Flood protection for Edinburgh3 October 2008
The Braid Burn flood prevention scheme in Scotland is designed to offer the city of Edinburgh a 1 in 200-year standard of protection. Clive Osbourne gives an insight into the project, which is due for completion in 2010
Flood prevention became the responsibility of the City of Edinburgh Council (CEC) after a major reorganisation of local government in Scotland in April 1996. Following severe floods in April 2000, CEC commissioned the Babtie Group to undertake a study to investigate possible flood alleviation options for the Braid Burn/Figgate Burn watercourse.
The outline proposals formed the basis of the Braid Burn flood prevention scheme. Faber Maunsell/AECOM was commissioned in February 2002 to undertake design and economic analysis for the scheme, submit the scheme to the Scottish Executive, and to prepare detailed designs and tender documents. The contract was subsequently awarded to Carillion Construction in May 2007 with work commencing on site in June 2007. The form of contract adopted is the NEC3 engineering and construction contract with Faber Maunsell/AECOM being commissioned for site and overall supervision.
The scheme is designed to offer a 1 in 200-year standard of protection to the residents and businesses of Edinburgh against flooding from the Braid Burn/Figgate Burn watercourse. An allowance, based on current projections, for climate change is built into the design. The aim of this allowance is to maintain this standard of protection against river flooding over the next 50 years.
The Braid Burn rises in the Pentland Hills to the southwest of Edinburgh, and flows in a northeasterly direction through the suburbs of the city before discharging into the Firth of Forth at Portobello, east of the city centre. The catchment has an overall area of 30.5km2 and is elongated in shape.
The headwaters of the catchment are rural steep uplands reaching a maximum altitude of almost 500m AOD. These headwaters contain three reservoirs: Torduff, Clubbidean and Bonaly. Torduff and Clubbidean reservoirs are owned and operated by Scottish Water (SW) whilst Bonaly reservoir is owned by CEC and operated by Pentland Hills Regional Park. The area draining to these three reservoirs constitutes around 8% of the overall catchment.
Hydrological models were developed for the various flow inputs to the main Braid Burn channel based on the procedures in the Flood Estimation Handbook (FEH). Reservoir routing calculations were included in the hydrological analysis to account for the effect of the three headwater reservoirs on flow in the watercourses downstream of them.
The resultant hydrological inputs were used as flow boundaries in the computational hydraulic model, which was built using ISIS – an industry standard one-dimensional hydraulic modelling software package. Following calibration and verification of the hydraulic models to historic flood events, the models were used as a design tool to test and refine the outline designs for the scheme.
The contract contains work to the Braid Burn covering 12 key geographical areas. Flood walls are generally of RC concrete with Formliner finish or sheet pile construction with brick or stone cladding above ground.
Some of the main characteristics of the work units down the length of the burn are as follows:
• Redford Road – The work here includes a new 2.7m dia. culvert, to replace the existing culvert, under Redford Road with associated headwalls, approximately 125m of flood defence works and replacement of the six non-listed bridges.
• Blackford Glen – The burn runs through a gentle valley bordered by a business and residential property. The works include bridge parapet replacement, installation of a flood drainage culvert and approximately 470m of flood defence works.
• Mayfield Road – This section is approximately 26m long and involves flood defence works to the residential property side. Ground stabilisation is also required beneath the existing wall and houses.
• Mid Liberton & Gilmerton Road – Here the burn runs in a trapezoidal channel, through a residential area bordered by grass banks making way to macadam paths and garden hedges. Flood defence works are to be undertaken to the whole of the 210m long section together with the installation of flood gates and the raising of the upstream parapet to Mid Liberton Bridge.
• Inch Park – The section of Braid Burn through Inch Park is 320m long with raised banks bordering generally level and low grass parkland. The resulting effect of the works means that the park will, under flood conditions, act as a significant storage area. The works also include river restoration and enhancement, flood defence works and installation of a debris screen and a vehicle access bridge.
• Cameron Toll Gardens – The work to this section includes 430m of flood defence works and wall strengthening works. This, together with the downstream work unit, creates a flood storage reservoir that comes under the requirements of the Reservoir Act 1975.
• Peffermill Playing Fields – This includes approximately 1200m of flood defence works and channel re-grading together with the installation of an outlet structure.
