20170219 Ver 1_Bridge to culvert Justification 400431_20170221

Guilford County Br# 400431
SR 2719
High Rock Road
Over Reedy Fork Tributary 1
The project consists of replacing 31 foot long Structurally Deficient Bridge that has a Sufficiency Rating
of 23 in Guilford County. The current structure was constructed primarily with timbers in 1963. Timber
structures typically have a life span of 50 years and this structure is 54 years old and has reached the
end of its life cycle.
The proposed structure consists of a precast double barrel box culvert; approximately 25 feet wide by 45
feet long by 10 feet high. 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 box culvert is the preferred structure of choice. A box culvert
grants a lower overall cost and greatly reduces the construction time while still maintaining the integrity
of the stream bed. Also, the 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. As
mentioned above, typically prefer culverts due to longer service life and reduced maintenance
vs a bridge.
The proposed double barrel 25 foot wide culvert and 31 foot 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.