Saturday, February 4, 2012

40 year or 100 year Flood? Either Way High Water Comes Often to Minto

First they called it a "40-year" event, then a "100-year." I'm pretty sure it was not a 100 year flood. Both the 1996 and 1964 floods, which have been characterised as 100 year events, were much higher.

Unfortunately, the City makes figuring flood elevations rather difficult! There are two gauge scales, and two vertical elevation scales (called a vertical datum). Without specifying a scale - perhaps experts are able to infer the proper one - a number is not very meaningful.

Figuring this out is a handful!

The details of my calculations are here.

The recent flooding raised the Willamette to about 138.5 feet, well below the 100-year mark of 143.7 feet you see here on this elevation detail of the Proposed Minto Bridge and Path.

But even the landing, at just under 132 feet, will often be under high water. The Willamette's flood stage is on this scale about 137 feet, so with water 5 feet under flood stage, the path landing on the Minto side will be submerged! The path and bridge will in many ways only be usable in fair weather.

Update, Feb 8th:

Well, the river it turns out is not in flood or very high water nearly as much as I feared. Using a table of daily mean river elevations from 1988 on, out of 8539 days of readings, only 142 days showed river elevations over 131 feet (about 21.5 on the graph), which would be the low point of the Minto path (I don't know what elevations there are already in the park, if that path system goes even lower, however). You can see maybe 15 peaks that go over this threshold in the 24 year period. So that's on average a week or maybe two, of high water each year. I was thinking it might be measured in weeks or months rather than days.

I wish the City would discuss this openly, though. I think it's reasonable for citizens to want to know how often they should expect not to be able to use a +/- $7M facility.

For more detail and discussion see this previous note.

Map from the SJ.

Update, December 16th, 2014

Here they are on one scale!

Table from "Historic Peakflows"
NOAA Northwest River Forecast Center

By this measure, the 2012 flood was distant from the 100 year events of 1996 and 1964 - and vastly smaller than the epic floods of the 19th century.

However, Minto Park experiences "high water" closures even in non-flood events. Even when the river is lower than park elevations, the swampy land is slow to drain, and standing water closes paths.

The City has still not addressed in a meaningful way the question of how often the paths and bridge will be closed seasonally.


me said...

It depends where you were. In Turner they saw more flooding than they did in '96. In Keizer they had less flooding than 96. The Southern parts of the Salem area had more rain and flooding than the Northern areas.

Salem Breakfast on Bikes said...

Thanks for the perspective. I have wanted to see more historical data on Mill Creek especially, but I suspect it has not had gauges for very long.

As regards South Salem, it will be interesting to see how matters with the school construction and other elements of the built environment shake out. It may be, especially in construction sequencing, that some unwise decisions were made: Stream restoration/mitigation perhaps should have come first.

Salem Breakfast on Bikes said...

In an article on the flooding near Gilmore Field and below South Salem High School, the City is calling it a "100-year Rain Event":

'"We were not in a floodplain," [Amelia Pennington] said. "In 1996, it didn't flood. The only reason it flooded this time was that part of the storm drain system was not maintained properly."...

City records show the grate at Gilmore Field and Yew Street SE was cleaned three times Jan. 18, the day before the worst of the flooding hit.

Sam Kidd, stormwater and wastewater collections services manager, said the maintenance on the culverts was as a good as it could have been. The stormwater system simply was overwhelmed by a unique rainstorm.

"A 100-year rain event for us is 3.95 inches of rain in a 24-hour period," Kidd said. "We ended up with 4.77."

He said the speed of the storm was different from past events — including the 1996 flood, when Kidd was a stormwater supervisor for the city — and would have overflowed the culvert in the best of conditions.'

Duck80 said...

I'm not so sure that the storm drain was overwhelmed as much as there was some sort of system failure. My fiance owns a house on Berry Street about 4 or 5 blocks north of Gilmore field and the storm drain was totally fine and hand plenty of room. It was the sewer system that was overflowing. Due to the system malfunction between the storm drain system and sewer system, the sewage backed up and had so much pressure that it broke the gaskets in our basement and flooded it with 5 feet of raw sewage. Then, City of Salem crew works refused to do any work until Risk Management came out and gave the go ahead for them to suction out the sewage (of which Risk Management never came). We had to pay for Rescue Rooter to pump out the sewage and guess where they were pumping it, pack into the storm drain since there was plenty of space. Additionally, we had them take video of the sewage coming out of the manhole in front of our property... That being the case, I find it hard to believe that the storm drain system was working properly.

Salem Breakfast on Bikes said...

Yikes! Sorry to hear about the sewage, and I hope you are able to find a satisfactory end to all.

Salem Breakfast on Bikes said...

updated with peaks on a single scale in a table from the noaa