Monday, May 23, 2016

Cape May at Utsch's Marine - We look at the bridges

Here's the bridge of interest, the east bridge on the Cape May canal
There was a small craft advisory out today for the trip up to Atlantic City. We just prefer not to be out in such conditions so we stayed over at the marina. Furthermore, we plan on staying over on Tuesday to let the Atlantic calm down some from all the winds. We like calm conditions for traveling on water.

Taken at high tide, about 53.5 ft, nowhere near 55 ft
I've received a number of questions about the two Cape May Canal bridges. They are listed at 55 ft of clearance at high tide on the nautical charts but they are not that high and could be much lower if the water level is running higher than normal. At the right and below are two photos, one at high tide and one at low tide on 5/23/2016. According to the height board on the bridge, you do not have 55 ft of clearance at high tide, in fact today there was a little less than 54 ft. Since every foot counts, how do you figure the clearance before entering the canal? There are two sources of water height variability, one from the tides of course and one from water level events such as a strong east wind that pushes water into the bay or heavy rains up river that raises the water level when the flow reaches Cape May.

Taken at low tide, about 57 ft plus a couple of inches
The tide part of the equation is easy, just look at tide tables. The second part is easily accessed although it's not generally known. NOAA maintains a water level monitor at the ferry terminal on the west end of the canal. So you combine the two sources of data for one reading giving the total water level including the tide and whatever influences come from wind or heavy rains.

For example, on 5/23/2016, high tide was at 9:47   EDT at 4.0 ft but the water level chart showed an additional 0.8 ft of water level due to all the heavy rains recently for a total of  4.8 ft above MLLW. At low tide a similar story is told. The low tide was at 3:24 pm at 0.4 ft but the water level was 0.7 above the predicted for a total of 1.1 ft above MLLW.. The missing number in all of this is the clearance under the bridge at low tide which I know from the experience of a dozen passages to be 58 ft at 0.0 MLLW when there's no adder due to wind or rain. You can also back into that 58 ft number by looking at the height boards and knowing the tide and water level per the NOAA station. Note in the above numbers I'm using the ferry terminal water levels for the adder to the tide but I'm using the Cape May harbor tide station for the predicted tides since it's much closer to the two bridges.

So how to figure? Take the tide table number, add in the water level above the predicted tide as shown on the NOAA site referenced above and subtract the the total from 58 ft, that will be the clearance under the  bridge. As an example, on 5/23/2016 the low tide was 0.4 ft at Cape May harbor and the water level delta per the NOAA station was +0.7 so the total above MLLW was 1.1 ft. Take the 58 ft number and subtract 1.1 ft for a total clearance of 56.9 ft. If your mast is less than 56.9 ft then you'll clear the bridge. During severe weather events (hurricanes, tropical depressions, etc.) the water level delta can be 2 to 3 ft! The NOAA water level station will tell you how much the delta is before you have to decide to go through.

Over the next few days I'll submit an article to in more detail since it seems to be a subject of interest to many. We'll still be in Cape May on Tuesday, perhaps I can get the article done then. The weather still looks good for Wednesday getting to Atlantic City and for Thursday in going on to Atlantic Highlands. There's a new tropical depression brewing in the Bahamas that may develop into a lot of wind and rain headed for the east coast. We hope to beat that weather system north, we hope, we hope.