Diversion Headworks & Dams

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).

Reservoirs

A reservoir is a body of water impounded by a dam to store water for later use.

Storage zones and levels

Capacity

• Area–elevation curve from contour surveys; capacity–elevation curve by integration using the trapezoidal formula $\Delta V=\frac{h}{2}(A_1+A_2)$, the cone formula $\Delta V=\frac{h}{3}\left(A_1+A_2+\sqrt{A_1{A}_2}\right)$ or the prismoidal formula.
• Safe (firm) yield — maximum quantity that can be supplied during a critical dry period; secondary yield — water available in excess of safe yield in wet years.
• Required storage for a given demand: mass curve (Rippl) method or sequent peak algorithm (see Runoff).

Reservoir sedimentation

Rivers carry sediment; when flow enters a reservoir, velocity falls and sediment settles — coarse near the head (forming deltas), fine near the dam; density currents can carry fine sediment right up to the dam.

• Trap efficiency ${\eta }_T$ — percentage of incoming sediment retained; depends mainly on the capacity–inflow ratio (C/I); Brune's curves relate them — large storage reservoirs trap almost all sediment.
• Life of a reservoir — the time until sediment fills a specified part of its capacity (e.g. dead storage filled, or live storage reduced to the point the reservoir no longer serves its purpose). Life is computed stepwise because trap efficiency falls as capacity falls.
• Distribution of sediment with depth — empirical area reduction method.

Control of sedimentation:

1. Catchment treatment — afforestation, soil conservation, check dams (most effective long-term).
2. Proper selection of dam site (low sediment yield).
3. Sluicing / flushing — releasing sediment-laden flood water through low-level outlets before it settles; drawdown flushing.
4. Sediment bypass tunnels; venting density currents.
5. Dredging or excavation (costly).
6. Vegetal screens at the head of the reservoir; design with adequate dead storage.

Reservoir losses: evaporation (major in shallow, wide reservoirs), seepage, sedimentation.

Dams — classification

Site selection

Sound foundation rock at reasonable depth; narrow valley opening into a wide basin upstream (small dam, large storage); suitable site for spillway; availability of construction materials; minimum submergence of valuable land, forests, habitations and mineral deposits; low sediment yield; accessibility; watertight reservoir basin; geological stability (no active faults, landslides).

Gravity dams

A gravity dam resists external forces by its own weight. It is the most durable type, requires little maintenance, suits almost any height, and can accommodate an overflow spillway section, but needs a strong rock foundation.

Forces acting

Wave height (Molitor–Stevenson):

($V$ = wind velocity in km/h, $F$ = fetch in km, $h_w$ in m.) Freeboard is commonly taken as about 1.5 $h_w$ above the maximum water level.

Load combinations

The dam is checked for combinations such as: construction condition (empty reservoir, with and without earthquake); normal operating condition (full reservoir, normal uplift, silt, ice); flood discharge condition (MWL, full uplift, tailwater); normal operating + earthquake; drains choked (extreme uplift). Permissible factors of safety are lower for extreme combinations.

Modes of failure

Elementary profile

The theoretical profile of a gravity dam is a right-angled triangle with the vertical face upstream, height $H$ equal to the water depth and base $B$.

The practical profile adds a top width (roadway, commonly about 14% of the height with a minimum), freeboard, and upstream/downstream batters near the base.

Galleries, joints and foundation treatment

• Galleries — for drainage of seepage (drain holes), inspection, grouting and instrumentation.
• Contraction joints (transverse) with shear keys and water stops; longitudinal joints in very large blocks.
• Foundation treatment: excavation to sound rock; consolidation grouting (shallow, to strengthen the foundation); curtain grouting (deep, near the heel, to reduce seepage and uplift); drainage holes downstream of the curtain; treatment of faults and seams (dental concrete).