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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />A vertical retaining wall of simulated rock would extend from the south <br /> <br />abutment of the improved Broadway bridge to the west abutment of the <br /> <br />improved Arapahoe Road bridge to protect an existing restaurant and <br /> <br />service station. <br /> <br />Marine Street and the Boulder High School. The existing Boulder High <br />School Athletic Facilities to the south of Boulder Creek would be relo- <br />cated in conjunction with this proposed excavation. A buried control <br />sill would be constructed approximately 700 feet upstream of the 17th <br />Street bridge to prevent streambed and bank erosion from moving pro- <br />gressively upstream (head-cutting) through the athletic field south <br />of Boulder Creek. <br /> <br />ARAPAHOE AVENUE TO 17TH <br />The existing Arapahoe Road bridge would be removed and replaced with a <br /> <br />three span bridge having a total length of 200 feet. The anticipated <br /> <br />water surface elevation associated with the estimated lOO-year discharge <br /> <br />of 12,000 c.f.s. would submerge the low chord of the proposed bridge by <br /> <br />l.9 Feet. This condition cannot be improved by increasing the length <br /> <br />of the proposed bridge because the water surface profile through the <br /> <br />proposed crossing is predominantly controlled by downstream conditions. <br /> <br />Freeboard could be obtained by raising the proposed bridge, however, <br /> <br />this would require extensive regrading of Arapahoe Avenue and adjacent <br /> <br />alleyways and other access. The proposed improvements do however sub- <br /> <br />stantially improve the flow conditions at this crossing by reducing the <br /> <br />maximum average velocity to seven feet per second. An existing building <br /> <br />located on the southwest corner of the existing Arapahoe Road bridge <br /> <br />would be removed in conjunction with the proposed improved crossing over <br /> <br />Boulder Creek. <br /> <br />The existing athletic field pedestrian bridge located at the east end of <br />the football field would be removed and replaced with a swing-away <br /> <br />bridge. <br /> <br />The playing field to the south of Boulder High School would be lowered <br />approximately six feet and the banks shaped to provide a smooth flow <br /> <br />transition to the l7th Street crossing. An existing multi-family unit <br /> <br />and City of Boulder building, both located northwest of the 17th Street <br />bridge, would be removed in conjunction with this improvement. <br /> <br />An apartment house located on l4th Street immediately north of Boulder <br /> <br />Creek would be removed and relocated in a more favorable location. <br /> <br />The 17th Street crossing would be lengthened approximately 70 feet to <br /> <br /> <br />the north to provide additional flood conveyance for the estimated lOO- <br /> <br /> <br />year discharge of 12,000 c.f.s. Three private residences downstream of <br /> <br />the 17th Street bridge to the north of Boulder Creek would be removed in <br /> <br />conjunction with improving the 17th Street bridge. <br /> <br />The athletic field to the south of Boulder Creek would be lowered <br /> <br />approximately five feet to provide additional flood conveyance and <br /> <br />prevent inundation to the north of Boulder Creek in the vicinity of <br /> <br />MODIFIED PLAN I I I APPROACH DOWNSTREAM FROM 17TH STREET <br />SELECTED BRIDGE IMPROVEMENTS AND RELATED CHANNEL MODIFICATIONS <br />The proposed improved crossing over Boulder Creek at 24th, 28th, and <br />30th Streets are as follows: <br /> <br />7 <br />