On May 18th, 2020, heavy rainfall in
Michigan raised the level of Wixom Lake - a man-made reservoir impounded
by Edenville Dam - higher than it had ever gone before. As the reservoir
continued to rise the following day, the dam suddenly broke, sending a wall of
water downstream. As it traveled along the Tittabawassee River, the flood wave reached and
quickly overpowered the Sanford Dam downstream. The catastrophic failure of the two dams impacted
more than 2,500 structures and caused more than 200-million-dollars in damage. The independent
forensic team charged with investigating the event released an interim report on the failures
in September 2021. The conclusions of the report include a discussion of a relatively
rare phenomenon in earthen dams. Let’s walk through the investigation to try and understand
what happened. I’m Grady, and this is Practical Engineering. In today’s episode, we’re talking
about the failures of Edenville and Sanford Dams. Edenville and Sanford Dams were two of four
dams owned by Boyce Hydro Power along the Tittabawassee River in Michigan. The dams were
built in the 1920s to generate hydroelectricity. Edenville Dam was constructed just upstream
of the confluence with the Tobacco River. It was an earthfill embankment dam
with two spillways and a powerhouse. The water impounded by the dam formed a reservoir
called Wixom Lake, nearly the entire perimeter of which was surrounded by waterfront homes. State
highway 30 bisected the dam along a causeway, splitting the lake between the two rivers with
a small bridge to allow water to flow between the two sections of the reservoir. Sanford Dam
downstream was a similar structure as Edenville, but not nearly as long. It consisted of an
earthen embankment, a gated spillway, an emergency spillway, and a powerhouse for the turbines,
generators, and other hydroelectric equipment. Edenville Dam, in particular, had a long
history of acrimony and disputes between the dam owner and regulatory agencies. Most dams that
generate hydroelectricity in the US are subject to oversight by the Federal Energy Regulatory
Commission (or FERC). But, Edenville Dam had its license to generate hydropower revoked in 2018
when the owner failed to comply with FERC’s safety regulations. Their report listed seven concerns,
the most significant of which was that the dam didn’t have enough spillway capacity. As a result,
if a severe storm were to come, the dam wouldn’t be able to release enough water to prevent the
reservoir level from climbing above the top of the structure, overtopping it and likely causing
it to fail. After losing the license to generate hydropower, jurisdiction over the dam fell to the
State of Michigan, where disagreements about its structural condition, spillway capacity,
and water levels in Wixom Lake continued. The days before the failure had already been
somewhat rainy, with small storms moving through the area. But heavy rain was in the forecast for
May 18th. The deluge arrived early that morning, and it didn’t take long for the water levels
in Wixom Lake to begin to rise. By 7 AM, operators at the dam had started opening gates on
both spillways to release some of the floodwaters downstream. Gate operations continued throughout
the day as the reservoir continued rising. At 3:30 PM, all six gates (three at each
spillway) were fully opened. From then on, there was nothing more operators could
do to get the floodwater out faster, and the level in Wixom Lake continued to creep
upwards. That night, the lake reached the highest level in its history, only about 4 feet or
1.3 meters below the top of the earthen dam. At daybreak on May 19th, it was already clear
that Edenville Dam was struggling from the enormous forces of the flood. Operators noticed
severe erosion from the quickly flowing water in the reservoir near the east spillway along
the embankment. Regulators and dam personnel met to review the damage, and a contractor was
brought in to deploy erosion control measures. And still, the water kept rising. By 5 PM, Wixom Lake had risen to within around a
foot (or 30 centimeters) from the top of the dam. As crews worked to mitigate the
erosion problems in other places, eyewitnesses noticed a new area of depression
on the far eastern end of the dam. This part of the embankment hadn’t been a significant
point of focus during the flood because it wasn’t experiencing visible erosion, but it was
apparent something serious had happened. Photos from a few hours earlier didn’t show anything
unusual, but now the top of the embankment sank down nearly to the reservoir level. Eyewitnesses
moved to the nearby electrical substation to get a better look at this part of the dam. Within
only a few moments, the embankment failed. Lynn Coleman, a Michigander and one of the
