Introduction to Hydropower

Introduction to Hydropower

Hydropower is defined as a renewable energy source generated by the movement of flowing water on the surface of the Earth.This energy is produced repeatedly without altering the water’s physical properties. In a hydropower plant, water isdirected to move turbines, which in turn run electric generators. The potential energy of water stored in a dam is firstconverted into the kinetic energy of flowing water in the penstock, and then into electrical energy via the turbine–generatorcombination.

Power & Energy Conversion

The power output of a hydropower system can be calculated using the formula:

P = ρ_w × g × Q × H × n (Watts)

• ρ_w – Density of water (approximately 1000 kg/m³)
• g – Acceleration due to gravity (9.81 m/s²)
• Q – Flow rate (m³/s)
• H – Effective head or height difference (m)
• n – Overall efficiency of the turbine–generator set

Energy is defined as the total power consumed over a period and is measured in kilowatt-hours (kWh). For instance,using 1 kW of power continuously for 1 hour consumes 1 kWh of energy.

Classification of Energy

Renewable Energy

Renewable energy sources are those that can be naturally replenished after use. They include:

• Hydropower
• Wind Energy
• Tidal Energy
• Solar Energy

Non-Renewable Energy

Non-renewable energy sources are transformed into unusable forms after consumption. Examples include:

• Fossil Fuels (natural gas, oil, and coal)

Classification of Hydroelectric Plants

Based on Elevation Difference (Head)

• Very Low Head (Head ≤ 15 m):
  Generally uses axial flow turbines (e.g., Kaplan Turbine).
  Example: Gandak Hydro Power Station.
• Low Head (15 m < Head ≤ 60 m):
  Utilizes Kaplan or Francis turbines.
• Medium Head (60 m < Head ≤ 150 m):
  Typically adopts Francis turbines.
• High Head (150 m < Head ≤ 350 m):
  Employs Pelton or Francis turbines.
• Very High Head (Head ≥ 350 m):
  Primarily uses Pelton turbines.

Based on Installed Capacity

• Micro-Hydropower: Up to 100 kW
• Mini Hydropower: 100 kW – 1000 kW
• Small Hydropower: 1 MW – 25 MW
• Medium Hydropower: 25 MW – 100 MW
• Large Hydropower: More than 100 MW

Hydropower Potential of Nepal

Nepal possesses tremendous hydropower potential due to its abundant water resources and dramatic topography. Key factors include:

• Annual Runoff: Approximately 175 billion cubic meters within Nepalese territory, rising to around 200 billion cubic meters when the Tibetan drainage is included.
• Topography: Ranges from the highest peaks (8848 m) to valleys below 100 m over a short distance (160–270 km), creating significant elevation differences.
• River Network: More than 6,000 rivers and rivulets cross the country.
• Average Annual Precipitation: About 1500 mm.

Potential Estimates

Nepal’s hydropower potential can be categorized into three types:

• Gross (Theoretical) Potential: This is the total theoretical capacity based solely on the physical availability of water and topography, estimated at approximately 83,000 MW.
• Technically Feasible Potential: This reflects the capacity that can realistically be harnessed using current technology, considering environmental and engineering constraints. Although exact figures vary, advances in technology make a large portion of the gross potential achievable.
• Economically Feasible Potential: Once economic, market, and infrastructure constraints are applied, the viable potential for development is around 42,000 MW.

Nepali Major River Basin Potential

Classification Based on Operation

• Isolated Plant: e.g. Micro hydropowers.
• Grid Connected: In Nepal, the Independent National Power System (INPS) connects all projects. The grid is owned by NEA and operated through a load center at Swichatar, Kathmandu.

Classification Based on Storage capacity

Run-of-River Hydropower

In run-of-river plants, electricity is generated as and when water flows in the river. When the river dries up or the flow falls below a certain predetermined minimum (i.e. the minimum flow required for the turbine), electricity generation stops.

Due to the large variation in river flow in Nepal, these projects are normally designed for the dry season flows. They are suitable for perennial rivers that have adequate discharge throughout the year. The minimum dry flow must be sufficient to run the turbine efficiently and economically.

For example: Bhotekoshi Hydroelectric Plant (36 MW), Indrawati-III (7.5 MW), Modi Hydropower Plant (14 MW), etc.

Examples

1. Kulekhani II: 2 × 16 MW, Francis Turbine, commissioned in 1986.
2. Kulekhani III: 2 × 7 MW, Francis Turbine, commissioned in 2019.
3. Upper Trishuli 3A Hydropower Station (60 MW): 2 units each of 30 MW; located in Rasuwa and Nuwakot districts; commissioned in 2019.
4. Trishuli Hydropower Station (24 MW): 6 units each of 3.5 MW and 1 unit of 3 MW; commissioned in 196? AD; utilizing Francis Turbine.
5. Devighat Hydropower Station (14.1 MW): 3 units; a cascade project of Trishuli Hydropower Station; commissioned in 1984 AD; using Vertical Francis Turbine.
6. Gandak Hydropower Station (15 MW): 3 units each of 5 MW; using Kaplan Turbine Generators; located in Nawalparasi district; commissioned in 1979 AD.
7. Modi Khola Hydro: 2 units each of 7.4 MW; located at Dimuwa in Parbat district.

Peaking Hydropower Projects of Nepal

1. Kali Gandaki 'A' Hydropower Station (144 MW):
   • 3 units, each with a capacity of 48 MW
   • Located in Beltari, Syangja
   • Commissioned in 2002 AD
   • Operates with 6 hours of daily peaking
2. Middle Marsyangdi Hydropower Station (70 MW):
   • 2 units, each with a capacity of 35 MW
   • Located in Lamjung
   • Commissioned in 2008 AD
   • Operates with 5 hours of daily peaking
3. Marsyangdi Hydropower Station (69 MW):
   • 3 units, each with a capacity of 26 MW
   • Located at Aanbukhaireni, Tanahu District
   • Commissioned in 1989 AD
   • Operates with 4 hours of daily peaking
4. Upper Tamakoshi Hydropower Station (456 MW):
   • 6 units, each with a capacity of 76 MW
   • Located in Dolakha District
   • Gross head: 822 m; Design discharge: 66 m³/s
   • Uses Pelton turbines
   • Operates with 4 hours of daily peaking
   • Live storage capacity: 1.2 million m³

Storage Type Hydropower

Storage plants have enough capacity to offset seasonal fluctuations in water flow and provide a constant supply of electricity throughout the year. Most rivers in Nepal show significant variations between the rainy (monsoon) season and the dry season. This variation often causes low power generation during the dry season when using run-of-river schemes.

To mitigate this, excess water during the rainy season is stored by constructing a dam and reservoir. This stored water can be used to generate electricity during the dry season (winter). Large dams can store several years’ worth of water and may be built in non-perennial rivers. In addition, the powerhouse may be located at the toe of the dam or separated from it.

For example: Kulekhani-I (60 MW) and Budhigand Hydroelectric Plant (650 MW) demonstrate the storage hydropower approach.

Kulekhani-I Hydropower Station (60 MW): Located in Makwanpur district, it features 2 units. The project uses a Rock Fill Earthen Dam with a height of 114 m and a total reservoir capacity of 85,300,000 m³, and is equipped with Pelton turbines.

Pump Storage Plants

Unlike conventional hydropower plants, pump storage plants reuse water. After water produces electricity by flowing through turbines, it is discharged into a lower reservoir located beneath the dam. During off-peak hours (when energy demand is low), some of this water is pumped into an upper reservoir. Later, during periods of peak demand, the stored water is released to generate additional electricity.

Pump storage plants generate power only during peak hours, providing extra capacity to meet high demand. However, these systems are comparatively expensive.

Methods of Fixing Installed Capacity of a Plant

1. Marginal Cost and Benefit Approach:

   Total Marginal Cost = Annual cost of installed electromechanical equipment (AC) + additional costs (e.g., O&M costs).

   Marginal Benefit = Total energy generated × energy rate.

2. Installed Capacity Optimization Approach:

   Determine the power corresponding to different percentage levels of time available discharge (from Q25 to Q75) and trade off between cost and revenue to achieve optimum benefit.

History of Power Sector Development in Nepal

Early Developments

• Farping (500 kW): A model project in Asia (investment: 7 lakhs; duration: 17 months), commissioned in 1911 AD.
• Sundarijal (640 kW): Achieved before World War II (1936 AD); used for water supply in the Kathmandu Valley after discharge from a tailrace.
• Morang Hydropower Company (MHC): Established during the tenure of Sri 3 Juddha Samser under the planning of Bijay Samser with Padma Sundar Malla as the Chief Engineer, in 1939 AD.
• Letang HPP (1800 kW): Constructed by MHC, mainly supplying the Biratnagar Jute Mill; commissioned in 1942 AD. Note that Letang HPP was never rehabilitated after it became defunct over time.
• MHC also established various diesel plants in different parts of the Terai.

Further Developments

• Panauti HPP (2.4 MW): Commissioned in 1965; noted for the construction of Nepal's first 33 kV transmission line.
• Phewa HPP (1090 kW): Commissioned in 1967.
• Trishuli Hydropower (24 MW): Commissioned in 1968.
• Subsequent projects carried out until 1989 include:
  • Sunkoshi (10 MW)
  • Tinau (1 MW)
  • Gandak (15 MW)
  • Kulekhani I (60 MW)
  • Devighat (14.1 MW)
  • Seti (1.5 MW)
  • Kulekhani II (32 MW)
  • Marsyandi (69 MW)
  (The total capacity achieved during this period is noted as "only 24" in the text, suggesting limited progress.)
• During the 8th Plan Period (1991/92–1996/97), additional projects were:
  • Andhikhola (5.1 MW)
  • Jhimruk (12 MW)
  • Chatrik (name partially given)
  These additions brought the capacity to around 269 MW, which still indicated very slow progress.

