8.3 Headworks of Storage Plants

8.3 Headworks of Storage Plants

Introduction to Headworks of Storage Plants

The headworks of a storage-type hydropower plant constitute the critical and complex system of structures responsible for impounding, regulating, and safely conveying water to the turbines. This subsystem is located upstream of the powerhouse and is defined by the presence of a substantial dam creating a reservoir. Its design and integrity are paramount, as failure can have catastrophic consequences. This section provides a detailed exploration of the components, types, design principles, stability considerations, and safety features of storage plant headworks, with a focus on dams, spillways, intakes, and associated control structures.

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1. Components of a Typical Storage Plant Headworks

A storage plant headworks is an integrated system comprising the following key structures:

1. Main Dam: The primary structure creating the reservoir by obstructing the river valley.

2. Intake Structure: The controlled entry point for water into the conveyance system (tunnel, penstock) leading to the powerhouse. It is equipped with gates and trash racks.

3. Spillway: The essential safety structure designed to safely pass extreme floods without overtopping the dam. It includes an energy dissipater at its toe.

4. Outlet Works (Bottom Outlet): A low-level conduit used for reservoir emptying, sediment flushing, and providing environmental releases.

5. Auxiliary/Service Spillway: A secondary spillway for extremely rare flood events.

6. Diversion Works: Temporary structures (cofferdams, diversion tunnels) used during construction to divert the river and create a dry work area for the dam.

7. Appurtenant Structures: Access roads, bridge, instrumentation (for monitoring dam health).

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2. Dams: Types, Functions, Selection, Design, Failure Modes and Remedies

2.1 Functions of a Dam

• Primary: Create a reservoir for water storage (for hydropower, irrigation, water supply).

• Secondary: Flood control, recreation, navigation, sediment control.

2.2 Types of Dams (Based on Construction Material and Design)

1. Gravity Dams:

   • Material: Mass concrete or masonry.

   • Principle: Resists horizontal water pressure by its own weight (gravity).

   • Shape: Triangular in cross-section, with a near-vertical upstream face and a sloping downstream face.

   • Advantages: Permanent, durable, can incorporate spillway. Suitable for narrow gorges with strong rock foundations.

   • Disadvantages: High cement consumption, susceptible to thermal cracking during curing.

   • Examples: Kulekhani Dam (Nepal), Grand Coulee (USA).

2. Embankment Dams (Earthfill & Rockfill):

   • Material: Compacted earth, rock, and an impervious core.

   • Principle: Resists forces by its shear strength and mass.

   • Types:

     • Homogeneous Earthfill: Single material.

     • Zoned Embankment: Impervious clay core with pervious outer shells (rockfill) for stability.

     • Rockfill with Concrete Face (RCCD): Rockfill main body with an upstream concrete slab as the water barrier.

   • Advantages: Can be built on weaker foundations, uses local materials, lower cost per volume.

   • Disadvantages: Requires more space, vulnerable to overtopping and internal erosion (piping).

   • Examples: Upper Tamakoshi Dam (Rockfill with CFRD), Nurek (Tajikistan).

3. Arch Dams:

   • Material: Concrete.

   • Principle: Curved upstream face transfers water pressure laterally to the canyon walls (abutments) by arch action.

   • Shape: Thin, doubly curved (in plan and elevation).

   • Advantages: Extremely economical in concrete for narrow, steep-walled valleys with strong abutment rock.

   • Disadvantages: Requires very strong, stable abutments; complex design and construction.

   • Example: Hoover Dam (USA), Idukki (India).

4. Buttress Dams:

   • Material: Concrete.

   • Principle: A thin upstream deck (slab) is supported at regular intervals by downstream buttresses (walls).

   • Types: Flat slab, Multiple arch.

   • Advantages: Less concrete than gravity dam, allows inspection between buttresses.

   • Disadvantages: More formwork, susceptible to ice damage in cold climates.

2.3 Selection of Dam Type

The choice depends on a multi-criteria evaluation:

1. Topography: Narrow gorge → Arch or Gravity. Wide valley → Embankment.

2. Geology/Foundation: Strong rock → Gravity, Arch. Weak/compressible foundation → Embankment.

3. Availability of Construction Materials: Abundant rock/earth nearby favors embankment; access to cement/aggregate favors concrete dams.

4. Climatic Conditions: Cold regions may favor concrete over earthfill (frost action).

5. Seismicity: Embankment dams generally perform better under strong shaking.

6. Cost and Time: Embankment dams often cheaper and faster to build for large volumes.

7. Purpose: Need for an overflow spillway may favor a concrete gravity section.

2.4 Design Principles

• Gravity Dam: Designed for stability against overturning, sliding, and excessive foundation bearing pressure. Must also maintain compressive stresses within the concrete.

• Embankment Dam: Designed for slope stability (upstream and downstream slopes), seepage control (via core and filters), and protection against overtopping (riprap on upstream face, grass on downstream).

• Arch Dam: Designed as a series of horizontal arches and vertical cantilevers, with stress analysis ensuring compressive stresses are within limits and abutments are stable.