bystanders, caught the whole thing on camera. Over the next two hours, all of Wixom Lake drained
through the breach in the dam. Water rushing through the narrow gap in the causeway washed
out the highway bridge, and all of the waterfront homes and docks around the entire perimeter of the
lake were left high and dry. As the floodwaters rushed through the breach into the river, the
level downstream in Sanford Lake rose rapidly. By 7:45, the reservoir was above the dam’s crest,
quickly eroding and breaching the structure. With the combined volumes of Wixom and Sanford Lakes
surging uncontrolled down the Tittabawassee River, downstream communities including Sanford,
Midland, and Saginaw were quickly inundated. Google Earth shows aerial imagery before, during,
and after the flood, so you can really grasp the magnitude of the event. More than 10,000 people
were evacuated, and flooding damaged more than 2,500 structures. Amazingly, no major
injuries or fatalities were reported. In their interim report on the event, the
independent forensic team considered a broad range of potential explanations for what happened at
Edenville Dam. Although the spillway for the dam was undersized per state regulations, this storm
event didn’t completely overwhelm the structure. The level in Wixom Lake never actually
went higher than the top of the embankment, so overtopping (one of the most common causes
of dam failure, including the cascading loss of the downstream Sanford Dam) was eliminated as
a possible cause of failure for Edenville Dam. The team also looked at internal erosion,
a phenomenon I’ve covered on this channel before that has resulted in many significant
dam failures. Internal erosion involves water seeping through the soil and washing it away
from the inside. However, this type of erosion usually happens over a longer time period
than what was witnessed at Edenville Dam. No water seepage exiting the downstream face of
the embankment or eroding soil was evident in the time leading up to the breach, ruling this
mechanism out as the main cause of failure. The forensic team determined that the actual
cause of the failure was static liquefaction, a relatively unusual mechanism for an earthen dam. Soils are kind of weird but don’t
tell that to geotechnical engineers. Because they are composed of many tiny particles,
they can behave like solids in some cases and liquids in others. Of course, most of our
constructed environment depends on the fact that soils mainly behave like solids, providing
support to the things we build on top of them. Liquefaction happens when soil experiences
an applied stress, like an earthquake, that causes it to behave like a liquid, and it
mostly happens in cohesionless soils - those where the grains don’t stick together, such as sand.
When a body of cohesionless soil is saturated, water fills the pore spaces between
each particle. When a load is applied, the water pressure within the soil increases,
and if it can’t flow out fast enough, it forces the particles of soil away from each other. A
soil’s strength is derived entirely from the friction between the interlocking particles. So,
when those grains no longer interlock, the ground loses its strength. Some of the most severe damage
from earthquakes comes from the near-instant transformation of underlying soils from solid to
liquid. Buildings sink into their foundations, sewer lines float to the surface, and
roads crumble without underlying support. Liquefaction typically requires cyclical
loading, like during an earthquake or extreme, sudden displacements to trigger
the flow. Gradual increases in loading will only cause the water within the soil to flow
out, equalizing the pore water pressure. But, some soils can reach a point of instability and
liquefy under sustained or gradually increasing loading conditions in certain circumstances.
This phenomenon is known as static liquefaction. A good analogy is the difference between glass and
steel. Both materials have a linear stress-strain curve at first. In simple terms, the harder you
push, the harder they push back. But both reach a point of peak strength, beyond which a soil
will fail or deform. Well-compacted sand is like steel. It fails with ductile behavior. If
you stress it beyond its strength, it deforms, but the strength is still there. In other words,
if you want to keep deforming it, you have to keep applying a force at its peak strength. On the
other hand, loose sand is like glass. If you push it beyond its peak strength, it fails with brittle
behavior, suddenly losing most of its strength. The independent forensic team took samples of
the soils within the Edenville Dam embankment and subjected them to testing to see if they were
liquefiable. The tests showed brittle collapse behavior necessary for static liquefaction.
