How do you design a spillway for a dam to prevent overtopping and erosion?

1 Spillway types

Depending on the site conditions, the design discharge, the available materials, and the cost, there are various types of spillways that can be used for a dam. These include overflow spillways located at the crest or top of the dam, chute spillways which are inclined channels that convey water to the downstream river or valley, shaft spillways which are vertical shafts connecting the reservoir to a horizontal tunnel or conduit that discharges water downstream, siphon spillways which are pipes using atmospheric pressure to create a siphon effect and draw water from the reservoir to the outlet, and fuse plug spillways which are low sections of the dam designed to fail and create an opening for water to escape when the reservoir reaches a critical level.

2 Spillway capacity

The spillway capacity is the maximum discharge that the spillway can handle without causing overtopping or damage to the dam or its surroundings. The spillway capacity depends on the design flood, which is the peak inflow that the dam is expected to withstand in a given return period. The design flood is usually determined by hydrologic analysis, using historical data, rainfall-runoff models, and probabilistic methods. The spillway capacity also depends on the hydraulic characteristics of the spillway, such as the geometry, the slope, the roughness, and the energy losses.

3 Spillway design

The spillway design involves selecting the type, location, and dimensions of the spillway that can meet the required capacity and safety criteria, while considering the structural stability, material durability, and environmental impacts. Typically, this process begins with establishing the design criteria and parameters, such as the design flood, the allowable headwater elevation, the freeboard, the tailwater elevation, and the erosion control measures. Then a preliminary analysis is conducted to select a spillway type and location based on site topography and reservoir configuration. Following this is a detailed hydraulic analysis to determine flow regime, velocity, pressure, and energy dissipation. Additionally, a structural analysis is done to ensure strength and stability of components and foundations. Finally, environmental impacts and mitigation measures are evaluated to assess water quality, sediment transport, fish passage, and aesthetics.

4 Spillway modeling

Spillway modeling is a useful tool for testing and verifying the spillway design and performance under various scenarios. Spillway modeling can be done using physical models or numerical models. Physical models are scaled replicas of the spillway that are built and tested in a laboratory or a field site, using water or other fluids to simulate the flow conditions. Numerical models are mathematical representations of the spillway that are solved and analyzed using computers and software, using equations and algorithms to simulate the flow conditions.

5 Spillway operation

Spillway operation is the management and control of the spillway discharge during normal and extreme events. Spillway operation involves monitoring and measuring the reservoir level, the inflow, and the outflow of the dam, using instruments and devices such as gauges, sensors, and cameras. Spillway operation also involves regulating and adjusting the spillway discharge, using mechanisms and devices such as gates, valves, and actuators. Spillway operation aims to optimize the water storage and supply functions of the dam, while ensuring the safety and security of the dam and its downstream areas.

6 Spillway maintenance

Spillway maintenance is the inspection and repair of the spillway components and structures to prevent or correct any defects or damages that may affect the spillway function and safety. Spillway maintenance includes routine and periodic tasks such as cleaning, painting, lubricating, testing, and replacing parts of the spillway. Spillway maintenance also includes emergency and corrective actions such as fixing cracks, leaks, or failures of the spillway. Spillway maintenance requires proper planning, scheduling, budgeting, and documentation of the activities and results.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?