Hydropower

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This section covers the classification of hydropower potential, the stages of hydropower development, and the history/policy framework of hydropower development in Nepal.

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Power Potential Classification Potential Type Meaning Gross (theoretical) potential The total power theoretically available from all the water resources of a region, computed from total head and flow with no consideration of losses, technical feasibility, or economics Technical potential The portion of the gross potential that could actually be harnessed given current technology and site constraints (topography, geology), after accounting for unavoidable losses and practical development limits Economic potential The portion of the technical potential that is economically viable to develop under prevailing cost and market conditions; generally the smallest of the three categories, and the figure most relevant to actual development planning Stages of Hydropower Development A hydropower project typically progresses through: reconnaissance study (broad identification of potential sites using available maps/data), pre-feasibility study (preliminary technical/economic screening to shortlist promising sites), feasibility study (detailed technical, economic, environmental, and social assessment of a specific site/scheme), detailed engineering/design (finalized design ready for construction), construction, and finally operation (commissioning and ongoing power generation).

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Hydropower Development in Nepal Nepal's hydropower development history dates back to the early twentieth century, with its first hydropower plant (a small facility near Kathmandu) marking the beginning of organized electricity generation in the country; development has since grown through numerous small, medium, and large-scale storage and run- of-river projects.

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The policy and legal framework governing Nepal's hydropower sector includes legislation and policy instruments addressing electricity generation/licensing, water resource use and allocation, and hydropower development promotion, which together set out the procedures, incentives, and regulatory requirements for developing hydropower projects in Nepal.

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This section covers the calculation of power and energy potential, methods of fixing installed capacity, the classification of hydropower plant types, their components, and reservoir regulation.

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Power & Energy Potential Hydropower is calculated as P = ρgQHη, where P is power, ρ is water density, g is gravitational acceleration, Q is discharge, H is the net head, and η is the overall efficiency (combining hydraulic, turbine, and generator efficiencies).

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Energy generated is the power integrated (summed) over the time the plant operates at that power level, commonly expressed in kilowatt-hours or gigawatt-hours over a given period (e.g. annually).

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Fixing Installed Capacity Installed capacity is commonly fixed using a flow duration curve (FDC), which shows the percentage of time a given discharge is equalled or exceeded at the site; a design discharge is selected at a chosen exceedance percentage (balancing higher capacity/energy against lower capacity utilization/plant factor).

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Load factor (average load / peak load) and plant factor (average power output / installed capacity) are used to evaluate how effectively the installed capacity is utilized over the operating period, informing the choice of design discharge and installed capacity.

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Types of Hydropower Plants Basis of Classification Types Head High head (typically >100 m), medium head, and low head (typically <30 m) plants, each generally suited to different turbine types Load served Base load plants (operate continuously at a fairly steady output) and peak load plants (operate intermittently to meet demand peaks, often with rapid start/stop capability) Structural arrangement Storage (reservoir) plants — substantial reservoir enabling significant flow regulation; run-of-river (ROR) plants — little to no significant storage, generation follows the river's natural flow (subject to short-term/daily peaking pondage in some designs); pumped storage plants — use surplus off- peak energy to pump water to an upper reservoir, later released through reversible pump-turbines to generate power during peak demand Components & Reservoir Regulation A storage plant typically comprises a dam and reservoir, intake, headrace (tunnel/canal/penstock as appropriate), powerhouse, and tailrace; a ROR plant typically comprises a diversion weir, intake, desilting/settling basin, headrace, forebay, penstock, powerhouse, and tailrace.

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Reservoir storage is divided into live storage (usable storage between minimum and maximum operating levels, available for regulating releases) and dead storage (storage below the minimum operating level, not usable for regulation, but accommodating sediment deposition over the project's life).

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Reservoir regulation follows operating rule curves that guide releases based on storage level, inflow forecasts, and demand, supporting daily, seasonal, or annual regulation of flow depending on the reservoir's size relative to the river's flow variability.