History of Power Sector Development in Nepal

After 1990, the Government of Nepal recognized the drawbacks of a monopolistic power generation and distribution system. In response, it embraced liberalization by inviting the private sector to participate in power development through the Electricity Act 1992 and the Hydropower Development Policy 1992.

Achievements During the 9th Plan Period (1997/98–2002/03)

The 9th Plan Period proved to be a milestone in hydropower development, with both private and public sector initiatives making substantial progress:

• Private Sector Projects: Khimti (60 MW), Bhotekoshi (36 MW), Indrawati (7.5 MW).
• Government/NEA Projects: Puwakhola (6.2 MW), Modi (14 MW), Kaligandaki A (144 MW).

The combined efforts resulted in an addition of 290 MW during this period, raising the total installed hydropower capacity to 537 MW. Overall, a progress rate of 46.2% was achieved, marking this period as one of the most successful eras for hydropower development in Nepal.

Planned and Proposed Hydropower Projects

Export of Hydropower

In an encouraging development, the Government of India (GoI) has granted concurrence for Nepal to export approximately 452 MW of power generated from various hydropower projects. The Nepal Electricity Authority (NEA) transmits this power via the Dhalkebar-Muzaffarpur 400 kV transmission line to the Day-Ahead Market of the Indian Energy Exchange (IEX), with an additional 40 MW supplied in real-time.

Hydropower Development Policy 1992 (Modified) 2001

In the wake of economic liberalization and open market reforms, the policies of 1992 were modified in 2001 to restructure the power system. These reforms addressed issues such as monopoly, transparency, cost, and electricity quality while ensuring that hydropower emerged as a key alternative to biomass and thermal plants.

Key Provisions

• Implementation of the Build, Operate, Own, and Transfer (BOOT) concept.
• Attraction of both national and international investments.
• Development of large storage and multi-purpose hydropower projects.
• Promotion of public-private partnership (PPP) models.
• Emphasis on hydropower as a viable alternative to biomass and thermal plants.
• Commitment to appropriate social and environmental safeguards.
• Provision for local benefits, including rural electrification, employment generation, and community development.
• Implementation of training programs to cultivate skilled manpower.

Licensing Framework

The licensing process within the policy is structured to facilitate hydropower projects of varying scales:

1. Study/Survey License: For projects up to 10 MW, issued within 60 days of submission and valid for up to 5 years.
2. Generation License:
   • For projects serving internal demand, the license is valid for 35 years.
   • For export-oriented projects, the license is valid for 30 years.
3. Transmission and Distribution License: Issued for 25 years, with renewable extensions of 10 years at a time as per prevailing laws.
4. For hydropower projects up to 1 MW, a formal license is not required; however, registration with the District Water Resources Committee is mandatory.

Investment and Power Purchase Agreements

Investment in hydropower initiatives may be executed through sole ventures or joint ventures involving both domestic and foreign investors. The associated Power Purchase Agreements (PPAs) outline tariff structures based on the project type:

• Reservoir Projects: Tariff rates vary between peak months (December to May) and off-peak months (June to November).
• Run-of-River (ROR) Projects: Tariffs are adjusted seasonally for wet and dry periods.
• Peaking ROR Projects: Adjusted tariffs are provided to account for seasonal output fluctuations.

Department of Electricity Development (DoED)

Initially established as the Electricity Development Center on July 16, 1993, under the Ministry of Water Resources, the organization was rebranded as the Department of Electricity Development in January 2000. The DoED is responsible for:

• Promoting private sector participation by awarding licenses for power generation.
• Conducting feasibility studies for small, medium, large, and storage-type hydropower projects.
• Monitoring ongoing and under-construction hydropower projects.
• Advising the Ministry of Energy on policy formulation, planning, and regulatory matters related to hydropower.

Water and Energy Commission Secretariat (WECS)

Established in 1975 under the Ministry of Energy, WECS functions as an advisory think tank to the government. Its primary objectives include:

• Formulating and supporting the development of policies and strategies in the water resources and energy sector.
• Providing recommendations and guidance for the development of irrigation, hydropower, and drinking water projects.
• Assisting in the establishment of policies addressing industrial water usage, flood management, inland navigation, fisheries, and recreational activities while ensuring environmental protection.

Relevant Legislative Acts

Foreign Investment and Technology Transfer Act, 2049 (1992)

• Requires permissions from the Department of Industries or the Department of Cottage and Small Industries.
• Provides income tax benefits and facilitates the transfer of foreign currency.
• Includes visa provisions for non-tourist, business-related travel.
• Establishes guidelines for dispute resolution and rule framing.

Water Resources Act, 2049

• Asserts the state's ownership over water resources.
• Requires licenses for using water beyond essential domestic, drinking, or irrigation purposes.
• Sets priority levels for water usage: drinking water, domestic use, irrigation, hydropower, industrial enterprises, and recreational activities.

Electricity Act, 2049

• Empowers the government to forge agreements with licensees on financial and technical matters.
• Details provisions regarding royalty payments, income tax benefits, and foreign exchange measures.
• Establishes the framework for assessing electricity charges and related fees.

Sediment Deposits in a Reservoir

Sediment deposition is a vital consideration for the design, operation, andmaintenance of reservoirs. Over time, sediments carried by inflowing water settle within the reservoir, reducing its effective storage capacity and potentially affecting operational efficiency.

Types of Loading

Sediment loading occurs in various forms, primarily characterized by the size and mode of transport of the sediment particles:

• Bed Load: Coarser sediments that move along the reservoir bed due to rolling or sliding.
• Suspended Load: Finer particles that remain in the water column and settle slowly.
• Wash Load: Very fine sediments that stay almost entirely in suspension and are generally transported out of the reservoir.

Factors Affecting Sedimentation

Several factors influence the rate and extent of sediment deposition in areservoir:

• Inflow Velocity: Higher velocities keep sediments in suspension longer, reducing the rate of deposition.
• Watershed Characteristics: The terrain, soil type, land use, and land cover determine the amount of sediment that reaches the reservoir.
• Reservoir Geometry: The shape, area, and depth affect how and where sediments settle.
• Seasonal Variations: Seasonal changes in rainfall and runoff patterns lead to episodically high sediment loads.
• Human Activity: Deforestation, agriculture, and construction practices can accelerate erosion and increase sediment inflow.

Sediment Control in Reservoirs

To mitigate the adverse effects of sediment deposition and extend thereservoir’s life, various control measures are implemented:

• Dredging: Mechanical removal of accumulated sediments to restore storage capacity.
• Flush Operations: Scheduled water releases which help mobilize and expel accumulated sediments downstream.
• Sediment Bypass Tunnels: Infrastructure designed to divert sediment-laden water away from the reservoir.
• Watershed Management: Land use practices such as reforestation and soil conservation to reduce sediment inflow.
• Design Modifications: Incorporation of sediment traps and erosion-control measures in dam design.

Life of a Reservoir

The useful life of a reservoir is largely determined by the rate of sedimentaccumulation. As sediments deposit over time, the effective storage capacitydiminishes, which may necessitate interventions such as dredging or othersediment management strategies to prolong the reservoir’s operational life.

Sediment Yield and Reservoir Trap Efficiency

Sediment Yield refers to the amount of sediment carriedby the inflowing water from the watershed, typically measured in tons peryear. The Trap Efficiency of a reservoir is the ratio ofthe sediment that the reservoir retains compared to the total sedimentinflow.

The following table summarizes these key parameters:

Capacity Inflow Ratio

The capacity inflow ratio is a critical metric for evaluating thesusceptibility of a reservoir to sedimentation. It is determined by comparingthe reservoir’s total storage capacity with the annual volume of water inflow.

• A low capacity inflow ratio indicates that even a small increase in sediment yield could significantly reduce the reservoir's capacity.
• A high capacity inflow ratio suggests greater resilience to sediment accumulation as the reservoir can accommodate more sediment before a notable reduction in storage capacity occurs.

Efficiency of Different Power Station

Hydropower Plant Components

This page presents the complete process of a hydropower project—from diverting the river water to the discharge back into the river. The diagram below shows the line components, and each component is explained with its functionality, premium features, common usage, and examples from Nepal.

