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1 <br /> <br />EXISTING HYDRAULIC CONDITIONS <br />For the limits of this study, FEMA has not performed any detailed floodplain analysis at this time. <br />The Army Corps of Engineers cross-sectioned the river in 1983 and 1984, prior to the flood event <br />in 1984. ALOE used this topographic information to develop the floodplain study completed in <br />' 1987. The tlow rates utilized in their report aze accurate; however, the river reconfigured during the <br />1984 storm event. <br />' The ACOE floodplain study and the HCE floodplain study are based on the 1929 USGS datum. <br />The benchmark used for the HCE floodplain study is the HARK point Ril-D'at elevation 5~ 17.01 <br />feet on the 1929 datum. The water surface elevations and cross sectional information in this (HCE) <br />' report are also based on the 1929 datum to enable a direct comparison to the ACOE study. <br />The aerial mapping was provided by Data Mapping in Apri12001. High Country Engineering, Inc. <br />' performed the underwater cross sectioning of the Colorado River in April and July of 2001. The <br />datum used for the aerial topography is based on the 1988 USGS datum (which is 3.79 feet higher <br />than the 1929 datum). The benchmark for the existing topography is the HARK point Ril-D at <br />' elevation ~~20.80 feet on the 1988 datum. Thus, the tloodplain study elevations are 3.79 feet lower <br />than the topography shown. <br />' The cross sections in the ACOE study were used as a base for the HCE study. The whole <br />numbered cross sections are from the ALOE study. Additional cross sections were surveyed in by <br />HCE at critical locations in the vicinity of the proposed gravel pit. <br />' The water limits were determined for the 50 year and 100 yeaz storm events. See Figure 3: Existing <br />Colorado River 100 Year Floodplain Limits per the ACOE study. Figure 4 shows the limits of the <br />' S0 and 100 year floodplains based on HCE's updated survey information. Table 2 provides a <br />comparison of the ~0 and 100 year water surface elevations for HCE's floodplain study and the <br />ACOE's floodplain study. Based on the difference in elevations shown in Table 2, the water <br />' surface elevations on the east end of the site are lower in the HCE study than the ACOE study. <br />Also, the water surface elevations from the proposed gravel pit site to the east end of the study area <br />are 1 to 5 feet higher than the ACOE study. This may be a result of the ] 984 storm on the Colorado <br />' River causing the channel to become more incised in narrow locations and at the borrow pit azea <br />adjacent to the City of Rifle. The borrow pit may have caused the channel to headcut directly <br />upstream of the pit area. Also, deposition may have occurred in the wider, braided areas. <br />1 Schmueser Gordon Meyer (SGM) and the City of Rifle prepared a study titled, "Floodplain Study <br />and Report of the Colorado River From Rifle to Mamm Creek". This study was based on aerial <br />' topographic mapping from 1993 and the channel information from the ACOE study. This study <br />did not incorporate new surveyed cross sections of the Colorado River (below the water surface). <br />Table 3 provides a comparison of the 50 and 100 year water surface elevations for HCE's <br />' floodplain study and the SGM's floodplain study. <br />Cross sections and the summary tables from the HEC-RAS analysis are provided in the <br />' Appendix. <br />