7.3 Diversion Headworks

7.3 Diversion Headworks

Introduction to Diversion Headworks

Diversion headworks are hydraulic structures constructed across a river to raise its water level and divert a portion of its flow into an off-taking canal for irrigation, water supply, or other uses. Unlike storage dams, their primary purpose is diversion, not storage. They consist of a complex of integrated structures working together to control water and sediment. The design involves ensuring structural stability against seepage and sliding, preventing unwanted sediment from entering the canal, safely passing floods, and dissipating the energy of overflowing water. This unit explores the components, seepage analysis theories, and key design elements of these critical structures.

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1. Components of Headworks

A typical diversion headwork comprises several interdependent structures, each serving a specific function. The main components from the upstream to downstream side are:

1.1 Main Structures Across the River

1. Weir or Barrage:

   • Weir: A solid, raised structure (masonry/concrete) over which river flow spills. Raises water level to the required Full Supply Level (FSL) of the canal.

   • Barrage: A low-crested structure with large, adjustable gates that can be lowered to the riverbed to pass floods and debris, or raised to pool water. Offers better flood control than a weir.

2. Under-Sluices/Scouring Sluices:

   • Located in the weir/barrage bays adjacent to the canal head.

   • Functions:

     • Create a deep channel in front of the canal head to draw clearer, sediment-depleted water.

     • Flush out sediment deposited in front of the canal head by periodic scouring.

     • Help pass low river flows without spilling over the crest.

3. Divide Wall:

   • A long, masonry/concrete wall constructed perpendicular to the weir/barrage.

   • Functions:

     • Separates the under-sluices from the rest of the weir bays.

     • Forms a still pocket (sand island) in front of the canal head, promoting sediment settlement.

     • Provides a straight approach to the under-sluices for effective scouring.

1.2 Structures for Canal Regulation and Sediment Control

1. Canal Head Regulator:

   • Located at the head of the off-taking canal.

   • Contains adjustable gates to control the discharge entering the canal.

   • Often incorporates a silt excluder in its floor.

2. Silt Control Devices:

   • Silt Excluder: A series of tunnels or conduits in the floor of the under-sluices/weir, upstream of the canal head, to exclude bottom silt-laden water and eject it downstream.

   • Silt Ejector (Silt Extractor): A structure in the canal bed, downstream of the head regulator, that ejects silt that has already entered the canal back into the river.

1.3 Protective and Ancillary Structures

1. Fish Ladder: Provides a graded passage for fish to migrate upstream across the weir.

2. Guide Banks (or Training Works): Earthen/stone banks extending upstream and downstream to confine the river flow to a fixed waterway, protecting the headworks from oblique attack and outflanking.

3. Marginal Bunds and Bank Protection: Protect the adjoining land from submergence and riverbanks from erosion.

4. Groynes/Spurs: Structures projecting from the river bank into the flow to deflect currents and protect the bank or guide the main current.

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2. Seepage Theories and Applications

Seepage (or percolation) of water under the foundation of a weir/barrage creates uplift pressure on the base slab (floor) and can cause piping failure (soil particles being washed out). Three classical theories are used to analyze this and design the length/depth of the impervious floor.

2.1 Bligh’s Creep Theory (1910)

1. Core Concept: Seepage water follows a creeping path (L) along the base contour of the structure. The length of this path determines the safety. Total creep length must be sufficient to cause enough head loss to keep residual uplift pressure safe.

2. Creep Length (L): Sum of horizontal (H) and vertical (V) contacts of water with the base.

   • Vertical cut-offs (sheet piles): Count their depth twice (down and up).

3. Bligh’s Coefficient of Creep (C): An empirical safety factor dependent on soil type.

   Soil Type	Bligh's Coefficient (C)
   Fine micaceous sand	15
   Coarse-grained sand	12
   Sand mixed with boulders	5-9
   Light sand and mud	18

4. Safety Criteria:

   • Against Piping/Undermining: $L \geq C \times H$ Where H = Maximum static head difference (upstream vs downstream).

   • Against Uplift: At any point under the floor, the residual uplift head $h_x = H - \frac{H}{L} \times (\text{Creep length to that point})$. The floor thickness $t$ must balance this uplift: $t = \frac{4}{3} \times \frac{h_x}{G-1}$ (where G = specific gravity of floor material ≈ 2.24 for concrete).

5. Limitations: Does not distinguish between horizontal and vertical creep (i.e., treats them equally). Lane later found vertical creep is more effective.

