TIDAL STIRRING IN WILLAPA BAY, WA, USA
A visualization of results from numerical model ing

by Neil Banas and Barbara Hickey (UW Oceanography)
neil @ ocean.washington.edu

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What is this?

Willapa Bay is an estuary on the southern coast of Washington state, with powerful tides that, at least in summer, are the main force drawing new ocean water into the bay and flushing out what's there. Understanding the tidal circulation helps us understand the supply of oceanic nutrients that drives the ecosystem, the pathw ays that oyster and crab larvae take in and out of and through the bay, and the potential for the spread of invasive species like Spartina.

The interactive display above is a window into the 3D circulation model that we have been running. Click on the map to drop simulated "drifters" into the bay and see where the tidal currents takes them.

This display shows only the subtidal (slower-than-tidal) circulation. The twice-daily in-and-out sweep of the tide happens between frames of the animation and is not shown. A particle that appears not to move in this animation really is one that travels with the tide but retu rns to its starting point. Areas where particles move farther are ones where the water doesn't return on flood the same way it left on ebb.

Note that this version of the model contains nothing but a regular semidiurnal (12.4-hour) tide. The full model adds realistic tides, rivers, wind, and variations in ocean salinity, all of which cause their own circulation patterns. The simple tidal model you're looking at is a good appro ximation to mid-to-late summer conditions, when the rivers are low.

Technical details (for the dedicated)

The full model is an implementation of GETM (Generalized Estuarine Transport Model), a 3D, finite-difference, primitive-equation model developed for estuaries in which wetting and drying of intertidal zones is significant. For more info on the tidal model, see the poster we presented at the 2004 Pacific Estuarine Research Society meeting.

For more info on our observations in Willapa Bay and a description of the seasonal cycle of cur rents and water properties, see this paper that recently appeared in the Journal of Physical Oceanography.

This interactive display was built using Processing (a toolkit more scientists should know about!). We used the full model to generate trajectories for particles launched at high slack at every grid cell, using the depth-averaged current field from a run with cyclical tidal forcing and no density variations (the neglected baroclinic currents are very small). The positions of these particles after one tidal cycle form a return map: a single grid of numbers that maps th e model grid onto itself, representing the rearrangement done by one tidal cycle. This return map is applied once per frame in the animations above. A slight random jitter (~ one grid cell per tidal cycle, or 1.5 m^2 s^-1 in diffusivity units) is added to keep particles from getting stuck unrealistically at fixed points.


Credits

This modeling work is supported by Washington SeaGrant, and the observations behind it by Washington SeaGrant and NOAA, through the PNCERS program.

For more information, or to give us a report on your experience with this web page (comments and suggestions are much appreciated!), email Neil at
neil @ ocean.washington.edu.