Component Explanations

               ┌─────────────────────────────┐
               │        RIVER/STREAM         │
               └──────────────┬──────────────┘
                              │
                              ▼
               ┌─────────────────────────────┐
               │   Diversion Structure       │
               │ (Diverts water from natural │
               │      river course)          │
               └──────────────┬──────────────┘
                              │
                              ▼
               ┌─────────────────────────────┐
               │        Gravel Trap          │
               │ (Removes gravel and debris) │
               └──────────────┬──────────────┘
                              │
                              ▼
               ┌─────────────────────────────┐
               │          Intake             │
               │ (Collects & regulates water,│
               │  prevents debris entry)     │
               └──────────────┬──────────────┘
                              │
                              ▼
               ┌─────────────────────────────┐
               │      Settling Basin         │
               │ (Slows water for sediment   │
               │        settlement)          │
               └──────────────┬──────────────┘
                              │
                              ▼
               ┌─────────────────────────────┐
               │  Headrace Conveyance        │
               │ (Channels water to fore bay)│
               └──────────────┬──────────────┘
                              │
                              ▼
               ┌─────────────────────────────┐
               │          Fore Bay           │
               │(Acts as a small reservoir   │
               │ to ensure constant head)    │
               └──────────────┬──────────────┘
                              │
                              ▼
               ┌─────────────────────────────┐
               │ Surge Tank / Head Tank      │
               │ (Absorbs pressure surges;   │
               │   prevents water hammer)    │
               └──────────────┬──────────────┘
                              │
                              ▼
               ┌─────────────────────────────┐
               │         Penstock            │
               │ (Channels high-pressure     │
               │ water to the turbines)      │
               └──────────────┬──────────────┘
                              │
                              ▼
               ┌─────────────────────────────┐
               │Anchor Block / Support Piers │
               │(Stabilizes and anchors the  │
               │         penstock)           │
               └──────────────┬──────────────┘
                              │
                              ▼
               ┌─────────────────────────────┐
               │         Powerhouse          │
               │(Houses turbines & generators│
               │to convert water energy into │
               │       electricity)          │
               └──────────────┬──────────────┘
                              │
                              ▼
               ┌─────────────────────────────┐
               │          Tailrace           │
               │(Discharges water back to the│
               │      river/stream)          │
               └──────────────┬──────────────┘
                              │
                              ▼
               ┌─────────────────────────────┐
               │        RIVER/STREAM         │
               └─────────────────────────────┘

Headworks of Storage Plant

Components of a Typical Storage Plant

1. Dam and Reservoir
2. Spillway
3. Trash Rack
4. Surge Tank
5. Penstock
6. Turbine
7. Powerhouse
8. Draft Tube

Inside a Hydropower Plant

1. Powerhouse
2. Power Lines
3. T t.nsfotmet (item unclear)
4. Generator
5. Control

Dam Components and Definitions

Spillway

It is the arrangement made (kind of passage) near the top of the dam for the passage of surplus or excessive water from the reservoir.

Abutments

The valley slopes on either side of the dam wall to which the left and right ends of the dam are fixed.

Gallery

A level or gently sloping tunnel-like passage (small room-like space) within the dam with a drain on the floor for seepage water. These passages provide space for drilling grout and drainage holes or to accommodate instrumentation for studying dam performance.

Sluiceway

An opening in the dam near the base provided to clear the silt accumulation in the reservoir.

Free Board

The space between the highest level of water in the reservoir and the top of the dam.

Dead Storage Level

The level of permanent storage below which water will not be withdrawn.

Diversion Tunnel

A tunnel constructed to divert or change the direction of water to bypass the dam construction site. The dam is built while the river flows through the diversion tunnel.

Purpose of Dam

A dam is generally most suitable in hilly areas where deep valleys offer extensive water storage. The stored water on its upstream side serves various purposes such as:

• Flood Mitigation
• Irrigation
• Water Supply
• Navigation
• Fishery and Wildlife Preservation
• Hydro-electric Power Generation
• Recreation

Types of Dam

A. Classification as per Function and Use

1. Storage Dam: Normally constructed to store excess flood water for various purposes such as irrigation, water supply, and hydropower. It may be made of concrete, earth, or rockfill (e.g., Kulekhani-1).
2. Detention Dam: Mainly constructed to control floods. A detention dam safeguards against possible flood damage on the downstream side. (No such dam in Nepal till date.)
3. Diversion Dam: Used to divert river water into canals, conduits, or other streams. Typically a weir or low-level dam is constructed to raise the water level for diversion (e.g., Bheri-Babai Diversion, Sunkoshi Marine Diversion).
4. Debris or Check Dam: A low-height dam constructed across a stream channel to conserve soil and retain sand, gravel, driftwood, or other debris.
5. Coffer Dam: A temporary dam constructed to isolate the construction area from river flow during dam building.

B. Classification as per Hydraulic Design

1. Overflow Dam: Built to allow the overflow of surplus water above its top. Only a few meters of its length act as the overflow section (e.g., Rigid Dam).
2. Non-Overflow Dam: Designed so that water is not allowed to overtop the dam (e.g., Earth dam, rockfill dam, Kulekhani-1 dam).

C. Classification as per Structural Design

1. Gravity Dam: A solid concrete or masonry dam that resists external forces through its own weight.
2. Arch Dam: A curved masonry or concrete dam with the convex side facing upstream. It resists a major portion of water pressure through arch action and generally has less self-weight than a gravity dam.
3. Buttress Dam: Consists of an upstream sloping deck supported by several buttresses or piers constructed of reinforced concrete and supported by struts or bracings.
4. Embankment Dams: Constructed from locally available soils, gravel, and sand that resist external forces through their shear strength. They are usually trapezoidal in section (e.g., Earth dam, earth and rock fill dam).

• High Dam or Large Dam: If the height of the dam is greater than 100 m.
• Medium Dam: If the height of the dam is between 50 m and 100 m.
• Low Dam or Small Dam: If the height of the dam is lower than 50 m.

D. Classification as per Material of Construction

• Rigid Dams: Dams made of rigid material (typically concrete).
• Non-Rigid Dams: Dams built of non-rigid materials such as earth, rock fill, etc. Examples include earth dams, embankment dams, and rock fill dams.

Selection of Site for Dam

• Foundation
• Topography
• Reservoir
• Catchment Area
• Spillway
• Construction Material
• Communication
• Environmental Conditions

Design of Dam

Forces Acting on a Gravity Dam

• Uplift Pressure: Due to water percolation under the dam.
• Effect of Vertical Acceleration (a_v):

  When vertical acceleration occurs, inertia forces act on the water.

  Formula:

  F = (W/g) x av g = W x av

  The inertia force acts opposite to the direction of acceleration. When the acceleration is downward, the inertia force acts upward (reducing the dam's effective weight), and when the acceleration is upward, it acts downward (increasing the effective weight). Hence, the modified weight of the dam is:

  Modified weight: W′ = W(1 ± a_v)

• Wave Pressure:

  Blowing winds generate waves on the reservoir surface that produce pressure toward the downstream side. The wave pressure depends on the height of the waves.

  Wave Height Equations:

  hw = 0.032WT + 0.763 - 0.271 km   hw = 0.032(TV for F > 32 km)

  Where:
  hw = height of water from crest to trough (in metres),
  V = wind velocity (in km/hr),
  F = fetch (length of water expanse in km).

  The maximum pressure intensity due to wave action is given by:

  pw = 2.4 x hw

  This pressure acts at (5hw/6) metres above the still water surface with a triangular distribution.

• Ice Pressure:

  In cold countries, ice formed on the reservoir may expand upon melting, causing a thrust on the dam’s face. This force acts linearly along the dam at the reservoir level. Its magnitude can vary from 250 to 1500 kN/m, with an average allowable value of around 500 kN/m².

Sliding Basis

To ensure safety against sliding, the frictional resistance μ(W – U) must be equal to or greater than the horizontal forces. Various forms of this relationship using parameters such as g, H, B, and other coefficients are provided.

μ(W - U) = P,   or,   g(W - U) = pK H B²   or   g B (G - K)²H,   or   B ≈ H / √[g(G - K)]  

Design of Earthen Dams

General Considerations

Earthen dams are typically more economical due to the use of local materials and unskilled labor. They can be constructed on relatively poor foundations with modern techniques like compaction, grouting, and foundation improvement. Continuous field observation and monitoring for deformation and pore water pressures are necessary during construction for any required modifications.

Demerits

• Risk of damage or destruction due to overtopping.
• Potential for internal erosion of the dam or its foundation.
• High maintenance cost.
• Not suitable for very wet climates.

Common Causes of Earthen Dam Failures

• Overtopping: Occurs when the free board or spillway capacity is insufficient.
• Erosion of the Downstream Toe: Caused by heavy cross-currents from spillway buckets or tail water. This can be mitigated by providing an appropriately pitched downstream slope or riprap protection.
• Erosion of the Upstream Surface: Waves at the water’s surface may erode the upstream face and cause slippage. Protective measures include stone pitching or riprap.
• Seepage Failure: Uncontrolled seepage through the dam body or foundation may lead to failure. This includes:
  • Piping through the dam body: Seepage initiating through weak soils, forming channels that transport material downstream.
  • Piping through the foundation: Occurring when permeable cavities or layers (of gravel or coarse sand) in the foundation lead to heavy seepage and erosion.
  • Sloughing of the downstream side: Saturation at the downstream toe can cause erosion and a subsequent slump or slide.

Design Criteria for Earthen Dams

• Top Width:
  • The top width should be a minimum of 3 m (often governed by roadway requirements).
  • Thumb rules:
    • For very low dams: W = H/5 + 3
    • For dams lower than 30 m: W = H/5
    • For dams higher than 30 m: W = 1.65(H + 1.5)^1/3
• Free Board: Typically 1.5 × hw.
• The phreatic (seepage) line must remain within the downstream face of the dam to prevent sloughing.
• Both upstream and downstream slopes should be protected using riprap, berms, or grass to withstand wave, rain, and erosive impacts.
• Adequate spillway and outlet capacities are required to avoid overtopping during the design flood.

