This Spillway Failed On Purpose

0:01 Hurricane Helene was one of the more

0:04 unusual tropical storms to hit the United States.

0:07 In late September 2024,

0:08 it made landfall on the gulf coast of Florida as a Category 4 hurricane.

0:13 We’re used to seeing storm damage on the coast from hurricanes,

0:17 but this time the worst damage was hundreds of miles inland.

0:21 As Helene tracked northward across the Appalachian Mountains,

0:24 it dropped a deluge of rainfall, swelling rivers,

0:28 destroying buildings, washing away bridges,

0:31 and ultimately causing more than 250 deaths in the US.

0:35 Places normally immune to tropical storms faced

0:38 flooding worse than anything in recorded history,

0:41 with some areas receiving more than

0:43 three feet (or 900 millimeters) of precipitation.

0:46 The worst of the rain was in a narrow

0:50 band centered roughly on Asheville, North Carolina.

0:52 Asheville’s primary source of water is

0:54 the North Fork Reservoir northeast of the city.

0:57 Built in the early 1950s,

0:59 North Fork Dam impounds a relatively pristine portion of the Swananoa River.

1:04 After some earlier major floods and six decades of service life,

1:08 the dam was starting to show its age,

1:11 so the City of Asheville embarked on a major rehabilitation project.

1:15 The project included a new auxiliary spillway to help manage

1:19 floods and make the dam safer under newer state regulations.

1:22 It was finished in October 2021, and three years later,

1:27 nearly to the day, Hurricane Helene hit the region.

1:30 When it did, a part of that brand new spillway blew out,

1:35 tumbling down the chute, unleashing a torrent of reservoir water downstream.

1:39 In other words, it worked exactly like it was designed.

1:43 I’m Grady, and this is Practical Engineering.

1:55 Nearly every dam has a spillway for a pretty simple reason:

1:59 every once in a while, a big storm comes along.

2:02 In most cases, it doesn’t make sense to build

2:04 a dam tall enough to absorb a once-in-a-lifetime flood,

2:08 and then keep that storage volume empty until one comes.

2:12 It’s not a good use of resources.

2:15 Even dams designed explicitly for flood control

2:17 that intentionally keep some or all of the reservoir

2:21 empty in anticipation of heavy rain usually aren’t

2:24 intended to store the largest of floods entirely.

2:27 Instead, we use spillways to discharge that water in a safe and controlled

2:32 way so that it doesn’t overtop the dam or cause damage to the structure.

2:36 I’ve done a bunch of videos about spillways

2:38 if you want to learn more after this.

2:40 One of the most fundamental decisions when it comes

2:43 to designing a spillway is whether to include gates.

2:46 The vast majority of dams around the world use uncontrolled spillways,

2:50 meaning there’s no way to make adjustments in real time.

2:53 Usually, some kind of weir sets the elevation

2:56 where the spillway engages and water naturally flows through.

3:00 Depending on the configuration of the dam and the type of spillway,

3:03 this might be the normal water level where the reservoir sits when it’s full.

3:08 Other dams have auxiliary spillways that don’t engage until a higher level.

3:12 In either case, once the water reaches the crest of the weir, it flows over.

3:16 A chute controls and directs the flow down,

3:19 and often a special pool or structure called

3:22 a stilling basin helps dissipate the energy in the water,

3:25 making it less erosive as it transitions into a natural channel downstream.

3:30 Most spillways are designed according to a simulated

3:33 extreme storm called the design flood.

3:35 In many cases, it’s the Probable Maximum Flood,

3:39 essentially the most extreme inflow that we think is meteorologically possible.

3:43 The flow through a spillway is proportional both

3:46 to its width and the height of the water, called the “head” by engineers.

3:50 So there are some tradeoffs here.

3:52 For a given design flood,

3:54 a smaller spillway means the reservoir is going to rise

3:57 higher as the water builds up waiting to get out.

4:00 This difference between the reservoir’s normal operating level and the maximum

4:04 level during the design storm is called the flood surcharge storage.

4:08 So, on top of the height you need to store the normal water in the reservoir,

4:13 you also need extra height up to the top of the surcharge storage,

4:17 plus usually some additional margin for waves.

4:19 If you widen the spillway, you can get more water out quickly,

4:24 decreasing the height of the surcharge pool

4:26 and reducing the need for a taller dam.

4:29 Smaller spillway, taller dam.

