How Does Water Gates Work?

Mar. 07, 2024

[Note that this article is a transcript of the video embedded above.]

In the heart of Minneapolis, Minnesota on the Mississippi River is the picturesque Upper Saint Anthony Falls Lock and Dam, which originally made it possible to travel upstream on the river past the falls starting in 1937. It’s a famous structure with a fascinating history, plus it has this striking overflow spillway with a stilling basin at the toe that protects the underlying sandstone from erosion. But there’s another dam just downstream, that is a little less-well-known and a little less scenic, aptly called the Lower Saint Anthony Falls Lock and Dam. Strangely, the spillway for the lower dam is less than half the width of the one above, even though they’re on the exact same stretch of the Mississippi River, subject to the same conditions and the same floods. That’s partly because, unlike its upstream cousin, the Lower Saint Anthony Falls dam is equipped with gates, providing greater control and capacity for the flow of water through the dam. In fact, dams all over the world use gates to control the flow of water through spillways.

If you ask me, there’s almost nothing on this blue earth more fascinating than water infrastructure. Plus I’ve always wanted to get a 3D printer for the shop. So, I’ve got the acrylic flume out, I put some sparkles in the water, and I printed a few types of gates so we can see them in action, talk about the engineering behind them, and compare their pros and cons. And I even made one type of gate that’s designed to raise and lower itself with almost no added force. But this particular type of gate was made famous in 2019, so we’ll talk about that too. I’m Grady, and this is Practical Engineering. On today’s episode, we’re talking about spillway gates.

Almost all dams need a way to release excess water when the reservoir is full. If you’ve ever tried to build an impoundment across a small stream or channel, you know how powerful even a small amount of flowing water can be. Modern spillways are often the most complex part of a dam because of the high velocities of flow. If not carefully managed, that quickly flowing water can quickly tear a dam apart. The incredible damage at Oroville Dam in 2017 is a striking example of this. Although many dams use uncontrolled spillways where the water naturally flows through once the reservoir rises to reach a certain level, gated spillways provide more control over the flow, and so can allow us to build smaller, more cost-effective structures. There are countless arrangements of mechanical devices that have been used across the world and throughout history to manage the flow of water. But, modern engineering has coalesced to variations on only a few different kinds of gates. One of the simplest is the crest gate that consists of a hinged leaf on top of a spillway.

A primary benefit of the crest gate is that ice and debris flow right over the top, since there’s nothing for the flow to get caught on. Another advantage of crest gates is that they provide a lot of control over the upstream level, since they act like a weir with an adjustable top. So, you’ll often see crest gates used on dams where the upstream water level needs to be kept within a narrow range. For example, here in San Antonio we have the RiverWalk downtown. If the water gets too low, it won’t be very attractive, and if it gets too high, it will overtop the sidewalks and flood all the restaurants. So, most of the dams that manage the flow of water in the San Antonio River downtown use steel crest gates like this one. Just up the road from me, Longhorn Dam holds back Ladybird Lake (formerly Town Lake) in downtown Austin. Longhorn Dam has vertical lift gates to pass major floods, but the central gates on the dam that handle everyday flows are crest gates. Finally, the dam that holds back Town Lake in Tempe, Arizona uses a series of crest gates that are lowered during floods.

Crest gates are attached to some kind of arm that raises or lowers the leaf as needed. Most use hydraulic cylinders like the one in Tempe Town Lake Dam. The ones here in San Antonio actually use a large nut on a long threaded rod like the emergency jack that comes in some cars. You might notice I’m using an intern with a metal hook to open and close the model crest gate, but most interns aren’t actually strong enough to hold up a crest gate at a real dam. In fact, one of the most significant disadvantages of crest gates is that the operators, whether hydraulic cylinders or something else, not only have to manage the weight of the gate itself but also the hydrostatic force of the water behind the gate, which can be enormous. Let’s do a little bit of quick recreational math to illustrate what I mean:

The gates at Tempe Town Lake are 32 meters or about 106 feet long and 6.4 meters or 21 feet tall. If the upstream water level is at the top of one of these gates, that means the average water pressure on the gate is around four-and-a-half pounds for every square inch or about 31,000 newtons for every square meter. Doesn’t sound like a lot, but when you add up all those square inches and square meters of such a large gate, you get a total force of nearly one-and-a-half million pounds or 660,000 kilograms. That’s the weight of almost two fully-loaded 747s, and by the way, Tempe Town Lake has eight of these gates. The hydraulic cylinders that hold them up have to withstand those enormous forces 24/7. That’s a lot to ask of a hydraulic or electromechanical system, especially because when the operation system fails on a crest gate, gravity and hydrostatic pressure tend to push the gate open, letting all the water out and potentially creating a dangerous condition downstream. The next kind of spillway gate solves some of these problems.

