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engineering report. The e~cisting structure (TELL-06-1.15-O1) is a reinforced concret~ <br />bridge, 40 feet in length with a roadway width of 28 feet. Single-span steel I-beam <br />founded on concrete abutments. The bridge was constructed in 1994. <br />Table 2. Bed Elevation History at Teller County Road 21 <br />slab <br />are <br />Elevation Source Year Elevation <br />(Wilson Local <br />Datum, Feet) Aggradati <br />Degradati <br />(Feet) n(+) <br />n (-) <br />FEMA 1988 92.1 <br />Lonco 1993 90.1 -2.0 <br />Foothills 5/29/94 90.6 +0.5 <br />Wilson & Co. 4/96 91.0 +0.4 <br />(data taken from Wilson & Co.,1996) <br />Wilson & Company (1996) reported that the 1988 FEMA elevation should be ~ iscarded, <br />apparently because the elevations between 1988 (FEMA) and 1993 (L.onco) indicated degradation, <br />not aggradation as stated in the FEMA ~1988) report. The 1988 may be in error; ho ever, an <br />alternate explanation is that the bed of Fountain Creek is dynamic, aggrading and ~ egrading <br />depending on the amount and timing of water and sediment discharge. <br />Based on their hydraulic analysis, Teller County Road 21 bridge will be overta'pped at a <br />Fountain Creek flow rate of 1850 cfs a roximatel a 17- ear recurrence interval. ~ilson & <br />, PP Y Y <br />Co an 1996 concluded that the overto in event would cause the roadwa emb~ent to <br />mP Y~ ) PP 8 Y <br />fail. A 17-year recurrence interval event has an approximately 6% chance of occurring in any one <br />year. However, as pointed out by Wilson & Co., continuing aggradation can increase ~he risk of <br />overtopping. Even if aggradation does not occur, the risk of having the 17-year flood c <br />years is 11.4% and in four years is 21.5%. <br />in two <br />5 <br />