What price reservoir safety? A practical use of disproportionality in dam risk assessments Available to Purchase

Two embankment dams form part of a series of reservoirs built at the turn of the nineteenth century to supply water locks that are used to ascend and descend over the summit of the Rochdale canal. The embankments are homogeneous with no engineered waterproof barrier and consist mainly of weathered clayey sandstone fill over peat/glacial till over bedrock. There is a history of instability during the early part of the dams' life, with buttress berms added. There have not been any recent incidents at either of the reservoirs and recent monitoring data suggest that there are no embankment stability issues and internal erosion processes are not currently manifested. Nevertheless, homogenous embankment dams are inherently vulnerable to internal erosion. Extensive risk analysis has been undertaken at both dams, which has provided the basis for identifying failure mechanisms that are estimated to have an individual risk that is within the Health and Safety Executive (HSE) ‘intolerable’ range (i.e. individual risk when conservatively estimated as the probability of failure is greater than 10⁻⁴ per annum). This paper describes the risk analysis used to evaluate potential remedial measures to reduce the risk to a tolerable limit using the ‘as low as reasonably practicable’ principle as recommended in HSE guidelines.