Prevention of Seepage through the Embankment

• Horizontal Drainage Filter: Provide a filter with a length L = 3H or 25–100% of the horizontal distance from the centre of the top width to the toe.
• Filter Drainage: An inclined filter towards the toe, with a height of 25–30% of the dam height.
• Chimney Drains: Extend upward into the embankment for effective drainage.

Modes of Failure and Stability Analysis

1. Overturning (Rotation) about the Toe: If the resultant of all the forces acting on a dam at any section passes outside the toe, the dam will tend to rotate and overturn about the toe. The ratio of the righting moments (anti-clockwise) to the overturning moments (clockwise) is called the factor of safety against overturning; a value generally greater than 1.5 is desirable.
2. Compression or Crushing: A dam may fail when the compressive stresses exceed the allowable stresses of the construction material. The vertical stress distribution at the base is given by the equation:
   p = Direct Stress + Bending Stress.Masonry and concrete gravity dams are designed so that no tension develops; however, under severe loading conditions, a limited amount of tensile stress (around 5 MN/m² under worst-case conditions) may be allowed. Tension cracks (for example, at the heel) may reduce the effective width of the base, shift the resultant force towards the toe, and lead to increased compressive stress and potential failure.
3. Sliding (Shear Failure): Sliding occurs when the net horizontal force (external forces) exceeds the frictional resistance along any potential sliding plane at the dam or its base. The frictional resistance is given by the product of the normal force and the coefficient of friction. A factor of safety against sliding (F.S.S.) greater than unity is required.

Foundation Treatment for Gravity Dams

Preparing the Surface

The entire loose soil is removed until a sound bedrock is exposed. The final surface is then stepped to increase frictional resistance against sliding. In addition, a shear key might be provided.

Grouting the Foundation

Foundation grouting is divided into two parts:

1. Consolidation Grouting: Shallow holes (called B holes) are drilled through the foundation rock, normally between 10 to 15 m deep and 5 to 20 m apart near the heel. A mixture of cement and water (grout) is forced into these holes at low pressure (about 30 to 40 N/cm²) prior to any concreting. This consolidates the foundation and later serves as a cutoff when high-pressure cement grouting is employed.
2. Curtain Grouting: Curtain grouting helps in forming the principal barrier (or curtain) against seepage through the foundation, thereby reducing uplift pressures. To accomplish this high‐pressure grouting, relatively deeper holes (called “A holes”) are drilled near the heel of the dam. The spacing of these holes may vary from 1.2 to 1.5 m. Initially, holes are drilled and grouted at about 10 to 12 m apart, and then the intermediate holes are drilled and grouted.

   The depths of the holes vary from 30 to 40% of the total upstream water head for strong rock foundations, and may be as much as 70% of the water pressure head for poor rock conditions. After drilling, a mixture of cement and water (grout) is forced into the holes under high pressure. The grouting pressure is maintained as high as possible without lifting the foundation strata. Typically, the foundation pressure during high‐pressure grouting is approximately 2.5 D N/cm² (where D is the depth of grouting in metres below the surface).

   Grouting is generally performed in stages (approximately 15 m depth stages) and is carried out only after a portion of the dam section has been laid. This grouting may be accomplished from the foundation gallery, other galleries within the dam, from the upstream face if feasible, or in certain special cases, from tunnels driven into the foundation rock below the dam.

Seepage Control in Earth Dam

1. Providing the Drainage Filter (Filter Media)

To reduce downstream pore water pressure, a drainage filter is provided. This typically involves:

• Rock toe protection
• Horizontal blanket
• Chimney drain

A multi-layer filter—commonly referred to as an inverted filter, horizontal blanket, or horizontal filter—is used in these applications.

2. Seepage Control Through the Foundation

• Impervious Cutoff: Concrete or sheet piles may be installed at the upstream end of the earthen dam. A cutoff installed to 50% of the foundation depth reduces discharge by 25%, and to 90% depth, discharge is reduced by 65%.
• Relief Wells and Drain Trenches: When large-scale seepage occurs through a foundation overlain by a thin layer, there is potential for water to boil up near the dam toe. Relief wells or drain trenches can be constructed through the upper impervious layer. The weight of the overlaying material helps resist upward pressure, preventing sand boil.

Spillways: Design and Energy Dissipation

A spillway is a hydraulic structure designed to safely pass excess water when the reservoir is full. It is often regarded as a safety valve.

Types of Spillways

According to Hydraulic Regime:

• Pressure Spillway: Includes shaft spillway, syphon spillway, and submerged spillway.
• Non-pressure Spillway: Other types not relying on pressure.

According to Function/Purpose:

• Main or service spillway
• Auxiliary spillway
• Emergency spillway

According to Regulation/Control:

• Controlled or gated spillway
• Uncontrolled or ungated spillway

According to Prominent Features:

• Overflow or overfall spillway
• Side channel spillway
• Chute or trough spillway
• Shaft spillway
• Siphon spillway
• Tunnel or conduit spillway
• Orifice or submerged type spillway

Ogee Spillway

An ogee spillway is an improved form of a straight drop spillway where the downstream face of the weir is constructed to mimic the shape of the lower nappe of a freely falling water jet (an ogee shape based on projectile motion principles).

Shaft Spillway

(Also known as Morning Glory or Bell Mouth Spillway)

• Features a vertical shaft followed by a horizontal conduit.
• Recommended when space is limited for other types of spillways.

Side Channel Spillway

• Similar in concept to a chute spillway, but the crest is located on one of its sides.
• Water spills from the crest, is deflected 90° and flows parallel to it.

Straight Drop Spillway

Consists of a low-height weir wall with a downstream face that is roughly or perfectly vertical. To prevent scouring of the downstream bed from the falling water jet, an artificial pool with a concrete apron and a low secondary dam is constructed downstream.

Chute Spillway (Trough or Open Channel Spillway)

• Discharges surplus water from upstream to downstream via a steeply sloped open channel.
• Preferred in situations where the river valley is very narrow.

Siphon Spillway

• Discharges surplus water downstream through an inverted U-shaped conduit.
• When the reservoir level rises above the normal pool level, water enters the conduit and discharges by siphonic action.

Energy Dissipation

The energy dissipators for spillways can be grouped into several categories:

• Hydraulic jump stilling basins
• Free pits and trajectory buckets
• Roller buckets
• Oscillation by spatial hydraulic jump (impact type energy dissipators)

Hydraulic Jump Type Stilling Basin

A stilling basin is provided at the toe of a spillway to dissipate the energy of excess water by forming a hydraulic jump. As the flow moves at a critical depth over the spillway crest, it becomes supercritical at the dam toe and then meets subcritical flow downstream. The basin typically consists of:

• Chute blocks
• Baffle blocks
• End sills

Roller Bucket

A roller bucket dissipates energy when the tail water depth is much greater than the post-jump depth. When a high-velocity sheet of water slides down the spillway, it is arrested by the tail water.

Deflector / Flip / Ski Jump / Trajectory Bucket

This energy dissipator is used when the tail water depth is insufficient to form a hydraulic jump (i.e., when the tail water depth is much less than the post-jump depth). It is suitable for situations where the foundation rock is strong enough to withstand the erosive impact of the jet. The bucket deflects the high-velocity jet into the air so that it strikes the river bed at a considerable distance, where energy is dissipated through air resistance, viscous effects, and turbulence.

Selection of Energy Dissipators

• Case 1: When the TW curve coincides with the Y2 curve — this condition is most favorable for jump formation. A simple concrete apron of length 5 × (Y2 – Y1) is generally provided.
• Case 2: When the TW curve is above the Y2 curve — a roller bucket is recommended.
• Case 3: When the TW curve is below the Y2 curve — a ski-jump bucket is used.
• Case 4: When the TW curve is above the Y2 curve at low discharge and below the Y2 curve at high discharge — a combination of energy dissipators is used (e.g., a hydraulic jump apron for low discharges and a flip bucket for high discharges).

Types of Stilling Basins

• USBR Stilling Basin Type II (for Froude numbers > 4.5)
• USBR Stilling Basin Type III (for Froude numbers > 4.5 & certain velocity conditions)
• USBR Stilling Basin Type IV (for Froude numbers between 2.5–4.5)

Gate Types and Location

Crest or Surface Type Gates

• Stop-log/Flash Boards: Made of timber, steel, or concrete beams that fit into end grooves between walls or piers to close an opening under unbalanced conditions. Commonly used in branch channels within irrigation systems.
• Vertical Lift Gates: Gates that move within a vertical groove between two piers. They are used to control flow over the crest of hydraulic structures and are usually equipped with wheels.
• Radial Gates: Hinged gates whose leaf (skin) is shaped as a circular arc with the center of curvature at the hinge or trunnion. Employed above weirs and dams.
• Ring Gates: Cylindrical drums that move vertically within an annular hydraulic chamber to control the peripheral flow of water from the reservoir through a vertical shaft.
• Sector Gates: A pair of circular arc gates hinged on a vertical axis within a lock. These are used in navigation locks where ships transit between reservoirs at different elevations.
• Flap Gate: Designed to remove floating debris from the river.

Deep Seated Gates

• Vertical Gate: Similar in function to crest-type gates but used for deep-seated purposes (such as controlling flow to a hydropower intake). They can feature roller wheels or operate as a sliding type.
• Deep-Seated Radial Gates: Low-level radial gates with sealing on the top and sides, typically located at sluices in the bottom portion of a dam.
• Cylindrical Gates: Gates in the form of a hollow cylinder placed in a vertical shaft. They are commonly used for intake towers or upstream of a dam to shut off water to penstocks/control valves in outlet works.
• Ring Follower Gates: Gates where the diameter of the circular opening is equal to that of the conduit. They serve as emergency gates and operate in either fully closed or fully open positions.

