In England the most common dam design is an earth embankment with an impervious clay core; these make up 83 per cent of the 2042 dams registered under the Reservoirs Act. So, what are the failure processes and problems associated with embankment dams?
Although no fatalities have been recorded for nearly 100 years, incidents which stop short of a full catastrophic failure are not uncommon. A report prepared for the Environment Agency in 2013 analysed the 76 incidents reported from 2004 to 2013. These incidents were most commonly related to overtopping and internal erosion. In 2013 reporting such incidents became mandatory and the Environment Agency publishes annual summaries with lessons to be learned from each incident.
Overtopping happens when water passes over the top (crest) of the dam rather than over or through the normal intended routes of valves, gates, sluices, tunnels, chutes, spillways etc. There are two main causes of overtopping. Firstly, when more water flows into the reservoir than can be discharged over the normal and emergency spillway, causing the water level to rise above the crest of the dam. Secondly, when water levels in the reservoir are so high that strong winds generate storm waves that overtop over the crest. Waves can also potentially erode the reservoir side of the dam causing a partial failure if there are insufficient protection measures in place.
Internal erosion happens when water passing through the dam body transports fine material out of the embankment eventually leading to the formation of a hole or “pipe” through the body of the dam. In some cases, pipes may start along a tree root or animal burrow and in the worst cases, these pipes may enlarge by progressive erosion until an open breach though the dam crest is formed. This failure mode is often termed a “piping” failure.
For an earth embankment dam, both overtopping and piping erode material from the surface or body of the dam. Initially this erosion is slow but catastrophic failure of the structure, with uncontrolled release of all the water in a reservoir, can eventually occur. This failure will lead to rapid inundation and poses a threat to life for the communities downstream of the structure.
Through decades of research looking at how earth embankments fail, we have identified the complexity of the processes of erosion, and also how long stages of the erosion process may take. The early phase of breach initiation happens when seepage through, or flow over, the embankment initiates erosion. Although little studied, this phase is of prime practical importance since the potential for catastrophic failure can be identified and prevented through appropriate emergency action and the potential for loss-of-life can be reduced or eliminated through evacuation of the hazardous zone downstream.
We can simulate breach formation using our simplified model (AREBA) or use more detailed approaches in EMBREA. The more detailed approaches can take account of construction with different zones of distinct material (e.g. two shoulders and an clay core) and uncertainties in knowledge of material properties.
The breach models show us how the flow out of a reservoir changes through the time it takes for the breach in an embankment to develop and the reservoir to drain in an uncontrolled fashion. These flow rates can form the starting point for flood simulations along the valley downstream (and in some cases also to a limited extent upstream) giving information for civil contingency planning such as maximum extent of inundation, maximum water velocity, time of first arrival, sequence of closure of evacuation routes, likely extent of structural damage to buildings, roads, bridges etc.