Radial crest gates, also known as Tainter gates, use a curved face connected to struts that converge downstream toward a hinge called a trunnion. A hoist lifts the gate using a set of chains or cables, and water flows underneath. My model being made from plastic means it kind of stays where it’s put due to friction, but full-scale radial gates are heavy enough to close under their own weight. That’s a good thing, because, unlike most crest gates, if the hoist breaks, the gate fails closed. The hoist is also mostly just lifting the weight of the gate itself, with the trunnion bearing the hydrostatic force of the water behind held back. These features make radial gates so reliable that they’re used in the vast majority of gated spillways at large dams around the world. If you go visit a dam or see a swooping aerial shot of a majestically flowing spillway, there’s a pretty good chance that the water is flowing under a radial gate.

The trunnion that holds back all that pressure while still allowing the gate to pivot is a pretty impressive piece of engineering. I mean, it’s a big metal pin, but the anchors that hold that pin to the rest of the dam are pretty impressive. Water pressure acts perpendicular to a surface, so the hydrostatic pressure on a radial gate acts directly through this pin. That keeps the force off the hoist, providing low-friction movement. But it’s not entirely friction-free. In fact, the design of many older radial gates neglected the force of friction within the trunnion and needed retrofits later on. I mentioned the story of California’s Folsom Dam in a prior video. That one wasn’t so lucky to get a structural retrofit before disaster struck in 1995. Operators were trying to raise one of the gates to make a release through the spillway when the struts buckled, releasing a wave of water downstream. Folsom Reservoir was half empty by the time they closed the opening created by the failed gate.

How did they do it? Stoplogs, another feature you’re likely to see on most large dams across the world. Just like all mechanical devices that could cause dangerous conditions and tremendous damage during a failure, spillway gates need to be regularly inspected and maintained. That’s hard to do when they’re submerged. The inspecting part is possible, but it’s hard to paint things underwater. In fact, it’s much simpler, safer, and more cost effective to do most types of maintenance in the dry. So we put gates on our gates. Usually these are simpler structures, just beams that fit into slots upstream of the main gate. Stoplogs usually can’t be installed in flowing water and are only used as a temporary measure to dewater the main gate for inspection or maintenance. I put some stoplog slots on my model so you can see how this works. I can drop the stoplogs into the slots one by one until they reach the reservoir level. Then I crack the gate open and the space is dewatered. You can see there’s still some leakage of the stoplogs, but that’s normal and those leaks can be diverted pretty easily. The main thing is that now the upstream face of the gate is dry so it can be inspected, cleaned, repaired, or repainted.

And if you look closely, it’s not just my model stoplogs that leak, but the gates too. In fact, all spillway gates leak at least a little bit. It’s usually not a big issue, but we can’t have them leaking too much. After all, there’s not much point in having a gate if it can’t hold back water. The steel components on spillway gates don’t just ride directly against the concrete surface of the spillway. Instead, they are equipped with gigantic rubber seals that slide on a steel plate embedded in the concrete. Even these seals have a lot of engineering in them. I won’t read you the entire Hydraulic Laboratory Report No. 323 - Tests for Seals on Radial Gates or the US Army Corps of Engineers manual on the Design of Spillway Tainter Gates, but suffice it to say, we’ve tried a lot of different ways to keep gates watertight over the years and have it mostly sealed up to a science now. Most gates use a j-bulb seal that’s oriented so that the water pressure from upstream pushes the seal against the embedded plate, making the gate more watertight. Different shapes of rubber seals can be used in different locations to allow all parts to move without letting water through where it’s not wanted.

In fact, there’s one more type of spillway gate I want to share where the seals are particularly important. Beartrap gates are like crest gates in that they have a leaf hinged at the bottom, but beartrap gates use two overlapping hinged leaves, and they open and close in an entirely different way. The theory behind a beartrap gate is that you can create a pressurized chamber between the two leaves. If you introduce water from upstream into this chamber, the resulting pressure will float the bottom leaf, pushing it upward. That, in turn, raises the upper leaf. The upstream water level rises as the gate goes up, increasing the pressure within the chamber between the gates. The two leaves are usually tied in a way that once fully open, they can be locked together. To lower the gates, the conduit to the upstream water is closed, and the water in the chamber is allowed to drain downstream, relieving the upward pressure on the lower leaf so it can slowly fall back to its resting position. It sounds simple in theory, but in practice this is pretty hard to get right.

I built a model of a bear trap gate that mostly works. If I open this valve on the upstream side, I subject the chamber to the upstream water pressure. In ideal conditions with no friction and watertight seals, this would create enough pressure to lift both leaves. In reality, it needs a little bit of help from the intern hook. But you can see that, as the water level upstream increases, the lower leaf floats upward as well. When the gates are fully opened, the leaves lock together to be self-supporting. Some old bear trap gates used air pressure in the chamber to give the gates a little bit of help going up. I tried that in my model and it worked like a charm. It took a few tries to figure out how much pressure to send, but eventually I got it down.

