20170185 Ver 1_Bridge to Culvert Justifaction 780283_1-17-2017_20170214

Rockingham County Br# 780283
SR 2686
Richardson Drive
Over Little Troublesome Creek
The project consists of replacing 42 foot long Structurally Deficient Bridge that has a Sufficiency Rating
of 49.5 in Rockingham County. The current structure was constructed primarily with timbers in 1939.
Timber structures typically have a life span of 50 years and this structure is 78 years old and has reached
the end of its life cycle.
The proposed structure consists of 2@ 12' X 7' reinforced concrete box culvert. This structure meets
the hydrological requirements of the drainage area without significantly altering the FEMA Flood
studies. 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. 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 24' wide culvert and 42' long bridge (with vertical 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.