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TECHNICAL MEMORANDUM FOR: COTTONWOOD CREEK PHYSICAL MAP REVISION . EASTER AVENUE TO INVERNESS DRIVE EAST Site Description The portion of Cottonwood Creek for which a Physical Map Revision is requested consists of approximately one half (0.5) stream mile, beginning immediately upstream of Inverness drive East (East Dry CrE!ek Road), and ending immediately downstream of the East Easter Avenue crossing. This reach includes sections J,K,L, and M of the existing Flood Insurance Rate Map (FIRM). The site location is presented in Figure 1. Proiect Backqround This reach of the Cottonwood Creek 100-year recurrence interval flood has been recently analyzed in detail, as part of the Cottonwood Creek Regional Detention Pond des i gn. Related documents i nc 1 ude the Cottonwood Crl!ek Reqi ona 1 Detent ion Pond Preliminarv Desiqn Report (;July 16, 1986, revised October 28, 1986), and the associated Addendum (March 19, 1987), prepared by Greenhorne and O'Mara, Inc., and the Cottonwood Creek Reqional Detentign Pond Final Desiqn Report [Phase llll (Jul y 8, 1987), prepared by Mull er Eng i neeri ng Company, Inc. The detent ion pond and channel improvements for this reach have been constructed. Hvdroloqic Analysis The original hydrographs from the Federal Emergency Management Agency's (FEMA) most recent Flood Insurance Study (FIS) could not be located by FEMA or the design engineers for the project. In order to model this reach of Cottonwood Creek most closely to the FIS, without the benefit of the original hydrographs, and to take advantage of the recent refinements in hydrology as ana"lyzed by Greenhorne and O'Mara (G & 0), the following methodology was utilized. The basin area determined by G & 0 was appl ied to the most recent FIS area.di scharge. frequency curves (see Figure 2). The peak di scharges for the 10-, 50" 100., and 500'year storms were read from this Figure. These peak rates were then used to adjust the hydrographs developed by G & O. The new hydrograph ordinates were calculated by determining the percentage of the peak flow rate that each of G & O's previous hydrograph ordinates represented. This percentage was then applied to the peak discharges determined from the FIS (Figure 2), to determine the new hydrograph ordinates, The 10. year hydrograph ordinates by G & 0 were util ized 'in modell ing the 10'YI,ar storm, and the 100'year ordinates were used for the 50-, 100-, and 500.year storms. No storm hydrographs had been determined by G & 0 for the 50- or 500.year storms, The original G & 0 hydrographs, and the hydrographs representing the area- adjustment according to Figure 2 are shown in the attached calculations.