There was a whole lot of dialogue within the media and elsewhere concerning the latest flooding in Seattle’s South Park neighborhood, and others have famous excessive water ranges on the Sound shoreline, together with Alki Seaside and Tacoma (see some footage under).
The origin of the flooding is described on this weblog.
To offer away the punchline, the flooding was the results of an uncommon astronomical setup producing King Tides, excessive low stress from a robust storm, average rainfall, and a human-caused vulnerability in South Park that ought to have been handled years in the past.
Though some are making unfounded claims about local weather change, world warming performed little or no position within the flooding occasion, one thing I’ll show to you under.
The South Park flooding, courtesy of the SDOT Weblog
Coastal flooding in Tacoma. Image courtesy of Jenni Wynkoop
King Tides
The important thing side of this occasion was the King Tide, very excessive high-tide ranges ensuing from an uncommon astronomical alignment of the Earth, Moon, and Solar.
As lots of , tides are the results of the gravitational attraction of the moon and solar on the oceans and huge our bodies of water. When the moon is nearer to the Earth (in perigee) tides are giant. When the solar and moon are aligned (as in full or new moon) the tides are increased. And when the Earth is closest to the solar (which happens in early winter!!) tides are increased.
This week all of those components had been close to optimum, producing a high-amplitude King Tide, with NOAA predicting King Tides reaching 12.9 ft above sea stage on December 26 and 27.
Beneath are the expected (blue) and noticed (crimson) water ranges for December 25-27 in Seattle from NOAA. The forecasts had been glorious on December twenty fifth however greatl underpredicted the precise most water stage (about 15 ft!) on Tuesday, December 27.
However why such a big error? International warming didn’t abruptly present up for a couple of days. It has to do with the low stress related to the incoming storm on Tuesday.
Low Stress Produces Excessive Water Ranges
It seems {that a} regional low-pressure middle brought about localized water stage will increase, one thing referred to as the inverse barometer impact (see determine).
Picture Courtesy of Swellnet
The ocean stage stress above western Washington was EXTRAORDINARILY low on Tuesday–the lowest of 2022. Need proof? Listed here are the pressures at SeaTac Airport for the interval.
Here’s a close-up view of the stress this week at SeaTac. Mama mia! All the way down to 978 hPa/mb on the twenty seventh. People that is very low stress for the inside of western Washington. Water ranges can be a lot increased consequently.
So we begin with an uncommon astronomical setup, with excessive tides we solely see a couple of occasions a 12 months. After which we get excessive low stress that occurs to happen on the King Tide day.
However what concerning the rainfall?
The most important low-pressure system/cyclone making landfall on Tuesday introduced reasonably heavy rain to the area, with 2-3 inches falling on the western slopes of the Cascades over December 26-27. 1-2 inches fell over the lowlands throughout the identical interval. Heavy however not distinctive.
This average rainfall drained into the Inexperienced/Duwamish River, which began out significantly BELOW NORMAL (see the Inexperienced’s river stage at Auburn under). The substantial rainfall resulted in water ranges rising to regular ranges on December twenty seventh and about 2 toes above regular on December twenty seventh. (notice that hole diamond symbols point out regular ranges)
So we had an uncommon astronomical configuration, a storm with unusually low stress, and average rain related to the storm. The astronomical and stress results resulted in a excessive tide about 3 toes above regular within the Sound.
Sea Degree Rise
How a lot would possibly sea stage rise, which is barely partly on account of a warming Earth, contribute to the excessive water?
Beneath is a plot of sea stage in Seattle over the previous 120 years. You’ll discover that sea stage has gone up about 2.07 mm per 12 months. That’s 4 inches throughout the previous 50 years. That definitely doesn’t clarify what occurred earlier this week. A really small contribution.
An attention-grabbing level is that sea stage rise will not be accelerating throughout the previous a long time, one thing one would count on if world warming was the dominant trigger. Additionally sea stage rise has been going up for a very long time…earlier than mankind may have been the trigger. Briefly, the contribution of world warming to regional sea stage rise is extremely unsure and positively small at this cut-off date (it should improve later within the century).
The Human Explanation for the South Park Flooding
The South Park space has flooded repeatedly throughout the previous century, which isn’t stunning: it’s positioned on the historic flood plain of the Duwamish/Inexperienced River. The “river” subsequent to South Park is a completely synthetic human-constructed channel.
However the massive drawback is that in excessive tide the stormwater drainage pipe for the realm CAN NOT DRAIN INTO THE RIVER. So considerable rain brings flooding.
That is a necessary flaw within the drainage engineering of the area and the Metropolis has recognized this for many years. Here’s a quote from a Metropolis of Seattle web site:
In the present day they’re establishing a pump station to cut back the issue.
Poorly informing or misinforming the residents of town about environmental challenges is counterproductive to fixing issues. Or in bringing “environmental justice” to residents of South Park.
