This Spillway Failed On Purpose

Hurricane Helene was one of the more unusual  tropical storms to hit the United States. In   late September 2024, it made landfall on  the gulf coast of Florida as a Category 4   hurricane. We’re used to seeing storm  damage on the coast from hurricanes,   but this time the worst damage was hundreds of  miles inland. As Helene tracked northward across   the Appalachian Mountains, it dropped a deluge of  rainfall, swelling rivers, destroying buildings,   washing away bridges, and ultimately causing  more than 250 deaths in the US. Places normally   immune to tropical storms faced flooding  worse than anything in recorded history,   with some areas receiving more than three  feet (or 900 millimeters) of precipitation.    The worst of the rain was in a narrow band  centered roughly on Asheville, North Carolina. Asheville’s primary source of water is the North  Fork Reservoir northeast of the city. Built in the   early 1950s, North Fork Dam impounds a relatively  pristine portion of the Swananoa River. After some   earlier major floods and six decades of service  life, the dam was starting to show its age,   so the City of Asheville embarked on a major  rehabilitation project. The project included   a new auxiliary spillway to help manage  floods and make the dam safer under newer   state regulations. It was finished in October  2021, and three years later, nearly to the day,   Hurricane Helene hit the region. When it did,  a part of that brand new spillway blew out,   tumbling down the chute, unleashing a torrent  of reservoir water downstream. In other words,   it worked exactly like it was designed. I’m  Grady, and this is Practical Engineering. Nearly every dam has a spillway for a  pretty simple reason: every once in a while,   a big storm comes along. In most cases, it  doesn’t make sense to build a dam tall enough   to absorb a once-in-a-lifetime flood, and then  keep that storage volume empty until one comes.    It’s not a good use of resources. Even dams  designed explicitly for flood control that   intentionally keep some or all of the reservoir  empty in anticipation of heavy rain usually   aren’t intended to store the largest of floods  entirely. Instead, we use spillways to discharge   that water in a safe and controlled way so that  it doesn’t overtop the dam or cause damage to   the structure. I’ve done a bunch of videos about  spillways if you want to learn more after this. One of the most fundamental decisions when it  comes to designing a spillway is whether to   include gates. The vast majority of dams around  the world use uncontrolled spillways, meaning   there’s no way to make adjustments in real time.   Usually, some kind of weir sets the elevation   where the spillway engages and water naturally  flows through. Depending on the configuration   of the dam and the type of spillway, this might  be the normal water level where the reservoir   sits when it’s full. Other dams have auxiliary  spillways that don’t engage until a higher level.    In either case, once the water reaches the crest  of the weir, it flows over. A chute controls and   directs the flow down, and often a special pool or  structure called a stilling basin helps dissipate   the energy in the water, making it less erosive as  it transitions into a natural channel downstream. Most spillways are designed according to a  simulated extreme storm called the design flood.    In many cases, it’s the Probable Maximum  Flood, essentially the most extreme inflow   that we think is meteorologically possible.   The flow through a spillway is proportional   both to its width and the height of the water,  called the “head” by engineers. So there are   some tradeoffs here. For a given design flood,  a smaller spillway means the reservoir is going   to rise higher as the water builds up waiting to  get out. This difference between the reservoir’s   normal operating level and the maximum level  during the design storm is called the flood   surcharge storage. So, on top of the height you  need to store the normal water in the reservoir,   you also need extra height up to the top of the  surcharge storage, plus usually some additional   margin for waves. If you widen the spillway,  you can get more water out quickly, decreasing   the height of the surcharge pool and reducing  the need for a taller dam. Smaller spillway,   taller dam. Wider spillway, smaller dam.   Both have costs, so it’s an engineering   balancing act. But with an uncontrolled  spillway, there’s no human intervention   needed at all. The spillway discharges water  when the reservoir reaches a certain elevation,   and that’s it. There’s a fixed relationship  between the reservoir level and the discharge   rate, called the spillway’s rating curve. But  sometimes you need more flexibility than that. Adding gates doesn’t increase the width of a  spillway, but it can change that second part of

the equation: the head. And it really only makes  sense for reservoirs designed to hold a permanent   pool of water, usually for irrigation or water  supply. Obviously, with an uncontrolled spillway,   you have to choose a crest height above the level  of that permanent pool or you would just lose all   your water. You can only use the height above the  crest to drive that water through the spillway.    Not true if you have gates. Opening a  gate instantly gets you a lot more head   above the spillway crest, providing greater  flow. That means, for a given design storm,   a gated structure can be a lot narrower. You  don’t need to rely on width to get the water   out. Of course, the gates are an added  expense, but there are situations where   that cost is offset by the reduced width of the  spillway. Plus you have a lot more flexibility.    Discharge is no longer fixed to the level  of the reservoir. You can adjust releases   based on season or downstream conditions or even  forecasted inflows, providing greater control. Those gates don’t only add  to a project’s overall cost;   they also add to the complexity. You have  moving parts, which means more wear and   tear and more maintenance. Gates rely on  hoists or hydraulics, seals, gearboxes,   and other specialized equipment where knowledge  and replacement parts aren’t always readily   available. The other thing is: they need  someone to open them when a storm comes. There are plenty of spillways equipped with  some level of automation, but in general you   want a real human brain in the decision tree.   