2.5 Failure Modes and Remedies

Failure Mode	Description	Common Remedies
1. Overtopping	Flood exceeds spillway capacity, water flows over dam crest.	Adequate spillway design (Probable Maximum Flood - PMF), freeboard provision, erosion-resistant crest protection.
2. Piping/Internal Erosion	Seepage within or under the dam transports fine particles, creating pipes.	Proper zoning with impervious core and graded filter zones, adequate cutoff (trench, grout curtain), drainage blankets.
3. Slope Instability	Failure of upstream or downstream slope due to seepage pressure or earthquake.	Flattening slopes, providing berms, installing drains to lower phreatic line, compaction to required density.
4. Foundation Failure	Shear failure or excessive settlement of foundation.	Thorough foundation investigation, excavation to sound rock, grouting, dental treatment of weak seams.
5. Structural Failure	Cracking or collapse due to excessive stress (concrete dams).	Proper design for loads (water, silt, ice, earthquake), contraction joints, post-cooling of concrete to control thermal stresses.
6. Conduit/Shaft Failure	Leakage or rupture of outlet works, penstock, or spillway conduit.	Robust design, high-quality construction, proper anchorage, and regular inspection.

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3. Stability Analysis of Gravity Dam

The analysis ensures the dam is stable under all possible loading combinations (usual, unusual, extreme).

3.1 Primary Loads on a Gravity Dam

1. Water Pressure ($P_w$):

   • Horizontal: $P_w = \frac{1}{2} \gamma_w H^2$ per unit length. Acts at $H/3$ from base.

   • Vertical (Uplift): Upward pressure in cracks/joints/foundation. Critical for stability. $U = \frac{1}{2} \gamma_w H B$ (simplified triangular distribution), where B is base width.

2. Self-Weight ($W$):

   • $W = \gamma_c \times \text{Volume}$. Acts through the centroid of the dam section.

3. Silt/Sediment Pressure ($P_s$):

   • Acts like earth pressure on upstream face. $P_s = \frac{1}{2} \gamma_s h_s^2 K_a$.

4. Earthquake Load ($P_{eq}$):

   • Modeled as inertia forces (horizontal and vertical) on the dam mass using seismic coefficients.

   • Also includes hydrodynamic pressure from the reservoir.

3.2 Stability Checks (per unit length of dam)

1. Factor of Safety Against Overturning:

   • Ratio of stabilizing moments about the toe to overturning moments. $FOS_{overturning} = \frac{\sum \text{Resisting Moments (about toe)}}{\sum \text{Overturning Moments (about toe)}}$

   • Requirement: Typically > 1.5 for usual loading, > 1.0 for extreme (e.g., earthquake).

2. Factor of Safety Against Sliding:

   • Two criteria:

   • Shear Friction Factor (SFF): $SFF = \frac{\mu \sum V + c A}{\sum H}$ Where $\mu$ is coefficient of friction, $c$ is cohesion, $A$ is base area, $\sum V$ is net vertical force, $\sum H$ is total horizontal force.

   • Sliding Factor (SF): $SF = \frac{\sum H}{\sum V}$ Must be less than the allowable value ($\approx 0.75 \mu$ for no cohesion).

3. Foundation Pressure (Stress) Check:

   • Ensure stresses at heel and toe are within safe limits for the foundation material.

   • The resultant force (R) should pass within the middle third of the base to prevent tension at the heel (for concrete dams).

   • Maximum toe pressure should be less than allowable bearing capacity of foundation rock. $p_{max,toe} = \frac{\sum V}{B} \left(1 + \frac{6e}{B}\right)$ $p_{min,heel} = \frac{\sum V}{B} \left(1 - \frac{6e}{B}\right)$ Where $e$ is the eccentricity of the resultant from the center of the base.

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4. Seepage Control and Foundation Treatment in Dams

4.1 Objectives of Seepage Control

• Reduce seepage loss from the reservoir.

• Control piping and internal erosion.

• Reduce uplift pressure under the dam.

• Prevent harmful seepage forces that can cause slope instability in embankment dams.

4.2 Methods of Seepage Control

1. Impervious Core (for Embankment Dams):

   • A central zone of compacted clay or other low-permeability material.

2. Cutoff:

   • An impervious barrier extending from the dam into the foundation.

   • Types: Concrete cutoff wall (diaphragm wall), Grout curtain (injected under pressure to fill rock fractures), Sheet pile, Slurry trench.

3. Drainage Systems:

   • To collect and safely convey seepage water, thereby lowering the phreatic line (the top of the seepage flow zone) within the dam or its foundation.

   • Components: Horizontal drainage blankets, chimney drains, toe drains, relief wells.

4. Upstream Blanket:

   • An extended layer of impervious material on the reservoir floor upstream of the dam to lengthen the seepage path.

4.3 Foundation Treatment

Essential to ensure a strong, stable, and watertight connection between the dam and its foundation.

1. Surface Preparation: Removal of topsoil, weathered rock, and loose material ("overburden") down to sound rock.

2. Dental Treatment: Excavation and backfilling with concrete to treat specific weak zones, faults, or shear zones in the rock foundation.

