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I ,I . I I data was used to redefine the bridge and road deck elevations and the bridge opening for the HEC-2 model. No as- built information was available for the l04th Avenue Bridge, so a combination of the updated Adams County mapping and the original FHAD bridge data were used to define that crossing. Bridge data for Baseline Road, 160th Avenue, Henderson Road, the Union Pacific Railroad crossings, Highway 224, 1-76, 1- 270, and York Street were based on a combination of the updated Adams County mapping and the original FHAD bridge data to define that crossing. AutoCAD SoftDesk™ was used to produce the updated road topography for the bridges. 4/8/05 to determine channel bed methods of interpolation and extrapolation were used Section 4 Hydraulic Analyses cross-sections, two different elevations: A factor significantly affecting the conveyance capacity of the floodplain and therefore the areal extent of the floodplain is the presence of gravel mining pits. Numerous pits currently exist, which were not present at the time of the 1977 FHAD. An example is provided in Figure 4-1, which shows the cross-section 28 geometry (between Henderson Road and First Creek). The large gravel pit in the west overbank could potentially provide a significant flow conveyance area and flood storage, and if modeled as is, HEC-2 would consider the area of this pit in calculating floodplain storage and conveyance. However, for the purpose of delineating flood boundaries, all pits were assumed to be full and provided no true conveyance capacity. This is consistent with what is recommended in HEC-2 documentation, namely gravel pit areas should not be considered effective in conveying flow (USACE 1991). The effective flow area for gravel pits was assumed to be limited to the area above the low point elevation of the pit embankment. Effective and ineffective areas were identified for all cross-sections and the HEC-2 model input was revised accordingly. I . The first method used the information from the Geomorphic Assessment (Stevens 1996) for the interpolation process. Oata from that report included cross-section geometry upstream of approximately HEC-2 cross-section 8 (upstream of 160th Avenue). This process included transferring the UOFCD annually surveyed cross-sections (which are not the same as the HEC-2 cross-sections) into the current model and determining the slope between the cross-sections. The HEC-2 cross-section thalwegs were then determined by linear interpolation between Stevens cross-sections. This procedure was used for all sections upstream of cross-section 7 where the MWRD survey data were not available. I I The second method was performed on HEC-2 cross-sections 1 through 7. The Geomorphic Assessment (Stevens 1996) data was extrapolated by assuming that the slope of the thalweg would follow the slope of the low flow water surface on the Adams County maps, which was depicted on the topographic mapping. The depth of water was estimated using the difference between the geomorphic cross-section elevations and the topographic water surface upstream of this reach. . I 'I /1 ? ,. which assumes the separated flows are still hydraulically connected (i.e., water surface elevations remain the same). However, if the flows paths remain separate over a long enough distance, the hydraulics can become "disconnected" (i.e., different water surface elevations can be anticipated) and a split flow analysis is warranted. The South Platte River in Adams County has instances of both split and divided flow. The modeling approach to these two flow patterns are discussed in Sections 4.3.3 and 4.3.4. I I Split flows and divided flows are another factor affecting the delineation of the floodplain. For areas where the floodplain becomes temporarily disconnected from the channel (such as flow around an island or levee), the hydraulics can be modeled as divided flow or split flow. If the flow paths separate over a relatively short distance and rejoin downstream of the split-causing feature, the hydraulics are treated as a divided flow analysis, ~"'''''.'''j............L..''''...T j~ : j . . . . . . . . ~.."_.....~.~..;..~..~..~.~ .~. ~t .~.~.~. t. . . . . ...... ..n..~...... .. ..~.. .....u...~ . . . . . . . . . . . . . . . .. .. . . . .. .. . . . . . . . . . . . . . . . . ......04... .~...... ...-:-...........~..__...._ ...: .. .__......~.... ....-:-_..........";....n......~. .......m.'.m ...J.m.m.'.......... ............,. .m..:.....,..........: mnm.nT .mh m.'hnnh .,0.0. 0.__". h.........r....n....T 1977DataOffsetby: ; ; :: -95 ft horiZDlJta11y ..........+-..... .......-j----....... ............~............~ 2.8 ft vertk:aJ1y . . .. . . ., . . .... . . .... ..........."!"".......: Tl" h..........t.. ~:~~~~~ nh.....T.h.....T. ..... ...........T" -lnelf~FlowBo~ .. . .. .. . .. -+- -+- . -+--+- 500 1000 3500 4CO) 5030 Channel geometry was then approximated by extending the banks of the channel from the 1998 topography down to the estimated channel bed elevations. 5020 5015 en 5010 E ~ s:: .2 m > .. jjj The 1998 mapping and survey are based on the NA VO 1988 datum, a vertical adjustment system using an msl elevation derived in the 1980s. NA VD 1988 has redefined the elevation above msl for several hundred thousand benchmarks across North America. The 1977 and 1985 FHAOs used the National Geodetic Vertical Datum of 1929 (NGVD29), which is based on a different msl elevation. Unfortunately, the difference between the two datums varies throughout Adams County. A review of benchmarks through the Study Area revealed that the 1977 FHAD elevation ranged from 2.44 feet to 2.99 feet lower than the current elevations, as shown in Table 4-1 49!;l; 4900 4900 -500 4005 Difference (ft) 2.47 2.40 2.53 2.44 2.84 2.99 2.99 Table 4-1 Comfl.arison of Benchmark Elevations for the 1929 NGVD and 1988 NAVD Datum Benchmark 1929 NGVD 1988 NAVD Name Elevation (ft msl) Elevation (ft msl) I SPR14 5133.13 5127.55 5087.44 5054.895 5022.29 4970.37 4960.30 5130.66 5125.15 5084.91 5052.46 5019.45 4967.38 4957.31 4500 Figure 4-1 South Platte River Cross-Section 28 (2,900 ft downstream of First Creek) :mo 1500 2tXK> 250J Station (F~ West to East) Benchmark Location On York Street Bridge over South Platte River On 1-270 Bridge Over South Platte River On 88th Ave. Bridge Over South Platte River On 104th Ave. Bridge Over South Platte River On Henderson Road Over South Platte River On 160th Ave. Bridge Over South Platte River On Baseline Road Bridge Over South Platte River John E. Chance & Assoc. 1996 SPR15 SPR17 SPR18 SPR19 SPR20 SPR21 Source: I I 'I The following steps Preparation 4.3 FHAD Model Channel and floodplain flow distances and roughness coefficients (Manning's "n") were updated based on current information developed for MWRO by COM (1999) from cross-sections 26 through 48. In addition, above and below this location the channel distances were changed using the updated topographic data. The rest of the Study Area, above and below cross-sections 26 through 48, assumed the same floodplain flow distances and roughness coefficients as in the FHAD modeling studies. Areas where the roughness coefficients changed over a given length were transferred onto cross-section plots and updated based on current topography. I was used as the starting point for development of the FHAD model . The main channel HEC-2 Master Plan model was imported into HEC-RAS v3.1.1 and then cross-sections and bridge geometry were examined to ensure the data were imported correctly The Master Plan model were taken in creating the FHAO model The 1977 FHAD did not include the McKay Road Bridge in the HEC-2 model. As-built information for this bridge was obtained from Adams County by COM and the bridge was added to the Master Plan model. Additionally, as- built information for the 88th A venue Bridge was obtained to refine the bridge data cross-section information. This I I CDWI 0611\40a92\PREP-FHAD\S4.DOC 4/5/05 CJe 4-2