Headworks of Run-of-River (ROR) Plants

General Arrangement of Components

The headwork of a ROR plant extends from the weir/barrage to the settling basin. Its primary purpose is to withdraw the required amount of sediment‐free water from the river.

General Requirements of a Functional ROR Headwork

• Withdrawal of the required amount of water (L, AT)
• Flood bypass capability
• Control of bed load
• Control of debris cover, ice, and trash
• Control of suspended particles
• Maintain minimum O/M ratio
• Minimize hydraulic losses
• Simple construction
• Prevent formation of air vortices

Intake

The intake is the hydraulic structure provided at the mouth (entrance) of a water conveyance system to withdraw water from the reservoir or river to the powerhouse.

Design Considerations

Points to be Considered

1. Ensure minimum head loss as water enters the water conducting system from the reservoir (or pool behind a barrage).
2. Prevent formation of vortices at the entry that could draw air into the system.
3. Minimize the entry of sediment into the water conducting system.
4. Prevent floating materials from entering the system.

Functions of Intakes

• Control the flow of water into the conveyance system via a gate or valve.
• Provide smooth, turbulence- and vortex-free entry to minimize head loss.
• Prevent entry of coarse river-borne materials such as boulders, logs, and tree branches.
• Exclude heavy sediment loads from entering the conveyance system.

Location of Intake

The optimum location for an intake depends on several factors including dam type, reservoir geometry, the volume of water to be diverted, topography, submergence, geotechnical conditions, sediment exclusion, and ice control when necessary. It must satisfy the following primary considerations:

• Adequate inflow
• Low silt inflow
• Least head loss
• Minimal environmental impact

Types of Intakes Based on Plant Layout

A) Run-of-River (ROR) Type Plants

In ROR plants, water is drawn from a fresh, continuous river flow without appreciable pondage. Typical configurations include:

• Side (lateral) intake
• Frontal intake
• Drop intake

B) Reservoir Intakes

• Dam intake
• Conveyance intake
• Re-entrant intake
• Shaft or Glory hole intake
• Tower intake

Design of the Intake Structure

1. Selection of Intake Type: Based on site and hydraulic conditions, choose an appropriate type such as side, frontal, trench, pressure, or non-pressure intake.
2. Determine Capacity: The design discharge should be 10–20% more than that of the turbine discharge.
3. Fix the Intake Invert Level: Based on the sediment content (bed load) and design experience, the invert level should be set at about 0.5 m (or more) above the under-sluice level, as per site conditions.
4. Satisfy Velocity Criteria:
   • The entrance velocity should be less than 0.6–0.8 m/s (up to 1 m/s for small systems).
   • Recommended criteria: Approach velocity = 1 m/s; Trash velocity = 0.6–0.75 m/s.
5. Determine the Number of Openings: (Fast Track considerations may apply.)
6. Account for Contraction Loss: Due to pier/abutment effects, the effective length is generally taken as 0.9–0.95 times the actual opening.
7. Design the Intake Opening/Orifice:

   The opening is typically designed as a broad-crested weir operating under submerged or free-flow conditions, based on the upstream and downstream water levels. It can also be designed using the orifice flow equation:

   Q = C × A × √(2gH)

   Where:

   • H = River water level (depth in front of the intake)
   • h = Canal water level (depth in the canal)
   • C = Constant depending on the opening shape (0.6 for sharp-edged, roughly finished openings; 0.8 for carefully finished openings)
8. Calculate Hydraulic Losses:
   • Trash rack loss
   • Entrance loss
   • Transition loss
   • Gate loss
9. Ensure No Air Entrainment: Air may be drawn into the system due to vortex formation (from hydraulic jumps, high velocity, or submergence effects). This can be checked using the relation:
   s / (v × d) > C
   
   • s = Submergence (minimum of 0.3–0.5 m)
   • v = Velocity in the conduit
   • d = Diameter of the conduit
   • C = 0.3 (symmetric approach) or 0.4 (asymmetric approach)
10. Provide Air Vents: To prevent cavitation, ensure that any entrapped air is vented properly.

Settling Basin (or Desander)

The main objective of a settling basin is to remove suspended particles of a particular size that can cause wear and tear on turbines and their accessories.

Classification of Settling Basins

a. Based on Flushing System

1. Periodic Type
   Advantages: No loss of water.
   Disadvantages: Generation loss during flushing.
2. Continuous Type
   Advantages: No generation loss.
   Disadvantages: Continuous loss of water during flushing.

b. Based on Prominent Feature

1. Conventional Type

• Sediment may be removed manually or with mechanical equipment after the basin is dewatered.
• Deposited sediment may also be removed by lowering the water level inside the basin and generating a swift free surface gravity flow (i.e. conventional flushing system).
• Two settling chambers are constructed for continuous operation.

2. Hooper Type

• This is a continuous flushing type wherein deposited sediment is removed continuously via a bottom opening.
• Example: Sunkoshi (10 MW, information incomplete)

3. Bieri Type

• A developed form of the hopper type designed to control the loss of water during flushing.
• It uses two horizontal shutters at the bottom – the lower one is fixed while the upper one moves horizontally.
• When the sediment accumulation reaches a desired level, the upper shutter moves over the lower shutter and the sediment is flushed out. Finally, the shutter is reset to stop the flushing operation.
• Example: Middle Marsyangdi (70 MW)

4. Serpent Sediment Sluicing System (S4)

• A developed form of the hopper type and Bieri type which addresses the major drawbacks:
  • Hopper type: Continuous loss of water.
  • Bieri type: Difficult operation of shutters.
• It incorporates a float unit (the "serpent") which covers the flushing channel at the basin bottom when filled with water.
• Once dewatered, the serpent becomes buoyant and flushes the sediment by opening the channel. When flushing is complete, the serpent fills with water, gradually sinks, and closes the channel.
• The S4 system requires no machinery or energy input as it operates under gravity.
  Examples: Aandhikhola (5.1 MW), Khimti (60 MW), Jhimruk (12.5 MW)

5. Split & Settle

• Recognizes that sediment concentration is much higher near the bottom of the channel compared to the upper layer.
• The dirtiest water (bottom layer) is diverted to a settling tunnel constructed parallel to the main conveyance.
• The relatively cleaner water from the upper layer is conveyed to the powerhouse.
• This concept saves the cost of a conventional settling basin and is especially useful when arranged underground.

5. Hydrocyclone

• Separates materials (sediments) that are denser than water using centrifugal force.
• Sediment particles follow a helicoidal path toward an orifice, providing a longer effective settling length relative to the separator dimensions.
• It typically removes particles ranging from 5 μm to 300 μm.
• The unit consists of a conical-shaped vessel joined to a cylindrical section with a tangential feed inlet. An extended overflow pipe, known as a vortex finder, prevents short-circuiting of the feed.
• A higher velocity flow is fed tangentially into the cylindrical section. This creates a forced vortex near the orifice and a free vortex in the outer periphery, building up a sediment concentration gradient across the vortex.

Professional Settling Basin Design Equations

1. Settling Velocity in the Laminar Regime (Stokes' Law)

For small, spherical particles in laminar flow (Re < 1), the settling velocity is given by:

Alternatively, using the kinematic viscosity \( \nu = \mu/\rho \) and the relative density \( s = \rho_p/\rho \):

Definitions:

• \(v_s\) = Settling velocity (m/s)
• \(g\) = Acceleration due to gravity (≈ 9.81 m/s²)
• \(\rho_p\) = Particle density (kg/m³)
• \(\rho\) = Fluid density (kg/m³)
• \(d\) = Particle diameter (m)
• \(\mu\) = Dynamic viscosity (Pa·s)
• \(\nu\) = Kinematic viscosity (m²/s)
• \(s\) = Relative density (i.e., \( \rho_p/\rho \))

2. Settling Velocity in the Turbulent Regime

For larger particles where inertial effects dominate (typically at higher Reynolds numbers), the settling velocity is estimated by:

Where \( C_d \) is the drag coefficient.

