Spillways and Overtopping Protection

Spillway HDR

Spillways are structures built into dams to provide a controlled release of flows from a dam into the downstream area, typically in the riverbed below. They are designed to accommodate major surge events from water and act as a sort of release valve for dams to protect them from overtopping and damaging or destroying the dam. The two main types of spillways are controlled and uncontrolled. Controlled spillways use mechanical structures or gates to control the flow of water on command, allowing for the full height of the dam to be used to store water. Uncontrolled spillways are designed to allow water to flow down a ramp or similar structure whenever the water level rises to the crest of the spillway, typically down a ramp or similar structure.

When designing spillways, there are a few important factors to consider. First and foremost, any uncontrolled discharge of surplus water needs to be automatic and not require any manual activation. The spillway also needs to be large enough in scope to dissipate the energy of water flowing from the dam to avoid any damage to the structure and foundation of the dam itself. The opening of the spillway should be large enough to accommodate the discharge of flood waters without allowing the upstream water levels to raise too high while also ensuring that the discharge of floodwater is never worse than it was prior to the construction of the dam. This ensures that the construction of the dam itself does not harm the surrounding landscape or disturb the lives of those living nearby.

Cement and concrete can be used in many different ways in spillways. Conventional concrete and RCC can be employed to build specifically shaped and sized ramps down dam faces or in other directions. Controlled spillways can even utilize full tunnel systems spanning underneath the dam and releasing the water further downstream. Some systems like soil-cement and RCC can act as a quick application to make a durable structure resistant to erosion and damage from flooding during extreme water events. Concrete mats and grout-filled mats can act in a similar sense to reduce erosion during runoff periods and maintain the structural integrity of the dam and surrounding landscape. Considering that water runoff coming down spillways can exceed speeds of 100 miles per hour, many spillways incorporate concrete “feet” or other similar structures to help distribute the force of water up into the air and reduce the initial impact of water at some of the more vulnerable points of erosion.

Roller compacted concrete (RCC) is a no-slump concrete that contains the same ingredients as conventional concrete: cement, aggregate and water. However, it is typically placed using bulldozers and compacted with vibratory steel drum rollers. The economic savings, long-term performance and speed of construction makes RCC an ideal material for protecting embankments and other water resource applications.