Remember that spillways are a critical safety   feature of a dam. The whole purpose is to  protect the structure so it doesn’t breach   during a flood, the consequences of which can  be catastrophic. On the other side of that coin,   opening floodgates can be dangerous  to people and property downstream.    Dams with gated spillways usually have elaborate  systems to warn people when making releases,   including lights, sirens, and sometimes even  emergency alerts sent to cell phones. A gate   opening when it shouldn’t can be almost as bad  as one not opening when it should have. There   are risks on both sides. That means nearly all  gated spillways require someone to be on call   24/7/365 to make sure operations go to plan. It  means checking the weather forecasts every day,   testing gates regularly to make sure they stay  operable, and having staff available mornings,   nights, weekends, and holidays in case of a  storm. It is a major obligation, especially   when you consider the decades or centuries-long  lifetimes of these structures. There cannot be   a single day when someone isn’t available  to handle a flood. For large organizations,   like federal agencies or water districts, it’s  definitely doable. Most of the largest dams   in the world have gated spillways with whole  teams of staff dedicated to their operation.    But it’s still a challenge, especially for small  owners like cities. So there is another option,   kind of in between controlled and uncontrolled  spillways, and its use is growing worldwide. Behold, a fuse plug spillway.   Let me put some water in this flume and show you  how this works… You can see water builds up on the   upstream side, but none is released yet. As soon  as the water rises above the plug, things happen   pretty quickly. The overtopping water erodes  the fuse plug down, quickly washing it away and   opening up a much larger area for the water to  flow. It’s basically a floodgate made of dirt.    Obviously, in my little demo, there’s not  really a reservoir, so the water level drops   pretty quickly back down. But you can imagine if  there was a larger volume of water to release,   the difference in flow rate before and after the  fuse plug washed out would be pretty dramatic. It’s funny because this is exactly what you  don’t want to happen at an embankment dam.    Overtopping is basically a worst-case  scenario precisely because of how that   erosion can cut through an embankment so quickly.   But that erosive force can be used in a beneficial   way on a spillway. It’s a little crude, but  the advantages are obvious. You don’t need a   person on site to operate gates, and there  are no moving parts. Plus, maintenance for   an earthen structure is a lot simpler than  for mechanical and electrical components.    Just like an electrical fuse is a small section  of wire that fails before the main wiring fails,   the fuse gate is like a mini-dam that  fails before the big dam is at risk. Of course, this takes some pretty careful  engineering. The materials you use for a fuse plug   have to be both sufficiently durable - able to  consistently hold water back for non-overtopping   reservoir levels - but also relatively erodible  so that they will wash out in a predictable and   controlled way when called upon to function.   Usually, this means a zoned embankment,   where part of the structure is pre-weakened  using erodible materials like sands, silts,   or fine gravel. Many fuse plugs include  a pilot channel or notch to give the

erosion a head start. So you tune both the  materials and the geometry of the fuse plug   so it performs as intended. And these are used in  quite a few dams. One of the most famous examples   is at Warragamba Dam in Australia that provides  the primary source of water for Sydney. You can   see that the service spillway in the center of  the dam still uses gates to control more frequent,   lower magnitude floods. But each bay of the  auxiliary spillway is equipped with a fuse plug   of earth and rock fill. The crests of each plug  are staged so they don’t all wash away at the same   time. As the reservoir gets closer and closer  to the top of the dam, more of the bays will   open up to increase the discharge capacity of the  spillway. But these structures aren’t foolproof. In 2003, the fuse plug spillway failed  at Silver Lake Basin, a reservoir in a   remote part of Michigan’s Upper Peninsula.   No one was hurt, but the event prompted the   evacuation of nearly 2000 residents. Bridges  were washed out, and the failure inflicted   millions of dollars of damage to the areas  downstream. When the fuse plug overtopped,   it eroded down as designed, but the erosion  didn’t stop. The foundation soil was just as   erodible, if not more, than the fuse plug, and  water continued to cut downward until most of   the lake had drained out. It’s a good case  study in why engineers only use soil erosion   as a failsafe measure in limited situations.   It’s hard to predict and hard to control. So   there’s a similar solution to this kind of  fusible spillway that avoids it altogether. I’ve removed the fuse plug in my demo and replaced  it with something a little more elaborate. I mounted a sliding bracket on the side of the flume  now. On one side is a float, and on the other is   a little arm. And I have a crest gate mounted to  the bottom. Let me get this set up and turn on the   water. You can see just like the fuse plug, this  holds back the water when the reservoir comes up.    And actually, this gate can allow water over the  top as it gets higher. But at a certain point,   my mechanism slides up (pushed by the float), and  the arm clears the top of the gate. When it does,   the gate folds down, quickly opening  up the spillway for a lot more flow. This has a major benefit over fuse plugs in that  it can release some water before it fully opens.    