Dam Drainage Systems: Spillways, Outlets, and Discharge Channels

Posted on Nov 25, 2024 in Geology

Types of Dam Drainage Systems

There are three types of dam drainage systems:

  1. Spillways: Allow the reservoir to release surplus water when full.

  2. Outlets: Drainage elements designed for specific reservoir demands, such as supply, irrigation, etc.

  3. Bottom Drains: Drain the reservoir for various reasons:

    • To create a cushion in the reservoir before a flood period.
    • To flush out bottom sediments and prevent reservoir silting.
    • For dam maintenance.

Main Parts of a Dam Drainage System

1. Spillway

Components:
  • Landfill: The flow control element. The lip can be fixed or regulated by gates.
  • Discharge Canal (Fast Channel): A canal system that rapidly returns water to the channel element.
  • Return to Channel Elements: Allow return flows to the channel, dissipating kinetic energy to prevent erosion and dam damage.
Weir (Part of the Spillway):

Controls the drainage rate and influences the capacity curve.

Elements:
  1. Approach area.
  2. Control section (strict profile or thick wall).
  3. Flow guide elements (batteries, power running boards).
  4. Regulatory elements (gates).
Landfill Classification:
  • Fixed Lip: No mechanisms that may be damaged.
    • Profile that fits the nappe: Based on adopting a profile that adapts to the underside of a well-aerated nappe on a thin wall. This allows upstream weir blade guidance without practical support. Such profiles are sized for a given flow. For lower flows, the film will be depressed, relying on the profile with greater than atmospheric pressure. For higher flow rates, it tends to take off, leading to depressions on the profile until the flow is large enough to allow effective blade separation.
    • Thick wall: If the threshold is flat with enough length, it establishes a system of almost parallel supply lines. Critical flow will occur on the threshold.
  • Regulated by Gates: Provides additional reservoir volume that can be emptied for flood control.

2. Discharge Channel (Rapid Discharge)

A fast-running canal system that returns water to the channel element. Design must account for surface waves from narrowing/widening and from batteries/abutments. Waves can be controlled using the floor (camber) and baffles to ensure no departures or dips from the theoretical flow path. Free-fall flow should be below the sill.

Energy Damping Elements:
  • Bowl: Dissipates hydraulic energy by the shoulder. Structurally expensive but safer.
  • Trampoline: Throws water onto a rock, lessening strain energy. Lower execution cost but requires good rock quality.
  • Submerged Trampoline: An intermediate solution with small prominence. Jet flow for higher flow rates ensures bump formation. The downstream depth must be greater than the draft needed for the bump. Consider the output curve in the river and adjust the bowl floor level. If the output curve is unknown, force a critical section at the bowl exit.

3. Bottom Outlets

Features:
  • Empty reservoir for flood control, sediment flushing, and dam maintenance.
Parts:
  • Protection grille mouth.
  • Transition mouth.
  • Cofferdam closure for repairs.
  • Ducts (independent of elbows and transitions).
  • Safety valves.
  • Control valves.
  • Chute.
  • Reinstatement work.

The mouth is flared to reduce head losses and includes a grid to retain large material. Entry speed between grid bars should be 0.5 m/s.

Hollow-Jet Valves (HOWELL-Bunge):

Opening/closing is done by a coaxial tube spool, ensuring flow doesn’t impede gate movement. Widely used for regulation, preferably discharging into the atmosphere. Submerged discharge requires a hood deflector for air into the flow.

Types of Channels

  • AVENUES (Evacuation Channels): Allow the fastest drainage, complement natural waterways, and must not be tampered with. Steep slopes, high flow rates, rapid and variable design.
  • WATER TRANSPORT Channels: Convey water from source to consumption point. Gentle slopes, low flow rate, adapted to contour lines, slow and uniform system (irrigation canals, supply channels, inter-basin transfer channels).
  • NAVIGATION Channels / HYDROELECTRIC Channels: Gentle slopes, low flow rate, allow vessel maneuverability, slow system. Pressure drop in pipes is always greater.

Indispensable to follow the mapped trace.