Spillways

A spillway is a structure built with a dam to safely pass surplus flood water from the reservoir to the downstream river when the reservoir is full.

Purpose of a spillway

• To protect the dam from overtopping (especially earth and rockfill dams) during floods.
• To keep the reservoir level below the maximum water level (MWL).
• To release water safely without eroding the dam toe or downstream bed.

Types of spillways

1. Free overfall (straight drop) spillway: water falls freely from a thin crest; used for low dams (arch dams) on firm rock.

 ~~~~~~~|\
        | \  nappe
        |  \
 _______|___\~~~~~

2. Ogee (overflow) spillway: crest shaped to fit the lower nappe of a sharp-crested weir; most common for concrete gravity dams.

 ~~~~~~~_
       /  \__
      |      \
      |       \___
 _____|___________\___~~

3. Chute (trough) spillway: water flows over a short crest into a steep open channel (chute) to the river; used with earth dams.

 reservoir  crest
 ~~~~~~~~~~|----\
                 \  chute
                  \______ stilling basin

4. Side channel spillway: crest is parallel to the channel; water spills sideways into a trough and then into a chute or tunnel; used in narrow valleys.

  ~~~~~~~ reservoir ~~~~~~
  ======== crest =========
  |   side channel  --->  | -> chute

5. Shaft (morning glory) spillway: water enters a circular funnel crest, drops through a vertical shaft and leaves through a horizontal tunnel.

   ~~~\    /~~~
       |  |  shaft
       |  |______
       |_________ -> tunnel

6. Siphon spillway: closed duct shaped like an inverted U; flow primes automatically when water rises above crest, giving a large discharge for small head rise.

     ____
   /  __  \
  ~|~|  |  \
   |_|  |   \__ outlet

7. Tunnel (conduit) spillway: flow is carried through a closed tunnel or conduit around or under the dam.
8. Labyrinth and stepped spillways are also used (longer crest in plan; steps to dissipate energy).

A spillway is a safety structure provided with a dam to pass surplus flood water from the reservoir to the downstream channel safely, so that the dam is not overtopped.

Classification of spillways

Ogee spillway

An ogee spillway is an overflow spillway whose crest and downstream face are shaped to follow the profile of the lower nappe of a fully aerated sheet of water flowing over a sharp-crested weir. The profile looks like an elongated "S" (ogee curve).

        upstream    crest
  ~~~~~~~~~~~~~~~~~~~__  water surface
                  _/  \__
  upstream face  |       \_  downstream
  (vertical or   |         \   profile
   sloping)      |          \  (x^n = K H_d^(n-1) y)
                 |           \
                 |            \___ reverse curve
 ________________|_______________\___~~~~~~ tailwater
                     bucket / stilling basin

Features:

• Upstream crest curve: compound circular curves (radii about $0.5H_d$ and $0.2H_d$) from the upstream face to the crest.
• Downstream profile: WES standard shape, $x^{1.85} = 2.0\,H_d^{0.85}\,y$ for a vertical upstream face, where $H_d$ is design head.
• Straight tangent portion following the dam slope, ending in a reverse curve (bucket) which turns the flow into a stilling basin or ski-jump bucket.

Discharge: $Q = C\,L\,H_e^{3/2}$ with $C \approx 2.2$ (SI) at design head.

Working and merits:

• Since the face fits the nappe, pressures on the surface are near atmospheric at design head, giving high discharge efficiency without damage.
• At heads above design head, negative pressures develop (cavitation risk); below design head, positive pressures reduce efficiency.
• It is simple, reliable, and suitable for concrete gravity and buttress dams; can be gated or ungated.
• Not suitable alone for earth dams, since water would flow over the dam body.

Spillway gates are movable barriers on the spillway crest that hold water above the crest level during normal times (increasing storage) and are opened to release floods in a controlled way.

