Bridge wakes????

[massnspace]

I am putting a theory out there….interested in comments.

I recently started leasing a slip at a marina on Lake Washington, Seattle. (Please see attached screenshot for a satellite view of the exact location). It is a relatively large lake that borders Seattle on the east.

I have been on the boat a lot, working on systems. I noticed right away that all the other boats in this marina had rubber line snubbers on all lines running from boat to dock…..and I found out pretty quickly why. “Monster wakes” seem to happen pretty frequently there. Quiet large.

At first I didn’t think much of it….I have been on boats at docks for many years and felt passing wakes many times. But one day a very large wake came, and it seemed to last for a very, very long time (several minutes). I tried to spy with my 14 power IS binoculars what kind of boat would do that. Scanned the lake and no boat seen.

Happened again. This time I stopped working and focused on it. Again, it was long (several minutes), and came in several distinct cycles (smaller for a half-minute, then much larger, then small, etc.). Again I looked and saw nothing. No boats anywhere.

And so it went. Large wakes for long perdiods; no other boat in sight.

My theory: The bridge is making the “wakes”.

The bridge south of the marina is a FLOATING bridge. (It carries the freeway I-90 east and westbound). I know that bridge is made up rectangle concrete modules that are held together with huge bolts. They float together, of course, but are allowed some flexibility in how each one individually moves. I think what it happening is the bridge is moving….say when the wind shifts from north to south, pulling and pushing itself left and right as it flexes with the forces. That movement is sending out oscillating waves into the lake.

My wife thinks maybe when several large trucks are all coming over the bridge together they depress the structure, thus creating “wakes” as the water moved has to go somewhere. And we both remembered that on an episode of “Ice Road Truckers” it was explained that when those huge trucks traveled over the “solid” ice they made a sort of depression, and that set of a sort of pressure wave under the ice, that could possibly damage the ice if too large. So the trucks, by law, had to keep a certain distance from the one ahead.

Another clue is that the wakes seem to be coming from the south…the bridge. Almost all boat traffic on that part of the lake moves north/south, which would send the wakes east and west….

Anyway. Any feedback on this is welcome. I am quite certain these are not just boat wakes, mainly based on the duration of the wave trains.

David
Seattle

[CaptTom]

Interesting. Could be a few things going on. One is resonance. Even fixed bridges move. If you've ever walked over a highway bridge you can feel it. See "Galloping Gertie." I would imagine the floating bridge might have certain frequencies which resonate and amplify the motion.

Or perhaps wakes or even wind waves are reflected off the bridge pontoons. Finally, look at the bottom contours around your marina. Those can also amplify waves.

[rwidman]

Ask your marina neighbors about the wakes. They have been there longer and should know about it.

[sailorboy1]

I bet it is more a matter of what caused the wake is long gone out of sight before the wake arrives in the marina. That is how it is in the marina I am at.

[massnspace]

Originally Posted by rwidman

Ask your marina neighbors about the wakes. They have been there longer and should know about it.

I did ask. Met with blank looks and told “Its a wake”.

I think most people just go about their lives and don’t think too deeply about things. But I really don’t think it is just a “wake”.

[massnspace]

Originally Posted by sailorboy1

I bet it is more a matter of what caused the wake is long gone out of sight before the wake arrives in the marina. That is how it is in the marina I am at.

But why is it so long in duration? Several minutes, with periods of smaller and much larger action. Every boat wake I have ever met in my life was gone in less than 30 seconds.

[JBP]

I think you have no choice but to hang out in the cockpit drinking beer to observe passing river and road traffic to discover the answer to your mystery.
(Or get a trail camera, but I think beer is a better idea.)

[sailorboy1]

Originally Posted by massnspace

But why is it so long in duration? Several minutes, with periods of smaller and much larger action. Every boat wake I have ever met in my life was gone in less than 30 seconds.

because it traveled a long way, the wake will last till its energy is given off to something else and sometimes a wake goes 1 direction and hits a bank and turns and goes back to join it's wake already going that way

but here is the thing, knowing will not change it. You are in a wake prone marina and the only way to fix is to move (I feel you)

[thinwater]

Is that a floating bridge?


https://en.wikipedia.org/wiki/Lacey_...emorial_Bridge


If so, interesting theory.

[massnspace]

Originally Posted by thinwater

Is that a floating bridge?


https://en.wikipedia.org/wiki/Lacey_...emorial_Bridge


If so, interesting theory.

Yes, floating bridge…

[thinwater]

Originally Posted by massnspace

Yes, floating bridge…

As others have done, place the snubbers on the dock side, away from the bridge. The goal is to keep the slack out of the lines on the wake side.

[Cheechako]

I have sailed on Lake WA and there is a "bridge effect" near the bridges for sure. Not to mention on a windy day it can be super rough on the windy side, near the bridge where the water is thrown back against the long fetch wind waves and flat calm on the lee side. Also throwing water into the traffic lanes. I imagine this is what is going on.