FLOOD05866 - Laserfiche WebLink

Home Browse Search

Blue River DRAfT There are only four locations on the Blue River where the flow breaks out of the main channel area during the IOO-year flood as analyzed. Three of these areas are a result of road crossings. At the downstream end of the study, the lOO-year flood overtops Valley Brook Road by approximately one foot. Further upstream, just above the Washington Avenue foot bridge, the flow could possibly break out along the east channel bank. More detailed survey would have to be performed to determine if the breakout will actually occur. At the downstream Broken Lance Drive crossing, the two 5' diameter RCP culverts cannot convey the entire lOO-year flood without overtopping the roadway about one foot. At the upstream Broken Lance Drive crossing, there is a possibility the the flow may exit the channel upstream of the crossing and flow to the north along the east side of Broken Lance Drive. The mapping in this area is insufficient to determine if this will actually happen. Of the four areas just described, the two areas which have the highest potential for damage are the area above the Washington Avenue foot bridge and the area above the second Broken Lance Drive crossing. In both these cases, the flow could actually leave general channel area and cause significant areas of shallow flooding. In the other two instances, the flow will remain confined to the general area of the roadway and the damage would likely be limited to the roadway and embankment. The greatest concern during a significant flood, would be plugging of culverts by debris. Although under assumptions that the culverts remain clear, Park Avenue (upper crossing), Ski Hill Road, and Watson street pass the IOO-year flood, with the presence of a minimal amount of debris, all these crossings would be inundated and the potential for diverting flows into developed areas would be significant. During a large flood, all culverts should be monitored regularly and appropriate equipment ready to remove any debris plugging the culverts. In the future, if any of the culvert crossings need to be replaced, structures with wider openings, such as box culverts should be used. In comparing this study with the FEMA study, several areas of significant differences were found. (Significant being defined as areas where the levels differ by more than one foot). For the entire reach from Valley Brook Road to the French Street bridge, the FEMA flood levels are usually lower by one to two feet. At the downstream end of the study, it appears that the FEMA study did not consider the effect of the Valley Brook Road crossing since the difference are up to 4 feet. Through the area of the Town Park, the topography has changed due to construction of the Park and realignment of the channeL Below the confluence with French Creek, a rock drop structure has raised the flood levels nearly 35