ROTHERHAM BOROUGH COUNCIL – REPORT TO CABINET 1. Meeting: Cabinet 2. Date: 29th July, 2009 3. Title: Ulley Reservoir All Wards 4. Directorate: Environment and Development Services 5. Summary The report outlines the progress and current estimate of costs for the rehabilitation of Ulley Reservoir, and seeks funding and approval to undertake the works. 6. Recommendations i. To reaffirm the Council’s intention to rehabilitate Ulley Reservoir at a construction cost estimated to be up to £3.8m. ii. To identify and commit additional funding as detailed in this report 7. Proposals and Details Background: Cabinet Members will recall the emergency incident at Ulley Reservoir, on Monday 25 th June 2007 following very heavy and prolonged rainfall in the catchment. A significant scour hole, some 50 m wide and up to 6 m deep, occurred to the downstream face of the dam embankment, caused by the failure of the spillway which runs down the left mitre of the dam embankment. The incident precipitated the evacuation of residents in Catcliffe, Treeton, Whiston and Canklow and the closure of the M1 Motorway between Junctions 32 and 34, a section of the Sheffield Parkway and other major roads. There was also a threat to an adjacent strategic natural gas installation and major electricity supply infrastructure. In October 2007 Members were asked to consider two main alternative courses of action open to the Council. The reservoir may be retained (rehabilitated) or removed or reduced to a level whereby it is no longer subject to the provisions of the Reservoirs Act (discontinued). This latter option had two variants associated with it; the basic works needed to remove from the authority any liability under the Reservoirs Act 1975, and a more expensive alternative to produce a different kind of amenity within the country park. The rehabilitation option was chosen at an estimated cost for the works of £2.5m. (Minute number 35 of the Council Meeting 10 th October 2007 refers). The scope of project was identified at the time as extensive works to the remaining spillway installation and associated stilling pond as it enters Ulley Brook. Other works included erosion protection to the embankment, the removal of redundant pipework, abandonment of the by-pass channels and the re-routing of the outfall scour pipe. Another area of work also required, where significant unknowns remained, was the repair to the existing embankment core including a possible slurry cut off wall to the top portion of the dam. The extent of this work would depend on the results of an extensive site investigation with boreholes and trial pits being excavated on the dam itself and immediate surrounds. This scope was confirmed in the ‘section 10’ principal inspection report published in November 2007; within this statutory report, completion by June 2008 was called for in respect of the abandonment of the Morthern bypass channel, with a December 2009 deadline for the other works. A significant level of uncertainty existed in the estimated cost, which was very much a budget estimate based on the best information available at the time. A commitment of over £3m was made in the Council’s capital programme, in recognition of the decision to rehabilitate; £2.5m for the design and supervision of the construction works and £0.5m to cover continued technical advice and support, generator hire, officer time and other activities such as surveys and model building associated with the rehabilitation process. Many of these functions were common to the other options available at that time, namely removal/reduction of the reservoir. Summary of Significant Changes to Initial Design Proposals The final design of the rehabilitation works has a number of elements which are significantly different from that envisaged at the outset of the scheme for a variety of reasons. The main areas of change are associated with the dam core, the new spillway and the wave protection on the dam. Other areas of increased cost include technical support costs and extent of environmental measures. These elements are outlined below. It had always been expected that some works would probably be necessary to improve the core of the dam, in particular, its upper portion which was installed during the 1960s. Investigation and consideration of construction methods has resulted in the replacement of the upper part of the core being proposed as opposed to ‘improvements’ to the existing one being undertaken. The ‘Section 10’ report, changed the status of the dam to a higher category which meant that it had to be designed for at a significantly higher rainfall event than before. In turn, this meant that the volume of water to be safely discharged from the reservoir could not be practicably handled by amending the existing spillway, as had originally been envisaged. The solution was to build a new, more efficient spillway and abandon the existing one. The wave protection on the upstream face of the dam wall has been subject to detailed examination. This has revealed the need to take up the existing protection and replace it rather than carry out more localised repair, as had originally been envisaged. As a result of increased investigation and design requirements, the extent and attendant cost of technical support has increased, by around £60k. Environmental protection measures have increased beyond that originally envisaged, for example the protection of grass snakes. Until revealed by specialist surveys, it was unknown that this species existed on the site in large numbers, necessitating the installation of a reptile fence costing in the region of £50k. Finally, since the construction costs were estimated nearly two years ago, there has been significant and unexpected inflation in construction costs. This increase now appears to have stabilised but has, nonetheless played a not insignificant part in increasing the overall scheme cost. Progress since October 2007 At the end of 2007/8, the pumps to maintain reservoir drawdown were purchased and an extensive site investigation was undertaken on site and reported on. Other localised repairs in response to the emergency situation were also completed. During 2008/9, over £200k of the fund was expended on the design of the works. Works to fill the Morthern bypass channel were completed on time at a cost of approximately £93k. A second site investigation was carried out in the autumn of 2008 to supplement that undertaken over the winter of 2007/8 and a model of the proposed new spillway was also built and tested at this time. Altogether, model building, ecological and protected species surveys and reports, topographical surveys, tree surveys, technical advice and the preparation of the planning application documentation, together with the costs of officer time has amounted to over £75k, mostly incurred during the financial year 2008/9. Continued fuelling and hire of the generators cost £64k. The outline design proposals were submitted for planning approval on 27 th March 2009. They include a new spillway at the approximate centre of the existing dam. The spillway width of 20m at the overflow weir is significantly larger than the existing spillway width at the weir of 12m. This increase is due to a re-evaluation of the likely maximum flow to be carried by the spillway. This maximum flow has increased due to the reclassification of the reservoir by the Inspecting Engineer from Category C to Category A, a re-measure of the catchment impermeable area, and the use of the latest hydrological advice for flood prediction. However, the new spillway utilises the shortest route to the receiving watercourse, in contrast to the existing spillway which sits to one side of the existing dam. The retention of the existing spillway until completion of the new spillway, ensures an overflow capacity exists at all times during the rehabilitation works, which could be brought into use in the event of a very severe storm during the construction phase. The proposals for the scour outlet pipe were discussed at length by the design team and the proposal utilises and reuses the existing scour tunnel wherever possible and the existing valve shaft. The new scour pipe will operate as a siphon at the inlet point, reducing the depth of excavation required. This solution for the scour is considered to offer both cost and technical benefits over the alternative of a replacement scour pipe laid in a new trench. The smaller disturbance to the dam and the existing ground reduces the potential for future seepage. Investigation of the existing wave protection on the upstream face of the dam has shown that this now all needs to be replaced; this was not allowed for when the original estimate of the cost of works was made. The site investigation and subsequent testing and monitoring have also shown the need for works to the core, the extent of which was not originally anticipated. They are confined to the upper parts of the core only, but do mean the complete replacement of the ‘plastic concrete’ that was installed in the late 1960’s. Alternative options to drive piles into the existing core were eventually discounted due to the possibilities of causing disturbance to the lower section of the core which is currently considered to be performing satisfactorily. The replacement section of the core will be in the form of a bentonite slurry wall. The Council’s partner civil engineering contractor Ringway have been involved with the design and buildability discussions throughout the design process. Ringway in turn have utilised their own partnerships with other group company members to ensure adequate expertise is applied to the buildability discussions and indeed for the delivery of some of the works. The proposals take full account of the site being a country park, and an environmental management plan has been prepared for the works.