20161183 Ver 1_670018 Bridge to Culvert Justifaction 9-8-2016_20161129

Orange County Br# 670018
SR1421, Lib Rd
Over UT to Back Creek
The project consists of replacing 18-foot long Structurally Deficient Bridge that has a Sufficiency Rating
of 40.96 in Orange County. The current structure was constructed primarily with timbers in 1956.
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 cast in place 16 feet by 8 feet high culvert. 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 16' wide culvert and 18' long bridge (with vertical timber/concrete 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.