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A winning defence
01 January, 2003
Attention to detail, flexibility and a pragmatic acceptance of risk were the secrets of success at the UK Environment Agency's recently completed £5.5M (US$8.8M) Walton-le-dale Flood Alleviation Scheme, believes design engineer George ArnisonURBAN flood defence schemes are particularly complicated and risky projects, often out of all proportion to their cost. The need for complex preparatory studies, public funding constraints, environmental sensitivities, restricted access and the involvement of a large number of third parties are just some of the non-technical issues that challenge such river engineering projects. As a result it can take a frustratingly long time for a scheme to proceed from concept to completion, risks are easily underestimated and difficulties are frequently encountered meeting budgets and target programmes. The Walton-le-dale flood alleviation scheme experienced all of these challenges, but after eight years of study, design and construction, the scheme was finally completed in November 2002 within budget, on programme and winning an award from the local community for its high quality of design and construction. BackgroundThe town of Walton-le-dale is situated around the confluence of the rivers Ribble and Darwen on the outskirts of Preston in Lancashire, UK. The Ribble is one of the largest rivers in the north west of England, draining a mixed catchment from the Pennines and North Yorkshire Moors into the Irish Sea. Under normal circumstances the river reacts slowly to rainfall and flood events are not frequent, however once the catchment has become saturated and the limited flood storage areas upstream of Walton filled, there is a threat of rapid and substantial inundation. As a result, in October 1994 the UK Environment Agency appointed consulting environmental engineers Black & Veatch Consulting (previously Binnie Black & Veatch) to study the feasibility of a flood alleviation scheme. Feasibility studyVery little factual data existed at the start of the project and the feasibility study was essentially an exercise in information gathering and analysis. The first key requirement was to develop a mathematical model of the river so that flood levels and return periods could be calculated. Extensive topographical and river section surveys were undertaken, and these together with historic hydrometric data were input into a transient-state computer model based on ONDA software. A 1-in-15 year flood event in January 1995 provided valuable calibration data. Further hydrographical studies included the development of hydrographs for a range of fluvial flood return periods and tidal curves for a similar range of tidal/surge events. Joint probability analysis of the two sets of data enabled combined design flood events to be defined.The second major area of work was to consider all possible flood alleviation options, which in turn required preliminary third party consultations, public displays and environmental appraisals, as well as site investigations and reviews of existing flood warning, development control and flood defence maintenance procedures. A wide range of options were considered but many rejected at an early stage as being either insufficiently effective or excessively expensive. These included:• Flood storage (eg. construction of upstream barrages or storage reservoirs).• Channel modification (eg. dredging or widening the river).• Catchment modification (eg. upland forestation and tributary management).• Managed retreat (ie. aligning flood defences remote from the river edge).• River diversion (ie. diverting the river or an overspill channel around Walton).It became clear that the only feasible means of alleviating the risk of flooding in Walton was to contain the flood flows within the river channel, utilising the existing flood plains. A number of flood defence options were therefore assessed in more detail. These ranged from simply repairing and maintaining the existing flood barriers (ie. a 'do minimum' option), to constructing new flood defences to 1:50, 1:75 or 1:100 year standards of defence. The costs, benefits and environment effects of each were assessed and compared to a baseline 'do nothing' option. In England and Wales, flood alleviation schemes are funded partly from local government (via Regional Flood Defence Committees) and partly from central government grants from the Department of Food and Rural Affairs (DEFRA). Accordingly, all schemes have to be justified in terms of strict government guidelines, and detailed economic benefit-cost analysis. The third major aspect of the feasibility study was therefore surveying and classifying the 600 properties considered to be at risk of flooding, and developing an economic model to assess the value of flood damages associated with not providing a flood defence, and then the economic benefits (defined as the flood damages avoided) associated with each of the options under consideration. The end result of the feasibility study was the identification of a 'preferred option' which was the construction of a series of new flood defence walls and embankments to a 1-in-75 year standard of defence. After 15 months of work, the first major hurdle in progressing the scheme was cleared when it's business case was successfully presented to various funding committees, and in January 1996 the EA commissioned Black & Veatch Consulting to proceed with the design.DesignThe design of the scheme was fundamentally influenced by the challenging and varied nature of the narrow riverside corridor along which the defences had to be constructed. The existing river banks were high, steep, overgrown and frequently of dubious ground quality. Access along the top of the river banks was frequently extremely restricted and only possible through domestic gardens or off public roads. Accordingly a range of design solutions were developed:• In the more open locations, earth embankments were used. These ranged from simple 1m high bunds remote from the river edge, to 7m high riverside embankments reinforced with geotextile and rock riprap. An imported granular cohesive fill material was specified to balance the requirements for impermeability, structural strength and durability.