Internal erosion

Piping is a related phenomenon and is defined as the progressive development of internal erosion by seepage, appearing downstream as a hole discharging water.

It can occur in sandy foundations, within the dam or levee, or in cofferdams under high flood pressures during construction, causing unraveling at the downstream face.

Experiments from Sellmeijer and co-workers have shown that backwards erosion initiates in a slot through the strata that overlays the eroding soil (e.g. through excavations or drainage ditches)[11][12] and then progress in many, smaller pipes (less than 2mm in height) rather than a single one.

The stability of the pipes is dependent on the head, and once this is larger than a critical value (0.3-0.5 of flow path length), the channel extends upstream.

Beyond this, at any head greater than the critical value, erosion progresses until eventually, the pipes break through to the upstream reservoir, at which point a breach occurs.

In order for backward erosion to occur, the dam or levee body must form and maintain a ‘roof’ for the pipe.

Suffusion leads to increased permeability in the embankment core, greater seepage velocities and possibly hydraulic fractures.

The type of filter required and its location is dependent on which zones of the dam are most susceptible to internal erosion.