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HomeMy WebLinkAbout20151251 Ver 1_Bridge to Culvert Justifaction 000146 10-28-2015_20151203 Alamance County Br# 000146 SR 2363 Beale Road Over Rock Creek The project consists of replacing 25 foot long Functionally Obsolete Bridge that has a Sufficiency Rating of 20.56 in Alamance County. The current structure was constructed primarily with timbers in 1955. Timber structures typically have a life span of 50 years and this structure is 60 years old and has reached the end of its life cycle. The proposed structure consists of a precast single span of 28 feet by 4.0 feet high crownspan culvert or equivalent. This structure meets the hydrological requirements of the drainage area without significantly altering the FEMA Flood studies. The use of this type of precast structure will reduce the construction time from 120 days to 90 days as compared to that of a bridge. Precast culverts have lower construction and maintenance cost over the life cycle of the structure as compared to a bridge. When site conditions allow a bottomless culvert is the preferred structure of choice due to the lower overall cost and minimum construction time while still maintaining the integrity of the stream bed. Current budget constraints require us to take the most economical option that meets environmental and safety concerns. Typically the length of a bridge will be much longer than that of a culvert. Increasing the bridge length creates several issues that must be overcome. If a bridge is utilized there must be at least four feet of clearance under the bridge to allow access for maintenance workers at the abutment. In areas where culverts are recommended this requires us to raise the roadway grade which also increases the impacts due to the increased fill slopes. Raising the roadway grade also affects the FEMA Flood Plain Mapping. In turn we cannot meet the requirements of the Memorandum of Agreement (MOA) with FEMA. If the new bridge cannot be sized to meet the requirements of an MOA then map revisions need to be performed for FEMA. First through a CLOMR (Conditional Letter of Map Revision) then a LOMR (Letter Of Map Revision). NCDOT works very hard to make sure the replacement structure meets the requirements of the MOA. Smaller streams in the Piedmont area are often best suited for culverts to minimize raising the roadway grade. Justification for bridge to culvert:  The basin drainage area and estimated discharges are compatible with a culvert structure. And as mentioned, typically prefer culverts due to longer service life and reduced maintenance vs a bridge.  The proposed 28' wide culvert and 25' long bridge (with vertical timber abutments) have similar hydraulic type openings and properties. Hydraulic function including backwater, outlet velocities, etc will remain similar.  The inlet and outlet channels look stable. So we would anticipate the same with culvert.