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I I I I I I I I I I I I I I I I I I I - 27 - River through Breckenridge is approximately ~~ which necessi- tated a maximum.interval between cross sections of 500 feet. It was necessary to run the HEC-2 program both for a super- critical flow regime through open stretches of the channel as well as a sub-critical flow condition. The water surface pro- file was a composite of the two conditions depending on which was found to be the control condition of the flood reach in- vestigated. Initially all bridges and crossing structures were considered free and clear of debris. This was done using the bridge and culvert routine incorporated in the HEC-2 model. Based on discussions with the town, as well as the Colorado Water Con- servation Board, it was decided that'all crossing structures should be considered fully plugged with debris due to the type of the structure. This analysis was performed using hand cal- culations with the weir overflow formula Q = CLH 3i2. In the channel reach between Village Road and Lincoln Avenue there is a split in flow due to inadequate channel capacity as well as due to the effect of the crossing at Lincoln Avenue being assumed plugged with debris. The overland flooding through the town caused by this condition was investigated by hand computations using Manning's formula. If a low lying area was surrounded on either side by floodwaters, the area was considered in the floodplain. Copies of I" to 100' mapping has been made available separately to the town of Breckenridge which indicates the Intermediate Regional Flood limits based on bridges completely plugged with debris as well as the floodplain limit assuming the crossing structure be replaced with adequate bridges and the channel is enlarged between Village Road and Lincoln Avenue. In the Recommendations section for new structures, the future water surface elevation after replacement of the obstructive crossing was made by comparison of hydraulic parameters downstream of the crossing with those upstream beyond the backwater effect due to the structure. The selected hydraulic parameters at the cross- " ing location were added to the bed level at the site taking into account potential erosion or stabilization that may occur due to the removal of the culvert structure. ~ Leonard Rice Consulting Waler Engineers. Inc.