Sure It's Safe...Until It Isn't. Don't be a Scour Critical Bridge

 

Source: https://hickmanbridge.com/#

It's a bridge. Just like tens of thousands of other bridges across the country. It was pretty modern when I moved to my little town in the Great Valley of California. It crosses the Tuolumne River about 12 miles upstream of Modesto, and it's a fairly important transportation corridor that carries a lot of traffic including heavily loaded gravel trucks from the quarries upstream. And it's home to dozens of White-throated Swifts, European Starlings, and apparently 6,000 bats!

But I've been here for 30 years, which means the bridge is more than half a century old, and...it has a problem. A serious problem, a major vulnerability. In fairness in case you are worried, there's no disaster coming later in the post. The bridge didn't collapse or anything. But time (and a couple of major floods) had revealed an important structural deficiency. It is a "scour critical" bridge. You really don't want a bridge that is scour critical. It is the underlying cause in more than half the bridge failures in the United States.

The picture above was apparently taken in 2017 when the river last flooded (I kind of miss the days of too much water compared to what we have now). Notice how the columns are all under water. There are some pretty powerful currents coursing around the bridge supports.

Now, take a look at the bridge in low-water conditions (above and below). The columns supporting the bridge are connected to concrete pads. The foundation extends 16 feet into the ground, which consists mainly of loose river sediment (although there may be some fairly solid sandstone too, but I don't have access to the geological reports from the engineers for this project).

Major floods in 1997 and in 2017 apparently scoured out much of the sediment surrounding the columns, and threatened to destabilize the entire bridge (I recall the bridge being closed for many days during both floods).

The problem became pronounced enough that an attempt was made to reinforce the foundation with concrete Tetris blocks (below). 

As I became aware of the problem, the old bridge began to look more and more delicate and fragile. I had been driving over the bridge for three decades without giving a second thought to the safety of the span, and did not learn the details of the threat until signs suddenly appeared on the river trail announcing the closure of a section of the trail for the next three years (in 2019), and a bunch of beautiful old oak trees were unceremoniously chopped down. They were beginning construction of a new bridge.

The construction crews quickly built a temporary trestle just above river level (betting against another 100-year flood during the construction period). It was strong enough to hold up to three gigantic cranes at once. The new bridge is going to be wider with bike lanes and a pedestrian walkway (the old bridge was rather terrifying to walk or bike across). But most important, the bridge will be supported by pairs of columns that will extend more than 100 feet into the riverbed and into the bedrock below. Scour will no longer be a serious threat.

The new bridge was constructed rather quickly all things considered, and is opening for business just three days from now. The old bridge was shut down permanently about 10 days ago. The photo below is a webcam image from November 17.

Source: https://app.oxblue.com/open/stancounty/hickmanbridge

The bridge is going to always have an odd alignment, as it would have been economically disruptive to remove the old bridge first and then build the new one over a three year period. I am now extremely curious how they will proceed with the demolition of the old bridge. There is nothing in the promotional materials that indicates how they will do it. It seems unlikely that they'll blow it up, considering the proximity of the new bridge to the old one. Their timeline suggests six months or so to accomplish the task. Geotripper will be there to report if anything interesting goes on!

When I photographed the bridges side by side the other day, I was struck by the degree to which the old bridge sags. I had never really thought about it before, wondering if that was originally part of the plan, or did it 'sink' a bit over the years. It bothers me a little not knowing the answer!

There was a huge political scuffle over the infrastructure bill that was recently passed into law over the objections of one major political party (they even want to punish the 13 members of their party who voted for it),. There are tens of thousands of structurally deficient bridges all across our nation that are 'safe until they aren't safe'. This particular bridge didn't collapse as it could have, but there are many, many others that are more dangerous than we collectively think. We drive over them every day, never knowing about the cracks in the concrete, and the rusting superstructure that holds the bridges together. This new investment in our country's infrastructure should have been implemented years ago, but I'm glad we got it through when we did. We deserve better from those who would 'serve' over us.

Liveblogging the Deluge: Reflections on the Events at Oroville Dam. What Will it Take to Change Things?

This picture is on the San Joaquin River, not the Feather. It's a metaphor for floods all around.

Let me be absolutely clear: this is a serious situation. Nearly two hundred thousand people have been evacuated as officials try to deal with two crumbling spillways at Oroville Dam as high river flows have filled the reservoir past capacity. The situation has briefly stabilized as the lake has been lowered to a level below the auxiliary ("emergency") spillway so temporary repairs can be started. But again, this is serious. If you live in the evacuation zone, follow the orders and get out.

I have a story that will seem pointless at first, but bear with me. I have a fence in the backyard. It was a nice fence at one time. But the years took their toll. Heavy storms would knock some of the slats loose, but I would nail them back in. Dry rot attacked some of the crossbeams, but chicken wire and bungee cords sort of sufficed to hold things together.

The years went by and it got worse. But there never seemed to be enough money, and there were always excuses to do other things instead. But the storms of January hit, and hit the fence hard. Whole sections just fell to pieces. The fence was totally destroyed and finally we had to act, whether we could afford it or not.

What does this small story from my life have to do with Oroville Dam?

From the Los Angeles Times

First, a recap...

Oroville Dam is the highest dam in the United States, at 770 feet, and the second largest reservoir in California with a capacity of 3.5 million acre-feet. It is a major component of the State Water Project, providing water for millions of people in central and southern California. It was completed in 1968.

