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Water Surface Profiles - Pre-project and post-project water surface profiles were computed by the standard step method using Manning's equation. Backwater profiles based on the Arkansas River 100-year flood, the SPF, and the flood of record were developed for the Arkansas River extending upstream from about 3.5 miles ea5t of Holly to about 1.0 mile west of Holly. Backwater profiles based on the Wild Horse Creek 100-year and SPF plus the coincident Arkansas River flow were developed for the Arkansas River and Wild Horse Creek extending upstream from about 3.5 miles east of Holly to the confluence with Wild Horse Creek, thcn upstream on Wild Horse Creek about 1.5 miles north of Holly. These water surface profiles are shown on Figures 2-8 (Plates 6-12 from the Corps' March 1977 Detailed Project Report). Note: The beginning station of the levee and the alignment at some stmctures and road ramps as shown on Figures 2-8 is slightly different than the final construction plans published by the Corps in April 1979 and shown on Figure 1. The final constmction plan porfiles did not show water surface profiles so they were not used in this report. These differences have a negligible effect on post-project water surface profiles and floodplain boundaries on the west and south sides of the Project. However, a significant ehange in the ending station of the levee was noted between the March 1977 Corps report and the April 1979 construction plans. The levee ended at station 232+50 feet in the March 1977 Corps report but ended at station 222+00 in the April 1979 construction plans, thus shortening the levee by 1050 feet on its east end. The shorter levee is also shown in the November 1981, O&M Ma.nual for the Holly Flood Control Project. This significantly changed the location of the post-project 100-year backwater floodplain boundary east of Holly as discussed under Floodplain Boundaries (Post-Project). Floodplain Boundaries (Pre-Project) - Pre-project floodplain boundaries were prepared by the Corps using the computed water surface profiles for the 100-year flood and the SPF. Pre- project floodplains for the Arkansas River 100-year flood and SPF are shown on Figure 9 (Plate 12 from Appendix 1 of the Corps' March 1977 report). Pre-project floodplains for the Wild Horse Creek 100-year and SPF plus coincident flow on the Arkansa5 River are shown on Figure 10 (Plate 13 from Appendix 1 of the Corps' March 1977 report). Floodplain Boundaries (Post-Project, Long Levee) - Post-project floodplain boundaries were prepared by the Corps using the computed water surface profiles for the 100-year Hood and the SPF. Post-project floodplains for the Arkansas River 100-year and Standard Project Flood are shown on Figure 11 (Plate 14 from Appendix I of the Corps' March 1977 report). These flood plain boundaries are based on the March 1977 plans for the longer levee southeast of Holly. The 100-year floodplain boundary on the east side of Holly would have been at approximate ground elevation 3381.7 feet (MSL) which is estimated to be the westward extension of the backwater east of Holly as determined by the summation of the energy head of the 100-year flood at the end of the longer levee. Post-project floodplains for the Wild Horse Creek lOO-year and SPF plus coincident flow on the Arkansas River are shown on Figure 12 (Plate 15 from Appendix I of the Corps' March 1977 report). These floodplain boundaries are also based on March 1977 plans for the longer levee southeast of Holly. These floodplain boundaries for Wild Horse Creek plus coincident Arkansas River flow are are further east of Holly and thus are less critical than the floodplain boundaries for the Arkansas River floods. 9