1. Vertical lift gates

• Plain sliding gates: steel plates sliding in grooves; large friction, so used for small openings.
• Fixed-wheel (roller) gates: steel gates on wheels running on rails in piers; less friction, common for medium openings.
• Stoney gates: run on a free train of rollers between gate and pier, further reducing friction for large gates. Lifted by hoists or gantry cranes; need tall piers and a hoist bridge.

2. Radial (Tainter) gates

• Curved skin plate (part of a cylinder) supported by radial arms on trunnion pins in the piers.
• Water pressure passes through the pin, so lifting force is small; most common for large spillways.

3. Drum gates

• Hollow segment of a cylinder hinged at the upstream edge, lying in a recess in the crest; raised and lowered by admitting water into the chamber below. Automatic operation; suits long crests.

4. Sector gates

• Like a drum gate but hinged on the downstream side; rotates into a recess in the crest using reservoir pressure.

5. Flash boards and stop logs

• Flash boards: temporary timber boards (0.5–1.2 m) on the crest, held by pins designed to fail during floods.
• Stop logs: horizontal beams placed in grooves between piers, removed by crane; used for maintenance or minor control.

6. Rolling (roller) gates

• Large horizontal steel cylinder rolled up inclined racks in piers; spans long openings.

7. Flap gates and automatic (fuse) gates

• Flap gates: hinged at the bottom; tilt down when water rises.
• Fuse gates / fuse plugs: units designed to tip over at a set flood level.

 Vertical lift        Radial (Tainter)
   |hoist|              trunnion
   ||   ||               o
 ~~||===||~~         ~~~/|
   ||   ||          skin/ | arms
 __||___||__        ___(  |___
   crest               crest

Selection depends on span and height of opening, head, cost, ease of operation, ice/debris, and availability of power for hoisting.

A spillway is a structure built as part of a dam to pass excess flood water safely from the reservoir to the river downstream, so that the reservoir level never rises above the maximum water level and the dam is not overtopped. It is the safety valve of a dam.

