Design of alluvial canal & headworks

Canal systems

A canal is an artificial channel that carries water from a river or reservoir to the fields.

Classification

Canal alignment

Principles of alignment: command the maximum area with the shortest length; avoid villages, cemeteries, temples, valuable property; minimise cross-drainage works and balance cutting and filling; avoid rocky and unstable strata; keep curves gentle; place the head where the river is stable.

Canal losses

• Evaporation — small (a few per cent) except in hot, dry, windy conditions.
• Seepage — the major loss: absorption (wetting soil above the water table) and percolation (flow reaching the water table). Depends on soil permeability, water table depth, wetted perimeter, canal age (silting reduces loss) and velocity.
• Seepage raises the water table in the command and is a main cause of waterlogging — lining reduces it (see Canal Lining).

Canal cross-section

• Side slopes — in cutting roughly 1:1 (depending on soil); in filling flatter, about 1.5:1 to 2:1.
• Freeboard — vertical distance between full supply level and top of bank; increases with discharge.
• Banks — retain water, carry service road and inspection path; top width depends on discharge and road requirement.
• Berms — horizontal strips at the full supply level or the natural ground level between the bank toe and water; strengthen banks, protect against erosion, allow future widening. In partial cutting, the berm width is chosen so that the silt deposited on the side slopes forms the stable regime slope (about ½ : 1).
• Service road / inspection road on the bank; dowla — small bund on the road side for safety.
• Spoil banks — excess excavated earth dumped along the canal when cutting exceeds filling.
• Borrow pits — pits from which earth is taken when filling exceeds cutting; dug inside the canal bed (preferably) or at a safe distance outside the bank.
• Balancing depth — the depth of cutting at which the earth from cutting exactly equals that required for the banks (economical section).

Sediment and regime

Alluvial canals carry silt. If velocity is too low, silt deposits (silting); if too high, the bed and banks erode (scouring). A canal is in regime when it neither silts nor scours over a hydrological cycle.

• Initial regime — only the bed slope and depth adjust; width fixed by banks.
• Final (true) regime — all variables (width, depth, slope, shape) adjusted to the discharge and silt.
• Permanent regime — protected (lined) banks and bed; no further adjustment.

Kennedy's silt theory (1895)

R.G. Kennedy's observations on the Upper Bari Doab Canal (Punjab): silt is kept in suspension by the vertical eddies generated from the bed; so the silt-supporting power depends on the depth of flow (not width).

Design procedure (for given $Q$, $n$, $m$, $S$): assume depth $y$ → $V_0$ from Kennedy → $A=Q/V_0$ → find bed width (with side slopes ½ : 1) → compute $R$ and the velocity from Kutter's formula → compare with $V_0$; repeat until they agree.

Limitations:

• Considers only eddies from the bed (ignores those from the sides).
• No equation for bed slope — slope taken from Kutter's formula, which itself has limitations.
• B/D ratio chosen arbitrarily; trial procedure lengthy.
• Silt charge and grade are expressed only through the arbitrary ratio $m$.
• Based on data from one canal system.

Lacey's regime theory (1930)

Gerald Lacey found that regime channels have definite relations among discharge, velocity, hydraulic radius and slope. Silt is supported by eddies generated over the whole wetted perimeter.

Regime conditions assumed: constant discharge; constant silt charge; channel flowing in incoherent alluvium of the same grade that it transports (scoured and deposited with equal ease); unlimited incoherent alluvium.

Design procedure: find $f$ → $V$ → $R$ and $A$ → $P$ → solve for bed width and depth with ½ : 1 slopes → bed slope $S$.

Drawbacks of Lacey's theory: true regime is rarely attained; the concept of incoherent alluvium is idealised; the equations do not treat silt charge and grade separately; the single silt factor cannot represent both; semi-circular shape idea and empirical constants based on limited data.

Kennedy versus Lacey

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).