How to construct a safe dam

Patel Dam Tragedy at Solai, Subukia Nakuru County on May 10, 2018 after the dam burst its bank. [Kipsang Joseph/Standard]

About 20 million litres of water, enough to satisfy the optimal demand for Nairobi and its environs for a whole day, broke the walls of Patel dam in Solai, Nakuru County.

Dimensions for the collapsed dam show it was roughly the size of a football stadium, but with stands built to five meters – about three times the height of an ordinary adult.

Considering that the dam was filled to the brim, a close estimate of the weight of the water is 20,000 tonnes.

Structures that stood on the path of the raging waters downstream were swept away in a trail of destruction that also claimed dozens of human lives.

It was one of the seven earth dams on the Patel farm that are now holding record amounts of water following heavy rainfall that has lasted more than six weeks.

Engineers say the collapse was a result of the earthen barriers soaking in too much water whose weight they could no longer hold.

Felix Gatumba, a structural engineer working in Kitui, told The Standard the strength of the dam might have been compromised after the water levels rose to the brim where the walls are thinnest.

“Water, like other liquids, exerts a lot of lateral pressure wherever it is contained, so it will always be pushing outward,” Mr Gatumba said.

He added: “It is clear that the bearing capacity of soil making the earth walls was compromised before it all crumbled.”

Earth dams, he explained, are built by excavating on the path of a river then piling lumps of soil to build the walls around a reservoir, which would arrest rain or run-off water.

The blockage, also known as the bank, should be strengthened by compacting soil.

It is advisable to carry out a test to determine if the soil in use has the required bearing capacity, and whether it is moist enough to ensure a strong bondage.

In the Solai case, the Patel dams harvest from the nearby seasonal Solai River, whose volumes have sharply increased with the sustained rainfall.

Since the walls are simply made by piling additional layers, the piles tend to be wider at the base and thinner as the dam grows taller.

“The risk of collapse when the water levels are low to medium is lower because the walls are thicker, meaning there is high absorption capacity,” Gatumba said.

But when the dam fills up and water begins overflowing, the thin brim is quickly compromised to give way, spilling over at the lowest point.

The entire weight of water is then exerted at this point to forcefully tear the wall all the way down.

“Once the water has found a point of weakness, it is not possible to contain the force with which it will find its way out,” the engineer said.

A properly built dam should have a spillway that is below the highest point of the wall, to evacuate excess water during flooding.

Engineers also advise that walls should have grass to increase stability of the dam and prevent erosion.

Walls should also be steeper on the outside than inside. A 33 degree slant is recommended for the inner side, against 20 degrees downstream.