Lakshmi: Why earthen dams fail

The destruction that accompanied the floods of early October raises the question: Why do dams fail?

Like all structures, dams have a life cycle: They gradually deteriorate over time and require maintenance to keep them safe while they hold back the water. Three types of processes can cause dam failure: hydraulic, seepage or structural.

Hydraulic failures occur when water that can’t be released from the spillway or gates flows over the face of the dam. This water can cause erosion of the downstream face of the dam, which can be easily seen as small gullies. Having grass on the downstream face slows down the water, enhances infiltration, provides an anchor to the soil and controls downstream erosion.

The upstream face of the dam also can be damaged by fast-moving water, especially if it is not reinforced with stones. The damage that is not visible to the human eye but happens on a continuous basis is seepage. Seepage is a continual process in earthen dams, but the seepage and associated pore pressure increase when the water levels are high. Seepage forms small channels within the dam that grow larger and eventually form a continuous channel from the upstream side of the dam to the downstream side underneath the dam. This channel is called piping through the foundation, and it will eventually cause the dam to collapse and fail.

Structural failure can occur due to faulty construction, steep slopes of the faces of the dam and poor maintenance.

Most of the dams that failed in the Midlands were built decades ago, when the area was rural and forests controlled surface runoff, promoting greater infiltration into the soil. Time has brought increased populations, the clearing of land and construction that removed the buffer effects of the vegetated areas. Urbanization has also increased soil erosion so that most lakes have more sediment in them — thereby decreasing their capacity.

Today dams are built with better drainage and protection for both the upstream and downstream faces.

But whether the dams are new or old, whether the rainfall is extreme or routine, a regular maintenance schedule is the most important way to ensure the proper drainage that safeguards earthen dams, and prevents the infrastructure from failing to provide protection at a critical time.

This story was originally published November 29, 2015 at 5:01 PM.