107+50. The point of concentration for area n would be at the outlet at station 290+00. The runoff hydrographs from the remaining subareas on the right bank "ere routed to key points along the interior drainage ditch to design the ditch and ditch structures to the outlet at station 473+00. The runoff hydrographs for the subareas on the north bank "\vere routed through existing drainage structures on State lligln.,ray 194 and furt11er routed to the levee outlet structure at station 352+DOi\I to design t~le interior drainage collection ditch from station 324+00N to station 351+00" and also from station 355+70N back to station 352+00N. Interior drainase data is tabulated in table 2. 7-18. Areas of easements, ponding excavations, ana outlet structure sizes \Jere varied to obtain the most feasible and economic combinations. Generally an increased cost of installing larger ontlet structures rpc;ultcu. in a savin[;s by reduced easements since stages in the retention pools di<l not allow spread beyond the designated areas (stage A). 'f~1is is cnaracteristic of Cue flat t~rrain. Hhen flmvs exceed the capacity of ti.le ponding area and inundate the flat surrounding area, easement costs increase sharply. The design pool elevations Here lm.Jered to raaintaj.n spread '.Jithin the limits of the cIesir;nnted areas 'uy routing the runoff resulting from the I-hour, 25-year recurrellC2 interval storn throuGh lareer outlets. Iiydrologic data for outlet structur2S and pondinc areas is shm;n on ?late 30. A comparison of the rcquirel! ponc.ia~ for ,'jt2Ges A (r,linor adverse effect~) and C (major adverse effects) is given belm-] . ',- 12