Bridge of Lions Vertical Clearance in Question (Statute Mile 778)
Here was the situation; we were approaching the Bridge of Lions in St Augustine at 7:45AM on 28 April 2010, with a tide level of +4.5′ and a listed bridge clearance at `low iron’ of 25′ above Mean High Water (MHW). According to NOAA, the MHW level for the St Augustine City Dock, which is next to the Bridge of Lions is 6.41′ (see NOAA link below). The tops of our VHF antennas were measured at 22′ above the water level. My calculations were:
Charted Bridge Clearance +25′
Plus Mean High Water +6.4′
Minus the Height of Tide -4.5′
Calculated Clearance 26.9′
Simply said, the listed bridge clearance 25′ is measured above MHW, and if the tide was at zero, the actual clearance would be 31.4′ (25′ + 6.4′); but we had a 4.5′ tide so the clearance should be 4.5′ less, or 26.9′ (25′ +6.4′ -4.5′).
Here is what we found as we approached the opening between the old temporary bridge, which is being demolished, and work barges from the north. Sue spotted a temporary tide board below the new bridge with red spray painted numbers indicating the current clearance was 22′ at low iron. We passed under the bridge with less than a foot above our antennas at the center of the bridge. I think the temporary tide board was correct, which would mean the actual clearance is closer to 20′ above MHW, and not 25′.
What am I missing? I contacted the Bridge of Lions and their contractor told
them that when I passed under the bridge the tide was extremely high and thus I
had less than the listed 25′ clearance.
MHW for the St Augustine City Dock:
http://tidesandcurrents.noaa.gov/data_menu.shtml?stn=8720576%20St.%20%20Augustine,%20FL&type=Datums
Bob and Sue Mimlitch
M/V Our Independence
Claiborne,
I should have said more about the difference between the listed bridge height on the charts and cruising guides, which is clearance at the center; versus the tide board which is clearance at low iron. A few bridges in Florida have signs that specify the number of feet of additional clearance at center.
I love Salty Southeast Cruiser’s Net. Thanks for all your efforts.
Bob Mimlitch
I have just spoken with Laurie Sanderson from the Bridge of Lions Rehabilitation Project and the bridge clearance is up for discussion at their meeting tomorrow. According to the contractor on site, the MHW clearance for the new bridge is indeed 22′ regardless of what the charts say. We will post more tomorrow after the meeting and more details are known.
Chuck
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