Tuesday, November 10, 2020

Little River SC, Tides and Bridges

We made it under the Snow's Cut Bridge yesterday morning with plenty of room to spare above the mast. The air clearance board at the bridge fender showed 67 feet clear, which matched my own tidal calculations to within about a tenth of a foot. Just to be sure, we let the catamaran ahead of us with a similar mast height go through first.

I now believe I have a pretty good understanding and methodology for navigating bridges on the the Intracoastal Waterway (ICW). That's a good thing, as we have dozens of bridges to pass through or under ahead of us.

The trick is to know both the tidal height at the time we pass under the bridge and the Mean High Water (MHW) level at the bridge. Almost all of the fixed bridges along the ICW have a minimum clearance above MHW of 65 feet. Intermezzo needs 64 feet to pass with a degree of comfort, just the top six inches of the VHF bending when it touches the bottom of the bridge girders with that clearance. So, if a bridge has a charted minimum vertical clearance of 65 feet and the tide is less than one foot above the MHW, we are good to go. I can lookup the MHW for any tide station at NOAA's website and my electronic charts give me the predicted tides at any time during the day. Voilá!

So, the reason we couldn't pass under the Snow's Cut Bridge on Sunday afternoon was because the high tide was almost 2 feet more than the MHW at the bridge, resulting in only 63 feet of vertical clearance, a height at which the mast would clear structurally, but rip off or damage all the appurtenances above its top.

After clearing Snow's Cut, we proceeded down the Cape Fear River towards Southport under overcast skies with occasional rain showers. At Southport we turned west into the dredged canal that we would follow for the rest of the day, interrupted only by more natural, meandering channels in the vicinity of ocean inlets. As we passed the Southport Marina where we stayed for a few days last July, we observed the aftermath of the damage caused by hurricane Isaisis, which we waited out in Martha's Vineyard in August. I'd seen photos of the destruction caused to the marina's docks and boats piled up on one another along the sea wall. Now, all the docks are gone! The marina is just a big open basin with nothing in it, only a fuel dock operating. I saw construction equipment, too, so I'm sure they are beginning the rebuilding process after removing all the damaged structures and vessels.

As we approached each of the bridges crossing our route, I would do a calculation on the fly to estimate the vertical clearance and then, when a faster boat passed us, called them on the radio and asked them to report back the height shown on the air draft boards at the bridge when they go there. That worked great, my calculations again being within a few tenths of a foot of actual. We passed under the last bridge with a clearance of 64 feet, our antenna scraping the underside of the bridge. The only thing I worried about was a passing boat's wake, which might cause us to rise and bump the mast top hard. Fortunately, the power boaters seemed aware of this and slowed down to let us pass by ourselves with flat water.

We docked at the Myrtle Beach Yacht Club just before 3 pm yesterday, a nice marina, good facilities, reasonably priced. 

The weather yesterday was very warm, a high temperature of 80 degrees F. Shorts, t-shirt and bare feet weather- Lisa is loving it. It's going to be sunny and warm today, too. Then it's rain for almost a week straight, the aftermath of yet another hurricane, Eta. So we're staying put today to do a land excursion in the nice weather, save the dreary days for moving the boat along.

The next stretch of the ICW goes through a stretch called "The Rockpile", a dangerous portion of the narrow canal where, according to the chart, "numerous rock ledges have been reported abutting the deep portion of the ICW channel. Mariners should use extreme caution to avoid grounding in this area." Commercial vessels use this channel which is too narrow between the rock ledges to allow vessels to pass. The commercial vessels reportedly announce themselves on the radio and wait in wider spots for recreational vessels to pass through before proceeding. I hope that is indeed the case.

We also have over a dozen bridges to negotiate on the next leg, so my tide calculations will be frequent and numerous. My preliminary figures indicate no problems, but I am concerned about the effect of rain on the water levels, which is not taken into account by tide stations. I believe I can get some hydrograph data from the Corps of Engineers if it rains hard enough to really worry me. I may be consulting with Rick Jorgenson, a good friend and engineering colleague who is an expert in hydrology, should the need arise!

What do non-engineers do to amuse themselves on the ICW?

Lisa watching the catamaran ahead of us clear the Snow's Cut Bridge

The Snow Cut Bridge air draft board (near lower center of photo) showing 67 feet clearance, close to my tidal calculations

The ICW west of Southport

A fishing vessel that didn't fare well near Lockwoods Folly Inlet