The gate basically acts like an uncontrolled  spillway until the reservoir reaches the literal   tipping point. It’s not an all-or-nothing thing  like the fuse plug. The other benefit here is   control. I can adjust the float or the arm to  change the exact point when this gate opens,   unlike an erodible structure that has some  inherent uncertainty around the amount and the   duration of flow required to wash it out. But you  might be thinking: “Grady, this is a mechanical   system with a sliding bearing and moving parts. ”  And you’d be exactly right. You’re not likely to   find a system exactly like this installed on a  dam. It’s not even that reliable in my model,   to be honest, so I wouldn’t trust it at full  scale. I’m just using it to show the fundamental   advantages because all of the fusible concrete  spillways that I know of around the world use a   proprietary system called Fusegates developed by  the company, Hydroplus. I didn’t want to step on   any of their patents by building a model in my  garage, but the way they work is pretty clever. Fusegates are concrete structures set on top of  a platform with a chamber built into the bottom.    An inlet connects the chamber to a prescribed  elevation above the gate. When the reservoir   reaches that target elevation, water flows into  the chamber, pressurizing it just enough that   the gate loses stability and tips downstream.   The benefits are the same as my demo: namely,   that you can discharge water before the gate  washes out, and the precise control you get over   when the gate tips. A lot of dams around the world  have been equipped with Fusegates. In the US,   a few high-profile projects include (of  course) the North Fork Dam in Asheville,   Canton Dam in Oklahoma, and Terminus Dam  that holds back Lake Kaweah in California. One important application of both fuse plugs and  Fusegates is extending the life of an existing   reservoir. I’ve talked about sedimentation in  a previous video, where a reservoir gradually   loses storage as it fills up with silt and  sand transported from upstream. There are no   easy fixes, and there are plenty of cases where  dams have to be decommissioned or removed because   they just don’t have enough storage anymore.   For dams that use uncontrolled spillways,   the volume typically reserved for flood surcharge  above the spillway crest is kind of an untapped   resource. So, there are projects where a fuse  plug or similar-type spillway is retrofitted   onto an existing dam to gain more storage without  sacrificing spillway capacity. In some cases,   this can save millions of dollars  associated with decommissioning

a dam and developing an alternative source  of water. But there are some downsides too. When a fuse plug or tipping spillway activates,  it’s a major endeavor to put it back. Unlike a   gate that you just close after the flood  is over, replacing a fusible spillway is   a construction project, which brings along all  kinds of complications, like hiring an engineer,   procuring a contractor, significant expenses,  and a lot of time. The time is important because,   until it’s replaced, you’ve lost a lot of storage  in your reservoir. The other disadvantage to these   systems is also what makes them useful in  the first place: there’s no human control.    It does make them safer; it also means that  there may be little warning when they activate.    In places with a lot of development downstream,  that’s a big deal, because dramatic and sudden   increases in water levels are dangerous.   That’s why these systems usually break up   the fusible structures into stages that give way  at different reservoir levels, smoothing out the   changes in flow as a flood passes through.   But even then, it can still cause problems. North Carolina came face-to-face with the  issue when Hurricane Helene hit in 2024. A   major impetus for the new auxiliary spillway  at North Fork Dam was a previous storm,   Hurricane Frances. Flows through the old spillways washed out   key pipelines that carry water from the treatment  plant at the dam into Asheville. In response,   the city built a new bypass line to provide  redundancy against failures. When Hurricane   Helene hit and tipped one of the Fusegates at the  auxiliary spillway, the surge eroded the channel   downstream, taking out not just the original  transmission lines but the bypass line too. So,   somewhat ironically, the flood left major  parts of the city without water for weeks. The water crisis in Asheville was just one  of the problems caused by Hurricane Helene   along its path. But I think it’s important to  recognize the tragedies that didn’t happen too,   one of those being that North Fork Dam was  never in any danger of breaching. Despite the   incredible rainfall, and despite the fact that  there was no way to control releases, the flood   passed through exactly as designed. The fusible  spillway tipped just when it was supposed to,   allowing more discharge during an extreme event,  and no one had to be there to push a button. You may have noticed that this funny   little bracket I made to automatically release my  floodgate is a little more professional-looking   than most of my demos. That’s because I didn’t  fabricate it myself. It came from today’s sponsor,   Send Cut Send. And, actually, I’ve been using them  a lot over the past year or so for a lot of the   demonstrations I build on the channel. I could  buy sheet material and cut all this out myself,   and I’ve done so much of that, but just  look at this. An entire idea from my head   shipped to my door. The quality’s better, the  cuts are way more precise than I would make,   and I don’t have a day's worth of  measuring, cutting, and cleaning up to do. They have a huge catalog of materials  they can cut, bend, countersink, and tap,   so the limit is practically what you can  dream up and put into CAD. Parts are made   in the USA and usually out the door in a few  days, which makes rapid prototyping a dream.    They just started offering CNC machining,  so I’m going to have to try that out soon.    I can’t recommend Send Cut Send enough. If you  have projects that use sheet goods, the link   below is going to give you a discount on your  first order. I really appreciate their support,   and I hope you give them a try. Thank you  for watching, and let me know what you think.