4:30 Wider spillway, smaller dam.

4:32 Both have costs, so it’s an engineering balancing act.

4:36 But with an uncontrolled spillway, there’s no human intervention needed at all.

4:41 The spillway discharges water when the reservoir

4:44 reaches a certain elevation, and that’s it.

4:46 There’s a fixed relationship between the reservoir level and the discharge rate,

4:51 called the spillway’s rating curve.

4:53 But sometimes you need more flexibility than that.

4:56 Adding gates doesn’t increase the width of a spillway,

4:58 but it can change that second part of the equation: the head.

5:02 And it really only makes sense for reservoirs

5:04 designed to hold a permanent pool of water,

5:07 usually for irrigation or water supply.

5:10 Obviously, with an uncontrolled spillway,

5:12 you have to choose a crest height above the level

5:15 of that permanent pool or you would just lose all your water.

5:18 You can only use the height above

5:21 the crest to drive that water through the spillway.

5:24 Not true if you have gates.

5:25 Opening a gate instantly gets you a lot more head above the spillway crest,

5:30 providing greater flow.

5:31 That means, for a given design storm, a gated structure can be a lot narrower.

5:36 You don’t need to rely on width to get the water out.

5:39 Of course, the gates are an added expense,

5:41 but there are situations where that cost is

5:44 offset by the reduced width of the spillway.

5:46 Plus you have a lot more flexibility.

5:49 Discharge is no longer fixed to the level of the reservoir.

5:52 You can adjust releases based on season

5:55 or downstream conditions or even forecasted inflows, providing greater control.

6:00 Those gates don’t only add to a project’s overall cost;

6:03 they also add to the complexity.

6:05 You have moving parts, which means more wear and tear and more maintenance.

6:10 Gates rely on hoists or hydraulics, seals, gearboxes,

6:14 and other specialized equipment where knowledge

6:16 and replacement parts aren’t always readily available.

6:19 The other thing is: they need someone to open them when a storm comes.

6:23 There are plenty of spillways equipped with some level of automation,

6:27 but in general you want a real human brain in the decision tree.

6:31 Remember that spillways are a critical safety feature of a dam.

6:35 The whole purpose is to protect the structure

6:37 so it doesn’t breach during a flood,

6:39 the consequences of which can be catastrophic.

6:41 On the other side of that coin,

6:44 opening floodgates can be dangerous to people and property downstream.

6:48 Dams with gated spillways usually have elaborate

6:50 systems to warn people when making releases, including lights, sirens,

6:55 and sometimes even emergency alerts sent to cell phones.

6:58 A gate opening when it shouldn’t can be almost

7:01 as bad as one not opening when it should have.

7:04 There are risks on both sides.

7:06 That means nearly all gated spillways require someone to be

7:10 on call 24/7/365 to make sure operations go to plan.

7:15 It means checking the weather forecasts every day,

7:19 testing gates regularly to make sure they stay operable,

7:22 and having staff available mornings, nights, weekends,

7:25 and holidays in case of a storm.

7:28 It is a major obligation, especially when you consider the decades

7:32 or centuries-long lifetimes of these structures.

7:35 There cannot be a single day when someone isn’t available to handle a flood.

7:41 For large organizations,

7:42 like federal agencies or water districts, it’s definitely doable.

7:45 Most of the largest dams in the world have gated

7:49 spillways with whole teams of staff dedicated to their operation.

7:53 But it’s still a challenge, especially for small owners like cities.

7:57 So there is another option,

7:58 kind of in between controlled and uncontrolled spillways,

8:02 and its use is growing worldwide.

8:05 Behold, a fuse plug spillway.

8:09 Let me put some water in this flume and show you how

8:12 this works… You can see water builds up on the upstream side,

8:15 but none is released yet.

8:17 As soon as the water rises above the plug, things happen pretty quickly.

8:22 The overtopping water erodes the fuse plug down,

8:25 quickly washing it away and opening up a much larger area for the water to flow.

8:31 It’s basically a floodgate made of dirt.

8:33 Obviously, in my little demo, there’s not really a reservoir,

8:36 so the water level drops pretty quickly back down.

8:39 But you can imagine if there was a larger volume of water to release,

8:43 the difference in flow rate before and after

8:45 the fuse plug washed out would be pretty dramatic.

8:48 It’s funny because this is exactly what you

8:51 don’t want to happen at an embankment dam.

