8.4 Headworks of Run-of-River (ROR) Plants

8.4 Headworks of Run-of-River (ROR) Plants

Introduction to ROR Headworks

In contrast to storage plants, Run-of-River (ROR) hydropower projects are characterized by minimal water storage and direct dependency on the instantaneous river flow. Their headworks are designed not for water impoundment, but for diversion, sediment exclusion, and safe conveyance of water to the turbines. Given that Himalayan rivers like those in Nepal carry very high sediment loads, especially during the monsoon, the design of the intake and sediment handling systems becomes the most critical and challenging aspect of ROR headworks. This section details the components, specialized design considerations, and sediment management strategies unique to ROR plants.

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1. Components of a Typical ROR Plant Headworks

The headworks of a typical ROR plant in a mountainous region consists of the following sequential structures:

1. Diversion Weir (or Barrage):

   • A low-height structure built across the river to raise the water level slightly and divert a portion of the flow into the intake.

   • Often includes a spillway section to pass floods and excess flow.

2. Intake Structure:

   • The entry point to the water conveyance system. Its primary functions are to divert water efficiently while excluding bed load and coarse sediment.

3. Gravel Trap:

   • A small settling chamber located immediately downstream of the intake, designed to remove very coarse bed load (cobbles, gravel) that enters the intake.

4. Settling Basin (or Desilting Basin):

   • The most critical component for sediment management. A long, wide channel or basin designed to settle out suspended sand-sized particles before water enters the headrace tunnel.

5. Flushing System:

   • Integrated with the intake and settling basin. Consists of flushing gates and flushing tunnels/conduits to periodically evacuate accumulated sediment back to the river.

6. By-Pass/Safety Spillway:

   • Located at the downstream end of the settling basin to safely divert excess water or flow during emergency shutdowns back to the river.

7. Headrace Channel/Tunnel Inlet:

   • The transition from the settling basin to the closed conveyance system (tunnel or penstock).

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2. Design of Intake for ROR Plants

The intake design for sediment-laden Himalayan rivers focuses on selective withdrawal to minimize sediment entry.

2.1 Key Principles

1. Location: Placed on the outer bank of a river bend where secondary currents push surface water (cleaner) towards the outer bank and bed load (heavier sediment) towards the inner bank.

2. Alignment: The intake axis should be oriented at an angle (30° to 45°) to the river flow direction to utilize the "ski-jump" effect, where heavier bed load particles skip over the intake opening.

3. Entry Velocity: Maintained low (typically 0.8 - 1.2 m/s) at the trash rack to allow bed load to settle out before entering and to prevent fish entrapment.

2.2 Essential Features

1. Trash Racks:

   • Coarse screens (bar spacing ~50-150 mm) to block large floating and submerged debris (logs, branches).

   • Inclined (10°-30° from vertical) for easier cleaning, often by a rake mechanism.

2. Bed Load Sluice (Scouring Sluice):

   • A low-level opening with a gate located just upstream or adjacent to the main intake.

   • Purpose: To create a localized high-velocity current that scours and carries bed load away from the intake entrance, diverting it downstream.

   • Operated intermittently, especially during high sediment flow.

3. Sill Level:

   • The elevation of the intake floor. Set 1.0 - 1.5 m above the riverbed level at the weir to prevent direct entry of bed load.

4. Approach Channel: A short, converging channel leading to the trash rack to streamline flow.

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3. Methods of Bed and Suspended Load Handling

Sediment is categorized by how it is transported:

• Bed Load: Coarse material (sand, gravel, cobbles) rolling, sliding, or saltating along the riverbed.

• Suspended Load: Fine material (silt, sand) carried in suspension by the water's turbulence.

3.1 Bed Load Handling Strategies

1. Exclusion at Intake (Upstream Handling):

   • River Training Works: Guide walls and groynes to stabilize flow alignment towards the intake.

   • Bed Load Sluice: As described above, the primary mechanical exclusion method.

   • Vortex Tube Settlers: A pipe with slots placed on the riverbed upstream of the intake; vortices suck in and eject bed load.

2. Trapping and Removal (Downstream Handling):

   • Gravel Trap: A small, deep chamber with a low velocity (<0.5 m/s) where coarse particles settle and are periodically flushed.

3.2 Suspended Load Handling Strategy

• Primary Method: Settling Basin (Desilting Basin).

• Principle: Reduce flow velocity so that the settling velocity of target particle sizes (e.g., >0.2 mm sand) exceeds the upward component of turbulent velocity, allowing particles to settle to the floor.

• Objective: To provide water with acceptable sediment concentration (<2000-5000 ppm, or often targeting removal of particles >0.2 mm) to the turbine to minimize abrasion.

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4. Design of Settling Basin: Practice and Concentration Approach

4.1 Settling Theory

The design is based on Stokes' Law for discrete particle settling in tranquil conditions. The key parameter is the settling velocity ($w_s$) of the target particle size (e.g., d=0.2 mm). $w_s = \frac{g(\rho_s - \rho_w)d^2}{18\mu}$ For practical purposes, $w_s$ is often determined from standard tables (~0.02 m/s for d=0.2 mm).

4.2 Standard Practice (Trial Settling Approach)

This is the most common method. The basin is sized so that a particle entering at the top of the inlet has just enough time to settle to the floor before reaching the outlet.

1. Design Parameters:

   • Design Discharge ($Q$): Plant discharge + flushing flow.

   • Target Sediment Size ($d$): e.g., 0.2 mm.

   • Settling Velocity ($w_s$): Corresponding to $d$.

   • Turbulence Factor ($k$): A factor (0.5-0.75) to account for reduced efficiency due to turbulence, short-circuiting, and inlet/outlet disturbances.