Ogee spillway

The ogee spillway is an overflow spillway whose crest and downstream face follow the shape of the lower surface of the nappe of water falling over a sharp-crested weir at the design head. Its profile is an S-shaped (ogee) curve.

 ~~~~~~~~~~~~~~~~~~__ 
                 _/  \_   crest
                |      \
   dam body     |       \  d/s profile
                |        \
 _______________|_________\____~~~~ TWL
                     bucket

• Upstream portion: compound curves joining the upstream face to the crest.
• Downstream profile: WES shape $x^{1.85} = 2\,H_d^{0.85}\,y$ ($H_d$ = design head), followed by a straight slope and a reverse curve/bucket that delivers water to an energy dissipator.
• Discharge: $Q = C\,L\,H^{3/2}$, with $C \approx 2.2$ (SI units).
• At design head the pressure on the face is nearly atmospheric, giving high efficiency with no cavitation. At higher heads negative pressure (risk of cavitation) and at lower heads positive pressure (lower discharge coefficient) occur.
• Widely used for concrete gravity dams and barrages; may be gated or ungated.

Five types of spillways

1. Ogee (overflow) spillway – crest shaped to the lower nappe; on the dam body itself.
2. Chute (trough) spillway – short crest followed by a steep open channel to the river.
3. Side channel spillway – crest parallel to a trough that leads to a chute or tunnel.
4. Shaft (morning glory) spillway – funnel crest, vertical shaft and horizontal tunnel.
5. Siphon spillway – inverted-U closed duct that primes automatically. (Others: free overfall, tunnel/conduit, labyrinth, stepped spillways.)

Most suitable type for Nepalese river sites

Nepalese rivers are steep, carry very high monsoon floods (often 10–50 times the dry-season flow), and transport huge loads of sediment, boulders, trees and debris. Valleys are narrow and deep, and many sites have sound rock but fragile slopes and high seismicity.

For these conditions a gated ogee (overflow) spillway on a concrete dam/weir, combined with large undersluices (low-level orifice spillways) is generally the most suitable choice, as used at Kali Gandaki A, Middle Marsyangdi, Kulekhani and Upper Tamakoshi headworks.

Reasons:

1. Large flood capacity: an ogee crest with radial gates can pass very large floods with a small rise in reservoir level, and it fits in a narrow gorge.
2. Passes sediment and debris: wide open gated bays and low-level sluices can be fully opened during floods to flush sediment and floating debris through the reservoir; enclosed types (siphon, shaft) choke with debris and logs.
3. Reservoir sediment flushing: low-level gates allow drawdown flushing, essential for keeping storage of small reservoirs in Nepal.
4. Concrete structure on rock: suitable for concrete gravity dams and barrages built on the rocky foundations of Himalayan gorges.
5. Energy dissipation: the ogee profile is easily combined with a stilling basin, ski-jump (flip) bucket or roller bucket, suited to the high tailwater variation.

Where an earth or rockfill dam is built (e.g. Kulekhani), a chute spillway on one abutment, or a side channel spillway in a narrow valley, is preferred, because water cannot flow over the dam body. Siphon and shaft spillways are generally not suitable in Nepal because of floating debris and sediment.

Spillways

A spillway is a structure built with a dam to release surplus flood water from the reservoir safely to the downstream river when the reservoir is full, protecting the dam from overtopping and failure.