California has finally ended the worst drought in the state's history with what may turn out to be the wettest year on record (there are still several months left in the season). A series of atmospheric river storms have dumped incredible amounts of rain and snow across the state, and rivers are swollen. Most of the state's reservoirs are nearly full, and operators are carefully draining what they can to avoid flooding now and in coming weeks as more storms roll through. But the big problems cropped up at Oroville Dam.

The main spillway early in the crisis. The emergency spillway is to the left.

When flows get too high, the lakes have to be drained through spillways which are supposed to be designed to accommodate the largest credible flood events. Oroville has such a spillway, but when flows ramped up earlier this week, a huge section of the concrete collapsed and water started tearing away at the adjacent hillside, threatening some of the major electrical transmission lines. It turns out that even though it was never meant to be used, a second spillway had been constructed. No effort had been made to prepare the slopes below for floodwaters, and the cement lip was never really properly reinforced.

Hard choices had to be made. The lake was filling fast, and the longer the main spillway was flowing, more and more concrete was being ripped away. The decision was made to let the lake drain over the auxiliary spillway and only yesterday morning the water started seeping over the edge. By the end of the day, the water was one or two feet deep across the entire structure, and if what I heard is accurate, the flow was around 10,000-15,000 cubic feet per second. For perspective, that's the current near-flood level flow of the Tuolumne River in my own backyard down the valley (below).

Tuolumne River flowing at 9,000 cubic feet per second

It was only a day before a big problem cropped up. A large hole had eroded from the spillway that threatened to cause its collapse. This was the event that precipitated the evacuation event, with warnings that the collapse could happen within an hour. I can't imagine what it must have been like to be living there and hearing such a warning.

What was about to happen? It wasn't a collapse of the entire dam. That's not really in the cards here. Had the spillway collapsed, the water would have starting flowing over the bedrock and loose soil beneath the concrete lip. The breach would have concentrated the flow in one spot, and even though the bedrock is solid metavolcanic rock, the smallest fissures and joints would have been exploited by the rushing water. The result would be a cut of several tens of feet, and that would have drained a significant portion of the reservoir (the top 30 feet out of the 770 feet). That would have been enough water to inundate a wide swath of the floodplain and adjacent areas downstream, and that is what prompted the evacuation order.

The operators of the dam ramped up the flow on the original spillway to around 100,000 cubic feet per second, and that was finally enough to start lowering the level of the lake. By 8 PM this evening the water dropped below the level of the auxiliary spillway, and repairs started immediately (six helicopters are dropping huge bags of boulders into the hole). The immediate crisis passed, but they are still depending on flow down a crippled spillway for the foreseeable future with a stated hope to drop the lake fifty feet beneath the emergency spillway. And we have more storms on the horizon.

What should we take from this perilous situation? My first thoughts as this event unfolded is that this dam and a great many others across the country are decades old. Concrete gets old and crumbles, steel reinforcement beams rust away deep within the concrete, and equipment suffers intense wear and tear. Congress and state legislatures are almost always generous with initial construction funds, but when the time comes for maintenance and renovation, suddenly the deficit becomes important and funds can't be found.

We have chosen to allow our population to grow beyond the capacity of our landscape to support us, and many times over we have chosen to place giant urban centers in places that make no sense (yes, I'm looking at you Las Vegas and Phoenix). We thus depend on gigantic projects bent on controlling nature to provide our water for irrigation and domestic use. We depend on them, and their failure would be catastrophic, both in lives lost, and in damaged infrastructure. And yet we refuse to provide enough funding for their upkeep.

I don't know what caused the failure of the original spillway at Oroville Dam, although I have some suspicions. I think I know the dynamics of what happened with the emergency auxiliary spillway. Whatever the reasons were, they were predictable to those knowledgeable in engineering design. They were in fact predicted in 2005 by environmental groups testifying during the relicensing of the dam according to a note by the California Water Research:

A single operational use or multiple operational uses (with failure to repair any preceding or cumulative damage) of the ungated spillway could result in a loss of crest control of Oroville Dam. A loss of crest control could not only cause additional damage to project lands and facilities but also cause damages and threaten lives in the protected floodplain downstream. An unarmored spillway is not in conformance with current FERC engineering regulations. (emphasis added.)

And that's where we are tonight. A dam in danger of spillway failure, as many as 180,000 people displaced suddenly from their homes, and a government unwilling to take care of infrastructure because it is just too expensive and inconvenient. This is a story that is going to be repeated over and over in coming years. The failure of the levees and the deaths of 1,800 people in New Orleans during Hurricane Katrina was a harbinger. I frankly don't think the events of this evening are going to be enough to convince legislatures to do the responsible thing. What will the tipping point be? The death of hundreds? The death of thousands?

Our country went collectively nuts after terrorists killed 3,000 people in the Twin Towers on 9/11 and we spent more than a trillion dollars on a useless war on terror. Fixing and investing in our failing dams and bridges makes so much more sense, and the benefits go far beyond just safety. It also means putting hundreds of thousands, maybe millions of people to work, and that will strengthen the economy immeasurably.

It's just like the way I put some people to work this week fixing my broken fence. They benefitted and so did I. And my yard is a much nicer place now.

UPDATE (2/13/17): A picture of the damage that was expected to impact the auxiliary spillway, from Peter Gleick on Twitter (@PeterGleick). I certainly looks like the jointed metamorphic rock I expected. This is similar to what happened at Don Pedro in 1997 when a 40 foot channel was carved in a few days.

Circled are people for scale.