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I 061 40892\PREP-FHAD\S4.DOC 4/5/05 cje 4-3 I COM I I I 4.3.2 Recent Channel Modifications Since the completion of the Master Plan model, a number of changes have occurred along the South Platte River, including the construction of the new E-470 Bridge north of the Adams County Fairgrounds. Further south, near McKay Road, the SACWSD recently completed an expansion to the Williams-Monaco Wastewater Treatment Plant. Data from these two projects were submitted to UOFCO as applications for LOMRs from FEMA. UDFCO in turn provided COM with these data for incorporation into this FHAD. Other projects undertaken along the river include the reconstruction of the Gardener's Ditch diversion structure at York Street, construction of a new grade control structure at Henderson Avenue, and the construction of MWRD's Reaeration Structure Number 4 (RS4) at the Riverdale Dunes Golf Course. Data for the York Street and Henderson A venue structures were obtained from UDFCD suggested that the east embankment may not confine the flow to the main channel, and this area should be modeled as divided flow for a conservative approach. Therefore, cross-sections 49 to 46 were modeled as The 2002 Master Plan HEC-2 model confined flow to the main channel immediately upstream of the 88th A venue Bridge (river station 818+00 to 778+00). It was believed that flow could not enter the east overbank of the South Platte River between river stations 818+00 to 778+00, unless the channel flow overtopped the east embankment. Backwater could get into this area, but the east overbank could not convey flow unless the embankment was overtopped. Blocked obstructions were used to model this situation along the east overbank throughout cross- section 48 to 44.9 (river station 818+00 to 778+00). I 'I I I Incorporation of Additional modifications made as part of the migration to HEC-RAS included revisions to ineffective flow areas, blocked obstructions, and contraction and expansion coefficients. X3 cards were used to restrict flow in the HEC-2 Master Plan model, with areas not included in flow calculations called "ineffective flow areas" (e.g., areas with large roughness coefficients, areas behind levees before overtopping, and filled areas). HEC-RAS has the ability to model levees, block out areas from the cross-section (blocked obstruction tool), and restrict conveyance to certain areas of the cross-section while using the remainder of the cross-section for storage (ineffective flow tool). When the HEC-2 model was imported into HEC-RAS, the X3 data were all imported as blocked obstructions; therefore, each cross-section with an X3 card in the HEC-2 model had to be examined to determine the appropriate tool to model the flow restriction. A majority of the X3 records were modeled using the ineffective flow tool. Flow was restricted using the blocked obstruction tool for cross-sections 6, 7, 35.9, and 38. The floodplain upstream of Clear Creek is primarily confined to the river channel, with some exceptions (UOFCO 2002). In particular, the floodplain is confined to the river channel around the Xcel Energy Cherokee Power Plant (west overbank between cross- sections 65.05-150.05) and the MWRD CTP (east overbank between cross-sections 65.05-150.05). In several reaches in the Study Area, the main channel is flanked by embankments that act as levees. The stability of these embankments is unknown, however, and they are not certified by the USACE, as required by FEMA if a levee is to be shown as providing protection from the 100-year flood event. Thus, it was recognized that it would be improper to use the levee tool in HEC-RAS to model the embankments, but allowing conveyance behind the embankments at all times would result in an overly-conservative floodplain width. After consultation with UDFCD and a representative from FEMAs mapping contractor, Michael Baker Jr., the following approach was devised to model the embankments. First, HEC-RAS was run with the embankments modeled as levees and the resulting base flood elevations applied to the main channel and to the unprotected overbank, if applicable. Next, the model was run again, this time "failing" the levee and allowing flow over the embankment. The base flood elevations resulting from this run were applied to the overbank area. This analysis was applied to the Brighton Reach (station 120+00 through 265+00) and to the reach upstream of 88th A venue (station 780+00 through 845+00). A gutter has been drawn to differentiate the overbank from the channel. I I I 4.3.1 to HEC-RAS The Master Plan model was migrated to HEC-RAS v3.1.1using the HEC-2 data import feature in HEC-RAS. This process imports cross-sectional geometry accurately; however, additional work is required at bridges and other hydraulic structures due to differences in modeling approach and input data format. Notably, the bridges in the Master Plan were modeled in HEC-2 using the "Special Bridge" method, which takes into account total pier width and calculates an approximate opening area. HEC-RAS has the ability to model bridges based on actual pier and deck geometry. The bridges within the Study Area were resurveyed to confirm pier and deck geometry and elevations and abutment side slopes. Bridge geometry is contained in Appendix C. In addition to the York Street split flow reach described above, flow leaves the main channel of the South Platte River at other locations along the study reach. The floodplain in the vicinity of 104th A venue carries a significant amount of flow and was modeled as split flow in the 2002 Master Plan model; however, after consultation with UDFCD, it was determined that the flow in this area remains hydraulically connected to the channel and therefore this area was modeled as divided flow. ~ivided flow also occurred in the vicinity of the Adams County Fairgrounds, between 136th Avenue extended and Henderson Road (station 290+00 through 380+00). 4.3.4 Modeling Divided Flow and Embankment Analysis I Migration from HEC-2 I The development of the HEC-RAS FHAD model is discussed in detail in the following sections. The split flow model predicts main channel water surface elevations (xs160.85 to 230.85) higher than the embankment elevation, and the overbank split flow water surface elevations lower than the embankment elevations, implying that water is flowing out of the main channel and into the west overbank. The gutter differentiating the main channel and overbank was drawn along the high ground, and the overbank split flow BFEs were extended east to the gutter, even though the ground elevation is higher than the BFEs immediately west of the main channel. . Half-foot and 1-foot flood way analyses were performed based on existing channel and floodplain characteristics I . The steady flow The separate HEC-2 split flow models were imported into HEC-RAS, and combined with the main channel model using a combination of junctions and lateral weirs I . The channel geometry was updated to reflect modifications to the river undertaken since the completion of the Master Plan model Flow Model As discussed in Section 4.2.2, split flows are a significant factor affecting the floodplain of the South Platte River in Adams County. In particular, the west embankment between Franklin Street and York Street (referred to as York Street) is identified as a split flow reach in the South Platte River Denver Metropolitan Area (Denver) FHAO (UOFCD 1985b). Split flow information was obtained from the Denver FHAO (UDFCD 1985b) and input into the HEC-RAS model. file was revised to include discharges for the 500-year flood event I . Bridges potentially affected by 100-year flows were resurveyed; modeled bridge geometry and elevation data were revised as needed 4.3.3 Developing the Split UDFCD. Since COM performed the modeling and design for RS4, these data were readily available These changes were incorporated into the FHAO model using the Import HEC-RAS Data option. I I . 4/8/05 in-house. Section 4 Hydraulic Analyses