2.2 Lane’s Weighted Creep Theory (1935)

1. Core Concept: A modification of Bligh’s theory that assigns more weight (resistance) to vertical creep paths than horizontal ones, recognizing their greater effectiveness in dissipating seepage energy.

2. Weighted Creep Length (L_w): $L_w = \sum L_{vertical} + \frac{1}{3} \sum L_{horizontal}$

3. Lane’s Weighted Creep Ratio (C_w): Revised safety factors.

   Foundation Material	Lane's C_w
   Very fine sand or silt	8.5
   Fine sand	7.0
   Coarse sand	5.0
   Gravel and sand	3.5-4.0
   Boulders with gravel/cobbles	2.5-3.0
   Clayey soils	3.0-1.6 (for soft to hard)

4. Safety Criterion: $L_w \geq C_w \times H$

2.3 Khosla’s Theory (or Method of Independent Variables)

1. Core Concept: A scientific, modern theory based on potential flow (Laplace equation). It treats seepage flow below a weir/barrage with intermediate pile lines.

2. Key Principles:

   • The floor is assumed to be of negligible thickness.

   • Exit gradient ($G_E$) is the critical parameter for safety against piping. It is the hydraulic gradient at the downstream end of the floor.

   • $G_E = \frac{H}{d} \times \frac{1}{\pi \sqrt{\lambda}}$ (where d = depth of downstream pile, λ is a function of floor geometry).

3. Safety Criteria:

   • Exit Gradient ($G_E$) must be less than the critical exit gradient for the foundation soil.

     Soil Type	Safe Exit Gradient
     Fine sand	1/6 to 1/7
     Coarse sand	1/5 to 1/6
     Shingle	1/4 to 1/5

4. Procedure: Uses Khosla’s curves (graphs) to compute:

   • Percentage pressures at key points (Pile toes).

   • Exit gradient.

5. Advantages: Most accurate and widely used today. Accounts for the actual flow net and the relative effectiveness of pile lines.

6. Comparison:

   • Bligh/Lane: Empirical, simpler, but less accurate. Suitable for preliminary checks.

   • Khosla: Analytical, accurate, mandatory for final design. Governs the depth and location of cut-off piles (sheet piles).

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3. Design of Silt Control Structures

Preventing excessive silt from entering the canal is crucial to avoid channel sedimentation. Three main devices are used, often in combination.

3.1 Silt Excluder

1. Location: Built into the floor of the weir/barrage, just upstream of the canal head regulator.

2. Principle: Utilizes the fact that bed load (coarser silt) moves near the riverbed. A series of tunnels or conduits with openings at the riverbed level are provided in the under-sluice bays. These tunnels skim off the bottom 20-30% of the flow (which is heavily silt-laden) and eject it downstream through the under-sluice gates, away from the canal head.

3. Design Features:

   • Number of tunnels: 3 to 10.

   • Tunnel floor is kept at riverbed level or lower.

   • Tunnel discharge is about 15-25% of canal discharge.

   • Operated during high silt periods (e.g., early flood season).

3.2 Silt Ejector (or Silt Extractor)

1. Location: Constructed in the bed of the canal itself, a few hundred meters downstream of the head regulator.

2. Principle: Allows silt that has entered the canal to settle in a depressed, enlarged chamber (settling basin). The clearer surface water proceeds downstream, while the settled silt is periodically flushed out through gates back into the river downstream of the weir.

3. Design Features:

   • Consists of a settling basin with lowered floor, followed by an escape regulator.

   • Operated intermittently (e.g., once a day) by opening the escape gates to create a high scouring velocity.

   • Requires a drop structure downstream to return water to the river.

3.3 Settling Basin (or Desilting Basin)

1. Location: Often a standalone, widened, and deepened section of the canal near its head.

2. Principle: Reduces flow velocity to allow suspended silt particles to settle out by gravity (Stokes' Law). Cleaner water is drawn off from the top.

3. Design:

   • Based on settling velocity of particles to be removed.

   • Basin Volume (V): $V = Q \times t$, where t = detention time.

   • Basin Length (L): $L = \frac{V \times v}{w}$, where v = flow velocity, w = settling velocity.

   • Requires periodic mechanical or hydraulic removal of deposited silt.

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4. Design of Weir/Barrage: Crest, Length and Thickness of Impervious Floor

4.1 Weir Crest Design

1. Shape: Often ogee-shaped (S-shaped profile). Designed to match the lower nappe of a free-falling water jet for efficient, stable overflow at design head.

