The heavy flooding of 2000 caused many local authorities to look carefully at their flood defences, none more so than the City of Edinburgh Council, which had assumed responsibility for flood prevention four years earlier.
Flooding occurred in two waterways – the Water of Leith and Braid Burn, both of which rise in the Pentland Hills to the south west of the city and run in a north easterly direction to the Firth of Forth. Flood studies were immediately commissioned for both rivers and, almost 10 years after the 2000 floods the first of the two schemes – Braid Burn – will be completed in 2010. Contracts for the Water of Leith Scheme are due to be awarded this year.
Braid Burn runs for 15.5km from the south western edge of Edinburgh at Redford Road to Portobello in the east, where it enters the sea. During the April 2000 floods, the burn burst its banks in various locations, flooding 250 homes and commercial premises. A study carried out for the council by Jacobs in 2001 identified 900 properties at risk at any time along the route of the burn. "During dry weather it looks like a little stream, but it is very responsive," explains the council’s assistant project manager Jim Grainger. "If we get a lot of rain in the hills it comes down very quickly."
And it is not just water running off the hills that makes the burn susceptible to flooding. It has a catchment area of 30km2, most of it heavily urbanised, which generates considerable amounts of surface water run-off. Following Jacobs’ study, the council appointed Bullen, now part of Faber Maunsell Aecom, to take the scheme to construction. This involved mathematical modelling, environmental assessment, civils design, consultation and obtaining the necessary consents.
The project required the Scottish government to grant the council a Flood Prevention Order giving it the powers to implement the scheme, as well as planning consent – a process that required consultation with people affected by the proposals. "The council has spent a lot of time and effort on consultation," says Faber Maunsell deputy project manager Jonathan Davies. "The Flood Prevention Order does give it very wide ranging statutory powers to implement this scheme, but it has taken the view that it would rather implement this with the cooperation and support of the people who are at risk of flooding."
With 900 homes and businesses at immediate risk, and many more affected by the construction work, consultation continues to be a major part of the project now it is under way. Carillion, which won the £18M construction contract in 2007, employs a full time stakeholder manager, Stuart Mackay, to liaise with residents and businesses. "The linear nature of the construction means that you only construct a couple of wall panels then you’re into another person’s land," says Mackay. “It is far more complex than any other type of construction because this is an artery that runs right through the city."
Faber Maunsell Aecom’s solution has been designed to protect against a 1 in 200 flood event plus an allowance for climate change. It involves building flood walls and embankments to protect the most vulnerable properties, and providing extra storage capacity by enabling existing open spaces – parks and playing fields – to be used as reservoirs for flood water. In all, 6km of flood walls will be constructed, and over 600,000m3 of storage capacity will be created. Where possible, the soft engineering option of embankments is preferred to walls, but where this is not possible, the walls are built of concrete or sheet piles and are clad in brick or stone to match adjacent structures.
Work is taking place in 14 locations along the route of the burn over the three-year contract period. The programme is being sequenced to ensure that construction puts no-one at greater risk of flooding than they are already. "The main hydraulic constraint is that the flood storage areas should be constructed first, and there are one or two sections where there are agreements that the work will be done at a certain time, but other than that we have tried to give the contractor as much freedom as possible to programme and sequence that work," says Davies.
A close working relationship on site has engendered a flexible approach to the sequencing that works in everyone’s favour, according to Faber Maunsell project manager Peter Senior. "We have been able to reduce risk and open up other areas that may originally have been later in the programme," he says. As a result, time lost at the start of the contract and during heavy flooding last summer is unlikely to impact on the completion date of September 2010.