Reservoir Design

Part of Water Systems

How to design and build storage reservoirs — from small household tanks to village-scale impoundments — that hold water reliably and safely.

Why This Matters

A water source that flows reliably at the right rate at all times is rare. Springs dry up in summer. Streams flood in winter. Rainfall is seasonal and unpredictable. A reservoir decouples supply from demand: it fills when water is abundant and empties gradually during dry periods, smoothing out the mismatch between natural supply and human need.

Reservoir storage is measured in days of supply. A household cistern might hold 3 days’ water. A village tank might hold 7 days. A seasonal agricultural reservoir might hold 3 months. The required storage depends on the length of the dry season, the variability of the source, and the consequences of running out.

Design involves three distinct challenges: sizing the reservoir correctly, containing the water without leakage, and managing the structure safely. A leaking reservoir is useless; a collapsing embankment dam is catastrophic. Both failures are preventable with sound engineering.

Sizing the Reservoir

Daily demand: Establish water consumption per day (see distribution network article — typically 15–50 liters per person per day).

Critical dry period: How many consecutive days can the source stop flowing? In a wet climate with a reliable spring, this might be 5–10 days. In a semi-arid region with seasonal rainfall, it could be 90–120 days.

Required storage volume: Storage = Daily demand × Critical dry period × Safety factor (1.3)

Example: 200-person village, 30 L/person/day, 14-day dry period: Storage = 200 × 30 × 14 × 1.3 = 109,200 liters ≈ 110 m³

Additional losses:

• Seepage (for earth reservoir): 5–15% per month
• Evaporation: 5–10 mm/day in hot climates (surface area × evaporation rate)
• Sediment accumulation: reserve 20% of volume for sediment over 10 years

For the example, a 150 m³ reservoir is prudent.

Types of Reservoir

Masonry or Concrete Tank (Covered)

For household to small-village scale (5–500 m³). Built above or partially below ground. Covered to prevent evaporation, algal growth, and contamination.

Advantages: Minimal seepage, easy to keep clean, accessible for inspection, no embankment failure risk.

Construction:

• Walls: stone masonry in hydraulic lime mortar (1 part lime : 2 parts sand, with pozzolan addition). Minimum 350 mm thick for tanks over 10 m³.
• Floor: 150 mm concrete or lime concrete, sloped to drain sump
• Render: Two-coat hydraulic lime render inside — scratch coat 1:3, finish coat 1:2. Must be crack-free and tight.
• Cover: Timber, stone slab, or cast concrete, with access hatch
• Overflow: Set 50–100 mm below top of walls; discharges to waste
• Draw-off: At lowest point, fitted with a valve

Testing for leaks: Fill with water. Mark the water level. Check after 24 hours. Acceptable loss: under 5 mm/day (from absorption into new masonry). After 7 days the masonry is saturated and loss should be near zero.

Earth Embankment Reservoir (Pond)

For larger volumes (500–10,000+ m³). An earthen embankment across a valley or hollow impounds runoff or stream flow.

Advantages: Lower cost per m³ for large volumes, local materials.

Disadvantages: Seepage, evaporation, algal growth, embankment failure risk.

Critical design rules for earth dams:

1. Never build on pervious foundation. The embankment rests on undisturbed subsoil. If the foundation is sand or gravel, the reservoir will drain through the base — a clay or rock foundation is required.
2. Freeboard minimum: The top of the embankment must be at least 0.5 m (small dams) to 1.0 m (larger dams) above the design flood level. Overtopping an earth dam erodes and collapses it catastrophically.
3. Spillway is mandatory: A formed overflow spillway protects the embankment from overtopping during floods. Stone-lined channel or concrete — never just a cut through the dam body.
4. Core trench: Dig a trench along the centreline of the embankment, fill with compacted clay. This interrupts any seepage path through the foundation.
5. Side slopes: Upstream face 2:1 (horizontal:vertical) or 3:1. Downstream face 2:1. Flatter for weak soils.
6. Crest width: Minimum 3 m for small dams, 5 m for larger.

Dam height limit without engineering analysis: Do not build earth dams higher than 3 m without formal engineering review. Higher dams store more water and carry higher failure risk. A 3 m dam can fail quickly but a 5 m dam failure can kill everyone downstream.

Lining Earth Reservoirs

Unlined earth ponds lose 20–50% of stored volume to seepage, depending on soil. Lining reduces this to 1–5%.

Clay blanket lining:

1. Excavate pond to design shape plus 200 mm extra depth
2. Source clay-rich subsoil from within the catchment
3. Place clay in 150 mm layers over the entire pond floor and sides
4. Compact each layer with a rammer or by foot-puddling (working thoroughly with feet until plastic and homogeneous)
5. Total minimum blanket thickness: 300 mm (two layers)
6. Protect the surface from cracking when dry: keep permanently wet, or add 100 mm of stone or gravel cover

Bentonite lining (if available): Bentonite is a highly expansive clay that swells enormously when wet, sealing pores. Mix 15–20 kg/m² into the top 150 mm of soil, compact. When wetted, the bentonite swells to form an almost impermeable membrane. Widely used in the 20th century for lining ponds.

Lime treatment: Where calcium-rich soil is available, mix hydrated lime (5–8% by weight) into the soil lining. Lime reacts with soil silicates to form cementite compounds, reducing permeability by 100–1,000×. Effective in many tropical soils.

Inlet, Outlet, and Spillway

Inlet: Water should enter gently to avoid eroding the reservoir bed. Discharge from a splash plate or spread over stones. For turbid inflows, direct water to a settling basin first.

Outlet (draw-off):

• Pipe through or under the embankment — NEVER cut a notch over the top
• Pipe must be embedded in compacted clay, not just laid in backfill — otherwise seepage travels along the pipe exterior and erodes the embankment from inside
• Control valve at the outlet end, accessible from outside the reservoir
• Anti-seepage collars (flanges welded around the pipe every 3 m) break up external seepage paths

Spillway:

• Capacity: able to pass the largest flood expected in a 50-year period
• For a small catchment (1–5 ha), this is typically 3–10× the average inflow
• Stone-lined cut through the original valley side (not through the embankment)
• Approach channel must be level-graded to prevent concentrated flow attacking the embankment
• Outlet of spillway must drop water away from the downstream embankment toe

Water Quality Management

Stored water deteriorates. Common problems and solutions:

Algae: Form mats on surface, consume oxygen at night, produce toxins. Prevention: minimize nutrient inflow (keep animals away from watershed), cover cisterns, shade tanks where possible. Treatment: copper sulfate (0.5–1 mg/L) kills algae but requires careful dosing — toxicity increases in small volumes.

Sedimentation: Turbid inflows deposit silt that reduces capacity. Maintain a settling basin (see settling basins article) at the inlet. Desilt annually — remove accumulated mud from the bottom of small tanks.

Contamination: Keep all human and animal waste sources at least 30 m uphill from the reservoir. Fence the area if animals graze nearby.

Mosquitoes: Exposed open-water reservoirs breed mosquitoes. Stock with mosquito fish (Gambusia) where available, or introduce other surface-feeding fish. Keep banks steep to minimize shallow margins.

Treat all water from open reservoirs before drinking — a covered, fenced cistern fed by a spring may be safe without treatment, but surface-water reservoirs almost always require filtration and disinfection.