Friday, January 20, 2012

Crowds on the Union St. Railroad Bridge Watch the Flood

Auspiciously, the clouds offered a rainbreak just as the floodwaters on the Willamette were cresting.

I wasn't the only person with the idea to come down to the river. The Union Street Railroad Bridge was full of people! And the way it gave citizens and visitors a connection to the river, even when in flood, was special.

The view straight down offered terrific scenes of the roiling, turbid waters. It was a little terrifying, actually. The water was so dirty and swift and powerful. The power was awesome - and maybe a little sublime.

The cinderblock bathrooms on the south side were surrounded by water and the soccer fields to the north were covered.

Water was almost all the way up the berm.

With all the conversation about the proposed Minto Bridge, here's the view across the slough, very near its likely alignment. The grass is the very top edge of the riverbank, immediately adjacent to the concrete walkway. Just a foot or two more, and Riverfront park would start to flood.

It's hard to parse out the engineer's drawings. They say the approved design is for a 100-year event with the elevation of 143.7 feet, and a deck elevation of 144.9 feet.

Unfortunately there are two measuring systems, and I could not find an easy conversion. I think 100-year event is roughly equal to the '96 flood at 35 feet, or maybe the '64 flood of 37 feet. The water level at the time of the photo was about 29 feet (not a 100 year event!). So lets say our current flood is 136 feet in elevation (144 feet - [37 feet - 29 feet]). Even floodwaters of 136 feet, 9 feet below the deck of the bridge (with inadequate clearance for boat navigation), will still render the island completely covered. There will be no path on the other side. In the distance you can just see the very top of the berm on top of which the bridge will land and which the path will follow.

Flooding is no reason not to build the path and bridge. Indeed, Union St. Railroad bridge is a huge asset here during this flood, offering safe views of the amazing water. Having another bridge will offer yet more connections for birding, recreation, and experiencing the grandeur, beauty, and terrifying power of the river.

But the flood also shows in important ways that the bridge and path will have a significant seasonality, unusable for stretches during the winter and rains. When fans of the bridge make claims about its patterns of use, it is important to be realistic about them.

1 comment:

Salem Breakfast on Bikes said...

Here's a link to the "Vertical Datum History" and a drawing.

I really can't make heads or tails of it. There are two numerical elevations at least, the 1988 NAVD and the 1929 NGDV, and two river gages, the pre-1966 and post-1966.

The NOAA gage references the 1861 flood at 47 feet, which corresponds to the pre-1966 gage. So when we say the crest was 29 feet, I believe we are reading off the pre-1966 scale. The difference between the pre-1966 scale and the 1988 NAVD is 109.5 feet. So that would give a flood crest of about 138.5 feet. That feels about right.

If you use the post-1966 gage scale, that difference is 115.1 feet, and would give a flood crest of 144.1 feet. This can't be right, though, not if the deck elevation is 144.9 feet. The deck is clearly supposed to be well above a line flush with the grassy bank edge of Riverfront park.

But I wonder if this is why the City revised its assessment and called this a "100 year event." If you mix up the scales, it would be easy to get the math and assessment wrong.

I hope to get clarification on this.