Storage of flood waters is being provided up to the 200-year return period level with an allowance for climate change at the Peffermill and Inch Park sports ground sites, where the flooding which occurred previously has now been restricted to the areas bounded by the new flood walls and embankments. This storage of flood water allows flood walls downstream to be at a lower height than would otherwise be the case. In both cases, as the stored volume is in excess of 25,000m3, the reservoirs must be designed and operated in accordance with the Reservoirs Act 1975.
The Peffermill reservoir site will be formed by a combination of reinforced concrete flood wall and earth embankments creating a reservoir capacity of 77,000m3. The flood walls around the Inch Park reservoir will create a flood reservoir of 190,000m3 capacity with a typical depth of 3.5m.
Before any significant piling works could commence in Inch Park a series of utility diversions were required. In particular about 550m of 630 dia. grey iron gas main had to be replaced both due to actual clashes with the route of the sheet piled wall, and for concerns of carrying out piling works close to the route of the old gas pipework.
To the left bank of the reservoir the route of the flood wall follows the existing boundary wall, and the above ground cladding to the sheet pile wall has been clad with the stone recovered from the existing wall.
Clay embankment overflow spillways will be provided to both reservoirs such that flood waters in excess of the 1 in 300-year events will overflow and replicate the current path of flooding downstream but with a much reduced frequency and magnitude.
Other locations along the length of the burn where storage of flood water can occur are included as follows. Although some of these locations provide storage greater than 25,000m3, they are naturally contained areas and do not therefore fall under the Reservoirs Act.
Minimising the effect of this scheme on the environment has been a fundamental consideration in the development of the project. A range of ecological surveys were undertaken as part of the environmental impact assessment process, in order to identify the baseline conditions and to assess the potential impact of the flood prevention works. These surveys included a general habitat survey and specific surveys to look for signs of protected species activity, including otter, water vole, birds, invertebrates etc.
Mitigation measures were included in the design to accommodate various species. For example, fish passes and otter ledges will be placed in some locations. Furthermore the contractor is prohibited from carrying out work in the burn during the brown trout spawning period.
A survey was also undertaken to record invasive species present in construction areas. It is vital that such species are eradicated as part of the works, and treatment of these invasive plants has been ongoing in recent years in advance of construction. A number of trees will need to be felled as part of the works, although extensive replacement planting is proposed. Detailed tree surveys were undertaken and bat surveys have also been undertaken during the summer months to identify the potential use of mature trees as summer roost sites, and to help identify appropriate mitigation measures.
The river restoration proposals were some of the earliest and most exciting aspects of the proposed flood prevention works. Techniques range from the removal of existing concrete channels and the creation of meanders to add diversity of flows/currents in Inch Park and Duddingston. These works will substantially benefit wildlife by restoring sections of the river to a more natural state.
The felling of existing trees along the route of the works has been minimised and in some locations full depth sheet piling has been omitted in the vicinity of tree routes and a section of shallower sheet pile wall has been provided spanning the roots, supported from pairs of H piles.
All work to be carried out by the contractor has to be in accordance with method statements approved by the Scottish Environment Protection Agency (SEPA). In this respect, pollution by construction materials and plant oil is being closely regulated.
Since the start of construction on site, the Carillion and Faber Maunsell/AECOM teams have worked together to look at options to reduce construction costs. A number of changes have been made to the contract design.
At the Inch Park reservoir site the flood wall was designed as a steel sheet pile wall with a decorative cladding of stone re-used from the existing boundary wall to the park site. In order to accommodate a sleeve for a gas main that had to be diverted, an in situ concrete section of the wall was designed. At an early date in the contract, excavation for this section of concrete wall encountered sand and gravel layers at approximately 4m depth. Extensive pumping was required to allow concreting to take place.
In the light of this experience the design of the wall to the right bank, which had been reinforced concrete with a down stand beam, was re-considered and the design was changed to driving a line of H piles on which the concrete wall could be founded at a higher level on a clay layer above the sands and gravel strata.
A number of existing road bridges along the burn are predicted to surcharge with the higher flows passing down the burn and the contract design required that the upstream parapets be demolished and replacement concrete parapets constructed, to be able to withstand possible impacts from floating debris. As a result of a value engineering and environmental risk assessment to the burn, it has been decided that separate upstream concrete protection structures will be constructed to withstand impacts of any floating debris.
Clive Osbourne works for Faber Maunsell/AECOM in Edinburgh, Scotland