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<br />I <br /> <br />. <br />:a <br /> <br />I <br /> <br />4/8/05 <br /> <br />I <br /> <br />I <br /> <br />'I <br /> <br />-- . - .. - _..--. - ..- --." ------ <br /> Resurveyed bridge Master Plan <br /> openin~ area bridge open area Difference <br />Bridge (ft ') (ft2) (%) <br />104th Avenue 2251 3223 43 <br />McKay Road 1913 1634 -15 <br />88th Avenue 2691 2868 7 <br />Upstream UPRR Bridge 10496 8317 -21 <br />State Highway 224 7166 9318 30 <br />1-76 7635 52340 586 <br />1-270 8624 7513 -13 <br />MWRD\LI 3899 <br />Downstream UPRR Bridge 5752 5341 -7 <br />York Street 3357 3065 -9 <br /> <br />C <br /> <br />Section 4 <br />Hydraulic Analyses <br /> <br />divided flow. Allowing flow in the overbank effectively increased the width of the channel by an order of <br />magnitude; the 100-year floodplain width changed from roughly 500 feet to 5,000 feet in this area. Increasing the <br />channel width (levee failure HEC-RAS run) resulted in lower water surface elevations. 100-year overbank water <br />surface elevations are substantially lower than the main channel water surface elevations in this area. <br /> <br />4.3.5 Bridge Geometry <br />The majority of the bridges on the South Platte River from Baseline Road to York Street were resurveyed to <br />confirm the bridge geometry and deck elevations in the Master Plan. A Sokkia LP30 Laser Level was used to take <br />elevation measurements; bridge geometry measurements, such as pier width and deck width, were taken using a <br />100-foot tape. <br /> <br />Value in table reflects record drawing obtained from UDFCD; the bridge was not <br />resurveyed as part of this effort. <br />Bridge not modeled in the Master Plan <br /> <br />(1 <br /> <br />(2) <br /> <br />Highways <br /> <br />County, and the Colorado Department of <br /> <br />measunng <br /> <br />Table 4-2 lists bridges surveyed using UDFCD, Adams <br />benchmarks (NA VD 88) <br /> <br />I <br /> <br />I <br /> <br />,~I <br /> <br />The bridge opening area is a geometric parameter used to solve the flow equations at bridges. The 1-76 Bridge was <br />not modeled precisely in the Master Plan because it was not expected to substantially affect the water surface <br />elevation (WSEL); decreasing the bridge open area by nearly 600 percent only raised the WSEL by 1.2 feet in the <br />vicinity of the bridge (station 905+00). Water surface elevations around other bridges, however, were affected by <br />the change in bridge open area. The greatest changes in water surface elevations occurred on the downstream side <br />of the Henderson Road Bridge (at station 379+24 the HEC-RAS WSEL is 2.9 feet higher than the HEC-2 WSEL) and <br />cross-sections downstream of the 88th Avenue Bridge (between station 777+65 through 775+70 the HEC-RAS <br />WSEL ranged from 2.7-3.5 feet lower than the HEC-2 WSEL) <br /> <br />- - <br />Elevation (ft msl) <br />4960.30 <br />4970.37 <br />5022.29 <br />5054.90 <br />5087.44 <br /> <br />5133.13 <br />5133.13 <br /> <br />Station Name <br />- <br />SPR 21 <br />- <br />SPR 20 <br />- <br />SPR19 <br />- <br />SPR18 <br />SPR 17 <br /> <br />SPR14 <br />SPR14 <br />South Platte River <br />Annual Cross-Section 29 <br /> <br />Table 4-2 Benchmark Information for Resurveyed Bridges <br />Bridge Benchmark <br />Baseline Road UDFCD <br />160th Avenue UDFCD <br />Henderson Road Adams County <br />104th Avenue UDFCD <br />88th Avenue CO Dept. of Highwavs <br />UPRR <br />(between York St <br />York Street <br />MWRD <br />Central Plant <br /> <br />UDFCD <br />- <br />UDFCD <br /> <br />1-270) <br /> <br />and <br /> <br />II <br /> <br />'I <br /> <br />I <br /> <br />4.3.6 Floodway Methodology <br /> <br />HEC-RAS has five different methods for performing a floodway analysis, each with its own target criteria. The <br />HEC-RAS floodplain encroachment procedures first calculate a natural water surface profile, using the existing <br />conditions geometry, then encroach on the floodplain until a target criteria is met (USACE 2002). The 1-foot and <br />O.5-foot flood way presented in this study were computed on the basis of equal conveyance reduction from each <br />side of the floodplain (Model 4). HEC-RAS Method 4 encroachment analyses was used to obtain initial <br />encroachment stations. The encroachment stations were then refined manually (Method 1). Floodway widths were <br />computed at cross-sections; boundaries were interpolated between cross-sections (USACE 2002) <br /> <br />5120.18 <br /> <br />Small discrepancies were found between the surveyed bridge data and the previously modeled geometry for these <br />bridges. Minor differences were found between pier width, deck thickness, low chord elevation, and high chord <br />elevation. The difference between the surveyed high chord elevations and the modeled high chord elevations was <br />less than 1 foot for all surveyed bridges, except the upstream Union Pacific Rail Road (UPRR) Bridge (station <br />1012+00), for which the difference was 2.9 feet. The largest differences were associated with the deck thickness and <br />low chord elevations, ranging from 0.3 feet to 2 feet. <br /> <br />UDFCD <br /> <br />Road <br /> <br />Access <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />Floodway modeling in the divided and split flow reaches required additional consideration. Floodways in the <br />divided flow reaches (Brighton and 88th Avenue) were modeled using the east embankment levee failure <br />geometry (as opposed to the east embankment levee intact geometry), resulting in conservative flood way widths. <br />UDFCD recommended not performing a flood way analysis for the York Street split flow reach because there is no <br />flood way south of 58th and York Street on the Adams County or Denver Flood Insurance Rate Maps (FEMA 1995 <br />and FEMA 1990) <br /> <br />The f100dway in the Adams County Fairgrounds reach between 136th Avenue extended and Henderson Road <br />required additional review and refinement because of divided flow in the area and the presence of several ponds. <br />After consultation with UDFCD and knowledge of the land use in the area, it was determined that the western <br />boundary of the flood way should generally align with the high ground on the west bank, rather than at the <br />location determined by the HEC-RAS model. <br /> <br />The MWRD CTP access bridge was not included in the previous hydraulic models. This bridge was surveyed <br />using the benchmark from the east side of UDFCD geomorphic cross-section 29. A LOMR application submittal for <br />the E-470 Bridge was recently reviewed and approved by UDFCD and record drawings were available; therefore, <br />this bridge was not surveyed. Elevation measurements were not taken on the following bridges because the bridge <br />decks will not impact the 100-year flows: State Highway 224 (886+00), 1-76 (902+50), 1-270 (951 +00), and the <br />upstream UPRR Bridge (1012+00) between 88th Avenue and State Highway 224. Bridge geometry (i.e., pier width, <br />deck thickness, and distance between abutments) were, however, measured for comparison with the modeled <br />geometry. These data were entered into the FHAD model and bridge opening areas were generated. Table 4-3 <br />shows the difference between the Master Plan and the resurveyed bridge opening area. <br /> <br />Difference <br />(%) <br />4 <br />-5 <br /> <br />Master Plan <br />bridge open area <br />J!ft <br />3550 <br />2940 <br /> <br />Table 4-3 Bridge Ol>.enin~ Area Comparison <br />Resurveyed bridge <br />openin~ area <br />J!rL <br />3401 <br />3081 <br />11895 <br />2770 <br /> <br />Bridge <br />Baseline Road <br />160th Avenue <br />E-470\lJ <br />Henderson Road <br /> <br />I <br /> <br />I <br /> <br />COlI <br /> <br />415/05 CJe <br /> <br />0611\40892\PREP-FHAD\S4.DOC <br /> <br />40 <br /> <br />3864 <br /> <br />4-4 <br />