2. Crest Level: Determined by the required Full Supply Level (FSL) of the canal and the head loss over the crest.

   • Crest Level = Canal FSL - Head over Crest (H).

   • H is calculated from the weir formula: $Q = C L H^{3/2}$, where C is the discharge coefficient, L is the clear waterway length.

3. Waterway Length (L): The total clear length over which water spills.

   • $L = \frac{Q}{C H^{3/2}}$

   • Must also satisfy the Lacey's regime width for alluvial rivers to ensure stability: $P = 4.75 \sqrt{Q}$ (where P is wetted perimeter ≈ waterway length).

4.2 Length of Impervious Floor (or Weir Floor)

1. Governed by Seepage Considerations: The total horizontal length of the concrete floor (upstream + downstream) must satisfy the required creep length from Bligh's, Lane's, or Khosla's theory.

2. Typical Proportions:

   • Upstream floor length: 1/3 to 1/4 of total floor length.

   • Downstream floor length: 2/3 to 3/4 of total floor length (more length is needed downstream where uplift pressure is higher).

3. Minimum Length (L_min) from Bligh: $L_{min} = C \times H$.

4.3 Thickness of Impervious Floor

1. Governed by Uplift Pressure: The floor slab must be thick enough to resist the net upward force (uplift pressure minus its self-weight).

2. Design Check (at any section): $t = \frac{4}{3} \times \frac{h_x}{G-1}$ Where:

   • $t$ = Required thickness.

   • $h_x$ = Residual uplift pressure head at that point (from creep theory or Khosla's % pressure).

   • $G$ = Specific gravity of floor material (2.24 for concrete).

3. Provision: Thickness is maximum at the downstream end of the floor (where uplift is highest) and tapers towards the upstream end. A nominal minimum thickness (0.8-1.0m) is maintained for construction and durability.

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5. Design of Energy Dissipaters

The water overflowing the weir crest gains considerable kinetic energy. If not dissipated, it will cause severe scouring downstream, endangering the structure. Energy dissipaters convert this kinetic energy into turbulence and heat.

5.1 Types of Energy Dissipaters

1. Hydraulic Jump Type Basin (Stilling Basin):

   • Most Common. Forces the formation of a hydraulic jump (a sudden transition from supercritical to subcritical flow) within a concrete basin.

   • Components: Chute blocks, baffle blocks (dentated sill), and an end sill to stabilize the jump.

   • Design Principle: The basin length and floor elevation are designed so that the sequent depth of the jump is contained within the basin. Uses conjugate depth relationship: $\frac{y_2}{y_1} = \frac{1}{2} (\sqrt{1+8F_1^2} - 1)$ where $F_1$ is the Froude number at the toe of the weir.

2. Roller (or Impact) Bucket:

   • Used where tailwater depth is insufficient for a hydraulic jump.

   • Types:

     • Solid Roller Bucket: Deflects the jet upwards to form a rolling boil, dissipating energy through internal turbulence.

     • Slotted Roller Bucket: Similar but with slots to reduce horizontal thrust.

   • Suitable for high head structures.

3. Ski-Jump (or Flip) Bucket:

   • Deflects the high-velocity jet up and away from the structure, throwing it into the air where it breaks up and falls into a plunge pool excavated downstream.

   • Used in rocky river beds where a scour hole is acceptable or even designed for.

5.2 Design Steps for a Stilling Basin (USBR Type II/III)

1. Determine Inflow Conditions: Calculate depth ($y_1$) and velocity ($V_1$) at the toe of the weir (supercritical flow).

2. Compute Froude Number: $F_1 = \frac{V_1}{\sqrt{g y_1}}$

3. Calculate Sequent Depth ($y_2$): Using hydraulic jump equation.

4. Check Tailwater Depth (TWL): The existing downstream river level (tailwater) should be equal to or slightly greater than $y_2$ for a free jump. If not, the basin floor must be lowered or raised.

5. Determine Basin Length ($L_B$): Based on $F_1$ and $y_2$, using empirical curves (e.g., USBR design charts). Typically $L_B \approx 4-6 y_2$.

6. Design Appurtenances: Size and position chute blocks, baffle blocks, and end sill per standard proportions.

Conclusion: The design of a diversion headwork is a synthesis of hydraulic, geotechnical, and structural engineering. It requires ensuring stability against seepage forces (via Khosla's theory), controlling sediment entry (via excluders/ejectors), safely passing floods (via barrage gates or weir crest design), and protecting the downstream riverbed from scour (via energy dissipaters). Each component is integral to the reliable and efficient long-term operation of the irrigation system.