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This section covers the components of a storage plant, dam types and stability analysis, seepage control and foundation treatment, and the design of intakes, spillways, energy dissipators, and gates.

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Types, Functions & Selection Dam types: gravity dams (resist water pressure primarily by their own weight, typically concrete/masonry), embankment/earthfill dams (built of compacted earth, suited to a wide range of foundation conditions), rockfill dams (built primarily of compacted rock with an impervious core/membrane), arch dams (transfer load to the valley sides via arch action, requiring a narrow, strong-walled valley), and buttress dams (a sloping upstream face supported by a series of buttresses).

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Selection factors include topography (valley shape/width), geology and foundation conditions, availability of construction materials locally, seismicity, and overall cost/construction schedule considerations.

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Failure modes: overtopping (inadequate spillway capacity during extreme floods), piping/seepage failure (progressive internal erosion along a seepage path), slope instability (for embankment dams), and foundation failure; remedies include providing adequate spillway capacity for the design/probable maximum flood, effective seepage control measures, proper compaction and foundation treatment, and regular monitoring/instrumentation.

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Stability Analysis of Gravity Dams A gravity dam's stability is checked against three primary modes: overturning (about the downstream toe, ensuring the resultant of all forces remains within the base, with an adequate factor of safety on the ratio of resisting to overturning moments), sliding (along the base or a weak plane, ensuring the resisting friction/shear force exceeds the driving horizontal force with an adequate factor of safety), and stress/compression at the base (ensuring the base is not subjected to unacceptable tensile stress, and that maximum compressive stress remains within the material's/foundation's safe limits).

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Forces considered in the stability analysis include the dam's self-weight, hydrostatic water pressure (upstream, and downstream if applicable), uplift pressure beneath the base, silt pressure (from sediment accumulated against the upstream face), and, where relevant, seismic (earthquake) forces.

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Seepage Control & Foundation Treatment Seepage control measures include cutoff walls/trenches (extending into relatively impervious strata to block seepage paths), grout curtains (a line of grouted boreholes creating a low-permeability barrier within the foundation), and drainage galleries (relieving uplift pressure by collecting and draining seepage before it builds up excessive pressure).

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Foundation treatment commonly includes consolidation grouting (improving the general foundation rock mass near the dam base) and curtain grouting (forming the seepage-cutoff barrier at depth), along with dental treatment (filling irregularities/weak zones in the foundation with concrete) as needed.

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Intake, Spillway, Energy Dissipators & Gates Component Key Design Points Intake Sized for the design discharge with an acceptable approach velocity, to minimize head losses and reduce the risk of vortex formation and debris/sediment entry; equipped with trash racks and control gates Spillway Provided to safely pass flood flows in excess of what can be stored/released through normal outlets; common types include the ogee (overflow) spillway (a smoothly curved crest profile following the shape of the underside of a free-falling jet), chute spillway, and side-channel spillway; sized for the design flood, often up to the probable maximum flood (PMF) for critical dams Energy dissipators Provided at the spillway toe to safely dissipate the high kinetic energy of the discharged flow before it re-enters the river, commonly a stilling basin (hydraulic jump) for moderate heads, or a flip bucket/ski-jump (projecting the jet away from the dam toe to dissipate energy in the air and at a plunge pool) for high-head dams Gates Radial (tainter) gates and vertical lift gates are common at spillway crests and intakes for discharge control; stop logs provide a simpler, often temporary/maintenance closure at various locations (e.g. intakes, outlet conduits)

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This section covers the components of a typical ROR plant, intake design, bed/suspended load handling, settling basin design, and sediment flushing.

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Components & Intake Design A typical ROR plant's headworks comprise a diversion weir (raises river level for diversion), an intake (admits water into the power conveyance system), a gravel trap (for coarse bed load), a settling basin (removes finer suspended sediment), and the headrace leading onward to the forebay and penstock.

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The intake is designed for the design discharge with an appropriate approach/orientation (often set at an angle to the river flow, with a skimmer wall/sill to exclude bed load and floating debris) and equipped with trash racks and control gates, similar in principle to an irrigation intake but with more stringent sediment- exclusion requirements to protect turbine components.

