Dam Safety Ltd - Achievements

Dam Safety Ltd, founded in 1998, specialises in dam safety engineering, particularly the performance of existing earth and earth-rock embankment dams. Our principal, Rodney Bridle, is a progressive and very experienced dam engineer, interested in applying new and old knowledge through technology to understand the behaviour of dams and levees and make them safe and economically effective.

He is a former Partner and Director in Watson Hawksley and Montgomery Watson (later MWH, now Stantec). Rod is also an Associate of the Geotechnical Consulting Group. He has worked variously on studies, design and construction of new dams at Derwent, Co Durham; Glen Finglas, Scotland; Empingham Dam, retaining Rutland Water; Queen's Valley Dam, Jersey; Freshwater Reservoir, Long Beach, USA; Fullerton Pollan Dam, Co Donegal, Ireland; Karangawan Dam, Sabah, Malaysia; Imang Dam, Brunei; Brentingby Flood Storage Dam, Melton Mowbray; River Nar Flood Storage Dam, near King's Lynn; and Stock Beck Flood Storage Dam, Kendal.

He was Chairman of the Review Panel with Dr Andrew Charles and Professor Richard Chandler for the raising of Abberton Dam in Essex. He was also chairman of the Review Panel for the concrete-faced rockfill Babagon Dam, Sabah, where the designers, NSW Public Works of Australia, were advised by (the late) J Barry Cooke, the pioneer and advocate of concrete-faced rockfill dam engineering.

In 1986 Rod was awarded the Telford Gold Medal, highest award of the Institution of Civil Engineers, for his paper with Professor Peter Vaughan and Howard Jones on Empingham dam. From 1999-2001, he was Chairman of the British Dam Society and the UK representative on the International Commission on Large Dams (ICOLD).

For 25 years he was an All Reservoirs Panel Engineer, appointed under the Reservoirs Act (1975) to inspect, supervise and certify safety measures and construction of new dams of all types. He inspected over 80 dams in Great Britain, Jersey, Isle of Man, Ireland, and Jamaica. He supervised construction of a filtered tailbay to guard against internal erosion along the culvert at Lower Bittell Dam completed in 1804 to supply water to canals around Birmingham. He also supervised the installation of a waterproofing membrane on the upstream face to improve the stability of the Val de la Mare Dam, Jersey, damaged by alkali-aggregate reaction.

Internal erosion

Rodney Bridle is the UK Member of the ICOLD Embankment Dams Committee and in recent years has been engaged with colleagues in writing ICOLD Bulletin 164 giving guidance on Internal Erosion in Existing Dams, Dikes and Levees. Internal erosion is a major cause of failures in all types of water-retaining earth embankments. The Bulletin deals largely with the mechanics of the four modes of internal erosion: concentrated leak erosion, backward erosion and piping, suffusion and contact erosion. This is new knowledge, making it possible for engineers to estimate the hydraulic load, usually expressed as a water level, imposed on an embankment that would cause failure by internal erosion.

To disseminate the new knowledge he has written papers and made many presentations, including at ICOLD Annual Meetings from 2013-2017, Internal Erosion Working Group Meetings since 2005, to US, UK and Polish Dam Societies and at a training seminar at the Canadian Dam Association Conference in 2013. Further information about internal erosion, and other dam issues, can be found in papers on the References page here.

Rod is a member of the BC Hydro Expert Engineering Panel, with Dr Kaare Hoeg (Norwegian Geotechnical Institute) and Professor Robin Fell (UNSW, University of New South Wales, Australia), reviewing the 'seepage control functions' (the ability to resist internal erosion) of large dams in Canada. He advised Dwr Cymru, Welsh Water, on internal erosion potential at their embankment dams. He is ready to do similar internal erosion reviews for other dam owners and engineers.

Load-based risk assessment

Internal erosion occurs when hydraulic loads and water levels are high during floods, and the annual probability of that water level can be derived from the flood hydrology. As risk is the product of probability and consequences, and consequences can be assessed by dambreak analyses, the new knowledge about internal erosion provides an alternative means of quantitative risk assessment based on estimated actual loads. Similarly, the probability of the water level that would cause overtopping to a depth sufficient to cause failure by scour and erosion of the downstream slope can also be derived from the flood hydrology. The probability of occurrence of the earthquake generating a Peak Ground Acceleration sufficient to cause failure by seismic shaking can be estimated from earthquake spectra.

This approach provides estimates of the probabilities of occurrence of the actual loads causing failure. It should improve on present methods, which often determine only the probability of failure, not the actual failure loads, or if the loads are determined, they are obscured by the application of load and resistance factors. The risk approach is particularly applicable to dams, where failures may cause many fatalities, and an acceptable probability of failure must be agreed. It can also be applied to flood management embankments which are expected to overtop during relatively modest floods (1 in 10-year, 25-year, 50-year events, for example). The level of resilience of structures and facilities that would be inundated should be the inverse of the probability of overtopping, high resilience is necessary for 1 in 10-year inundation, less resilience for rare inundations, 1 in 100-year, say. Rod would be pleased to work with dam owners and engineers to develop the actual load risk approach further.

Dams, ecology and sustainability

Arising from his acknowledged engagement with the World Commission on Dams and his interests in sustainability, Rodney Bridle advised IUCN (World Conservation Union), Geneva, on implementation of environmental projects associated with dams. Advised by specialist ecologists, he also worked closely with Natural England to develop dam safety remedial works acceptable to them on an existing embankment dam on a Site of Special Scientific Interest. In recognition of the conflicting needs of vegetation management on the surfaces of the dam and the SSSI, the dam footprint was removed from the SSSI. Invitations to participate in similar assignments would also be welcomed.