Gates

Spillway gates are movable steel barriers placed on the spillway crest between piers. They store water above the crest during normal operation (increasing storage and head) and are opened during floods to control the outflow. Common types: vertical lift (fixed-wheel, Stoney), radial (Tainter), drum, sector, rolling gates, flash boards and stop logs.

Types of spillways used in hydropower projects

1. Free overfall (straight drop) spillway: water drops freely over a thin crest; used for low arch or thin dams on hard rock, with a plunge pool or apron below.
2. Ogee (overflow) spillway: crest shaped to the lower nappe of a sharp-crested weir; high discharge coefficient ($C \approx 2.2$); most common on concrete gravity dams and barrages.
3. Chute (trough) spillway: short crest followed by a steep lined open channel to the river; used with earth and rockfill dams; placed on an abutment or saddle.
4. Side channel spillway: crest along the side of a channel; water spills laterally and turns 90° into a chute or tunnel; useful in narrow valleys where a long crest is needed.
5. Shaft (morning glory) spillway: water enters a funnel-shaped circular crest, falls down a vertical shaft and exits through a horizontal tunnel (often the diversion tunnel). Suitable where there is no room for other types.
6. Siphon spillway: inverted U-shaped closed conduit; once primed it gives large discharge for small rise in water level, keeping the reservoir level nearly constant.
7. Tunnel (conduit) spillway: closed tunnel through the abutment or a conduit through the dam carries the flood.
8. Labyrinth spillway: zigzag crest in plan gives a long crest in a narrow width.
9. Stepped spillway: steps on the downstream face dissipate energy along the chute.

  Ogee         Chute            Shaft
 ~~__        ~~|--\           ~~\   /~~
   / \_          \  \            | |
  |    \__        \__\___        | |____
  |_______\~~      stilling      |______ tunnel

Given: crest length $L = 60$ m, crest elevation $= 133$ m, reservoir water surface $= 135$ m, coefficient of discharge $C_d = 0.74$.

Head over crest:

$$H = 135 - 133 = 2\ \text{m}$$

Formula (flow over a weir/overflow spillway, approach velocity neglected):

$$Q = \frac{2}{3}\,C_d\,\sqrt{2g}\;L\,H^{3/2}$$

Substitution:

$$\begin{aligned} Q &= \frac{2}{3} \times 0.74 \times \sqrt{2 \times 9.81} \times 60 \times 2^{3/2} \\ &= 2.185 \times 60 \times 2.828 \\ &= 370.8\ \text{m}^3/\text{s} \end{aligned}$$

(The equivalent spillway coefficient $C = \tfrac{2}{3}C_d\sqrt{2g} = 2.185$, which is the usual range of about 2.2 for ogee crests in $Q = C L H^{3/2}$.)

Answer: discharge over the spillway $Q \approx 370.8$ m³/s.

Types of spillway gates

1. Vertical lift gates – plain sliding, fixed-wheel (roller) and Stoney gates
2. Radial (Tainter) gates
3. Drum gates
4. Sector gates
5. Rolling (roller) gates
6. Flap gates
7. Flash boards and stop logs

Radial (Tainter) gate

A radial gate has a curved steel skin plate, which is a part of a cylinder, supported by radial arms that converge to trunnion pins fixed on the piers. The gate rotates about the trunnion to open or close.

              hoist rope
               |
     ~~~~~~~~~ |\   skin plate (curved)
   reservoir   | \
   ~~~~~~~~~~~ |  \ ---- arm ----o trunnion
               |  /               (on pier)
               | /  ---- arm ---/
   ____________|/__________________
         crest   sill    flow -->

