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ig i <br />~~i <br />Southwestern Portland Cemeat <br />SHB AGRA Project E92-7075 <br />Haul Road Bridge Floodplain Analysis <br />Page 3 <br />result of alterations to the main channel. The change in computed surface water elevation as a <br />result of this discharge change was then determined. <br />4.0 RESULTS OF ANALYSIS <br />Results of the three computer runs made for the main channel section aze summarized in Table <br />1, which presents the computed water surface elevation, total channel flow, width of water <br />surface and cross sectional area of flow at channel sections extending 2000 feet downstream of <br />the proposed haul road bridge to 4900 feet upstream of the bridge. Complete listings of the <br />output from the analyses aze available upon request. Table 1 also presents the change in water <br />surface elevation and flow quantity relative to the initial channel conditions considered in the <br />Love and Associates analysis. <br />As presented on Table 1, the refinement in channel geometry resulting from the addition of <br />channel Sections 108 and 109 results in a decrease in water surface elevation of 0.07 feet at <br />Section 110 which is immediately upstream of the two new channel sections and an increase in <br />flow of 21.8 cubic feet per second (cfs). At Section 107, which is located immediately <br />downstream of the Section 108 and 109, the refinement results in a decrease in water elevation <br />of 0.47 feet and a decrease in flow of 184.7 cfs, relative to results of the Love and Associates <br />analysis. This change reflects the effect of refining the definition of the discharge conditions to <br />the south channel. Accordingly, the results indicate an additional 184.7 cfs is diverted to the <br />south channel floodplain between Section 110 and 107, relative to the Love and Associates <br />analysis. <br />Run 3 analyzes the effects of the proposed haul road bridge, with all other conditions as <br />incorporated in the Love and Associates floodplain analysis. As presented in Table 1, the <br />proposed 116 foot long, two span bridge causes an additional 373 cfs to be diverted to the south <br />channel above that determined in the floodplain delineation study. This results in a surface water <br />profile in the main channel downstream of the haul road bridge that is at a lower elevation than <br />that computed for the undeveloped condition. Figure 7 presents the surface water profile in the <br />vicinity of the proposed haul road bridge. As shown on Figure 7 and summarized in Table 1, <br />the computed water surface is 0.8 feet lower at the North 51st Street bridge as a result of the <br />haul road bridge and the corresponding increase in water diverted to the south channel. <br />The effects of the increased flow in the south channel as a result of the haul road bridge are <br />summarized in Table 2, along with the initial conditions developed by Love and Associates for <br />the floodplain delineation. Figure 8 presents the computed surface water profiles for the initial <br />conditions as well as with the haul road bridge. An increase in flow to the south channel of 373 <br />cfs was made to reflect the diverted volume indicated by the Run 3 analyses of the main channel. <br />The increased flow that is diverted to the south channel as a result of the haul road bridge causes <br />a maximum change in computed water surface elevation of 0.22 feet. The computed change in <br />surface water elevation should be considered conservative in that Love and Associates defined <br />the south channel flow area with lateral limits to restrict the width the flow channel can spread <br />with increased flow. Accordingly, the defined channel is essentially a rectangular cross section <br />AGRA <br />Earth & Envimnmente/Group <br />