2. Key Dimensions:

   • Surface Area ($A_s$): Dictated by overflow rate. $A_s = \frac{Q}{k \cdot w_s}$ This ensures the theoretical upward flow velocity is less than the settling velocity.

   • Length ($L$) and Width ($B$): $L = \frac{v \cdot H}{k \cdot w_s}$ Where $v$ is the horizontal flow velocity (0.2 - 0.3 m/s to prevent re-suspension), and $H$ is the water depth (typically 4-8 m). $B = \frac{Q}{v \cdot H}$

   • Check for L/B Ratio: Typically 4:1 to 10:1 to ensure plug flow and minimize short-circuiting.

4.3 Concentration Approach (Camp's Method)

This method accounts for the continuous removal of sediment along the basin length, leading to a logarithmic decrease in concentration. $\frac{C_{out}}{C_{in}} = \exp\left(-\frac{w_s A_s}{Q}\right)$ Where:

• $C_{in}$, $C_{out}$ = Inlet and outlet sediment concentrations.

• $w_s$ = Settling velocity.

• $A_s$ = Surface area of basin.

• $Q$ = Discharge.

This approach is more rigorous and directly relates basin size to desired outlet concentration.

4.4 Practical Design Features

1. Inlet Structure: Designed to distribute flow evenly across the basin's width and depth (using diffuser walls or multiple ports) to minimize short-circuiting.

2. Outlet Structure: A collecting channel with orifices or weirs at the surface to draw off the cleanest water from the top layer.

3. Floor Slope: A steep longitudinal slope (1-5%) towards the flushing tunnel inlet to facilitate sediment sliding during flushing.

4. Number of Basins: Often two or more in parallel to allow one basin to be taken out of service for flushing while the other remains operational.

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5. Estimation of Sediment Volume in Settling Basin

To size the flushing system and determine frequency, the volume of sediment deposited must be estimated.

5.1 Input Data Required

• Sediment Inflow Concentration ($C_{in}$): Average or peak concentration in kg/m³ or ppm, obtained from sediment rating curves ($C = aQ^b$).

• Trap Efficiency ($\eta_t$): Fraction of incoming sediment trapped. For a well-designed basin targeting size d, $\eta_t$ can be estimated from the ratio $w_s/(Q/A_s)$ or using empirical curves (e.g., Brune's curve).

• Sediment Density ($\rho_{dep}$): Density of deposited sediment in the basin (approx. 1300-1600 kg/m³ for loosely packed sand/silt).

5.2 Calculation of Sediment Inflow Rate

$\text{Sediment Inflow Rate (kg/s)} = C_{in} \times Q$ $\text{Deposition Rate (kg/s)} = \eta_t \times C_{in} \times Q$

5.3 Estimation of Deposit Volume

1. Volumetric Deposition Rate: $V_{dep,rate} (m^3/s) = \frac{\eta_t \cdot C_{in} \cdot Q}{\rho_{dep}}$

2. Total Volume per Period (e.g., daily during monsoon): $V_{dep,total} = V_{dep,rate} \times \text{Time Period}$

3. Basin Storage Volume: The active sediment storage volume of the basin is the space between the operating floor level and a level above which sediment would interfere with operation or be drawn into the outlet.

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6. Flushing of Deposited Sediment and Estimation of Flushing Frequency

Flushing is the process of evacuating accumulated sediment from the basin using a sudden release of high-velocity water.

6.1 Flushing Methods

1. Pressure Flushing (Slulce Flushing):

   • Mechanism: A flushing gate at the downstream end of the basin is suddenly opened. The stored water in the basin creates a pressure gradient, generating a high-velocity current that scours and carries the sediment through a flushing tunnel back to the river downstream of the weir.

   • Applicability: Most common method for ROR plants. Requires the basin to be taken offline (dewatered).

2. Vacuum Flushing:

   • Uses a siphon system to create a sudden drawdown and flushing current. Less common.

6.2 Design of Flushing System

• Flushing Tunnel Conduit: Sized to carry the flushing discharge (typically 2-3 times the plant discharge, $Q_f \approx 2-3Q$) under pressurized flow.

• Flushing Velocity: Must exceed incipient motion velocity for the deposited sediment (typically > 3-4 m/s).

• Flushing Gate: A robust, quick-opening gate (often radial or slide gate) capable of withstanding abrasive flow.

• Outlet Energy Dissipation: Required at the flushing tunnel outlet to protect the riverbed from scour.

6.3 Estimation of Flushing Frequency

Flushing frequency is a balance between operational continuity and sediment management efficiency.

1. Basis: The basin is flushed when the accumulated sediment volume reaches a pre-determined limit (e.g., 50-75% of the active storage volume), before it starts affecting trap efficiency or enters the headrace.

2. Calculation: $\text{Flushing Frequency} = \frac{\text{Allowable Sediment Storage Volume (m}^3)}{V_{dep,rate} (m^3/\text{day})}$ The result is in days.

3. Operational Considerations:

   • During the peak monsoon, flushing may be required daily or even multiple times a day.

   • In the dry season, flushing may be infrequent or not required.

   • Flushing duration is typically short (10-30 minutes) but must be sufficient to remove the majority of deposited sediment.

4. Flushing Water Requirement: The volume of water used for flushing represents lost generation. This "flushing loss" is an important economic penalty and must be minimized through optimal basin and flushing design.

Conclusion: The headworks of a Run-of-River plant is fundamentally a sediment management system. Its success hinges on the integrated design of the intake, settling basin, and flushing works to continuously separate water from the abrasive sediment it carries. Mastery of settling theory, coupled with practical knowledge of flushing hydraulics and sediment estimation, is essential for designing ROR plants in sediment-rich environments like Nepal, ensuring both the longevity of mechanical equipment and the reliable production of energy.