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Bed Load & Suspended Load Handling Bed load (coarser particles moving along/near the riverbed) is managed primarily by excluding it at the intake (via a properly designed sill/skimmer wall) and trapping any that enters in a gravel trap, which is periodically flushed.

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Suspended load (finer particles carried in suspension throughout the flow depth) is managed by the settling basin, designed to reduce flow velocity sufficiently that particles above a specified design size settle out before the water proceeds to the headrace/penstock/turbine, protecting turbine runners and other components from abrasive wear.

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Settling Basin Design Approach Basis Practice (empirical) approach Uses established empirical/practical design rules and past project experience to proportion the settling basin's dimensions for a target particle removal size, without a detailed sediment- transport-based derivation Concentration approach Bases the design on the allowable sediment concentration permitted downstream (e.g. to protect turbine components from excessive abrasive wear per design/manufacturer criteria), working back to determine the required trapping efficiency and hence basin dimensions Sediment Volume, Flushing & Flushing Frequency The volume of sediment expected to accumulate in the settling basin over an operational period is estimated from the expected sediment inflow rate (based on river sediment data/sediment rating curves) and the basin's trapping efficiency for the relevant particle sizes.

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Flushing of deposited sediment from the settling basin is carried out periodically via bottom sluice gates, using either continuous flushing (a small continuous flow maintained through flushing channels/conduits during operation) or intermittent flushing (periodic, often more thorough, flushing operations that may require a brief interruption or reduction of power generation).

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The flushing frequency is estimated by comparing the rate of sediment accumulation against the basin's available sediment storage capacity before trapping efficiency (or basin performance) drops below an acceptable level, determining how often flushing operations must be carried out to maintain design performance.

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This section covers hydraulic tunnels and their lining, the design of forebays and surge tanks, penstock and pressure shaft design, and hydraulic transients (water hammer).

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Hydraulic Tunnels Cross-sections: circular sections are the most hydraulically efficient and structurally sound, particularly well-suited to tunnels operating under high internal pressure; horseshoe or D-shaped sections are often used for low-pressure/free-flow (gravity, part-full) tunnels, where their shape suits typical excavation and structural support methods.

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Hydraulic design sizes the tunnel based on acceptable velocity limits (balancing head loss against tunnel size/cost), using friction-loss relationships (e.g.

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Manning's or Darcy-Weisbach formulas) to estimate head loss for a given size, roughness, and design discharge.

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Tunnel lining (typically concrete, sometimes steel-lined for high pressure) provides structural support to the excavated opening, reduces hydraulic friction losses (a smoother surface than bare rock), and helps control seepage; tunnels may be fully lined, partially lined, or left unlined in sections of good-quality, self-supporting, low-permeability rock, depending on rock quality and internal pressure.

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Forebay & Surge Tanks A forebay is a small reservoir/basin located at the end of the headrace (canal/low-pressure tunnel) and the start of the penstock, providing a transition and buffer storage to accommodate flow fluctuations from load changes, allowing any remaining sediment to settle, and housing the penstock intake with trash racks.

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A surge tank is provided on a pressure conduit/tunnel system to absorb pressure fluctuations (water hammer effects) arising from rapid load changes (e.g. sudden turbine load rejection or acceptance), by providing a free water surface located close to the powerhouse, which shortens the effective length of conduit subject to the full water hammer pressure rise.

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Types of surge tanks: simple surge tank (a plain vertical shaft with a free water surface), restricted-orifice surge tank (a throttled connection to the tunnel damps oscillations more quickly), differential surge tank (a central riser combined with an outer chamber, providing rapid initial pressure relief plus damping), and chamber (gallery) surge tank (enlarged chambers at the top/bottom of the shaft increasing available surge volume for a given height).

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Penstocks, Pressure Shaft & Water Hammer A penstock is a pipe (commonly steel, sometimes HDPE, concrete, or FRP for lower pressures) conveying water under pressure from the forebay/surge tank to the turbine; it is sized for the design discharge/an economical velocity, with wall thickness designed to resist the internal pressure, including the transient pressure rise from water hammer.

