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sawmill Gulch \)R~f1 Sawmill Gulch has several complicated flood plain issues. As previously discussed, there is the question of whether the lOa-year discharge as defined by FEMA is low. In addition, due to the channel having been relocated several times, in some places the channel is actually perched above lower lying areas of its flood plain. Consequently, when the channel banks are overtopped, the flood can spread out for a significant distance. Finally, the accuracy of existing studies is somewhat suspect. There is considerable disagreement between the 1980 Breckenridge Town Topographic Mapping and the FEMA topographic mapping. This is not unusual since much of the area is heavily vegetated. Since many of the FEMA cross sections were taken from the topographic mapping they may be inaccurate. Additionally, the channel often does not show up on the mapping so that the FEMA cross sections may have a flat bed one hundred or more feet wide. This leads to flows possibly spreading out further in their model than would be the actual case. Finally, use of the Town topographic maps is not the solution either. Several large discrepancies were found between the Town mapping and cross sections surveyed for this study. A complete description of the flooding problems and assessment of the existing flood plain definition follow. The designated discharge of 120 cfs can be adequately conveyed through the existing storm sewer from upstream of Park Avenue to the Blue River. However, between cross sections 8.0 and 8.1 the flow may break out of the channel. This is not a certainty since the accuracy of the topographic information is unsuitable to make definitive conclusions. Because of the possibility of this breakout, a large area of land has been mapped as potential sheet flow areas. A similar situation exists near cross section 11.0. Upstream of cross-section 11.2, the flows remain in the channel for the 120 cfs lOa-year flood up to Four o'Clock Road. However, the portion of the relocated channel downstream of the Tyra access road culverts is much flatter than other sections of the channel. As a result, sediment deposits are building up and could eventually create flooding problems at the culvert or downstream. There is a potential flow break out at the 4 O'Clock culverts. A portion of the flow could leave the channel just upstream of the culvert and follow the south side of 4 o'Clock Road. Additional survey information would be needed to determine if this would happen. In addition, the existing culvert is collapsing beneath the roadway, probably due to inadequate cover. If the damage continues, then a break out of flow would be certain. This could result in a considerable amount of damage to developed areas downstream. upstream of Four O'Clock Road, the channel flows through wetlands and becomes confined on both sides by steep slopes. There is no development in this area and consquently no 38