Thread by @highlyanne

So these dams on the Tittabawassee River have quite a history with them & were known problems in the region. I'll thread some discussion and resources here. #Midland

All of the dams were about 100 years old and hadn't had significant upgrades for most of that time. The dams were owned by Boyce Hydro, but a local Four Lakes Task Force was in the process of purchasing them to make repairs. ourmidland.com/news/article/D…

"...the concrete surfaces of the spillways have deteriorated, and one of first things the FLTF plans to do after the closing of the purchase agreement is to repair and replace the concrete on the spillways..." ourmidland.com/news/article/D…

Edenville Dam was a particular problem. FERC actually revoked its hydropower generation license in 2018 for history of failure to make repairs and upgrades. Biggest problem was spillway capacity. bridgemi.com/michigan-envir…

“Current spillway capacity can only pass about 50 percent of the PMF (possible maximum flood) of the Edenville Dam,” federal regulators wrote in a 2018 report. bridgemi.com/michigan-envir…

So we've got degraded spillway concrete and at least one below-standard spillway. We've seen over and over again how spillways can be the weak link on dams during extraordinary floods. Think Oroville in California a few years ago: en.wikipedia.org/wiki/Oroville_…

Or Johnstown, PA in 1889, where removal of an emergency spillway and restriction of flow over the main spillway were likely primary causes of a huge tragedy en.wikipedia.org/wiki/Johnstown…

[I kept thinking about Johnstown last night and being so so so glad that Michigan does not have the same topography as western PA.]

Here's a map of the four dams on the Tittabawassee River. The only one I haven't seen reported as failing is the upstream-most Secord Dam. four-lakes-taskforce-mi.com/special-assess…

The second dam, Smallwood Dam, I've seen scattered reports as having failed, but having a hard time finding news coverage. But here's some bad look drone footage from yesterday:

Water coming through the powerhouse and over a spillway on the side of the dam. But erosion at the toe of the dam next to the gates is very concerning and is a possible mode of dam failure in a situation like this. If you undercut the toe, the dam can collapse downward.

Have confirmation that Smallwood failure was announced on emergency alert last night, just can't find it in print for sure. But here's what Smallwood Dam looked like before yesterday.

Dams are more than just the concrete gates. They are mostly earthen embankments. The image of Smallwood Dam illustrates that nicely.

The emergency spillway is part of the dam with a lower elevation lip. Careful grading and reinforcement are supposed to keep it from eroding

Water isn't designed to ever go over the top of the main dam, just through the gates and if needed over the spillway.

Moving downstream we get to the Edenville Dam, which is apparently the largest and definitely the most problematic of the lot. That's the one with helicopter footage that caught attention last night.

If you watch the footage, you can clearly see water flowing over the dam and not just passing through the gates.

This dam is a lot bigger and more complicated than Smallwood. It's got two sets of gates and two outlets. And it's not easy to ID the emergency spillway in the Google Earth image.

Here's the annotated image of Edenville Dam, pre-failure. I'm still not 100% confident on the emergency spillway location and would welcome corrections.

But one thing is clear from looking at Smallwood and Edenville up close, these aren't concrete spillways. They are earthen. Which isn't atypical - and can be OK, but it means the reported of degraded spillway concrete upthread is about concrete under the gates. Not in spillways.

Also, I think I've seen that the road (30) that crosses the reservoir and dam collapsed. Folks along the reservoir upstream are safe, but collapsed roads complicate everything in emergencies & the aftermath.

Finally, we move downstream to Sanford Dam. Here the dam is smaller again and the spillway is easier to ID.

One thing striking in the 2018 Google Earth images of Edenville and Sanford is that there was clearly work actively going on to repair maintain the dams and spillways. You can see recent land clearing and heavy equipment in the images.

Guessing that this is the reaction of Boyce Hydro to loosing Edenville's FERC license and trying to sell the dams to the local task force.

Also, note that areas immediately downstream of the dams are forested (or at Sanford parkland), which is really really good in terms of minimizing the immediate loss of life in the event of dam failure. I always feel sick when I see communities just downstream of dams.

I've got to take a break for a while. Meetings!

Keep Sanford and Midland & further downstream in your thoughts and keep sharing info with me. I'll RT when I can. Still looking for definitive print proclamations of Sanford and Smallwood failures.

And I'd really love someone to point me to the plans for the Edenville dam and spillway. I'll find it on my own if I need to, but meetings...