Step by step21 March 2011
The failure of some stepped masonry spillways in the UK prompted the Environment Agency to commission research into the design, maintenance and safe operation of such structures
Most stepped masonry spillways in the UK have stood the test of time. The average British dam has experienced over 100 years of successful operation, and many of their spillways predate the introduction of concrete when masonry was used. However, the failure of several stepped masonry spillways over the past few decades has necessitated clarification of the safety of such structures.
In June 2007 failure of a stepped masonry spillway at Ulley dam threatened the security of the main dam. A major concern was that the uncontrolled release of 600,000m3 of water would have serious consequences downstream. This was reminiscent of the failure of a similar spillway at Boltby dam in June 2005. There are also thought to have been other less well publicised failures dating back to 1985. While in 2007 it became apparent that major flood events had affected a number of other masonry spillways in the UK to a lesser extent.
Research into stepped spillways seemed to have focused on the hydraulic regime of the flow, concentrating on the floor regions. However it became clear at Ulley and Boltby that significant failure had occurred to the masonry walls, and that internal hydrodynamic effects had been a major contributory factor to the failures.
Following incident reviews of Ulley and Boltby dams, it was recommended that research into such dynamic forces was required to clarify the matter. The Environment Agency awarded a contract to MWH UK to carry out such research, supported by CRM Rainwater Consultancy and the Building Research Establishment. United Utilities and Yorkshire Water Services also agreed to make results available from model testing they had carried out on stepped masonry spillways.
Entitled Guidance for the Design and Maintenance of Stepped Masonry Spillways, the project was carried out from October 2008 to April 2010. It focused on shallower stepped spillways in the order of 1v to 3h, associated with UK embankment dams. Examples were collected of industry experience of operating such spillways and of distress and/or remedial measures. This was supplemented by a programme of hydraulic model testing both to clarify how flow depths can be calculated in such spillways. It also helped to improve understanding of the localised hydrodynamic pressure fluctuations which spillway walls and inverts may be subjected to.
Various results and conclusions were drawn from this work. For example, in addition to nappe and skimming flow regimes, the research was able to identify very high flows which may ride on the top of enlarged vortices, hardly touching the spillway steps. While it was also noted that high levels of turbulence mean that absolute flow depths are difficult to quantify.
Research into pressure distribution also showed that if high pressures are injected into open textured masonry in high pressure zones (so that they create back-pressures behind the masonry blocks in low pressure zones), the block in the low pressure zone can be subject to removal. Furthermore model testing also demonstrated that there can be considerable turbulence and pressure fluctuations during such flows. Such potential pressure differentials on typical UK spillways could reach 5-10m of water head. The high levels of turbulence within high pressure zones can also be sufficient to dislodge blocks within those zones.
The integrity of mortar pointing was also highlighted as being vitally important. In high pressure and high velocity zones, the research indicated that it was more important to ensure that vertical mortar pointing (normal to the flow regime) was intact, than horizontal mortar pointing (parallel to the flow regime). It was also noted that there may be some benefit in selectively omitting some of the vertical pointing on the vertical faces of steps to permit drainage relief. While mortar pointing in general should be in a good condition, finished either flush with the masonry blocks or with a bucket handle profile.
Regular inspections and routine maintenance of masonry spillways are crucial. This should include the removal of any weeds and associated root growth in and around the spillways. Indeed the presence of any trees within 10m of the spillway should be noted, and if thought to have the potential to cause or be causing structural damage, their removal should be considered.
Although it is likely that most masonry spillways will be in a reasonable condition at inspections, remedial measures will sometimes be required. These can include:
• Reinstating pieces of displaced masonry.
• Stitching masonry blocks together using stainless steel ties.
• Local patching with concrete.
• Pressure pointing to restore the integrity of the surrounding mortar matrix.
• In extreme cases where movement or sulphate attack is found to be severe or widespread, demolition and rebuilding of the spillway.
Maintenance and inspections
As the Environment Agency is keen to point out, most stepped masonry spillways in the UK have performed successfully and stood the test of time for more than a century. However, it believes that their use will continue to be acceptable provided that maintenance works and inspections are undertaken on a regular basis, combined with careful remediation measures when required. This is of particular importance when the spillways are located along the mitre of an embankment where a collapse of the sidewalls could endanger the dam.
The Guidance for the Design and Maintenance of Stepped Spillways (SCO80015) was commissioned by the Environment Agency as part of the joint EA/DEFRA Flood and Coastal Erosion Risk Management Research and Development Programme. Copies of the report are available from http://publications.environment-agency.gov.uk