3. Particle Reynolds Number

4. Drag Coefficient (\(C_d\)) Correlations

The drag coefficient is expressed with the following empirical correlations:

• For \( Re < 1 \):
  $$ C_d = \frac{24}{Re} $$
• For \( 1 \le Re \le 1000 \):
  $$ C_d = \frac{24}{Re\left(1 + 0.15\,Re^{0.687}\right)} $$
• For \( Re > 1000 \):
  $$ C_d = 0.44 $$

5. Empirical Settling Velocity Formula

In practical designs, an empirical relationship (with \(v_s\) in cm/s and \(d\) in mm) is sometimes used:

Where the coefficient \(a\) is determined by particle size:

• For \( d > 1 \) mm, \( a \approx 36 \)
• For \( 0.1 \, \text{mm} < d \le 1 \) mm, \( a \approx 44 \)
• For \( d \le 0.1 \) mm, \( a \approx 51 \)

6. Basin Flow and Dimensions (Continuity Equation)

Assuming steady, uniform flow in the settling basin:

Solving for the basin width \(B\):

Where:

• \(Q\) = Design discharge (m³/s)
• \(h\) = Basin depth (m) (typically 3–4 m)
• \(v\) = Mean flow velocity (m/s)

7. Sediment Load and Deposition

Sediment Load:

Where:

• \(T\) = Detention time (s) (e.g., 6 hours ≈ 21,600 s)
• \(C\) = Sediment concentration (kg/m³) (Note: 1000 ppm ≈ 1 kg/m³)

Volume of Sediment Deposit:

• \(\rho_s\) = Sediment density (≈2600 kg/m³)
• \(F\) = Packing factor (typically ≈0.5)

Deposition Depth (for a basin with plan area \(L \times B\)):

Inlet and Outlet Transition Guidelines

• To minimize secondary flows, maintain approach channel velocities between 1.1 and 1.3 m/s.
• Employ a gradual inlet expansion: horizontally, an expansion ratio of about 1:5 is recommended and vertically about 1:2.
• If space constraints exist, short inlet transitions may incorporate guide walls or baffle locks to achieve uniform flow conditions in the basin.
• Outlet transitions typically allow more abrupt changes; horizontal and vertical ratios around 1:2 and 1:1 are common.
• For flushing operations, design the basin’s bed slope between 1:20 and 1:50 using pressurized flow principles.
• In concrete conduits, it is recommended that the flow velocity be kept between 1.75 m/s and 2.5 m/s.

Hydraulic Tunnels

A hydraulic tunnel is an underground water conduit formed by excavation without disturbing the surface. Water can be conveyed through tunnels beneath high ground or mountains in rugged terrain where constructing a surface line is difficult.

Advantages

• Less environmental effect (reduction in land acquisition, forest clearance, etc.)
• Adopts the shortest possible route so head loss is minimal.
• Optimal space consumption and the natural landscape is not disturbed.
• Less seismic effect – reduction in ground motion amplitude with increasing depth, reduction in seismic coefficient, increased modulus of elasticity with depth, and smaller excavation dimensions relative to seismic wavelength.

Disadvantages

• Normally high construction cost
• High construction risks
• The construction period is normally long

Based on purpose or function, hydraulic tunnels may exist in the following forms:

• Interconnection tunnel of reservoirs
• Diversion tunnel during construction
• Spillway tunnel
• Power tunnel
• Tailrace tunnel
• Navigational tunnel
• Silt flushing tunnel

Pressure and Non-Pressure Tunnels

Non-Pressure Tunnel: The tunnel in which the flow occurs as a free surface exposed to the atmosphere. The flow is open channel (e.g., spillway, diversion, tailrace tunnels).

Pressure Tunnel: The tunnel in which the flow occurs under pressure (e.g., headrace tunnel).

Hydraulic Design of Tunnels

Free Flow Tunnel (Non-pressure Tunnel)

The hydraulic design of a free flow tunnel is executed similar to that of a canal. Manning's frictional factors are used to compute the head loss along the tunnel length.

For example (approximate representation):

hf = (n² * v² * L)  

Pressure Flow Tunnel

The hydraulic design of a pressure flow tunnel is computed akin to pipe flow:

hf = (f · l · v²) / (2 · g · d)  

The head loss is computed using the Darcy–Weisbach friction factor. Discharge through a pressurized tunnel is calculated by the continuity equation (Q = A × v), where:

• Unlined tunnel: max velocity of about 2 to 2.5 m/s
• Lined tunnel: max velocity of around 4–5 m/s

Based on sediment load considerations (suspended or bed load), a recommended velocity is about 2.5 m/s.

Size and Shape of Tunnel

The geometric design (shapes) of tunnels includes:

• Circular Section: Suitable when the tunnel is subjected to high internal pressure but the surrounding rock is not of high quality. Structurally optimal, although excavation can be challenging.
  Example:

  Cross-sectional area (A) = 0.785 × D²Perimeter (P) = 3.14 × D

• D-shaped Section: Suitable for tunnels in good quality rock where external pressure exceeds internal pressure. Provides more working space.
  Example:

  Cross-sectional area (A) = 0.9293 × D²Perimeter (P) = 3.57 × D

• Horse-Shoe Section: A compromise between circular and D-shaped sections. Structurally strong to withstand external rock and water pressures, suitable for moderately good rock conditions, and often preferred for construction.
  Example:

  Cross-sectional area (A) = 0.8293 × D²Perimeter (P) = 3.267 × DHydraulic radius = 0.2538 × D

In cases where the rock is stratified, soft and closely laminated (e.g., sand, stones, silts, micaceous schists) and where high external pressure and tensile stresses exist, a sheared section may be considered:

• Area (A) = 0.854 × D²
• Perimeter (P) = 3.313 × D

Tunnel Alignment

• The alignment should be as straight as possible (to be economical and minimize head loss).
• Short alignment is preferred because bends increase both construction cost and head loss.
• The tunnel should be easily accessible near the entrance and exit with minimal weathered, loose, or fractured layers.

After excavation, tunnel lining is applied to increase the hydraulic capacity, strength, and to reduce resistance and losses.

Advantages of Tunnel Lining

• Acts as a structural unit, transmitting part of the load to the surrounding rock, thereby enhancing stability.
• A lined tunnel has much lower hydraulic resistance, minimizing losses and saving energy/revenue.
• Higher water velocity can be achieved compared to an unlined tunnel allowing smaller sections to carry better discharge.
• Seepage losses through the tunnel are significantly reduced.

Tunneling Methods

1. Heading and Benching
   • An old technique where only part of the tunnel cross-section (the heading) is excavated first and then enlarged to the required size.
   • Heading may be initiated at the top, middle, or bottom.
2. Full Face Blasting
   • Entails uniform excavation of the entire tunnel section.
   • Suitable for tunnels passing through strong ground rocks.
3. Tunnel Boring Machine (TBM)
   • Uses mechanical excavation for the full tunnel face.
   • Latest technology suitable for long tunnels and accessible sites (e.g., roads).
   • More expensive compared to other methods but offers fast, safe, and efficient construction in weak to moderate rock conditions.
4. Drilling and Blasting

   (A method mentioned though detailed steps are not included.)

5. Cut and Cover Method
   • The oldest tunneling method which involves digging a trench, constructing the tunnel, and then restoring the surface.
6. New Austrian Tunnel Method (NATM)
   • A construction method and design philosophy that utilizes the strength of the surrounding soil to reinforce the tunnel structure.
   • Offers flexibility in design and excavation through ongoing monitoring.
   • Best suited for short-range (< 2 km) tunnels in regions with variable soil conditions.

Support in Tunnels

Tunnel support is required because the excavated rock tends to drop out of the roof. The time between blasting and the onset of roof collapse (without support) is known as the Bridge Action Period (ranging from minutes to weeks).

• Rock bolts (spot or pattern bolting) of varying lengths are provided.
• Steel ribs (girders) or bolt/cable systems may also be applied to support the tunnel.

Size of Tunnel

• The tunnel size is fixed based on functional requirements such as discharge capacity and transportation needs.
• The minimum diameter is determined considering transport, excavation, and hauling distance:
  • For circular tunnels: greater than 2.0 m
  • For other tunnels: greater than 1.9 m in width and 2.1 m in height

Flow in Tunnels

Free Flow Tunnel

Manning's formula is used to calculate flow in a free flow tunnel:

Q = (1/n) * A * R^(2/3) * S^(1/2)  

Where:

• A = Tunnel cross-sectional area
• R = Hydraulic radius
• S = Slope
• n = Roughness coefficient (typically 0.012–0.018 for tunnels)
• hf = Head loss in the tunnel

Tunnel Lining

After excavation, tunnel lining is applied to increase hydraulic capacity, reduce resistance, enhance strength, and decrease losses.

Note: An empirical formula to calculate the thickness of the tunnel lining (in mm) is:

Thickness = 82 * D  

where D is the tunnel diameter in meters.

• Lining is done in cases of high internal pressure, low-strength strata, or when an increased discharge capacity is desired.

Main purposes of tunnel lining include:

• Providing the correct tunnel section
• Withstanding soil pressures
• Reducing friction losses
• Preventing water percolation
• Supporting loose rock slabs from blasting

Types of Tunnel Lining

1. Shotcrete lining
2. Precast concrete segment lining
3. Cast-in-place concrete lining
4. Steel liner plates
5. Fiber-reinforced polymer (FRP) lining
6. Brick or masonry lining
7. Geotextile or geocomposite lining
8. Tunnel lining with waterproofing membranes

Forebay (Head Pond) Design

A forebay (or head pond) is the upstream reservoir of a hydropower facility. It stores and regulates water before it is drawn into the pressure conduit (penstock) and subsequently to the turbine.

Estimation of Drawn Head

The drawn head is the effective vertical distance that water travels—from the forebay water level to the turbine's tailwater level—after deducting losses. It is estimated as:

H_d = H_f − H_t − Losses

Where H_f is the forebay water level, H_t is the tailwater level, and losses include friction and minor losses in the conveyance system.

Minimum Submergence Condition

To avoid cavitation at the turbine intake, a minimum water depth must cover the intake (i.e. a minimum submergence condition). Design guidelines usually specify a submergence ratio (for example, greater than 2.0) to ensure smooth flow without air entrainment.

Design of Surge Tank

A surge tank is a hydraulic structure placed between a slightly inclined pressure conduit (or tunnel) and a steeply sloping enstock (pressure shaft). It is designed to control hydraulic transients by acting as a water storage device or pressure neutralizer.

