Storage of Agricultural Water in Earth Dams

Credits: Biovision-Infonet

Cut-Off Drain

Cut-off drains deliver rainwater run-off from roads onto farmland where it sometimes creates erosion and deep gullies. This potentially destructive practice can be changed to a gain for the farmers by diverting the water into ground tanks, small earth dams or land for seasonal irrigation.  

Here a cut-off drain diverts run-off water from a road into a natural and shallow depression on the lower side of a road called a pan.

A pan can be made into a pond by deepening the reservoir and place the excavated soil as a dam wall (embankment) with two spillways (overflows) on the lower side of the reservoir. Ponds are small earth dams.

Where road contractors have excavated murram for road construction and left ‘borrow pits’ or ‘murram pits’, these can be converted into pans or ponds by digging a trench to divert water from a road into the pit.

Usually these pits have water-tight (impermeable) floors through which water cannot leak into the underground. 

Charco ponds and Charco dams are half-ball shaped (hemi-spherical) excavations where the soil is placed as a dam wall around the excavation, except at the inflow channel which has two spillways for safe discharge of surplus water. Charco ponds and dams are viable in flat land.

Hillside ponds/dams have a semi-circular dam wall made of the soil excavated for the water reservoir. A stony spillway is built onto each end of the dam wall. These dams are designed to be constructed on rolling land and hill sides.

Valley dams are straight dam walls built across narrow points in valleys. A wide spillway lined with stones is built at each end of the dam wall to discharge surplus water safely.

Due to global warming, many valley dams have been damaged by extraordinarily big thunder storms exceeding the design criteria of the highest rainfall in the last 50 years.

Excavation dams should be circular or oval excavations where the excavated soil is used for building the dam walls whose sides should slope at least 45 degrees.

The excavation dam in the above photo was a waste of money and labour. The sides are too steep and therefore collapsing. The soil is too porous and can therefore not hold any water. 

The photo above shows a series of well designed and constructed excavation ponds have been filled with run-off water from roads. The embankments should be stabilised with grass.