8:53 Overtopping is basically a worst-case scenario precisely because of how

8:58 that erosion can cut through an embankment so quickly.

9:01 But that erosive force can be used in a beneficial way on a spillway.

9:05 It’s a little crude, but the advantages are obvious.

9:08 You don’t need a person on site to operate gates, and there are no moving parts.

9:13 Plus, maintenance for an earthen structure is

9:16 a lot simpler than for mechanical and electrical components.

9:20 Just like an electrical fuse is a small section

9:23 of wire that fails before the main wiring fails,

9:26 the fuse gate is like a mini-dam that fails before the big dam is at risk.

9:31 Of course, this takes some pretty careful engineering.

9:34 The materials you use for a fuse plug have to be

9:37 both sufficiently durable- able to consistently

9:40 hold water back for non-overtopping

9:43 reservoir levels- but also relatively erodible so that they will wash

9:47 out in a predictable and controlled way when called upon to function.

9:51 Usually, this means a zoned embankment,

9:54 where part of the structure is pre-weakened using erodible materials like sands,

9:59 silts, or fine gravel.

10:00 Many fuse plugs include a pilot channel

10:02 or notch to give the erosion a head start.

10:05 So you tune both the materials and the geometry

10:08 of the fuse plug so it performs as intended.

10:11 And these are used in quite a few dams.

10:13 One of the most famous examples is at Warragamba Dam

10:16 in Australia that provides the primary source of water for Sydney.

10:20 You can see that the service spillway in the center

10:23 of the dam still uses gates to control more frequent, lower magnitude floods.

10:27 But each bay of the auxiliary spillway is equipped

10:30 with a fuse plug of earth and rock fill.

10:33 The crests of each plug are staged so they don’t all wash away at the same time.

10:38 As the reservoir gets closer and closer to the top of the dam,

10:41 more of the bays will open up

10:44 to increase the discharge capacity of the spillway.

10:47 But these structures aren’t foolproof.

10:49 In 2003, the fuse plug spillway failed at Silver Lake Basin,

10:53 a reservoir in a remote part of Michigan’s Upper Peninsula.

10:56 No one was hurt, but the event prompted the evacuation of nearly 2000 residents.

11:01 Bridges were washed out,

11:03 and the failure inflicted millions of dollars of damage to the areas downstream.

11:08 When the fuse plug overtopped,

11:10 it eroded down as designed, but the erosion didn’t stop.

11:14 The foundation soil was just as erodible, if not more, than the fuse plug,

11:19 and water continued to cut downward until most of the lake had drained out.

11:23 It’s a good case study in why engineers only

11:26 use soil erosion as a failsafe measure in limited situations.

11:30 It’s hard to predict and hard to control.

11:33 So there’s a similar solution to this kind

11:36 of fusible spillway that avoids it altogether.

11:39 I’ve removed the fuse plug in my demo

11:42 and replaced it with something a little more elaborate.

11:46 I mounted a sliding bracket on the side of the flume now.

11:49 On one side is a float, and on the other is a little arm.

11:52 And I have a crest gate mounted to the bottom.

11:54 Let me get this set up and turn on the water.

11:57 You can see just like the fuse plug,

11:59 this holds back the water when the reservoir comes up.

12:02 And actually, this gate can allow water over the top as it gets higher.

12:06 But at a certain point, my mechanism slides up (pushed by the float),

12:10 and the arm clears the top of the gate.

12:13 When it does, the gate folds down,

12:15 quickly opening up the spillway for a lot more flow.

12:19 This has a major benefit over fuse plugs

12:21 in that it can release some water before it fully opens.

12:24 The gate basically acts like an uncontrolled spillway

12:27 until the reservoir reaches the literal tipping point.

12:30 It’s not an all-or-nothing thing like the fuse plug.

12:33 The other benefit here is control.

12:36 I can adjust the float or the arm

12:38 to change the exact point when this gate opens,

12:41 unlike an erodible structure that has some inherent uncertainty around

12:45 the amount and the duration of flow required to wash it out.

12:48 But you might be thinking: “Grady, this is a mechanical system with a sliding

12:53 bearing and moving parts.” And you’d be exactly right.

12:56 You’re not likely to find a system exactly like this installed on a dam.

13:00 It’s not even that reliable in my model,

13:03 to be honest, so I wouldn’t trust it at full scale.

