Constructing ground tanks

Ground tanks are a type of dam excavated below the natural surface of the land which is filled by surface runoff collected via diversion drains. 

These dams are common in western NSW and can be a cost-effective way to store water, but they require careful planning and construction.

Design considerations

Soil type

• Drill a pilot hole to test the area's suitability before construction.
• The soil must contain at least 20% clay to seal properly and hold water.
• Use a field ribbon test or lab analysis to confirm clay content.
• A flat ribbon of at least 30 mm long indicates enough clay content.
• The pilot hole should show gravel or sand seams, which cause leakage and should be avoided building a ground tank in.

Evaporation losses

• Evaporation losses vary across the landscape due to climatic conditions, such as:
  • temperature
  • humidity
  • wind.
• Shallow tanks with large surface areas lose more water.
• Deep tanks with small surface areas lose less water

Constructing a ground tank

It is important to consider what the water requirements of stock are that will use the ground tank's water, as well as what the lay of the land is to determine:

• catchment area
• embankment requirements
• suitable spillway areas.

Stock water requirements

When determining tank size, stock and domestic requirements the evaporation loss needs to be considered. 

There are several recommended tank sizes across western NSW when considering stock requirements and evaporation losses based on dry sheep equivalent (DSE) water consumption estimates. Domestic household requirements are not taken into account.

Designing a ground tank

Catchment area

The yield of a catchment (run-off) is the amount of rainfall that runs off the surface and can be diverted to and collected in ground tanks. The yield depends on site characteristics, e.g. hard, rocky catchment area can yield more than a grassy catchment area.

Use this formula to calculate the minimum catchment size:

• Tank volume (m³) / average annual rainfall (m) × catchment yield % = minimum catchment size (m²)
• Minimum catchment size (m²) / 10,000 = minimum catchment size (ha)

For example:

A landholder intends to construct a 5 mega litre (ML) ground tank in the Cobar area where the average 
annual rainfall is 394 mm with a yield of 2%. This would be worked as:

• 5,000 m³ / 0.394 m x 0.02 = 634,518 m²
• 634,518 m² / 10,000 = 63 ha (rounded down)

Therefore, in this example from Cobar, 63 ha is the minimum catchment size needed to fill a 5 ML ground tank.

Diversion drains (tank drains)

Once a catchment area and size has been selected, diversion drains can be constructed to collect and divert run-off from the catchment area into the ground tank. 

These must be properly surveyed and constructed to prevent erosion and ensure runoff reaches the tank.

Diversion drain slopes

A slight slope in the diversion drain is required to ensure water flows into the ground tank without causing erosion. A slope of 0.2 – 0.3% is ideal for bare earth channels. 

• 0.2% should be used on highly erodible soils.
• 0.3% should be used on more robust soils. 

The slope of the diversion drain determines the speed of water flow in the channel. If the slope is too steep, the water flows are too fast, causing the channel to erode. Eroding diversion drains cause excess 
sedimentation into the ground tank, which reduces capacity over time.

A laser level can be used to measure changes in land height and ensure correct slope percentage.

To determine a slope of 0.2%:

• survey the drain commencing at the inlet of the ground tank
• continue on a rising grade of 5 cm every 25 m (which equals to 20 cm rise every 100 m). 

Diversion drain banks

The bank of the diversion drain ensures water flows down the drain and into the ground tank. The height of the bank is dependent on the length of the diversion drain and the area of the intercepted catchment. Bank height is measured from natural ground level not the channel base.

• As a rule of thumb, diversion banks which are less than 1 km long and have a catchment area less than 100 ha, should have a minimum settled bank height of 60 cm.
• If the catchment area is greater than 100 ha, a larger diversion drain will need to be constructed, with a wider channel and a minimum settled bank height of 80 cm.
• If diversion banks are longer than 1 km, the bank height closer to the ground tank should be increased. This is required to accommodate the larger volume of flows from the increased area of catchment.

Embankments (tank banks)

• An embankment contains the water in the ground tank.
• Freeboard refers to the height of the embankment above the top water level (spillway level).
• Adequate freeboard of the embankment is required to account for any surcharge during heavy inflow, wave action and clearance.
• The freeboard of the embankment should be a minimum of 1 m.

Spillway (by-wash)

• A spillway provides an outlet for excess water to be disposed of without causing damage to the tank 
  or surrounding area.
• A spillway should be located on low, broad areas to allow water flows to spread out and lose energy.
• A good rule of thumb is to create the spillway at 3 times the width of the inlet. 

For example:

If the inlet is 20 m, the width of the spillway should be 60 m. This may be created with 2 spillways of 30 m.