They also reviewed construction records and photographs where no compaction equipment was
seen. The team concluded that as the level of Wixom Lake rose that fateful May evening, it
increased the hydraulic load on the embankment, putting more stress on the earthen structure than
it had ever been asked to withstand. In addition, the higher levels may have introduced water
from the reservoir to permeable layers of the upper embankment (as evidenced by the
depression that formed before the failure), increasing seepage and thus increasing the pore
water pressure of saturated, uncompacted, sandy soils within the structure. Eventually, the peak
strength of the embankment soil was surpassed, and a brittle collapse resulted, liquefying
enough soil to breach a downstream section of the dam. A few seconds later, lacking
support from the rest of the structure, the dam’s upstream face collapsed, and
all of Wixom Lake began rushing through. Edenville Dam was built in the 1920s before most
of our current understanding of geotechnical engineering and modern dam safety standards
existed. Most dams are earthen embankment dams, but modern ones are built much differently
than this one was. Embankments are constructed slowly from the bottom up in individual layers
called lifts. This lets you compact and densify every layer before moving upward, rather than
just piling up heaps of loose material. We use gentle slopes on embankments to increase long-term
stability since soils are naturally unstable on steep slopes. We have strict control over the
type of soil used to construct the embankment, constantly testing to ensure
the properties match or exceed the assumptions used during design. We
often build an embankment of multiple zones. The core is made of clay soils that are highly
impermeable to seepage, while the outer shells have less stringent specifications. We include
rock riprap or other armoring on the upstream face so that waves and swift water in the reservoir
can’t erode the vulnerable embankment. And, we include drains that both relieve pressure
so it can’t build up within the soil and filter the seepage to prevent it from washing away
soil particles from inside or below the structure. Edenville Dam actually did have a primitive
internal drainage system made from clay tiles, but many of the drains in the area of the failure
appeared to be missing in a recent inspection. Although it seems like an outlier, the story of
Edenville and Sanford Dams is not an unusual one. There are a lot of small, old dams across the
United States built to generate hydropower in a time before everyone was interconnected with
power grids. Over time, the revenue that comes from hydropower generation gradually declines
as the maintenance costs for the facility and the danger the dam poses to the public both
increase. However, the reservoir created by the dam is now a fixture of the landscape, elevating
property values, creating communities and tourism, and serving as habitat for wildlife. You end up
with a mismatch of value where most of the dam’s benefits are borne by those who don’t incur any
responsibility for its upkeep or liability for the threat it poses to downstream communities. Even
owners with the best intentions find themselves utterly incapable of good stewardship. Combine all
that with the fact that the regulatory authorities are often underfunded and lack the resources to
keep a good eye on every dam under their purview, and you get a recipe for disaster. After all,
there’s only so much you can do to compel an owner to embark on a multimillion-dollar
rehabilitation project for an aging dam when they don’t have the money to do it and
won’t derive any of the benefits as a result. Since the failure, the dam owner Boyce Hydro filed
for bankruptcy protection, and the counties took control of the dams with a nonprofit coalition of
community members and experts to manage repair and restoration efforts. Of course, there’s a lot more
to this story than just the technical cause of the failure, and the final Independent Forensic Team
report will have a deeper dive into all the human factors that contributed to the failure. They
expect that report to be released later in 2021. Dams are inherently risky structures,
and it’s unfortunate that we have to keep learning that lesson the hard way. Thank you
for watching, and let me know what you think!
Grady does an excellent job on all his videos. Lots of great information in this one,
Love this, strain softening for newbies.
the end is the most key, who fixes these when they just become landscaping features of huge hazard potential?
the rich neighbors? it seems like it in this case
Fun fact, you can see my house in this video
I LOVE GRADYS VIDEOS
This seams to be the same phenomenon as the tailings dan failure in Brazil a few years ago. It slumps away just like the one in the video.