By Jon Horrocks
Three years to the day after the failure of one of the spillways at Ulley Dam, a celebration was held to recognize the completion of the rebuilt structure. The rehabilitation work costt £3.8 million (US$6 million) and included a new spillway in the center of the dam wall.
During the emergency response and the subsequent rebuilding of the dam, owner Rotherham Metropolitan Borough Council (RMBC) learned several valuable lessons that are applicable to other dam owners.
Ulley Dam is an earth embankment dam that was completed in 1873 to provide drinking water for the city of Rotherham in South Yorkshire, UK. The dam is 16 meters high and 205 meters long, with a central puddle clay core, and impounds a reservoir with a capacity of 580,000 m3. RMBC purchased the dam in 1986 and opened the reservoir and the surrounding park for public recreation.
On June 24 and 25, 2007, 92.4 mm of rain fell in the Rotherham area. Total outflow down the two spillways engaged during the flood was about 10.1 m3/s, with 6.1 m3/s flowing down the spillway that failed on June 25. This spillway, on the left abutment of the dam, is 1.8 meters wide and is the first spillway to begin operating when water levels rise. The masonry walls of this spillway were partially destroyed. The high flood flows also caused scour of the toe and downstream shoulder of the embankment, threatening to breach the dam itself. About 700 local residents were evacuated, and the M1 motorway was closed for 48 hours as a precaution.
The dam contains no penstocks, stoplogs, or other equipment that could be used to control the flow of water into the spillway. RMBC contacted Ove Arup and Partners Ltd. on June 25 to provide advice. Arup was appointed the supervising engineering firm for the dam under the 1975 Reservoir Act. Supervising engineer Dave Crook immediately came to the site. Working with RMBC staff, he gave advice about the condition of the dam. RMBC's emergency procurement division obtained a crane, an 8-tonne skip, and about a dozen intermediate bulk containers full of stone. The spillway was blocked using the skip packed with large sandbags and the intermediate bulk containers. In parallel with the plugging work, the council lowered the water level in the reservoir first using high-volume pumps provided by the fire service and then using commercial submersible pumps.
Crook then contacted Jim Claydon, an engineer with the All Reservoir Panel. This panel consists of civil engineers qualified to design and supervise the construction and alteration of, to inspect and report, to supervise and certify the carrying into effect of recommended safety measures, and to act for the purposes of Section 16 (emergency powers) on all reservoirs to which the act applies. Claydon contacted JN Bentley Ltd. of Skipton, North Yorkshire, to stabilize the dam by packing the scour hole with about 2,500 tonnes of coarse limestone fill.
Investigating the problem
Arup identified the cause of the incident as the failure of one of the spillways. The masonry walls to the lower spillway on the left abutment had been partially destroyed, allowing erosion of the embankment toe, raising stability concerns.
Arup personnel then performed geotechnical and hydrological investigations to determine the extent of rehabilitation work needed. Terrestrial light detection and ranging (Lidar) equipment was used to monitor the downstream face of the dam for signs of continuing movement. In addition, a Section 10 inspection under the Reservoirs Act was undertaken by Claydon.
Claydon made nine recommendations to remediate the structure and ensure its future safety:
— Recalculate the probable maximum flood (PMF) to take into account the catchment area occupied by impermeable surfaces;
— Conduct a site investigation to determine the condition of the dam core and undertake remedial work as needed;
— Construct one or more spillways to pass the PMF;
— Repair a hole in the crown of the former diversion tunnel;
— Install a larger-diameter scour pipe discharging downstream from the dam;
— Remove redundant pipework;
— Fill the Morthen spillway down the right side of the dam with stone to support the toe of the embankment;
— Repair the wave protection on the upstream face to a level determined by the modified overflow; and,
— Engage mining specialists to provide a statement on the coal that had been mined under the dam, the coal remaining, and the likelihood of further settlement of the dam and appurtenant structures.
Determining a solution
In September 2007, council members agreed to rehabilitate the facility rather than discontinue or abandon the
reservoir. In June 2008, RMBC awarded a contract to Arup to design and manage the rehabilitation work.
A review of historical records indicated the dam contained a puddle clay core with a filter zone on each side. To determine how to rehabilitate the dam and core, Arup personnel built a computer model of the embankment and used this model to identify potential seepage paths and assess the properties of the fill materials for their susceptibility to internal erosion.
|The scour of the toe and downstream shoulder of Ulley Dam (bottom left) resulted from high flood flows. The flows also partially destroyed the masonry walls of a spillway located on the left abutment of the dam. (Photo courtesy Environment Agency)|
Because of the combination of hydraulic fracture risk, possibility of dispersive clays, and potential seepage paths through the clay core and concrete placed in 1969 to raise the dam height, Arup assumed that seepage through the core was possible. Thus, personnel performed an analysis of internal erosion on the clay core and then on select fill and shoulder fill successively downstream. The analysis indicated that if seepage occurred through the core and select fill material into the shoulder fill material, without fines being eroded, the ability of the shoulder fill material to resist internal erosion would govern the risk of embankment failure by erosion. Thus, Arup decided to limit the depth of remedial work to the clay core to just below the base of the concrete core extension, to minimize development of further problems during remedial work (such as creation of voids during excavation or piling).
Arup determined that a bentonite cement slurry wall would be most appropriate to achieve a seal with the
existing puddle clay and provide the same permeability as the original core. This option also could accommodate any small ongoing settlement of the embankment. A key into the puddle clay core would be required.
|After the flood, contractors worked to fill the scour hole and stabilize Ulley Dam, giving owner Rotherham Metropolitan Borough Council time to determine how to rehabilitate the structure. (Photo courtesy Ove Arup and Partners Ltd.)|
An optimal spillway length of 20 meters was identified to safely discharge the peak PMF outflow of 125 m3/s. Arup studied three locations for the new spillway: the right abutment, left abutment, and middle of the dam. The third location was considered the best option in both technical and cost terms. A triangular shape to the spillway was adopted to provide a good coefficient of discharge at all heads.
