Shy on time this week. Here's an update on a research question I'm working on.
"Estuaries are low-pass filters," said Ana Vila-Concejo at a talk I attended up at Bodega Marine Lab in June. An estuary is the region where a river empties into the ocean, characterized by brackish water and complex interactions between the saltwater- and freshwater- side dynamics. What she was talking about is how the mouth of an estuary filters signals coming from the ocean.
If you stick a pressure sensor underwater near an estuary mouth, the timeseries will tell you a lot. You can see the water levels go up and down on long timelines (i.e. 14 days for spring-neap tidal cycles—synched to lunar phase—and 6.2 hour fluctuations for tides themselves) as well as short timelines (infragravity waves, swell, boat wakes—from a few minutes to a few seconds).
Water's ability to carry sediment is positively related to the amount of energy in the flow; for this reason, when flowing water slows down, it tends to dump sediment. This is why surfers love river mouths: river water suddenly decelerates when it hits the ocean, and the river drops its sediment. This sediment gets shaped by waves into sandbars, which then produce surfable waves. The sandbar's shallowness makes waves break, which effectively destroys the energy in the wave. It is only the longer-period fluctuations that do not break, and thus "get past" the sandbar, snaking further up the estuary. Hence: an estuary is a low-pass filter.
But it is exactly the longer-period fluctuations (tides, specifically) that modulate how "deep" underwater a certain sandbar is, a control on the stringency of its filtering. At high tide, with the sandbar further underwater, it removes less high-frequency signal; at low tide, with the sandbar closer to the surface of the water, only the tides may get through.
I'm looking at a set of pressure sensor timeseries collected in Tomales Bay—a lovely, straight embayment that follows the San Andreas Fault line—to see how the mouth of the bay filters different frequency bands, across different tidal levels, at a few locations in the Bay. Exactly how the filtering happens is dependent on the unique bathymetry around each sensor. I'll be looking for patterns.
Above is what installing a sensor looks like. On a lucky, gorgeous day! I am seriously interested in estuaries, please ask me about them.
Tidally varying,
Lukas
"Estuaries are low-pass filters," said Ana Vila-Concejo at a talk I attended up at Bodega Marine Lab in June. An estuary is the region where a river empties into the ocean, characterized by brackish water and complex interactions between the saltwater- and freshwater- side dynamics. What she was talking about is how the mouth of an estuary filters signals coming from the ocean.
If you stick a pressure sensor underwater near an estuary mouth, the timeseries will tell you a lot. You can see the water levels go up and down on long timelines (i.e. 14 days for spring-neap tidal cycles—synched to lunar phase—and 6.2 hour fluctuations for tides themselves) as well as short timelines (infragravity waves, swell, boat wakes—from a few minutes to a few seconds).
Water's ability to carry sediment is positively related to the amount of energy in the flow; for this reason, when flowing water slows down, it tends to dump sediment. This is why surfers love river mouths: river water suddenly decelerates when it hits the ocean, and the river drops its sediment. This sediment gets shaped by waves into sandbars, which then produce surfable waves. The sandbar's shallowness makes waves break, which effectively destroys the energy in the wave. It is only the longer-period fluctuations that do not break, and thus "get past" the sandbar, snaking further up the estuary. Hence: an estuary is a low-pass filter.
But it is exactly the longer-period fluctuations (tides, specifically) that modulate how "deep" underwater a certain sandbar is, a control on the stringency of its filtering. At high tide, with the sandbar further underwater, it removes less high-frequency signal; at low tide, with the sandbar closer to the surface of the water, only the tides may get through.
I'm looking at a set of pressure sensor timeseries collected in Tomales Bay—a lovely, straight embayment that follows the San Andreas Fault line—to see how the mouth of the bay filters different frequency bands, across different tidal levels, at a few locations in the Bay. Exactly how the filtering happens is dependent on the unique bathymetry around each sensor. I'll be looking for patterns.
Above is what installing a sensor looks like. On a lucky, gorgeous day! I am seriously interested in estuaries, please ask me about them.
Tidally varying,
Lukas