Bridge Clearance  - Complicated by the Tides

 

When I was going on my first boating adventure to the salt water from the WAC I was wondering how I would know exactly how much clearance Rascal would have when we pass under the Burlington Northern Railroad bridge just west of the locks.  Once you get out of the locks you are in salt water and there are tides taking the water surface up and down.

 

We start by looking up the clearance on the NOAA chart.  The clearance is reported in feet above "Mean High Water"(MHW).  The chart says 44 feet clearance.

 

We then need to figure out just how high the water is going to be at various times of the day..

 

How do we compute the tide height sometime between low and high?  We use the rule of 12ths.

 

Using the rule of 12ths. 

During first hour after high water the water drops 1/12th of the full range.

During the second hour an additional 2/12th.

During the third hour an additional 3/12th.

During the fourth hour an additional 3/12th.

During the fifth hour an additional 2/12th.

During the sixth hour an additional 1/12th.

Notice that the tide starts to drop slowly, then it picks up speed and drops faster. The maximim rate is halfway between high an low tides. Then it starts to slow down as it gets near low tide. It's the rule of 12ths!

 

These tide times are in Local Time, and all heights are in Feet referenced to Mean Lower Low Water (MLLW).

 

Saturday 5/19/07   -  06:09 High tide measuring 10.8 from MLLW.

                                   13:22 Low tide measuring -3.3from MLLW. 

The tide difference is 14.1 feet between high and low.  Tide height difference divided by 12 = 1.175 ft.

The time difference is 7 hours and 13 minutes.   Time difference divided by 6 =  72.17 minutes.

Subtracting 1 or 2 or 3 twelveths of height for each 69.5 minutes lets us construct the following chart:

 

Time      Height

06:09      10.8   =   high tide

07:21       9.6   =   high tide minus 1/12th        

08:33       7.3   =   minus additional 2/12th

09:45      3.8=   minus additional 3/12th

10:57      0.2   =   minus additional 3/12th

12:09      -2.1   =   minus additional 2/12th

13:21       -3.3   =   minus additional 1/12th   This is a very low tide.

 

So, we now know how high the tide is in Elliot Bay at various times between 06:09 and 13:22 Saturday the 19th of May. All this is computed as Mean Lower Low Water (MLLW). But darn! the bridge height was reported on the chart as Mean High Water (MHW). The bridge is 44 feet above MHW.

 

For now, let’s say there is 10.4 feet difference (in Eliott Bay) between MHW and MLLW. This number is found on the NOAA chart. Thus the bridge is 54.4 feet above MLLW.

We can add to our previous little chart and show the distance from the water to the bottom of the bridge.

 

Time      Bridge Height

06:09      54.4 - 10.8 = 43.6 clearance at high tide

07:21      54.4 - 9.6   = 44.8         

08:33      54.4 - 7.3   = 47.1 

09:45      54.4 - 3.8=   50.7

10:57      54.4 - 0.2   =  54.2

12:09      54.4 + 2.1   =   56.5

13:21      54.4 + 3.3   =  57.7

.

So for this trip on this particular saturday morning we don't need to worry about putting Rascal or Charlotte under that Railroad bridge. Then on Sunday we'll be returning mid way through the tidal exchange. Saturday night at 22:02 the high is 11.9 and that would be cutting it close. You'd want to do some calculations or better yet, give it one long and one short toot and wait for the bridgemaster to open the darn thing.

 

Keep in mind that larger sailboats have taller masts. If you are on Frenesi, Gaia, or Strider these calcs would carry a little more importance.

 

Note 1:  Wave and Swell can be up to 2 to 4 feet.  You have to add this to your mast height.

Note2: There are sometimes height boards at the base of the bridge that tell you how much clearance you have. Trust them or not.

 

Scott V.

 

Years ago when I was putting this together I asked Rebecca Linde for her thoughts:

   

"I make the students calculate from the tide data.  There are other bridges in kb skipper waters that will not have height boards, so I figure it is a skill that they will do well to have practice with.

 

The MHW data is found somewhere on the chart that the bridge is located on, usually a table with several locations. I have them select the nearest station (I think Elliot bay is the station we use for BN bridge). The vertical clearance for the bridge is given for MHW, which is about a10 foot tide at BN.  Then I have them calculate what the tide condition will be at the time of arrival at the bridge (using rule of 12's).  I also stress that they need to take into consideration any wave/wake/swell conditions that they may experience (which also applies for University bridge with only 42' clearance) considering large boat wakes that may be experience while transiting under the bridge, and to have it opened if it is going to be close.  Spackle's mast was thrust into the I‑90 bridge when a powerboat wake lifted the boat up a couple of feet.

 

Of interest 2 trips ago, we did the calculation for the bridge, which should have allowed us to transit through without opening it, but when we arrived the height boards showed less water than our calculations... so remember that the MHW station is a different location than the bridge itself, and tides do change differently from place to place... more reason to be extra careful when doing calculations, and advance towards the bridge with care.

 

I did actually count the boards that were showing to the level that said 42' on the bridge, and those calculations don't work either.  I know that Frenesi fits under the bridge when 5 boards show, but I counted10 boards to 42' mark.

 

Although I know that Charlotte is shorter than Frenesi, so therefore would fit if 5 boards were showing..... I would never, never tells a student to rely on such a calculation or to ignore a posted height on a club boat.  To address such issues, I have told students that when they have their own boats, they may check heights on bridges, as Norm Ahlquist has on Frenisi.  But in part my strong bridge opinions come from my personal experience of being aboard 2 different boats that have hit bridges, and the personal terror I have experienced since.... and I was neither driving nor in charge for either event!  I think my heart still stops when passing under 100 foot tall bridges.

 

And ---------------------

 

Yes there is a bit of variance in the datum ( difference between MLLW and MHW ).  You need to look on the chart itself to find what the "local" MHW is.  There is usually a small table in one corner or other (or stuck in the middle of a land mass), which will list MHW MHHW MLW MLLW; it will often include several stations that exist on the particular chart.

 

Once you find the table on the chart, identify the station closest to the bridge you are planning to pass under.  I believe that Elliot Bay is the nearest station for BN Bridge.

 

When the tide is at MHW it will have the vertical clearance noted on the chart next to the bridge.  As, University Bridge states 42' vertical clearance, and has not tide, it is always 42'.  The wise men who decided to set bridge clearances choose to set them on the chart using MHW.  Using MLW would potentially get a few more in trouble who failed to do the calculation for tides higher than zero.

 

So if MHW closest to BN Bridge is 10.4', then the clearance is as stated on the chart at a tide of 10.4'.  As the tide level goes up, greater than 10.4, the amount of clearance gets less.  So if the BN Bridge has 44' clearance at 10.4' tide (Now really I'm just guessing numbers so no quotes on the math), then at a 12.4 tide, there would only be 42' clearance.  Considering a moderate allowance for wave action (non-busy day at the locks) of 2' chop, I would be inclined to open the bridge, as 2' of elevation on a wave would technically put Charlotte's mast into the bridge. On a day when there is heavy boat traffic at the locks, I might be inclined to add a larger margin for wake.  I think the largest wake I have experienced in there was 4'!

 

There is certainly some grace worked into the bridge heights, but not knowing what it is, nor if there is any consistent formula, I have chosen to ignore whatever excess the bridges include in their estimations...  I am a firm believer in sticking to the safe side of hard places.

 

Same idea as the tide goes out. Anything less than the 10.4 tide would increase clearance under the bridge.

 

So, how's that for more answer than you ever really wanted ;)   Rebecca"