Functions of a Surge Tank

• Protects the conduit system from high internal pressures.
• Assists the hydraulic turbine by improving regulation characteristics.
• Stores water to raise pressure under conditions of pressure drop.

Design Conditions & Components

In designing a surge tank, engineers consider factors such as:

• Maximum Upsurge/Downsurge: The surge height is estimated using transient analysis. One simplified formula is:
  ΔH = (a × ΔV) / g
  where a is the speed of the pressure wave, ΔV is the change in velocity, and g is the acceleration due to gravity.
• Tank Dimensions: The surge tank must have a cross-sectional area much larger than the penstock. Its height is designed to accommodate the maximum predicted surge.
• Additional Components:
  • Inlet and outlet structures for controlled water flow.
  • Vent pipes to manage air pressure fluctuations.
  • Instrumentation ports for monitoring water levels and pressures.
• Types of Surge Tanks: Commonly used types include vertical surge tanks, inverted siphon surge tanks, and open surge basins. The choice depends on site conditions and design requirements.

Design of Penstock Thickness

The penstock is the pressure conduit that delivers water from the forebay to the turbine. Its wall thickness must be adequate to withstand internal pressure, dynamic loads (such as those from water hammer), and environmental wear.

Basic Thickness Calculation

A commonly used design formula is:

t = (P × D) / (2 × σ × F)

• t: Required wall thickness
• P: Design pressure
• D: Penstock diameter
• σ: Allowable stress of the material
• F: Safety factor as per design codes

In addition to static loads, dynamic effects from water hammer and allowances for corrosion are important considerations.

Hydraulic Transients (Water Hammer)

Water hammer is a sudden pressure surge resulting from the abrupt stoppage or change of flow in a pressurized pipe system. This phenomenon is typically caused by rapid valve closures or pump stoppages/failures.

Water Hammer Explained

When water flow is suddenly obstructed, the kinetic energy of the moving water is converted into a pressure pulse. This pulse can produce a loud "hammer knocking" noise as the water mass collides with the obstruction (such as a valve or pump) and the internal walls of the pipe.

Consequences of Water Hammer

• Pipe and pump casing rupture
• Vibration and high noise levels
• Excessive displacement of pipes
• Vapor cavity formation (cavitation)
• Environmental pollution
• Loss of life and economic damage

Conditions That Cause Water Hammer

Water hammer often occurs during events such as:

• Turbine or pump failure and startup
• Sudden closure of a valve
• Failure of flow or pressure regulators

Pressure Surge Calculation

The pressure surge from water hammer can be estimated by:

ΔP = ρ × a × ΔV

• ΔP: Pressure surge
• ρ: Density of water
• a: Speed of sound in water (adjusted for pipe elasticity)
• ΔV: Change in water velocity

Hydro-mechanical Equipment

Hydraulic Turbines

A hydraulic turbine is a mechanical device that converts the potential energy contained in an elevated body of water (a stream or reservoir) into rotational mechanical energy. Its primary function is to drive an electric generator.

Types of Hydraulic Turbines

A) According to the Energy Conversion

• 1. Impulse Turbine: Often uses a forebay and high-gross head. The water’s potential energy is converted into kinetic energy via a nozzle.
• 2. Reaction Turbine: Part of the energy conversion occurs before the water enters the runner. After entering, water undergoes changes in both velocity and pressure.

Difference Between Impulse and Reaction Turbines

B) According to the Direction of Flow

• Tangential Flow Turbine: The water strikes the runner in the direction of the wheel (e.g., Pelton wheel turbine).
• Axial Flow Turbine: The water flows in a direction parallel to the shaft axis (e.g., Kaplan turbine, propeller turbine).
• Radial Flow Turbine: The water strikes the runner radially (e.g., older Francis turbine designs).
• Mixed Flow Turbine: The water enters radially and leaves axially.

C) According to Name

• Pelton wheel turbine
• Francis turbine
• Kaplan turbine
• Deraiz turbine

Design parameters of Pelton wheel turbine:

1. Velocity of jet: At inlet \({V_1} = {C_V}\sqrt {2gH} \) where Cv = coefficient of velocity = 0.98 - 0.99.

2.Velocity of wheel: \(u = \phi \sqrt {2gH} \) where φis the speed ratio = 0.43 - 0.48.

3. Angle of deflection: 165 °unless mentioned.

4.Pitch or mean diameter: D can be expressed by \(u = \frac{{\pi DN}}{{60}}\)

5.Jet ratio: \(m = \frac{D}{d}\) (12 in most cases/calculate), d = nozzle diameter or jet diameter.

6.Number of buckets on a runner: \(Z = 15 + \frac{D}{{2d}}\) (Tygun formula) or,\(Z = 5.4\sqrt m \) , m = 6 to 35.

7.Number of Jets: obtained by dividing the total rate of flow through the turbine by the rate of flow through a single jet. The number of jets is not more than two for horizontal shaft turbines and is limited to six for vertical shaft turbines.

8. Size of bucket: length of bucket L = 2.5d, width of bucket B = 5d , depth of bucket Db = 0.8d.

Turbine Efficiency and Speeds

Turbine Efficiency

Hydraulic Efficiency: The ratio of the power developed by the runner to the net power supplied by the water at the turbine entrance.

Mechanical Efficiency: The ratio of the power available at the turbine shaft to the power developed by the runner (differs by the mechanical losses).

Volumetric Efficiency: The ratio of the quantity of water actually striking the runner to the quantity supplied. This accounts for water that may slip through without doing work.

Overall Efficiency: The ratio of the power available at the turbine shaft to the power supplied at the turbine entrance.

Turbine Speeds

1. Runway Speed:

   If the external load on the machine suddenly drops to zero (sudden rejection) and the governing mechanism fails, the turbine races to the maximum possible speed known as the runway speed.

   Turbine Type	Runaway Speed (% of Normal)	Minimum Acceptable Head Variation (% of Design Head)	Maximum Acceptable Head Variation (% of Design Head)
   Impulse	170–190	65	123
   Reaction	(Data not clearly specified)	50	150

2. Specific Speed (Ns):

   The specific speed of a turbine is defined as the speed of a geometrically similar turbine that would develop unit power under unit head conditions. (For example, the specific speeds for turbines developing 1 HP under 1 m head are approximately:)

   The specific speed of a turbine is in the range of

   Turbine	Ns
   Pelton wheel	10 - 60
   Francis	60 - 300
   Kaplan	> 300

3. Synchronous Speed:

   When the turbine is directly connected to the generator, its speed must be synchronous. This is generally given by the equation:

   n = 120f / N

   where n is the turbine’s rotational speed (in rpm), f is the electrical frequency (in Hz, e.g., 50 Hz), and N is the number of generator poles.

4. Speed Factor or Speed Ratio (φ):

   This is the ratio of the peripheral speed of the buckets or vanes at the nominal diameter to the theoretical velocity of water under the effective head acting on the turbine. In formula form:

   φ = (Peripheral Speed) / √(2gH)

The classification of the turbine based on the basis of operating head

Draft Tube

A draft tube is an integral part of low head turbines with large through flow (e.g., Francis and Kaplan turbines). It is an airtight diverging conduit whose cross-sectional area increases along its length, connecting the runner exit to the tail race level. Its primary functions include:

• Decreasing the pressure at the runner exit to below atmospheric, thereby increasing the effective head.
• Recovering a portion of the exit kinetic energy that would otherwise be lost in the tail race.
• Fixing the turbine in position above the tail race to ensure proper installation.

Types of Draft Tube:

Various forms of draft tubes are available. There are mainly 4 types of draft tubes, and those are:

• Conical draft tube
• Simple elbow draft tube
• Moody spreading draft tube
• Elbow draft tube with a varying cross-section
• Bell-mouth draft tube

Governing of Hydraulic Turbines

• With a constant generator speed (e.g., N = 120), the turbine speed tends to decrease when the load is increased—and vice versa.
• The governor regulates the amount of water flowing through the turbine in proportion to the load, maintaining nearly constant turbine speed.
• For reaction turbines, the governor controls the guide vanes and wicket gates.
• For impulse turbines, the governor controls devices such as spear valves and nozzle openings.

Electro-mechanical Installation

Generator

An electric generator is a machine that converts mechanical energy into electrical energy. Usually, this energy is obtained from a rotating shaft (the armature). The generated electricity is used for power transmission at commercial, industrial, or domestic levels. In most systems, current is supplied at 50 Hz. A generator consists primarily of:

• Stator: Contains the stationary magnetic poles.
• Rotor: Contains the rotating armature.

Hydropower Generator

The hydropower generator is the main equipment used to produce electric energy in a hydropower station. In a “water-to-wire” system, the turbine acts as the prime mover, converting the water power into mechanical energy which is then transformed into electrical power.

Types of Hydropower Generators

• Horizontal Type: Generally used for medium and small capacity units (often driven by Pelton turbines), with maximum speeds up to 1500 rpm.
• Vertical Type: Common in large and medium units (typically paired with Francis or Kaplan turbines) with maximum speeds reaching up to 750 rpm.

Types of Electrical Generators

• Synchronous Generator: Equipped with a DC excitation system (rotating or static) and a voltage regulator to manage voltage and phase angle. They can operate in isolation because the excitation is not grid dependent, though these tend to be more expensive.
• Asynchronous Generator: Typically a squirrel-cage induction motor without voltage regulation capability; its speed is directly linked to the system frequency and it draws excitation current from the grid.