RMBC commissioned CRM Rainwater Drainage Consultancy of Bolton to construct a physical model of the proposed spillway arrangement based on the initial designs by Arup. This model was used to carry out a hydraulic test of the spillway and stilling basin design. Results from the model confirmed that these two structures would be able to withstand the new PMF of 134.8 m3/s.
Ulley Dam is located in an area of extensive coal mining. Thus, the spillway needed to be designed to allow for future settlement. The design is articulated, with several movement joints along its length. Each joint features a leakage detection system that consists of a perforated pipe laid the length of the joint that passes via an access chamber into a collector drain that takes flows to the toe of the embankment. If seepage appears from the collector drain, the access points can be opened up to determine the origin of the water.
Arup also designed a new emergency drawdown system for Ulley Dam. The original drawdown system on the right abutment discharged into the Morthen spillway, which was considered to be unsafe after the Section 10 inspection. Claydon recommended that this spillway be filled in to protect the integrity of the embankment slope. Filling in the spillway would make the discharge point for the drawdown system unusable. The drawdown system will lower the reservoir level by 1 meter per day under Q10 conditions. Q10 is the inflow into the reservoir from the upstream catchment that is exceeded 10 percent of the time.
New locations considered for the drawdown system included on or near the line of the existing drawdown system and next to the proposed spillway. Because of several factors, including the fact that the existing system was in unknown condition, Arup decided the upstream tunnel could not be reused. Instead, Arup elected to install a pipe in a trench along the line of the existing tunnel that connected to the wet/dry shaft in the dam. The pipework then would be routed through the existing downstream tunnel to discharge into the stilling basin.
Rehabilitating the structure
Rehabilitation work was carried out by Ringway Ltd./Vinci Construction UK Ltd. This included installing the new spillway, fitting a new scour pipe, carrying out erosion protection to the embankment, and improving the clay core of the dam.
The spillway had to be cast on a 19-degree slope. Steel mesh form work was installed vertically at regular intervals between the top and bottom layers of reinforcement, held in position with z-bars. This helped control the slump of the concrete. Reinforcing mesh was installed on top of the main reinforcement on thickened benched areas of the spillway to prevent concrete from flowing downhill. The new spillway contains 8,000 tonnes of reinforced concrete.
To install the new scour pipe, Hydra-Ject Valve Services Ltd. of Bolton shut the inlet valve into the wet shaft on the crest, allowing the tunnel to be drained. The Morthen spillway was then infilled and temporary pumps were used to regulate the reservoir level. The new scour pipe has twice the capacity of the original one and can release about 40,000 m3 of water per day. This will allow the reservoir to be drained at a rate of 1 meter per day if necessary.
A bentonite cement slurry was used to replace the concrete added to the dam core in 1969. The old concrete was excavated from the core, and the bentonite was keyed into the existing core. To ensure a watertight interface between the bentonite and the new spillway, the outer face of the spillway structure was sloped and incorporated a water bar to channel water away.
The wave protection on the upstream face of the embankment was completely replaced. The new rip-rap is a three-layer construction. On the outside, there is a rock armor comprising dense local sandstone. This is founded on a coarse filter, which in turn is founded on a fine filter.
The council is now working to refill the 35-acre reservoir. The rate of filling is dependent on rainfall in the area, and refilling is expected to be complete by late September 2010.
Already, the lessons learned from the incident at Ulley Dam are being disseminated. Sir Michael Pitt visited the site after the initial incident, and the information gathered was incorporated into his contribution to the Department for Environment, Food and Rural Affairs flooding review and subsequently the Floods and Water Management Act 2010.
Further information is being made available via a post-incident review for the Environment Agency and guidance soon to be published for the design and maintenance of stepped masonry spillways.
|The new spillway at Ulley Dam contains 8,000 tonnes of reinforced concrete. The dam itself also was rehabilitated through the installation of a new scour pipe, reinforcement of the dam core, and replacement of the upstream wave protection. (Photo courtesy Environment Agency)|
There were many lessons learned from this incident regarding emergency preparedness, spillway design, and dam remediation. These include:
— Immediately identify possible mitigation measures. As soon as RMBC identified the cause of the deterioration, the council set about determining ways to reduce the rate of this deterioration. The need to provide an alternative route for the water led to the decision to simultaneously block the channel and begin emergency pumping operations.
— Keep a record of events as they unfold, using both notes and photographs. Things move quickly, and there will be questions after the event. It may be difficult to find the time to write full notes, record measurements, and photograph the site while investigations are under way.
— Develop a physical model to validate the design. During modeling of the spillway design, engineers determined that the proposed stilling basin did not have sufficient tailwater to contain the hydraulic jump. Modifications were made to increase the length of the stilling basin and the size of the baffle and chute blocks.
— Involve the contractor early. RMBC involved Arup in the meetings during which options were discussed, which provided useful input into the design process. In addition, this arrangement allowed the design and tendering schedules to overlap and gave the contractor the opportunity to review the documents and conduct discussions about the requirements for each element of the work.
— Take time to investigate all the options. Initially, it was anticipated that solutions to the three main problems could be found very easily. However, as more information was collected, no clear-cut solutions presented themselves. Significant consideration was required to determine the pros and cons of individual solutions. The act of going through each option carefully and rigorously provided a result that will ensure the safety of the dam in the future and provides the best value for the dam owner.
Jon Horrocks is associate director of the Leeds office for contractor Ove Arup and Partners Ltd.