A spillway is a structure built into or alongside a dam that safely releases surplus water when reservoir levels rise above normal. Without one, water would flow over the top of the dam itself, eroding the structure and potentially causing catastrophic failure. Every significant dam in the world has at least one spillway, and many have two or more designed for different conditions.
How a Spillway Works
The basic job is straightforward: move excess water from the reservoir to the river below without damaging the dam. During heavy rain or snowmelt, water levels rise. Once the reservoir reaches a set threshold, water begins flowing through the spillway, either automatically (by gravity) or through gates that operators open. The water travels down a channel, chute, or tunnel and rejoins the natural watercourse downstream.
What makes this harder than it sounds is energy. Water falling from the top of a tall dam picks up enormous speed and force. A spillway isn’t just a slide for water. It’s an engineered system that controls where that energy goes so it doesn’t chew apart the riverbed, the dam’s foundation, or the spillway itself.
Types of Spillways
Spillways come in several designs, each suited to different dam types and terrain.
- Overflow (ogee crest): The most common type, shaped like a smooth, curved lip that water pours over. Many include crest gates, which are movable barriers that let operators adjust how much water flows through. You’ve likely seen photos of these on large concrete dams.
- Chute: A steep concrete channel that carries water down a slope, often built into the hillside beside an earth dam rather than through the dam itself. Each chute is custom-fitted to the terrain and foundation beneath it.
- Morning glory (shaft): A funnel-shaped opening in the reservoir that looks like a giant drain. Water spirals down into a vertical shaft and exits through a tunnel at the base. These are used where there isn’t room for a surface channel.
- Side channel: The crest runs roughly parallel to the discharge channel, forming a trough that collects water and routes it into a tunnel or chute. This design works in tight spaces where a standard overflow wouldn’t fit.
The choice depends on factors like the dam’s material (concrete vs. earth), the shape of the valley, how much water needs to pass through, and the geology of the surrounding rock.
Service Spillways vs. Emergency Spillways
Most dams have a primary (service) spillway designed for regular use. It handles the routine rises in water level that happen with seasonal storms and normal rainfall. Federal guidelines call for service spillways to exhibit “excellent performance characteristics” for sustained flows up to a 100-year flood event, meaning a flood with a 1% chance of occurring in any given year.
Many dams also have a secondary or emergency spillway. This is a backup path, often a separate channel that routes water into a secondary watercourse. It only activates during extreme events when the primary spillway can’t handle the volume alone. Emergency spillways are sometimes as simple as a reinforced low point on the dam’s rim that allows controlled overtopping before things get truly dangerous.
Engineers design spillway capacity around something called the Inflow Design Flood, which represents the worst-case scenario a dam must safely handle. For high-hazard dams (those where failure could cause loss of life), this is typically set at the Probable Maximum Flood: the most severe combination of rainfall and runoff conditions reasonably possible for that drainage basin. For low-hazard dams, the standard is often the 100-year flood.
Controlling the Energy of Falling Water
Water rushing down a spillway can reach speeds above 65 feet per second. At the bottom, all that energy has to go somewhere. Left unchecked, it would scour deep holes in the riverbed, undermine the dam’s foundation, and erode the banks downstream. Engineers use several structures to absorb or redirect this force.
A stilling basin is a flat concrete pad at the base of the spillway, often fitted with rows of concrete blocks (called chute blocks or baffle blocks) and a raised sill at the downstream end. These features force the fast-moving water to slow down abruptly, creating turbulence that dissipates energy before the water reaches the natural channel. The end sill directs remaining bottom currents upward and away from the riverbed. Riprap, which is loose stone laid along the banks and channel floor, protects areas just beyond the stilling basin from erosion caused by residual surges.
Flip buckets take a different approach entirely. Rather than slowing the water down, a curved ramp at the base of the spillway launches it into the air like a ski jump. The water lands far enough downstream that any scouring happens at a safe distance from the dam and its structures. Flip buckets don’t actually dissipate energy. They simply move the impact zone away from anything that could be damaged.
Cavitation: The Invisible Threat
At very high velocities, spillways face a counterintuitive problem. When water flows faster than about 45 to 65 miles per hour across a concrete surface, tiny imperfections in the surface can cause the water pressure to drop so low that vapor bubbles form. When those bubbles collapse against the concrete, they release intense, focused force that chips away at the surface over time. This process, called cavitation, can destroy even thick reinforced concrete.
For moderate speeds, engineers manage cavitation by keeping spillway surfaces extremely smooth and using high-strength concrete. But above roughly 45 to 65 mph, the surface tolerances needed to prevent cavitation become impractical. The solution is to deliberately inject air along the spillway surface using devices called aerators. These are ramps, offsets, or grooves built into the spillway floor that lift the water slightly and allow air underneath. A thin layer of air bubbles cushions the concrete from the collapsing vapor bubbles, dramatically reducing erosion.
What Happens When Spillways Fail
The 2017 Oroville Dam incident in California is the most prominent recent example of spillway failure in the United States. During heavy rains, a crack developed in the main spillway chute. Water penetrated the crack and eroded poorly weathered bedrock beneath the concrete, causing a massive section of the spillway to break apart. Nearly 190,000 people were evacuated as operators scrambled to manage flows through the damaged structure and an unpaved emergency spillway that also began eroding.
Investigation found the root cause was insufficient quality of the bedrock foundation under the spillway chute. Internal erosion beneath the concrete, driven by water seeping through the crack, hollowed out the support structure until the surface collapsed. The incident highlighted that even a well-designed spillway can fail if the foundation it sits on isn’t sound, and it triggered a nationwide reassessment of spillway conditions.
Inspection and Oversight
In the United States, the Federal Energy Regulatory Commission oversees dam safety for hydroelectric projects. Engineers inspect dams during construction and continue regular inspections throughout the dam’s life. Operators of dams with spillway gates must file annual certificates verifying that the gates have been tested and are functioning properly. Regulations under 18 CFR Part 12, updated in 2022, set the legal framework for these safety requirements. State dam safety programs cover non-federal, non-hydroelectric dams, though the rigor of these programs varies significantly from state to state.
Spillways are among the most scrutinized components of any dam because their failure directly threatens the dam itself. A spillway that can’t pass enough water forces the reservoir to rise, and an overtopped dam, particularly an earthen one, can erode and fail within hours.