3. Grouting:

   • Consolidation Grouting: To improve the general strength and stiffness of the foundation rock mass by filling fine cracks. Done in a pattern over a large area.

   • Curtain Grouting: To create a deep, impervious barrier (cutoff) under the dam axis. Done in a single or multiple lines of deep holes.

4. Drainage Grouting/Holes: Installation of downstream drainage holes to relieve uplift pressure.

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5. Design of Intake, Spillway and Energy Dissipaters

5.1 Intake Structure for Storage Plants

• Location: Usually in the upstream face of the dam or in a separate tower.

• Design Features:

  1. Trash Racks: Coarse screens to block debris. Inclined for easier cleaning.

  2. Gate Slot: For bulkhead/emergency gate and service/head gate.

  3. Bell Mouth Entrance: Streamlined to minimize head loss.

  4. Anti-Vortex Device: Prevents air-entraining vortices from forming at the intake, which can cause vibration and cavitation.

• Elevation: Set above the Minimum Drawdown Level (MDDL) to avoid air entrainment and cavitation, and sufficiently high to minimize sediment entry.

5.2 Spillway Design

The spillway is sized to pass the Design Flood (e.g., Probable Maximum Flood - PMF) without raising the reservoir above the Maximum Water Level (MWL).

1. Types:

   • Ogee (Overflow) Spillway: Common on concrete gravity dams. Has an S-shaped crest profile (ogee) designed for efficient flow at the design head.

   • Chute (Open Channel) Spillway: A steep, open channel on the dam abutment.

   • Side Channel Spillway: The spillway crest is alongside the discharge channel; water spills over and turns 90° into the channel.

   • Shaft (Morning Glory) Spillway: A funnel-shaped crest leading to a vertical then horizontal shaft/tunnel. Used when space is limited.

   • Siphon Spillway: Uses siphonic action to start flow automatically at a set water level.

2. Design Parameters:

   • Design Flood Discharge (Q): Determined from hydrological studies.

   • Crest Length (L) and Crest Shape: Determine the spillway capacity using the weir equation.

   • Crest Gates: Can be added to increase storage without increasing crest height.

5.3 Energy Dissipaters

Used at the spillway outlet to safely dissipate the high kinetic energy of the flowing water and prevent downstream scour.

1. Hydraulic Jump Basin:

   • Forces a jump from supercritical to subcritical flow within a concrete-lined basin.

   • Stilling Basin: A depressed basin with baffle blocks and end sill to stabilize the jump.

2. Bucket-Type Dissipaters:

   • Ski Jump Bucket: Deflects water into the air as a jet, dispersing energy through aerial diffusion and plunge pool impact. Used where rock is very strong.

   • Flip Bucket: Similar, with a more pronounced upward curve.

3. Plunge Pool: A natural or excavated pool where a free-falling jet impacts and dissipates energy. Requires sound rock.

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6. Gates: Types and Locations

Gates are movable barriers used to control the flow of water.

6.1 Based on Function

• Service Gate: Used for normal, regular operation (e.g., to start/stop water flow to turbines).

• Emergency/Guard Gate: Installed upstream of the service gate. Closed only in emergencies or to allow maintenance of the service gate.

• Regulating Gate: Used to precisely control the flow rate (e.g., in spillways, outlet works).

• Bulkhead Gate (Stop Log): Used for temporary closure during inspection/repair. Placed in slots manually or with a crane.

6.2 Common Types of Gates

1. Radial (Tainter) Gate:

   • Curved face plate hinged at the ends. Raised/lowered by hoists.

   • Advantages: Hydraulic forces pass through the pivot, requiring lower hoist capacity. Good for spillways.

   • Location: Spillway crests, intake openings.

2. Vertical Lift (Slide/Sliding) Gate:

   • Flat or slightly curved plate that slides vertically in guides.

   • Advantages: Simple, good sealing.

   • Disadvantages: High hoist capacity required (friction + weight).

   • Location: Intakes, bottom outlets, diversion tunnels.

3. Roller Gate:

   • Similar to vertical lift gate but with rollers to reduce friction.

4. Drum Gate:

   • Hinged at the sill, rotates into a recess in the spillway crest.

   • Provides an unbroken crest when closed. Used for automatic crest raising.

5. Flap Gate:

   • Hinged at the bottom, opens downstream. Used for small outlets and tidal barriers.

6. Butterfly Valve (for conduits):

   • A disk rotating on a shaft inside a pipe. Used for isolation and regulation in penstocks and outlets.

6.3 Gate Selection Factors

• Size of opening (span, height).

• Head (pressure) acting on the gate.

• Operating frequency (regular vs. emergency).

• Space availability.

• Cost and maintenance requirements.

Conclusion: The headworks of a storage plant represent a pinnacle of civil engineering, requiring the synthesis of hydrology, geotechnical engineering, structural mechanics, hydraulics, and construction management. A deep understanding of dam types and their stability, coupled with meticulous design of spillways, intakes, and control gates, is essential to create a safe, efficient, and long-lasting structure that responsibly harnesses the power of stored water.