• Reinforced concrete cantilever retaining walls were used, varying in height between 1m and 3m. Sliding was usually found to be the critical mode of potential failure and this was overcome using deeply embedded bases, shear keys or laterally loaded raking piles.• Where the river bank was particularly steep and construction space was particularly constrained, reinforced concrete walls (typically 2m to 3m high) were founded on continuous steel sheet piles foundations.• A range of pre-cast, mass concrete block, gravity walls up to 1m in height were developed particularly for road-side locations where speed of construction and minimising disruption were deemed to be important issues.The ever changing nature of the locations, topography and height requirements meant that frequent changes of section and design solution were necessary, standardisation was difficult and particular attention had to be paid to interfaces between structures and detailing. Significant emphasis was also placed on the aesthetics of the scheme so that the works would enhance the previously neglected riverside environment. All the reinforced concrete walls were clad in local stone, and exposed concrete elements such as copings and pre-cast blocks were constructed using coloured, high quality pre-cast concrete. Designs for widespread planting of native bulbs, shrubs, hedges and trees were undertaken by Landscape Architects. The opportunity was also taken to renovate a local monument and to improve riverside footpaths and viewing areas. Carefully focusing on sympathetic materials and design details enabled significant environmental improvements to be made for relatively minor impacts on construction costs.The design stage for Walton-le-dale (which also included further site investigations, a detailed environmental assessment, widespread consultations, updating and re-presenting the scheme's business case and the production of contract documents) took over four years to complete. This gives an indication both of the complexities of the scheme and of the difficulties encountered.CostsPreparing accurate pre-construction estimates for urban flood defence schemes is notoriously difficult due to the uniqueness of each project, the dominance of 'method related' costs and high levels of engineering risk. With the stringent constraints imposed in the UK on publicly funded projects, this can present the EA with serious budgetting problems which can adversely affect progress on site. On Walton, a detailed appraisal of the costs and risks of each phase of the works, taking into account difficulties of access, restrictions in working room, buildability and methods and scale of operation, was undertaken at the end of the design stage. As shown in the table below - these proved to be relatively accurate. The £0.66M (US$1.04M) cumulative value of the 60 separately identified residual risks (approximately 1/3 of which related to ground conditions) equalled approximately 20% of the pre-construction capital cost estimate, highlighting the amount of uncertainty still remaining at the end of design. However the EA's willingness to accept this risk and allow adequate contingency funds in the scheme budget to cover it, enabled the project to avoid many of the financial problems and stresses suffered by other similar schemes.ConstructionFollowing a competitive tender, the Engineering and Construction Contract (ECC) option A (lump sum) contract was awarded to Harbour & General Works in April 2001, and site work started in June 2001.Difficulties encountered on site included delays due to the UK foot and mouth epidemic, prolonged periods of wet weather and high river levels, as well as overcoming the practical difficulties of difficult site access and restricted working space in residential areas. A number of design changes were required (most notably where it was found that the banks were steeper than anticipated from the surveyed sections) or allowed to generate savings (such as a relaxation of the embankment fill specification to recycle available materials). Additional works were instructed as a result of savings generated elsewhere. However despite delays and additional works, the positive attitude of all the parties meant that Harbour & General were able to re-sequence their programme to maintain the overall 17 month work period. The works were completed within budget and on programme in November 2002.A key foundation to the successful administration of the contract proved to be the realistic forecast of construction costs which resulted from the design stage cost estimation and risk valuation processes. Having secured an adequate budget, the ECC Project Manager was able to administer the contract fairly without undue pressure from account managers. This fundamentally contributed to good working relationships on site, encouraged flexibility, and helped ensure that the parties were able to act in the 'spirit of mutual trust and cooperation' that forms the central tenet of the ECC form of contract.Success rewardedThe considerable thought and hard work invested in the design and construction of the Walton scheme was recognised when the EA, Black & Veatch Consulting and Harbour & General were awarded Certificates of Merit by the local Civic Trust, in recognition of the high quality of design and construction. George Arnison BEng CEng MICE is a Principal Engineer with Black & Veatch Consulting, and was the Design Engineer for the scheme.