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A pressure shaft serves a functionally similar role to a penstock but is a bored/excavated shaft or tunnel through rock (often steel-lined at high pressure), typically used where the conveyance route passes through rock rather than being an above-ground pipeline.

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Hydraulic transients (water hammer) are sudden pressure rises or falls in a pressure conduit caused by a rapid change in flow velocity, such as valve/gate closure or sudden turbine load rejection/acceptance; they are analysed using water hammer theory (relating pressure rise to the pressure wave celerity and the rate of flow change, e.g. via the Joukowsky/Allievi relationships), and are critical considerations in penstock/pressure shaft wall thickness design and in sizing the surge tank.

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This section covers hydro-mechanical equipment, turbine types and selection, preliminary turbine design, scroll cases and draft tubes, generators, governors, pumps, and powerhouse arrangement.

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Turbine Types & Selection Turbine Type Characteristics Pelton (impulse) Used for high head, comparatively low discharge; a jet of water impinges on bucket-shaped vanes on the runner at atmospheric pressure (no pressure drop across the runner); suited to low specific speed applications Francis (reaction, mixed flow) Used for medium head, medium discharge; water enters radially and exits axially through the runner while under pressure throughout, with pressure dropping across the runner; suited to a moderate range of specific speed Kaplan (reaction, axial flow) Used for low head, high discharge; axial flow through the runner with adjustable (or fixed, in a propeller-type variant) blades, allowing good efficiency over a range of flow conditions; suited to high specific speed applications Performance Characteristics & Specific Speed Performance characteristics (efficiency vs. discharge, efficiency vs. head, and power output curves) describe how a turbine performs across its operating range, guiding turbine selection and operational planning.

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Specific speed (Ns) is a key non-dimensional (or dimensional, depending on convention) parameter that classifies/selects the appropriate turbine type for a given head and power: low specific speed values correspond to Pelton turbines (high head), intermediate values to Francis turbines (medium head), and high specific speed values to Kaplan turbines (low head).

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Preliminary Design, Scroll Case & Draft Tube Preliminary design of Francis and Pelton turbines involves determining basic dimensions (e.g. runner diameter, number of buckets for Pelton, or runner vane/blade configuration for Francis) based on the design head, discharge, and the specific speed selected for the site.

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A scroll case (spiral casing) surrounds a reaction turbine's runner (Francis/Kaplan), distributing the incoming flow uniformly and smoothly around the runner's periphery to ensure even flow entry and efficient energy conversion.

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A draft tube, used with reaction turbines, is a gradually expanding conduit downstream of the runner that recovers residual kinetic energy from the water leaving the runner (by decelerating the flow, converting velocity head to pressure recovery) and allows the turbine to be set above tailwater level while still recovering the head difference down to the tailrace.

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Generators, Governors & Pumps Generators used in hydropower plants are typically synchronous generators, rated by parameters including rated power, voltage, and power factor, converting the turbine's mechanical rotational energy into electrical energy synchronized to the grid frequency.

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Governors regulate turbine speed/output by automatically adjusting gate opening (Francis/Kaplan) or needle/jet deflector position (Pelton) in response to load changes, maintaining stable rotational speed and, hence, stable grid frequency.

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Pumps (e.g. in pumped storage schemes, or for dewatering/auxiliary purposes) are characterized by performance curves relating head to discharge and efficiency to discharge, analogous in concept to turbine performance curves but describing energy input rather than energy extraction.

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Powerhouse Powerhouses may be surface (above ground, generally simpler/cheaper where geologically feasible), underground (excavated within rock, often used for very high head/large schemes or where surface siting is constrained), or semi-underground.

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The general arrangement houses the turbine-generator units and associated auxiliary equipment (governors, transformers, switchgear, cranes for erection/maintenance); overall dimensions are determined by the equipment's size and layout, crane requirements for installation/maintenance, and necessary operational clearances.