2009-08-01_HYDROLOGY - M1981112 - Laserfiche WebLink

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2 summer flow. However, instream flow requirements and interstate compacts maintain a base flow for much of the year. The U.S. Geological Survey (USGS) operates a stream flow gage on the River near Fort Morgan (USGS 06759500) which has flow data from 1943-1958 and from 2002 to the present. This stream gage is located immediately below Colorado Highway 52 about 500 feet downstream of the project site. The Colorado Division of Water Resources operates a stream gage on the River near the town of Weldona, CO, which has records since 1952. The Weldona gage is about eight miles upstream of the project site. Composite hydrographs from the two gages are shown in Figure 4. The graph indicates that there is reasonable coincidence between the two gages over most of the range of flows, although diversions exist between the gage locations. Bijou Creek enters the River three miles upstream of the project site (five miles below the Weldona gage). Bijou Creek is a small, sand bed stream with an average flow of 16 cfs except during large rainfall events, which can generate enormous discharge into the River. Because of extensive upstream development in the River basin, increases in water consumption and construction of reservoirs in recent decades, the 1943 to 1958 period of record for the Fort Morgan gage was not used in hydrologic analysis (Figure 4, Sheet 1). For the gaged period of 2002 to 2009, bankfull discharge with a mean annual duration of 14 days was determined to be 1,200 cfs for the Fort Morgan gage and 1,210 cfs for the Weldona gage. However, the Weldona gage has a 14-day duration flow of 2,090 cfs for the period of 1990 to 2009. This difference is likely due to the extensive drought experienced in the River basin during most of this decade, and appears to skew the estimate of bankfull discharge for the 2002 to 2009 period (Figure 4, Sheet 2). In order to develop a more accurate estimate of bankfull discharge at the project site, the Weldona gage data was used to synthesize and extend to 20 years a flow record from 1990 to 2009 for the Fort Morgan gage. Based on this data, a bankfull flow value with 14-day duration was determined to be 2,025 cfs in the project reach. This estimate was confirmed to be reasonable using hydraulic calculations for surveyed cross-sections and the physical bankfull channel (OMWL) indicators in the project reach. Table 1 shows a comparison of flow duration values for the two gages over the two periods of record analyzed. Spring runoff in 2009 included flow above 2,025 cfs from June 3 to July 1, a total of 29 days or 15 days longer than average. Most of that period (24 days) saw flow between 3,000 cfs and 4,000 cfs through the project reach. As illustrated by Figure 5, four cross-sections of the River in the areas of excavation and fill were completed. Figures 6, 7, 8, and 9 illustrate the surveyed Cross-Sections A-A', B-B', C-C' and D-D'. Included on the Figures are the water surface elevations at each cross-section at the time of the survey when the River discharge was 570 cfs at the Ft. Morgan gage. Figures 6, 7, 8, and 9 also show the calculated water surface elevation at bankfull discharge of 2,025 cfs. The bankfull discharge elevation coincides well with Ordinary High Water Line (OHWL) features observed in the field and picked up in the survey data. Bankfull channel geometry parameters are included in Table 2. 2.4.2 FEMA Floodplain The entire project area falls within the 100-year floodplain on the River as designated by the Flood Insurance Rate Map prepared by FEMA (1989). Figure 3 illustrates the south boundary of the 100- year floodplain. The flood zone is designated Zone A (no base flood elevations determined), has a width of about 3,600 feet and extends 1,600 feet north and 800 feet south of the project area. The published 100-year flow estimate is 114,000 cfs in the project reach (US Army Corps of Engineers, 1977). The highest flow event recorded at the Fort Morgan gage was 84,300 cfs on May 31, 1935. These flood waters came principally from Bijou Creek.