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Table 3-6 Runoff Volumes at Selec:ted Locations I Runoff Volume (acre-feet)- Loc:ation I 2-Vea. S-Vea. I Ill-Vea. I 5O-Vea. I lOO-V... I SOO-V... Upper Box Elder Creek. Existing Development Conditio.. Upstream Study Limit 100 140 920 4,700 6,800 10,700 Immediately Upstream of Coyote Run Confluence 100 140 1,300 6.300 9,300 14,500 Immediately Downstream of Coyote Run Confluence 100 ISO 1,700 7.600 11,000 17,200 Future Development Conditions Upstream Study Umit 810 1,200 2,300 6,100 8,300 12,200 Immediately Upstream of Coyote Run Confluence 1,900 2.900 4,300 9,600 12,600 18,000 Immediately Downstream. of Coyote Run Confluence 2,800 4.100 5,800 12,000 15,500 21,800 Coyote Run Existing Development Conditions Interstate 701US 36 I 3 I 5 I 260 I 750 I 1,000 I 1.600 Immediately Upstream of Box Elder C....k Confluence I 3 I 10 I 400 I 1,300 I 1.700 I 2,700 Future Development Conditions Interstate 701US 36 I 420 620 I 830 I 1,400 I 1.700 I 2.200 Immediately Upstream of Box Elder C....k Confluence I 900 1,200 I 2,400 I 2,400 I 2,900 I 3,800 'Runoff volumes pertain to a 50- to 75-square-mile area adjustment of rainfall. The AS reports in Adams and Arapahoe Counties were based on regional approaches, primarily on USGS Water Supply Paper 1680 (USGS, 1968), 1?is si~p~fied ap?roach provides for flood estimates in different regional areas of the Missoun River Basm upstream of Sioux City, Iowa. The approach, in Region 10 (where Box Elder Creek is located), was based ?n discharge records from 6 to 22 years (from 1940 to 1962) for five gages on Cherry, Kiowa, and Bijou Creeks (none of these watersheds are as narrow as the Box Elder Creek wate~hed), Discharges are shown to vary directly with watershed area, a phenomenon that conflIcts with this study's results and the COE results for the narrow Box Elder Creek watershed. If the method is applied based on the adjacent hydrologic region to the west (Region 9, where Plum Creek is located), the 100-year discharge for the 200 square mile Box Elder Creek watershed is 6000 cfs, compared to 26,500 cfs, In other words, the procedure produces significantly different results for creeks like Plum Cree~ ~d Cherry Creek, even though the creeks are adjacent and both drain from the Palmer DIVide. The Arapahoe County AS study used the Water Supply Paper 1680 results verbatim, while the Adams County AS study adjusted the discharge, settling on a value about half of the Water Supply Paper 1680 100-year discharge. 3.8.2 Comparison to Previous Studies Stream gage information does not exist on Box Elder Creek, precluding the use of a gage analysis to estimate discharges for various return periods, It is questionable whether gage information from other watersheds in the region could be appropriately applied to Box Elder Creek because of the unusually long and narrow shape of the Box Elder Creek watershed. The results of this study for existing development conditions were compared to previous hydrologic analyses of Box Elder Creek and nearby watersheds. These analyses included the COE Section 22 Study upstream of Bootleg Reservoir (COE, 1989), the Adams County AS (FEMA, 1989), the Arapahoe County AS (FEMA, 1993), the Running Creek study upstream of Elizabeth (WRC, 1979), and the Beebe Draw Study (WWE, 1992). The comparison of existing development condition 100-year discharges between this study and the Beebe Draw study is shown in Figure 3-1. The comparison of peak discharges between this study and the other Box Elder Creek studies is shown in Figure 3-2. The UDFCD estimate of the 1973 flood discharge at Bootleg Reservoir, possibly the flood of record, is also shown on Figure 3-2, This event was produced by rainfall approaching the order of magnitude of the NOAA 100-year precipitation (NOAA, 1973), The 1973 flood discharge estimate, although not totally definitive, correlates reasonable well with the results of this study and the COE study, Because it was based on an actual flood event with recorded rainfall depth data, the 1973 estimate provides a reference point against which the reasonableness of the simulated hydrology can be checked, Because this hydrologic analysis is consistent with standard UDFCD hydrologic criteria, is in close agreement with the detailed COE modelling effort, and correlates reasonably well with the 1973 flood discharge estimate, the project sponsors accepted the results of this hydrologic analysis for use in subsequent phases of the Box Elder Creek Outfall Systems Planning Study and the Coyote Run FHAD, One-hundred-year existing development discharges estimated in this study compare very favorably to the discharges developed in the Beebe Draw Study (WWE, 1992) and the COE study (COE, 1989). The Beebe Draw results pertain only to tributary areas less than 2000 acres), The results of this study and the COE study for Box Elder Creek are lower than the discharges for comparable areas identified in the Adams and Arapahoe County FIS reports and the Running Creek report, as shown in Figure 3-2, Besides this study, the COE study was the only analysis that used detailed rainfall/runoff modelling procedures (COE, 1989), The COE analysis used a modified version of the Storm Water Management Model (SWMM) runoff block to generate and route hydro- graphs from 238 individual subwatersheds. COE applied a uniform, areally adjusted 6-hour design storm similar to the design storm used in this study. COE also conducted a sensitivity analysis regarding storm size and centering, The COE report notes that regional approaches may not be directly applicable to Box Elder Creek because of its unusual (long and narrow) watershed shape, 3-5 DENlOOI768I.WP5