Hydraulic Structures & Spillways

Spillways

A spillway passes surplus flood water from the reservoir to the river downstream safely, so that the reservoir never rises above the maximum water level and the dam is never overtopped.

• Spillway capacity is fixed by the inflow design flood routed through the reservoir (surcharge storage reduces the peak outflow) — see Floods — Estimation & Flood Routing.
• Spillway failure (inadequate capacity) is a major cause of dam failures, especially of earth dams.

Components

1. Entrance (approach) channel.
2. Control structure — crest (weir, orifice or gates) that regulates outflow.
3. Discharge channel (conveyance) — the face of the dam, a chute, tunnel or shaft.
4. Terminal structure — energy dissipator.
5. Outlet channel — returns water to the river.

Types of spillways

Notes on special types

• Shaft spillway — at low heads the flow is crest controlled ($Q=C(2\pi R)H^{3/2}$); at higher heads the flow becomes orifice/tube controlled and discharge increases very little with head. Needs debris protection and air vents; vortices must be prevented.
• Siphon spillway — discharge $Q=C_{d}a\sqrt{2gH}$; the crown (summit) pressure is negative, so the height of the crown above the downstream water is limited to avoid cavitation; an air vent (deprimer) at FRL stops siphon action when the water level falls; problems include clogging by debris, vibration, sudden surges and ice.

Ogee spillway

Crest profile

The downstream profile follows the lower surface of the nappe from a fully ventilated sharp-crested weir, so that pressures on the surface stay close to atmospheric.

Discharge

• Head greater than design head — the nappe tends to leave the surface → negative pressures on the crest, higher discharge coefficient, but risk of cavitation and vibration.
• Head less than design head — the nappe clings to the surface → positive pressures, lower discharge coefficient.
• Design heads are therefore often chosen a little below the maximum head so that negative pressures at maximum flood remain within safe limits.

Diversion headworks

Diversion headworks (canal headworks) are structures built across a river at the head of a canal to divert the required supply into the canal.

Purposes:

1. Raise the water level in the river so the canal can command the area.
2. Regulate supply into the canal.
3. Control silt entering the canal.
4. Provide some pondage to meet daily fluctuations.
5. Reduce fluctuations in the river supply level.

(Storage headworks — dams — store water for use over longer periods.)

Site selection

River section narrow and well defined with stable, high banks; good foundation close to the surface; river reach straight (so the canal can take off from the outer/concave side where silt entry is less); availability of construction materials and access; the canal should command the area without long unproductive reaches; minimum submergence of valuable land.

Weir versus barrage

Components of diversion headworks

1. Weir or barrage — main structure across the river.
2. Undersluices (scouring sluices) — gated openings with crest at a lower level than the weir, adjacent to the canal head regulator. They keep a clear, deep channel near the regulator (still pond), scour silt deposited in front of the regulator, pass low floods, and maintain a defined river channel.
3. Divide wall — a masonry or concrete wall at right angles to the weir axis separating the undersluice bays from the weir proper; forms the still pond and prevents cross flows.
4. Fish ladder (fish pass) — a stepped or baffled channel with low velocities (commonly about 2 m/s or less) that lets fish migrate upstream past the structure; usually in the divide wall.
5. Canal head regulator — regulates supply into the canal, controls silt entry, shuts out floods.
6. Silt excluder — a system of tunnels in the undersluice pocket upstream of the head regulator; the lower silt-laden layers pass through tunnels to the downstream side, so relatively clear top water enters the canal.
7. Silt ejector (silt extractor) — a device in the canal, a short distance downstream of the head regulator, which removes silt that has already entered the canal through tunnels/vortex tubes.
8. River training works — guide banks (to guide the river centrally through the structure and prevent outflanking), marginal bunds (to prevent flooding of upstream areas due to afflux), spurs/groynes.

Types of weirs

• Masonry weir with vertical drop — raised crest with vertical downstream face and a cistern; for hard foundations.
• Dry stone (rock-fill) slope weir — masonry crest wall with sloping rock-fill aprons; where stone is abundant.
• Concrete sloping glacis weir — crest with a sloping downstream glacis on which a hydraulic jump forms for energy dissipation; standard for modern weirs and barrages on permeable foundations.

Failure of weirs on permeable foundations

By subsurface flow

• Piping (undermining) — seepage beneath the floor emerges at the downstream end; if the exit gradient is high, soil particles are carried away, a pipe forms backwards under the floor and the structure collapses.
• Direct uplift — seepage pressure under the floor lifts or cracks it if the floor weight is insufficient.

Remedies: longer seepage path (longer floor, deeper sheet piles/cut-off walls), downstream inverted filter with loaded weight, adequate floor thickness.

By surface flow

• Hydraulic jump on the glacis causes suction/uplift — floor thickness must resist it.
• Scour on the upstream and downstream sides — deep cut-off walls and launching aprons (loose stone) protect against scour; depths are related to Lacey's normal scour depth.

Bligh's creep theory

Bligh assumed that seepage water creeps along the contact of the base profile with the subsoil (vertical and horizontal paths counted equally), and that head is lost uniformly along this path.

Bligh's coefficient $C$ (indicative): fine micaceous sand 15; coarse-grained sand 12; boulders, gravel and sand mixed 5–9.

Limitations: horizontal and vertical creep treated alike; no distinction between inner and outer faces of piles; exit gradient (the real cause of piping) not considered; uniform head loss assumed; ignores the effect of pile depth and position.

Lane's weighted creep theory

Lane analysed about 200 dams and found vertical paths more effective than horizontal ones:

The weighted creep length must satisfy $L_w\ge C_{w}H$ (Lane's coefficients). Used mainly for dams on permeable foundations; it still does not account for exit gradient explicitly.

Khosla's theory

A.N. Khosla (with Bose and Taylor, 1936) studied seepage below weirs in Punjab using the theory of flow nets (conformal transformation) and field observations.

Key conclusions:

1. Outer faces of end sheet piles are much more effective than inner faces and horizontal floor lengths.
2. Intermediate piles shorter than the end piles are ineffective except for local redistribution of pressure.
3. Undermining starts at the tail end — the exit gradient must be kept within safe limits.
4. A downstream end cut-off (pile) is essential.

Method of independent variables

A complex profile is split into simple standard profiles, each solved independently:

1. Straight horizontal floor of negligible thickness with a sheet pile at the upstream end.
2. Straight floor with a pile at the downstream end.
3. Straight floor with an intermediate pile.
4. Straight horizontal floor with no pile (depressed floor effect).

The pressures are then corrected for:

1. Mutual interference of piles — correction $C=19\sqrt{\frac{D}{b^{\prime }}}\frac{d+D}{b}$ (%), where $b^{\prime }$ = distance between piles, $D$ = depth of the pile whose effect is considered, $d$ = depth of the pile at which the correction is applied, $b$ = total floor length.
2. Floor thickness — pressures at the top of the floor found by linear interpolation along the pile depth.
3. Slope of the floor — correction for sloping glacis (tabulated values), positive for down slopes and negative for up slopes in the flow direction.

Floor thickness from the corrected uplift: $t=\frac{h}{G-1}$ (with an appropriate factor of safety), where $h$ = residual uplift head at the point above the water on the floor (downstream floor near the jump is designed for the worst condition — no water flowing, pond full).