Reminder: Bridge Boards Indicate Least Vertical Clearance and Charts Indicate Maximum Clearance
This reminder comes to us from Skipper Rohrkemper who relates a bridge incident in which the bridge tender incorrectly stated that the clearance boards were reading the vertical clearance at the center of the span. USCG Officer Rousseau’s response is quite clear: clearance boards give the least vertical clearance available, normally found at the outer edge of the span, and charts indicate the maximum vertical clearance above MHHW at the center of the span. With a closed vertical clearance of 20ft, the Wrightsville Beach Bascule Bridge crosses the AICW at Statute Mile 283, southwest of marker #125.
Approaching the Wrightsville Beach/ S.R. 74 bridge southbound on 10/30/2014, at high tide at Masonboro Inlet, the clearance signboard read 14′. The bridge tender confirmed that the signboard was measuring clearance at the center of the span and not at the lower shoulders. A December, 2013 version of Chart 11541 lists the clearance as 20′ (at center). Was the bridge tender wrong? Tide was high but not 6′ above MHHW.
Paul Rohrkemper
Larry,
I have looked into the Wrightsville Beach question. The clearance boards are most likely reading correctly. The gauges show the vertical clearance of the bridge at the lowest point across the channel 13.5 feet at MHW (outer edges of channel). The Chart shows the clearance of the bridge at the center of the spans. We normally do not mark this clearance on the chart. It is normally what the clearance is at the lowest part of the bridge across the channel 13.5 feet at MHW. Probably at some point in the past it was requested to mark the center and highest point (This point would only be in that exact spot of the bridge). Legally the bridge owner still needs to show the clearance gauges for the lowest clearance across the channel. NCDOT is going to check to make sure the boards are reading correctly. Hope this clears up this question.
Regards,
Jim Rousseau
Bridge Program Manager
United States Coast Guard District 5
Click Here To View the Cruisers’ Net’s North Carolina Bridge Directory Listing For SR 74 Bridge
Click Here To Open A Chart View Window, Zoomed To the Location of SR 74 Bridge
Comments from Cruisers (1)
Ah, but when the water level cuts through the center of the digits, is it indicating 13′ or 12’6″? Some clearance boards have large index marks at the center of the digits, leading one to interpret mid-digit water level as the even-foot clearance. Some only have tiles with numbers: if I need 64 feet (for example), do I need the water to be at the bottom of the 64 tile, or at its center? Or does it vary?
Sorry if this is a duplicate–flakey connection.