Valley tanks

Definition

A man-made valley tank can be defined as an excavated barrier that stops or restricts the flow of water or underground streams.

Valley tanks have different designs depending on purpose for its construction. 

Water reservoirs  can provide water for activities such as;

• Irrigation
• Human consumption
• Livestock
• Industrial use
• Aquaculture.

Valley tanks for irrigation

These are tanks excavated specifically to provide water for irrigation.

Design features

• Length should be longer than the width to prevent failure of open walls.
• Edges should be slanting to prevent collapsing in of the edge walls
• It should have an outlet to pass the excess water
• It’s recommended to always protect the tanks by fencing
• It should have an appropriate depth to reduce on evaporation losses

How big should the tank be?

Valley tank sizes may depend on factors like;

To mention

Excavation methods

Two methods are  available;

1. Manually excavated tank

This is where people provide labour for excavation.

Advantages of this method

• Some areas can have softer soils in this case manually excavated tanks can be cheaper
• some areas have good water recharge capacities, smaller tanks can be excavated hence reducing on costs
• some areas are inaccessible to machines which is not the case to people.

Limitations may include

• some areas are hard to be excavated by local people
• local labour takes time to mobilize and may not be consistent.
• local people can not make slanting edges increasing chances of wall failure.
• this method takes much more time compared to machine excavation.

2. Machine excavated tanks

This is where machines especially excavator (excavator with a longer arm is recommended for irrigation water) and dozers are used.

Limitations

– Expensive

– some areas may not be accessible to machines

Advantages of valley tanks over other artificially constructed water sources

• Can provide sufficient water to irrigate over 5 acres
• Cheaper to be excavated compared to borehole drilling and some shallow wells
• Cheaper and convenient in terms of water drawing compared keeping water flows and pressure constant
• Does not need sophisticated equipment to determine areas where they can be constructed

Limitations

Valley tanks are limited to areas especially valleys otherwise lining can be needed

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Some field photos

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