It’s not just my model bear trap gate that’s finicky, though. Despite the huge benefit of not needing any significant outside force to raise and lower the gates, this type of system has never been widely used.

This chamber between the leaves is the perfect place for silt and sand to deposit. They were also quite difficult to inspect and maintain because you had to dewater the entire chamber and reroute flows. And because they weren’t widely used, there were never any off-the-shelf components, so anytime something needed to be fixed, it was a custom job. The world got to see a pretty dramatic example of the challenges associated with maintaining old bear trap gates in 2019 when one of the gates at Dunlap Dam near New Braunfels, Texas completely collapsed.

This dam was one of five on the Guadalupe River built in the 1930s to provide hydropower to the area. But over nearly a century that followed, power got a lot cheaper, and replacing old dams got a lot more expensive. Since the dam wasn’t built with maintenance in mind, it was nearly impossible to inspect the condition of the steel hinges of the gate. But that lack of surveillance caught up with the owner on the morning of May 14, 2019 when a security camera at the dam caught the dramatic failure of one of the gate’s hinges. The lake behind the dam quickly drained and kicked off a chain of legal battles, some of which are still going on today. Luckily, no one was hurt as a result of the failure. Eventually, the homeowners around the lake upstream banded together to tax themselves and rebuild the structure, a task that is nearly complete now more than three years later. Of course, there’s a lot more to this fascinating story, but it’s a great reminder of the importance of spillway gates in our lives and what can go wrong if we neglect our water infrastructure.

Storm surge gates and flood barriers are fixed installations that allow water to pass in normal conditions and have gates or bulkheads that can be closed against storm surges or high tide to prevent flooding. They can close the sea mouth of a river, the sea mouth of a waterway or a tidal inlet. These barriers are major infrastructure systems. Their implementation can be complemented with other grey and green storm surge and flood protection measures, such as dikes, seawalls and beach nourishment.

The implementation of an advanced flood forecast system and of an early warning system is required to ensure the prompt activation of storm surge gates and flood barriers before the storm surge or the flooding event actually occur. In normal conditions, storm surge gates and flood barriers allow the free passage of water, enabling regular navigation and natural water exchange in tidal inlets.

Storm surge gates and flood barriers are built to protect highly vulnerable urban areas and infrastructure where storm surges and sea flooding could have major impacts. Due to their poor flexibility and the high direct and indirect associated costs, storm surge gates and flood barriers must be accurately designed. This design should take into account projected changes in sea level and in storminess due to climate change, since the beginning of the planning stage. A long-term adaptive management plan of the structure and of other complementary strategies against flooding in the face of climate change can favour the success of the measure, avoid possible failures, and minimise environmental impacts. Due to their high costs and potential impacts, storm surge gates and flood barriers are relatively rare. They are used to protect particularly vulnerable and precious areas. Most known examples in Europe include:

The
Thames Barrier
(operative since 1983), London, can close off the Thames River just east of the City of London, at a point where the river is about 520 metres wide.
The
six storm surge barriers operated in the Netherlands
by the Ministry of Infrastructures and Public Works (Rijkswaterstaat) to protect the most vulnerable parts of the country from flooding. The largest barriers (the Eastern Scheldt Barrier and the Maeslant Barrier are part of the Delta Works and are located at the southern North Sea coast. If the water level rises to a dangerous level, the barriers close. The water is then prevented from flowing inland via rivers or estuaries.
The Venice barriers (also called the
‘Mose’ system
) are built across the three outlets of the Lagoon of Venice to the Adriatic Sea. The system is composed of four barriers with 78 flap-gates covering a total length of 1.6 km. It became operational, though in a test phase, since autumn 2020.

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The
St. Petersburg barrier
(completed in 2011, Neva Bay - eastern part of the Gulf of Finland) is part of a large flood prevention facility complex to protect the city from flooding, with an overall length of 24.5 km.

Additional Details

Reference information

Adaptation Details

IPCC categories

Structural and physical: Engineering and built environment options

Stakeholder participation

Due to the complexity of the engineering solutions, the significant costs of construction and maintenance, and the possible expected environmental impacts, proposals for storm surge barriers require wide and prolonged stakeholder and public participation. Moreover, these structures generally require an Environmental Impact Assessment procedure that, according to the EU EIA Directive, must ensure the right to access information and to participate in the environmental decision making. Similarly, the EU Floods Directive, and the EU Water Framework Directive establish public participation processes that may refer to these projects as well.