Working and features:

• The skin plate is curved concentric with the trunnion, so the resultant water pressure passes through the trunnion pin. Water pressure therefore produces no moment about the pin, and the hoist only lifts the gate's own weight and overcomes pin and seal friction.
• The gate is raised by rope or chain hoists or hydraulic cylinders; water flows under the gate.
• Rubber seals at the sides and bottom make it watertight; no deep slots in piers are needed, so flow is smooth.

Advantages: small hoisting force, simple and reliable, good for large openings (up to about 20 m × 20 m), easy automatic control, cheaper than vertical gates of the same size.

Disadvantages: piers must be long to hold the trunnions; trunnion anchorage carries large concentrated loads; requires accurate fabrication.

Function of a spillway

A spillway safely passes surplus flood water from the reservoir to the downstream river. Its functions are:

• to prevent overtopping and failure of the dam during floods;
• to keep the reservoir below the maximum water level;
• to release water without eroding the dam toe or riverbed (with an energy dissipator).

Vertical lift gate

A vertical lift gate is a rectangular steel gate that moves vertically in grooves (slots) in the piers.

     hoist / gantry crane
     ______|______
    | |    |    | |  pier
    | |=========| |  gate (raised to open)
 ~~~| |  skin   | |
    | |  plate  | |
 ___|_|_________|_|___ crest
      wheels in pier slots

• Types: plain sliding gate (slides on steel faces, high friction), fixed-wheel gate (wheels on axles run on rails in the piers), Stoney gate (free roller train between gate and pier).
• The full water thrust is carried by the wheels to the piers, so the lifting force must overcome gate weight + friction of wheels/seals; a high hoist bridge or gantry crane is needed.
• Advantages: simple, compact in the flow direction, can be lifted out completely for maintenance, suits stop-log/emergency use.
• Disadvantages: large hoisting force, tall piers and hoist structure, slots disturb flow and may cause cavitation.

Radial (Tainter) gate

A radial gate has a curved skin plate (a segment of a cylinder) carried by radial arms on trunnion pins anchored in the piers.

            hoist
              |
   ~~~~~~~~~  |\  curved skin plate
   ~~~~~~~~~  | \___ arms ___ o trunnion
              | /         _/
   ___________|/_________/_____
       crest         flow -->

• Water pressure on the curved plate passes through the trunnion, so no moment about the pin; the hoist lifts only the gate weight and friction—very small hoisting force.
• It rotates upward to open; flow passes under the gate.
• Advantages: light hoist, smooth flow (no deep slots), suits large spans, easy automatic control; most used on large spillways.
• Disadvantages: long piers needed for trunnions, heavy concentrated trunnion loads.

Cavitation is the formation of vapour bubbles in a flowing liquid where the local pressure falls to the vapour pressure of the liquid, followed by the violent collapse of these bubbles when they move into a region of higher pressure. The collapse causes very high local pressures (hundreds of MPa), noise, vibration and pitting (erosion) of the boundary.

Cavitation over spillways

1. Cause – negative pressure: On a spillway the flow velocity is very high (20–40 m/s on high dams), so the pressure head $p/\gamma = H - V^2/2g$ can fall below atmospheric. When the pressure drops to the vapour pressure (about −10 m of water gauge), vapour cavities form.
2. Operation above design head: an ogee crest designed for head $H_d$ follows the nappe; when the actual head exceeds about 1.33 $H_d$, the nappe tends to spring away from the surface, creating sub-atmospheric pressure on the crest and leading to cavitation.
3. Surface irregularities: offsets, joints, protruding aggregate, rough patches, gate slots, sudden changes in slope or curvature cause local flow separation and low pressure zones in the high-velocity flow.
4. Gate slots and piers: sharp corners at gate grooves and pier ends generate vortices with low pressure cores.
5. Location: most damage occurs on the lower chute, the toe/bucket, baffle blocks of stilling basins and downstream of gate slots, where velocities are highest.

Effects

• Pitting and erosion of concrete; large holes can form in a few hours of high flood flow (e.g. damage at Glen Canyon dam spillway tunnels, 1983).
• Vibration and noise; reduction in discharge capacity.

Prevention

• Design crest for adequate head (design head close to maximum head) and smooth profiles.
• Strict surface tolerances; remove offsets and protrusions; use high-strength concrete or steel lining.
• Aeration: provide aerators (ramps, grooves, offsets) on chutes to supply air to the flow; 5–8% air concentration near the boundary almost eliminates cavitation damage.
• Streamlined gate slots and pier noses.
• Cavitation index check: keep $\sigma = \dfrac{p - p_v}{\rho V^2/2}$ above the critical value for the surface.

Cavitation erosion in spillways

Cavitation erosion is the damage (pitting and removal of concrete) on a spillway surface caused by the repeated collapse of vapour bubbles. In high-velocity flow over the spillway, pressure falls to the vapour pressure at low-pressure points (crest operating above design head, surface offsets, gate slots, curves); vapour bubbles form and then collapse near the surface, producing very high impact pressures that gradually erode the concrete. It is prevented by smooth surfaces, correct crest design and air entrainment (aerators).

Energy dissipators

Water leaving a spillway has high kinetic energy, which would scour the riverbed and endanger the dam. Energy dissipators destroy this energy before the water enters the river.

1. Hydraulic jump type stilling basins A hydraulic jump is formed on a concrete apron where supercritical flow changes to subcritical, dissipating energy in turbulence. USBR standard basins: Type I (simple apron), Type II (chute blocks + dentated end sill, high dams), Type III (chute blocks + baffle blocks + end sill, $V < 18$ m/s), Type IV (oscillating jump).

 spillway
   \
    \___   jump     
        \_/~~~\~~~~~~~~ TWL
  ________|_|_|___|__
  chute  baffle end sill
  blocks blocks

2. Bucket type dissipators (used when tailwater is too high for a jump or too low)

• Ski-jump (trajectory/flip) bucket: water is thrown high into the air and falls far from the dam into a plunge pool; used where tailwater is low and rock is sound (common in Nepal).

   \
    \      _..--''--.._
     \____/  jet        '.
      bucket             \ plunge pool
 _________________________\__~~~

• Solid and slotted roller bucket: submerged bucket where tailwater is high; water forms surface and ground rollers that dissipate energy.

 ~~~~~~~~~~~~~~~ TWL
   \    (roller)
    \___  ___
        \/   | lip
 ____________|____

3. Plunge pool / free fall: jet from a free overfall or flip bucket falls into a deep pool, which absorbs the energy.

4. Impact and baffle type: baffle blocks, impact walls, stepped spillways and dentated sills break up the flow.

Selection depends on tailwater rating curve versus jump-height curve, rock quality and economy.