There has been a lot of discussion in the media and elsewhere about recent flooding in the South Park neighborhood of Seattle, and others have noted high water levels on the Sound Coast, including Alki Beach. and Tacoma (see some pictures below).
The origin of the flood is described in this blog.
To ignore the bottom line, the flooding was the result of an unusual astronomical setup that produced King Tides, extreme low pressure from a strong storm, moderate rainfall, and man-made holes in South Park. which should have been handled many years ago.
Although some are making unfounded claims about climate change, global warming plays a very small role in the flood event, something i will demonstrate to you below.
Flooding in South Park, powered by SDOT Blog
Coastal flooding in Tacoma. Image courtesy of Jenni Wynkoop
king tide
The key aspect of this event was the King Tide, a very high tide due to the unusual astronomical alignment of the Earth, Moon, and Sun.
As many of you know, tides are the result of the gravitational pull of the moon and sun on the oceans and large bodies of water. When the moon is closer to the Earth (at perihelion), the tides are large. When the sun and moon are aligned (like a full moon or a new moon), the tides will be higher. And when the Earth is closest to the sun (which happens in early winter!!), the tides are higher.
This week all factors are near optimalproduces a high-margin King Tide, with NOAA predicting King Tide to reach 12.9 ft above sea level on December 26 and 27.
Below are the predicted (blue) and observed (red) water levels for December 25-27 in Seattle from NOAA. The forecasts were all great on December 25, but predicted too low the actual maximum (about 15 ft!) on Tuesday, December 27.
But why such a big error? Global warming does not suddenly appear in a few days. It relates to the low pressure associated with the upcoming storm on Tuesday.
Low pressure creates high water level
It turned out that a center of low pressure in the region caused the local rise in water levels, known as the phenomenon. inverse barometer effect (see picture).
Image courtesy of Swellnet
Sea level pressure over western Washington was VERY low on Tuesday–the lowest in 2022. Want proof? Here are the pressures at SeaTac Airport during this time.
Here’s a close look at this week’s pressure at SeaTac. Mommy! Down to 978 hPa/mb on the 27th. Guys, this is very low pressure inland west of Washington. As a result, the water level will be much higher.
So we started with an unusual astronomical setup, with high tides that we only see a few times a year. And then we get the extremely low pressure that occurred on King Tide day.
But what about rainfall?
A low-pressure system/major storm that made landfall on Tuesday brought moderate rainfall to the area, with 2-3 inches falling on the western slopes of the Cascades Dec. 26-27. 1-2 inches have decreased on the lowlands during the same period. Heavy but no exception.
This moderate rainfall flows into the Green/Duwamish River, which begins significantly BELOW NORMAL (see Green River water levels at Auburn below). Heavy rainfall resulted in water levels rising to normal on December 27 and about 2 feet above normal on December 27. (note that hollow diamond icons indicate normal levels)
So we had an unusual astronomical configuration, a storm with unusually low pressure and moderate rain accompanying the storm. The astronomical and pressure effects result in the tides being about 3 feet higher than normal in the Sound.
Sea level rise
How much sea level rise, which is only partly due to global warming, contributes to rising water levels?
Below is a chart of sea levels in Seattle over the past 120 years. You will notice that the sea level has increased by about 2.07 mm per year. That’s 4 inches for the last 50 years. That certainly doesn’t explain what happened earlier this week. A very small contribution.
An interesting point is that sea level rise has not increased rapidly over the past decades, which one might expect if global warming were the main cause. Also, sea level rise has been going up for a long time… before humanity could be the cause. In short, the contribution of global warming to sea level rise in the region is highly uncertain and certainly small at this point (it will increase later in the century).
Human causes of flooding in South Park
The South Park area has been flooded several times over the past century, which is not surprising: it is located on the historic flood plain of the Duwamish/Green River. The “river” next to South Park is a completely man-made canal built by man.
But the big problem is that during high tide, the rainwater drainage pipe for the area CANNOT GET OUT TO THE RIVER. So significant rain brings flooding.
This is a fundamental flaw in the area’s drainage engineering, and the City has known this for decades. Here is a quote from a City of Seattle website:
Today they are building a pumping station to mitigate the problem.
Poor or misinformed city dwellers about environmental challenges are counterproductive in solving the problem. Or in bringing “environmental justice” to South Park residents.