The construction phase requires considerable consultancy with engineers, local communities, NGOs, local authorities and representatives of policy sectors that can be affected by the measure (e.g. fisheries, maritime transport, tourism, etc.). A strong political support and wide public consensus, together with a long-term vision, is needed to ensure success in the implementation of such complex measures.

Success and Limiting Factors

Storm surge gates and flood barriers provide a high degree of protection of low lying coastal areas by providing a physical barrier against flooding. In particular, they are used to protect highly vulnerable and precious coastal urban and infrastructure areas. Existing gates and barriers (Netherlands, UK, Venice, St. Petersburg) have provided effectiveness against storm surges. The use of mobile barriers, instead of fixed structures, allows waterways to remain open during normal conditions. They allow to limit the (environmental, social, economic) impacts related to a permanent closure. Success examples of mobile barriers in the world are shared through I-Storms, the international network for storm surge barriers. It aims at facilitating knowledge exchange and collaboration of experiences of barrier planners and operators facing similar challenges.

One key limiting factor of the storm surge gates is the high capital and maintenance costs since significant investment is required to build these structures and to continually maintain them. The environmental impact of such measures is another key issue to be considered. The construction of mobile barriers can cause large modifications of natural environments and related environmental impacts must be properly assessed and minimised in the design phase. If too frequently operated, mobile gates and flood barriers can limit water exchange in estuarine and lagoon habitats.

Another important issue is the extent to which these barriers will remain viable in the face of future climate change and sea-level rise. In the case of London, the Thames Barrier is expected to continue to protect the city to its current standard up until 2070. The Thames Estuary 2100 Plan was designed to be adaptable to different rates of sea level rise and changes affecting the estuary. The plan identifies different options for improving or replacing the Thames Barrier. Full review and update of the Plan is scheduled every 10 years.

Other limiting factors are related to the capacity of the forecasting systems to early predict in a reliable way the flooding event, thus allowing to activate the procedures of gates closing on time. The time needed to close the barriers can vary according to both specific technical aspects and to complex management issues of the whole area. It can imply the interruption of navigation, port services and other activities. Continuous investment in research and technological innovation is essential to improve the reliability and precision of forecasting systems and their use under operational conditions.

Finally, the technical failure of the system (e.g. a barrier not closing properly) can be perceived as a large risk by the public. Acceptance of the work by the public and stakeholders can be fostered by an overall transparency in the decision-making process. , Proper stakeholder engagement, public consultation and informative workshops are proven means for transparent process settings.

Costs and Benefits

Storm surge gates and flood barriers provide a high degree of protection for urban settlements and infrastructure against seaward storm surges and related flooding. Compared to fixed gates, this type of infrastructure provides a more flexible solution. It allows waterways to be open in normal conditions for the natural water exchange and the movement of aquatic species as well as for human activities such as shipping and fisheries.

Large capital and maintenance costs are required to design, build, and maintain storm surge gates and flood barriers. Investment in monitoring hydrological parameters, flood forecasting and warning systems must also be ensured, to improve robustness and precision of information needed for the activation of the system in a prompt manner.

The construction of the Thames Barrier cost GBP 535 million in 1982 (about GBP 1.7 billion or EUR 2.5 billion in 2007), according to the UK Environment Agency. Operational costs are about GBP 8 million a year (about EUR 9.5 million in 2013 prices). Building the Mose system (including four mobile barriers at the Venice lagoon inlets) cost EUR 5.49 billion, according to official estimates. The estimate also includes two additional activities, i.e; the requalification of the facilities of the Venice Arsenal for the maintenance and operation of the MOSE system, and the requalification works needed to improve the integration of the mobile barriers within the lagoon environment.

Legal Aspects

The EU Floods Directive provides a legal framework for flood actions and defence. As major infrastructure systems, storm surge gates and flood barriers are likely to be part of the flood protection plan required by the Directive, which undergoes a strategic environmental assessment (SEA Directive). As coastal works, storm surge gates and flood barriers fall into Annex II of the Environmental Impact Assessment: Member States decide whether projects in Annex II should undergo an EIA procedure, either on a case-by-case basis or in terms of thresholds and criteria.

Implementation Time

The construction of these complex and often largescale engineering solutions is a long process which must be preceded by a detailed modelling, assessment, and design phases. Normally it takes more than 15 years.

Life Time

Storm surge gates and flood barriers have a long life expectancy (more than 50 years). Continuous maintenance is needed to ensure their full life-time and proper functioning without risks. Monitoring of potential effects on the environment is also essential.

Reference information

References:

UNEP-DHI (2016). Managing climate change hazards in coastal areas. The coastal hazard wheel decision-support system: Catalogue of hazard management options. United Nations Environment Programme & Lars Rosendahl Appelquist ISBN: 978-92-807-3593-2

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