13:06 I’m just using it to show the fundamental

13:08 advantages because all of the fusible concrete spillways

13:11 that I know of around the world use

13:13 a proprietary system called Fusegates developed by the company, Hydroplus.

13:17 I didn’t want to step on any of their patents by building a model in my garage,

13:23 but the way they work is pretty clever.

13:25 Fusegates are concrete structures set on top

13:28 of a platform with a chamber built into the bottom.

13:31 An inlet connects the chamber to a prescribed elevation above the gate.

13:36 When the reservoir reaches that target elevation, water flows into the chamber,

13:40 pressurizing it just enough that the gate loses stability and tips downstream.

13:45 The benefits are the same as my demo: namely,

13:49 that you can discharge water before the gate washes out,

13:53 and the precise control you get over when the gate tips.

13:56 A lot of dams around the world have been equipped with Fusegates.

14:00 In the US, a few high-profile projects include

14:03 (of course) the North Fork Dam in Asheville, Canton Dam in Oklahoma,

14:08 and Terminus Dam that holds back Lake Kaweah in California.

14:13 One important application of both fuse plugs and Fusegates

14:17 is extending the life of an existing reservoir.

14:20 I’ve talked about sedimentation in a previous video,

14:23 where a reservoir gradually loses storage as it

14:26 fills up with silt and sand transported from upstream.

14:29 There are no easy fixes, and there are plenty of cases where dams have to be

14:34 decommissioned or removed because they just don’t have enough storage anymore.

14:38 For dams that use uncontrolled spillways,

14:40 the volume typically reserved for flood surcharge above

14:44 the spillway crest is kind of an untapped resource.

14:47 So, there are projects where a fuse plug or similar-type spillway is

14:52 retrofitted onto an existing dam to gain

14:55 more storage without sacrificing spillway capacity.

14:58 In some cases, this can save millions of dollars associated

15:02 with decommissioning a dam and developing an alternative source of water.

15:06 But there are some downsides too.

15:08 When a fuse plug or tipping spillway activates,

15:11 it’s a major endeavor to put it back.

15:14 Unlike a gate that you just close after the flood is over,

15:17 replacing a fusible spillway is a construction project,

15:20 which brings along all kinds of complications, like hiring an engineer,

15:25 procuring a contractor, significant expenses, and a lot of time.

15:28 The time is important because, until it’s replaced,

15:31 you’ve lost a lot of storage in your reservoir.

15:35 The other disadvantage to these systems is also

15:38 what makes them useful in the first place: there’s no human control.

15:42 It does make them safer;

15:43 it also means that there may be little warning when they activate.

15:49 In places with a lot of development downstream, that’s a big deal,

15:53 because dramatic and sudden increases in water levels are dangerous.

15:57 That’s why these systems usually break up the fusible

16:00 structures into stages that give way at different reservoir levels,

16:04 smoothing out the changes in flow as a flood passes through.

16:08 But even then, it can still cause problems.

16:11 North Carolina came face-to-face with the issue

16:13 when Hurricane Helene hit in 2024.

16:16 A major impetus for the new auxiliary spillway

16:18 at North Fork Dam was a previous storm, Hurricane Frances.

16:22 Flows through the old spillways washed out key pipelines that carry

16:26 water from the treatment plant at the dam into Asheville.

16:29 In response, the city built a new

16:32 bypass line to provide redundancy against failures.

16:34 When Hurricane Helene hit and tipped one

16:37 of the Fusegates at the auxiliary spillway,

16:39 the surge eroded the channel downstream,

16:42 taking out not just the original transmission lines but the bypass line too.

16:47 So, somewhat ironically,

16:48 the flood left major parts of the city without water for weeks.

16:53 The water crisis in Asheville was just one

16:55 of the problems caused by Hurricane Helene along its path.

16:59 But I think it’s important to recognize the tragedies that didn’t happen too,

17:03 one of those being that North Fork Dam was never in any danger of breaching.

17:08 Despite the incredible rainfall,

17:09 and despite the fact that there was no way to control releases,

17:14 the flood passed through exactly as designed.

17:16 The fusible spillway tipped just when it was

17:19 supposed to, allowing more discharge during an extreme event,

17:23 and no one had to be there to push a button.

17:27 You may have noticed that this funny little bracket I made to automatically

17:32 release my floodgate is a little

17:34 more professional-looking than most of my demos.

17:37 That’s because I didn’t fabricate it myself.

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