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Southweetem PorUaud Cement <br />SHB AGRA Project E92-7075 <br />i S ' nau~ xoaa tfnage hiooapiam rmatyeie <br />'_ B' Pege 2 <br />3.0 ANALYSIS OF IMPACTS OF PROPOSED BRIDGE <br />3.1 Main Channel <br />An analysis was made of the impacts of the proposed haul road bridge on the capability of St. <br />Vrain Creek to wnvey the design 100 year flood event. In making the analysis, HEC-2 program <br />input data from the Love and Associates analysis was obtained from the Boulder County <br />Transportation Department. The data included stream flow data, channel cross sections, channel <br />hydraulic characteristics, bridge characteristics and the split flow data used in defining the <br />proportion of flow into the main and south channels. The analysis was made using HEC-2, <br />Version 4.6.2 and considered only suberitical flow conditions as was done by Love and <br />Associates in their floodplain assessment. <br />Three runs of HEC-2 were made to evaluate the impacts of bridge construction on the flow in <br />the main channel. The first utilized the Love and Associates data as provided by Boulder County <br />to confirm the accuracy of the input data by comparing the output obtained with that reported <br />by Love and Associates. The second run included two new channel sections (Section 108 and <br />109) that were input to define the main channel cross section between Section 107 and 110 as <br />indicated on Figure 1. Figures 2 and 3 present the channel cross sections input for Sections 108 <br />and 109. The geometry of the split flow section was revised to incorporate the two new sections. <br />This run provided an assessment of the impacts of the additional refinement of channel <br />characteristics on the computed water surface profile and the distribution of flow through the <br />split channel section with all other conditions being the same as the Love and Associates <br />analysis. <br />A third run was made that included the effects of the proposed haul road bridge. The HEC-2 <br />normal bridge routine was used to model the constriction caused by the bridge and approach <br />fills. With the normal bridge routine, neither pressure flow under the bridge or overtopping of <br />the bridge is considered, but overtopping of low sections of the approaches are considered. The <br />channel configuration and bridge geometry included in the analysis is shown on Figures 2 <br />through 6 which are profiles of Sections 108, 109, 109.1, 109.2 and 109.3 developed for this <br />analysis and added to the Love and Associates input data. <br />3.2 South Channel <br />Two runs were made to analyze flow conditions in the south channel. The first run was made <br />with unaltered input data provided by Boulder County to verify the accuracy of the input data. <br />The second run was then made to evaluate the effect of the haul road bridge on flow in the south <br />channel. To assess this effect, the discharge to the south channel was modified based on the <br />results of the third run of the main channel analysis. The input discharge at the upstream end <br />of the south channel was adjusted by the difference between the channel flow at the North 51st <br />Street bridge as determined in the second and third runs for the main channel. This difference <br />in discharge would be the quantity of water diverted either to or from the south channel as a <br />AG RA <br />Earth & Emironmenta/Group <br />