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I I I I I I I I I I I I I I I I I I I directly into the format of the HEC-2 input. These sections were obtained from the same photography flown for the topographic mapping used in this study (Reference 2). The digitized sections were supplemented with additional cross sections as needed. All bridge and culvert data were obtained from actual field measurements. Two areas of the Study Reach complete the hydraulic analysis. in the following paragraphs. required additional effort in order to The analyses of these areas are described Overbank: Channel: .040 to .065 .040 used throughout Washington Street - Washington Street crosses the floodplain with a road surface el evated two to four feet above the surroundi ng 1 and, and a skew angle which would force the primary flood flow to follow the road along part of the crossing. Upstream of the crossing, this configuration would create a flow situation composed of flow both parallel to the road and crossing the road embankment. The modeling of this area involved use of the split flow option contained in the HEC-2 computer program. This option allows incremental flow splits from the primary flow to be determined between adjacent cross sections carrying the primary flow. The quantity of the flow split is determined by the weir equation. The water surface of the primary flow provides the head to force flow over the weir section defined between the primary flow cross sections. Roughness coeffi ci ents, as values of Manni ng' s "n," for the channel and overbank areas were estimated by field inspection of the study area and procedures outlined by Chow (Reference 1). Typical ranges for these values are as foll ows: In general, the hi gher "n" val ues were used to account for obstruct ions, such as trees, bushes, and weeds within the floodplain. Expansion and contraction coefficients were employed within the HEC-2 model to account for the turbulent losses due to changes in the flow velocity. For this study, values of 0.3 and 0.5 were selected for contracting and expanding flow near bridges. Where the velocity changes were more 9radual, values of 0.1 and 0.3 were used. These values are similar to those recommended in the HEC-2 User's Manual (Reference 8). Several bridges cross the floodplain in a skewed orientation, causing the digitized road profiles to represent unrealistically large flow areas. This problem was corrected by projecting the digitized road profiles onto cross sections perpendicular to the direction of the flood flow at the road crossings. Affected crossings include 144th Avenue, York Street and 156th Avenue. Implementation of the split flow option required two hydraulic models for the Washington Street crossing. One model described the flow upstream of the road embankment, and the other descri bed the flow downstream of the embankment. Cross sections in this area exhibit two flood elevations on the Fl oodp 1 ai n Maps; one upst ream of, and one downst ream of, the road embankment. This situation necessitates an explanation of the data tabulated in Table VII for these cross sections. Tabulated floodplain and floodway widths pertain to the sum of the upstream and downstream increments at each cross section. However, tabulated velocities and thalweg elevations at these cross sections pertain to either the upstream or the downstream hydraulic model, depending on which contains the established channel and thus the primary flood flow. Flood discharges and water surface el evati ons are tabul ated separately for the upstream and downstream portions of each of these cross sections. The wide floodplain encountered in this study contains areas which would be flooded to a significant depth, but which would provide little conveyance of the fl ood water. These i neffect i ve flow areas were removed from the cross sections by use of the encroachment option within the HEC-2 computer program. ColoradD Boulevard - Colorado Boulevard crosses downstream from the Union Pacific Railroad Bi g Dry Creek immedi ately crossing. The railroad 12