Last reviewed 16 Sept 2026 · 13 min read
Reservoirs
A reservoir is a body of water impounded by a dam to store water for later use.
| Type | Purpose |
|---|---|
| Storage (conservation) reservoir | Stores excess water in wet seasons for use in dry seasons (irrigation, water supply, hydropower) |
| Flood control reservoir | Stores flood water temporarily and releases it gradually: retarding basin (ungated outlets — automatic) or detention basin (gated outlets — controlled) |
| Distribution reservoir | Small storage within a water supply system to meet hourly fluctuations |
| Balancing reservoir | Downstream of a main reservoir to balance releases (e.g. from a power house) |
| Multipurpose reservoir | Serves several purposes |
Storage zones and levels
| Level / zone | Meaning |
|---|---|
| Dead storage | Below the minimum pool level / MDDL (minimum drawdown level) — not available by gravity; reserved for sediment |
| Live (useful) storage | Between MDDL and full reservoir level (FRL) / normal pool level |
| Surcharge storage | Between FRL and maximum water level (MWL) — uncontrolled, occurs during floods above the spillway crest |
| Bank storage | Water held in the permeable banks — released as the level falls |
| Valley storage | Natural storage in the river valley before the dam; net storage created = total − valley storage |
Capacity
- Area–elevation curve from contour surveys; capacity–elevation curve by integration using the trapezoidal formula , the cone formula or the prismoidal formula.
- Safe (firm) yield — maximum quantity that can be supplied during a critical dry period; secondary yield — water available in excess of safe yield in wet years.
- Required storage for a given demand: mass curve (Rippl) method or sequent peak algorithm (see Runoff).
Reservoir sedimentation
Rivers carry sediment; when flow enters a reservoir, velocity falls and sediment settles — coarse near the head (forming deltas), fine near the dam; density currents can carry fine sediment right up to the dam.
- Trap efficiency — percentage of incoming sediment retained; depends mainly on the capacity–inflow ratio (C/I); Brune's curves relate them — large storage reservoirs trap almost all sediment.
- Life of a reservoir — the time until sediment fills a specified part of its capacity (e.g. dead storage filled, or live storage reduced to the point the reservoir no longer serves its purpose). Life is computed stepwise because trap efficiency falls as capacity falls.
- Distribution of sediment with depth — empirical area reduction method.
Control of sedimentation:
- Catchment treatment — afforestation, soil conservation, check dams (most effective long-term).
- Proper selection of dam site (low sediment yield).
- Sluicing / flushing — releasing sediment-laden flood water through low-level outlets before it settles; drawdown flushing.
- Sediment bypass tunnels; venting density currents.
- Dredging or excavation (costly).
- Vegetal screens at the head of the reservoir; design with adequate dead storage.
Reservoir losses: evaporation (major in shallow, wide reservoirs), seepage, sedimentation.
Dams — classification
| Basis | Types |
|---|---|
| Function | Storage dam; diversion dam (weir/barrage); detention dam; debris dam; coffer dam (temporary enclosure for construction in dry) |
| Hydraulic design | Overflow dam (water spills over the crest); non-overflow dam |
| Material / structural behaviour | Rigid — gravity (concrete/masonry), arch, buttress, steel, timber; non-rigid — earth, rockfill |
Site selection
Sound foundation rock at reasonable depth; narrow valley opening into a wide basin upstream (small dam, large storage); suitable site for spillway; availability of construction materials; minimum submergence of valuable land, forests, habitations and mineral deposits; low sediment yield; accessibility; watertight reservoir basin; geological stability (no active faults, landslides).
Gravity dams
A gravity dam resists external forces by its own weight. It is the most durable type, requires little maintenance, suits almost any height, and can accommodate an overflow spillway section, but needs a strong rock foundation.
Forces acting
- Self-weight — acts through the centroid of the section; the main stabilising force.
- Water pressure — horizontal acting at above the base; plus the vertical weight of water on any upstream batter; tailwater on the downstream side.
- Uplift — due to seepage through the foundation and dam body. Without drains: varies linearly from at the heel to (tailwater) at the toe. With a drainage gallery, uplift at the drain line is commonly taken as tailwater head plus one-third of the difference between reservoir and tailwater heads, varying linearly to the heel and toe; uplift assumed to act over the full base area.
- Silt pressure — ().
- Wave pressure — maximum at about above still water; total (kN/m) acting at above still water level.
- Earthquake forces — horizontal and vertical inertia of the dam, and hydrodynamic pressure of water (Westergaard/Zanger approaches), as per IS 1893.
- Ice pressure (cold regions) and wind pressure (minor).
Wave height (Molitor–Stevenson):
( = wind velocity in km/h, = fetch in km, in m.) Freeboard is commonly taken as about 1.5 above the maximum water level.
Load combinations
The dam is checked for combinations such as: construction condition (empty reservoir, with and without earthquake); normal operating condition (full reservoir, normal uplift, silt, ice); flood discharge condition (MWL, full uplift, tailwater); normal operating + earthquake; drains choked (extreme uplift). Permissible factors of safety are lower for extreme combinations.
Modes of failure
- Overturning about the toe: — commonly required 1.5 or more (without earthquake). In practice the no-tension condition usually governs before overturning.
- Sliding:
- Sliding factor (should exceed about 1.0)
- Shear friction factor ( = shear strength of the joint/foundation) — commonly required 3 to 5 depending on load combination.
- Compression (crushing): maximum stress at the toe must not exceed the permissible stress of concrete/masonry/foundation.
- Tension: masonry and concrete dams should have no tension; the resultant must lie within the middle third of the base ().
Normal stresses at the base:
Principal stress at the toe (full reservoir): (downstream face at angle to vertical).
Elementary profile
The theoretical profile of a gravity dam is a right-angled triangle with the vertical face upstream, height equal to the water depth and base .
- No tension (resultant at the middle-third point):
- Without uplift:
- With full uplift ( = uplift intensity factor, 1 for full):
- No sliding (friction only):
- = specific gravity of dam material (≈ 2.4 for concrete).
Limiting height (low vs high gravity dam):
( = allowable compressive stress.) A dam taller than this is a high gravity dam — the downstream face is flattened near the base and the upstream face given a batter to keep stresses within limits.
The practical profile adds a top width (roadway, commonly about 14% of the height with a minimum), freeboard, and upstream/downstream batters near the base.
Galleries, joints and foundation treatment
- Galleries — for drainage of seepage (drain holes), inspection, grouting and instrumentation.
- Contraction joints (transverse) with shear keys and water stops; longitudinal joints in very large blocks.
- Foundation treatment: excavation to sound rock; consolidation grouting (shallow, to strengthen the foundation); curtain grouting (deep, near the heel, to reduce seepage and uplift); drainage holes downstream of the curtain; treatment of faults and seams (dental concrete).