Factors that Determine Tidal Height

The astronomical circumstances that create king tides are only one factor affecting tidal height in our region. Besides sea level rise, weather, topography and bathymetry are other factors that contribute to tidal height. When more than one of these factors is combined tidal height can compound creating dramatic water levels.

Weather

It’s probably not surprising that weather affects tidal height. Many of us have observed the waves rolling onto the shore as a storm approaches. If a low-pressure system – which typically brings in the cloudy, rainy conditions – moves inland, we may see higher tides than predicted. High-pressure systems can depress sea levels, so if one moves into the area we may see tides that are lower than predicted.

Local wind direction and speed also affect tidal height. If the wind is blowing from the sea to the land it is called an “onshore wind.” This can greatly increase tidal heights because it causes the water to pile up onto the shore. Onshore winds are often associated with winter storms and known as a “storm surge.” Winds can also be blown away from the shore, causing lower than predicted tide levels. This is called an “offshore wind.”

 

Topography

Washington is a geologically varied area surrounded by steep rocky coastlines, sandy shores, bays and estuaries. The variation in our shorelines means that the tides might have different intensities at different locations along our coastlines.  In narrow-mouthed basins that are connected to the ocean, tides often rise higher than in wide bays and harbors. One example of this is that tidal heights are much higher in Olympia than they are further north in Puget Sound. As the water makes its way through the Strait of Juan de Fuca and down to Olympia, it splashes at the end of the basin, increasing water levels.

 

Bathymetry

Tidal ranges vary at different points of coastline due to seafloor features. The varying depths, caused by sills and ridges on the sea floor, can restrict or enhance tidal flows. One example in Puget Sound is Hood Canal, where a sill restricts tidal circulation.