Pump

A pump is a device that moves fluids—liquids or gases (or sometimes slurries)—by mechanical action. Pumps operate by mechanisms (typically reciprocating or rotary) and are used to perform mechanical work on a fluid.

Classification of Pumps

• Rotodynamic Pumps: Energy increase is achieved by a combination of centrifugal force, pressure energy, and kinetic energy. These are commonly known as centrifugal pumps.
• Positive Displacement Pumps: These move fluid by trapping a fixed amount and then forcing (displacing) that trapped volume into the discharge pipe.

Component Parts of a Centrifugal Pump

A centrifugal pump typically consists of the following parts:

• Suction Pipe/Port
• Casing
• Impeller
• Delivery (Discharge) Pipe

Reciprocating Pump

A reciprocating pump is a device that moves a fluid by displacing it with a piston or plunger executing a reciprocating motion in a closely fitting cylinder. The amount of liquid pumped equals the volume displaced by the piston.

Pumps designed with disk pistons can create pressures up to 25 bar, while plunger pumps can generate even higher pressures. The discharge rate from these pumps is almost entirely dependent on the pump speed.

The overall efficiency of a reciprocating pump is about 0 to 200% higher than that of a comparable centrifugal pump.

Although reciprocating pumps for industrial applications have become nearly obsolete due to their high capital and maintenance costs compared to centrifugal pumps, small reciprocating pumps (such as cycle pumps, football pumps, village well pumps, and pumps used as hydraulic jacks) still find wide application. This pump type is best suited for relatively small capacities and high heads and is very useful in oil drilling operations.

Main Components and Working of a Reciprocating Pump

The main parts of a reciprocating pump are:

• Cylinder
• Suction Valve
• Suction Pipe
• Piston
• Delivery Valve
• Delivery Pipe
• Connecting mechanism operated by a power source such as a steam engine, diesel engine, or an electric motor.

Revision Notes Hydropower Engineering

• Hydropower plants have the least fuel requirement for the same operation.
• In the context of Nepal, the hydro plant is the most reliable and suitable energy source.
• Hydro generators deliver high efficiency over a wide range of load conditions.
• The most reliable power is hydroelectric power.
• For the same power output, a hydel plant has minimum operating charges.
• The hydel plant is the most economical source of power.
• A hydroelectric power plant is a conventional source of energy.
• Continuous power supply is not an advantage of a hydroelectric power plant.
• Electricity regulation is approved by the Government of Nepal.
• The flow duration curve at a given head of a hydroelectric plant is used to determine the total power available at the site.
• The coincidence factor is the reciprocal of the diversity factor.
• When the operating cost of a power plant is high but it provides great flexibility, a peak plant is used.
• In remote, isolated areas with only one micro hydropower plant, the capacity should be equivalent to the peak load.
• The load factor is the ratio of average demand to maximum demand.
• The load curve shows the variation of load on a power station with respect to time.
• The annual depreciation of a dam in a hydropower plant is about 0.5% to 1.5%.
• The power needed to uplift water can be calculated as P = nvQH.
• If installed capacity is equal to the peak load, then the ratio of capacity factor to load factor is 1.
• The pondage in a hydropower station is temporary storage used to meet peak demands.
• The power available continuously is known as firm power.
• If the operating cost is high but the plant offers great flexibility, it is used for peak load only.
• If 20% of a reservoir’s capacity is designated as dead storage in a 30 M.cum reservoir and the average annual silt deposition is 0.1 M.cum, the useful life of the reservoir is approximately 60 years.
• The power available continuously during all seasons in a hydropower project is known as firm power.
• The Pharping hydropower plant was constructed in 1911 AD with a capacity of 500 kW.
• A gravity dam is most suitable when the foundation is strong.
• Contraction joints are commonly used in the construction of arch dams.
• The recommended top width of a low earthen dam is 0.2H + 3.
• The core of an earthen embankment is important for controlling seepage.
• The middle 1/3 rule is used in tension design.
• For no tension in the dam, the resultant force must pass through the middle third of the dam section.
• Morning glory is the special flared inlet of the shaft spillway of a large dam project.
• In modern times, fixed gates are mostly used in hydraulic structures.
• The energy dissipater provided when the jump height exceeds the tail water depth is known as a ski jump.
• A coffer dam is a temporary structure built to enclose a worksite.
• Sloughing is the process of progressive removal of soil from the downstream face.
• A homogeneous embankment-type earth dam is suitable only on an impervious foundation.
• Dead storage in a reservoir is defined as the storage between the reservoir bed level and the minimum pool level.
• The most commonly used vertical lift gate today is the fixed wheel gate.
• A farm pond is used for water storage.
• The bottom portion of a concrete or masonry gravity dam is usually stepped to increase shear strength.
• The maximum permissible eccentricity for no tension at the base of a gravity dam is B/6, where B is the dam’s base width.
• The central core of a zoned embankment-type earth dam controls seepage.
• Undermining and uplift are causes of failure in hydraulic structures on pervious foundations.
• When the reservoir is empty, the maximum vertical stress is: at the heel = 2W/B and at the toe = 0.
• The middle third rule is applicable in designing structures under combined direct and bending stresses, especially for rectangular sections.
• Based on hydraulic design, dams are classified as overflow and non-overflow dams.
• The safety valve of a dam/reservoir is the spillway.
• Energy dissipation downstream of a sloping glacis is achieved by a hydraulic jump.
• For a given discharge, the efficiency of a sedimentation basin can be increased by enlarging its surface area.
• The governor is used to control the mean speed.
• The efficiency of a settling tank can be increased by increasing its surface area.
• A trash rack is used to prevent the entry of debris.
• A submerged intake is located at the bottom of a river.
• The wrong statement is: Surge tanks are totally closed to avoid entry of unwanted objects into the penstock.
• The function of a surge tank is to reduce pressure effects.
• The function of a surge tank is to relieve water hammer pressures in the penstock pipe.
• The forepoling method is used for tunneling in soft ground.
• For tunneling in rock terrain, the correct sequence of operations is: (2) Marking the tunnel profile, (3) Setting up and drilling, (1) Removing foul gases, (4) Checking misfire, (5) Mucking (i.e., sequence 23145).
• The recommended shear friction factor against sliding is between 3 and 5.
• Heading and benching methods of tunneling are used in hard rocks.
• Side drainage is not used as tunnel drainage.
• The claim that “removal of muck from the heading is very easy” is not correct for the heading and benching method of tunneling.
• Cast iron linings are suitable for shield-driven tunnels, particularly in subaqueous regions.
• In a high-head hydropower plant, the velocity of water in the penstock is about 7 m/s.
• The water hammer effect in a hydropower plant is prevented by the surge tank.
• The surge tank in a hydroelectric power plant prevents water hammer in the penstock.
• The power generated by unit head under unit discharge conditions is defined as unit power.
• The formula for the jet velocity in a Pelton turbine is v = C_v√(2gH).
• A powerhouse is used to protect the turbine and runner from damage.
• Governors in power plants directly control the flow of working fluids.
• The use of a draft tube in a reaction turbine helps convert kinetic energy to pressure energy.
• The Pelton turbine is an impulse turbine.
• The pressure at the inlet and outlet of the draft tube should not be less than one‐third of atmospheric pressure.
• A pumped storage plant uses a reversible turbine that operates at relatively high efficiencies, reducing overall cost.
• The Kaplan turbine is a low-head axial flow turbine.
• In an impulse turbine, water’s pressure energy is converted to kinetic energy through a nozzle adjacent to the runner blade.
• The power output from a hydroelectric power plant depends on three parameters: head, discharge, and system efficiency.
• The energy of water entering a reaction turbine is partly pressure energy and partly kinetic energy.
• Draft tubes are not used in Pelton turbines.
• A centrifugal pump acts as the reverse of an inward radial flow reaction turbine.
• In a hydropower plant, the correct flow sequence is: reservoir → penstock → surge tank → turbine.
• A flow duration curve is a graph showing discharge versus the percentage of time that discharge is exceeded.
• The power of a centrifugal pump is proportional to D³.
• Reversible turbines and pumps are very suitable for pumped storage plants.
• A penstock in a hydroelectric power plant is a conduit connecting the forebay to the turbine’s scroll case.
• A high‐head turbine is typically a Pelton turbine.
• The hydraulic efficiency of a turbine is (fluid power transferred to the runner) divided by (power available at the turbine inlet).
• The head loss in a penstock pipe of given length increases with increasing velocity.
• Importing electricity from another country to reduce load shedding is not beneficial for hydropower development policy.
• The governor in a hydropower plant regulates the flow rate of water striking the runner to control turbine RPM.
• The hydrostatic pressure along the phreatic line within a dam section is equal to atmospheric pressure.
• In a hydropower plant, net head is calculated as gross head minus head losses.
• In a Francis turbine runner, the number of blades is generally between 16 and 24.
• A storage-type hydropower plant is suitable for meeting both base load and peak load demands.
• The process of laying and compacting earth in layers using a power roller under optimum moisture content for constructing an earthen dam is known as the rolled fill method.
• Trash racks are designed to prevent floating and submerged debris from entering.
• In a reaction turbine, the gross head is the difference between the head race and tail race levels.
• A trash rack is not required at the entrance of a drum gate installation.
• The axis of a gravity dam is defined as the line through the crown of the dam on the upstream side.