20161083_400254_Bridge to Culvert Justifaction 10-17-2016_20161103

Guilford County Br# 400254
SR 3121
Ferguson Road
Over Creek

The project consists of replacing 17 foot long Structurally Deficient Bridge that has a Sufficiency Rating
of 26.82 in Guilford County. The current structure was constructed primarily with timbers in 1957.
Timber structures typically have a life span of 50 years and this structure is 59 years old and has reached
the end of its life cycle.
The proposed structure consists of a double barrel 10 X 7 cast in place culvert with bevels. Seals will be
constructed in each of the barrels. This structure meets the hydrological requirements of the drainage
area without significantly altering the FEMA Flood studies.
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 20' wide culvert and 17' 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.