Curious Waves

We spent some time motoring and also a little time with the sails up in light winds in the SF Bay today.

As we approached Raccoon Straight, two and a half hours after high tide, it was evident based on our progress and confirmed by instrumentation, that we were passing through reasonable ebb tide currents at 1.5-2 knots. As we entered the strait, the outflow of the Sacramento River, the difference between boat speed snd speed over ground remained about 1.5-2 knots. No real surprises there but I will say I expected a slight increase in current. Perhaps we had already experienced the additive effects by the time I began to take note. The wind was light at 1-2 knots on our bow.

Curiously, once we were about one-third of the way through the strait, we encountered standing waves that I’ve seen before with strong ebb currents but, mostly, when the wind was blowing in the direction opposite the flow of the the water.

In this case, the river current and the ebb tide current were additive and there was no wind blowing against but instead just a knot or two in the same direction. The depth was about 90 feet. The strait was not particularly narrow at this point.

Check out this video taken just as we had passed through most of the disturbance in the waters surface.

Raccoon Strait. Almost three hours past high tide during an ebb current just under 2 knots with wind direction of current at 1-2 knots.

I wanted the video capture as I had planned to do some research to better understand this phenomenon. I know similar things can happen with the Gulf Stream and other ocean currents. Again, I know this can be magnified by an opposing wind. I also know that an uneven or shallow bottom can result in similar occurrence. I have seen tidal bores with a wave when a tide opposes a river current. But, I couldn’t understand this observation. I’ve passed through the strait numerous times in different tidal flows and have seen minor disturbances in the surface of the water that can be understood but this one was beyond me.

I spent some time doing on-line research to better understand and to be able to predict this phenomenon in the future. As it turns out, this topic has been greatly studied. It’s rather complex and not fully understood, at least as far as I can tell, and seems different in different geographic locations. Clearly, the disturbances are probably related to the additive effects of the ebb tide current and the river current and not their opposition….. that would have made more sense to me. At any rate, let me hear your thoughts.

Here are a few interesting links.

https://journals.lib.unb.ca/index.php/ihr/article/download/23504/27277/0

https://eos.org/research-spotlights/when-rivers-and-tides-collide

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