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<br />Section 4.0
<br />Hydraulic Analysis
<br />
<br />profile drawings (Sheets 40 through 89) show the lO-year and lOO-year water surface
<br />prof1les, Sheets 90 through 100 show representative floodplain cross sections and Sheets
<br />101 through 105 depict cross sections at all existing bridges and major culvert crossings.
<br />
<br />4.1 Methodology
<br />
<br />Detailed floodplain and floodway data are provided in Appendix N. At each cross section
<br />used for the hydraulic analysis, information is shown regarding discharges, water surface
<br />elevations, velocities, and widths for floodplains and floodways.
<br />
<br />A hydraulic analysis was undertaken to estimate water surface elevations for the 10-year
<br />and lOO-year flood events. The analysis was performed using the U,S, Army Corps of
<br />Engineers' HEC-2 Water Surface Profile program, Baseline hydrology peak discharges
<br />for future development conditions (documented in Section 3,0) were used for the analysis,
<br />
<br />4.3 General Overview of Flood Impacts
<br />
<br />Cross section data for the hydraulic analysis were developed from a digital terrain model
<br />representing existing topography along the stream corridors, The terrain model was based
<br />on a three-dimensional file of 2-foot ground contours provided by Landmark Mapping,
<br />Cross section data at bridges and major culverts were obtained from field measurements
<br />provided by the mapper,
<br />
<br />Few existing structures are impacted by the lOO-year floodplain, This is due primarily to
<br />the sparse development in the study area, The Town of Watkins, for the most part,
<br />appears to be built on ground that is slightly higher than the Coyote Run and Box Elder
<br />Creek floodplains to the west and east, respectively,
<br />
<br />Manning's rouglmess coefficients for the channel and overbank areas were estimated by
<br />field inspection following procedures outlined in several standard references (Chow, 1959)
<br />(USGS, 1987) (Simons and Senturk, 1977). A value of 0.030 was used for the ribbon of
<br />sand comprising the central channel section of Box Elder Creek. Overbank values for
<br />Box Elder Creek ranged from 0,40 to 0,60, A value of 0,040 was used for channel and
<br />overbank areas in Coyote Run and in the tributary drainageways, Expansion and
<br />contraction coefficients were based on the recommendations provided in the HEC-2 User's
<br />Manual.
<br />
<br />However, the Coyote Run and Upper Box Elder Creek floodplains will likely have a
<br />significant impact on future development plans in the area, Much of the mainstream flood
<br />plain is located in flat bottomland areas and is widespread, often exceeding 1,000 feet in
<br />width, Large areas of shallow flooding are predicted as a result of spills from mainstream
<br />channels,
<br />
<br />Existing bridges and culverts in the study area are generally undersized with respect to
<br />future development peak discharges, although the Box Elder Creek bridges at 1-70, US 36,
<br />and the railroad have adequate capacity. Cross-drainage structures on tributary channels
<br />generally consist of small culverts or depressed sections of roadway and are undersized
<br />with respect to future development discharges,
<br />
<br />A number of shallow "spills" from the main floodplain were estimated to occur, producing
<br />areas of shallow flooding hydraulically disconnected from the floodplain, These areas are
<br />indicated on floodplain maps and their depths estimated, For the purposes of delineating
<br />the downstream floodplain, baseline peak discharges were not estimated to be diminished
<br />by the spills,
<br />
<br />Average flood velocities typically range from 3 to 11 feet per second (fps). The higher
<br />velocities are typically found in the relatively steep, confined channels in the southern
<br />portion of the study area, The channels in the study area are erodible and may undergo
<br />changes in shape, width, and direction during the flood events. Therefore, flood hazards
<br />may exist outside the floodplain limits indicated in this report.
<br />
<br />The hydraulic analysis included estimation of two floodways. The floodways were based
<br />on encroachments resulting in maximum increases in the energy grade line of 0.5 and 1.0
<br />feet. The floodway analysis was initiated using the "automatic" encroachment option
<br />provided in the HEC-2 model. This was followed by manually adjusting encroachments
<br />and rerunning HEC-2 to yield more uniform floodway widths and stable water surface
<br />profile results,
<br />
<br />4.2 Results
<br />
<br />The results of the hydraulic analysis are shown on drawings and in tables, Drawings
<br />include plan view maps, profiles, and cross sections, The drawings are provided in
<br />Appendix 0 at the end of this report, Thirty-eight plan view maps (Sheets 2 through 39)
<br />indicate limits of the 100-year floodplain, An index map is provided as Sheet 1. Fifty
<br />
<br />DENlOO17682.wPS
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<br />4-1
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