Application of the internal erosion toolbox to Rochdale canal dams. Part 1: Introduction and erosion initiation

Abstract

Portfolio risk assessments (PRAs) have indicated that some dams built at the turn of the nineteenth century to supply water for the Rochdale canal are particularly vulnerable to internal erosion. The canal is served by a series of dams built between 1794 and 1857. United Utilities (UU), who now own the dams, have recognised that the PRA was a high-level screening tool, and that a more rigorous method was required to provide a more detailed assessment of all potential failure modes. This led to the development of a more robust approach using the unified method of risk analysis (internal erosion toolbox), which utilises a generic event tree approach to assessing potential seepage and piping failure paths. This paper describes some lessons learnt in applying the toolbox to canal dams. It identifies a number of issues that application of the toolbox posed and the research undertaken to overcome these in order to make credible assessments of probability of failure so as to identify dams with an unacceptable level of risk and to help UU to prioritise its capital expenditure on reservoir remedial works. The paper is in two parts: Part 1 consists of an introduction to the dams under consideration and the toolbox. It then considers how to overcome bias in estimation of the probability of initiation of internal erosion. Part 2 considers ways to assess the time for development and likely detection of internal erosion.