Considerately shoring up the flood defences of Louth in Lincolnshire – VBA Ltd Joint Venture
The VBA Ltd joint venture team is future-proofing the market town of Louth in Lincolnshire from flooding, being the first flood control project in the country to use the Hydroslide – a new technology in flood defence which controls water deluge.
The joint venture comprising VolkerStevin, Boskalis Westminster and Atkins is part of the Environment Agency’s Water and Environment Management (WEM) Framework.
The Louth Flood Alleviation Scheme will reduce the risk of flooding to the town of Louth by the construction of two dams which are homogenous earth-filled embankments. The embankment of the north storage area will measure 150m and the one in the south will measure 200m. These storage areas will reduce the risk of flooding to 355 local properties from the River Lud.
Outlining the project, VBA VolkerStevin Site Manager, Matthew Parkinson said:
“There have been several recorded flood events within Louth, with the primary source of flooding being associated with heavy rainfall. However, it has also been known to flood as a result of surface water and combined overflow systems (this is the responsibility of LCC and Anglian Water respectively).
“Risk to life, property and infrastructure are predominantly concentrated in the town of Louth itself, where the most severe flooding events occurred in 1920 and most recently in 2007. This scheme will see two flood storage areas constructed outside the town. The first will be located off the A157 and the second off Halfpenny Lane, both adjacent to the A16.
“Both of the Louth Flood Alleviation schemes are part of a ‘Design and Build’ framework agreement for the Environment Agency. The beauty of schemes like these is that the construction process is discussed and planned alongside the design. This means that as a site team and a design team, we can work in collaboration to produce a final design that not only meets the client’s requirements, but meets the needs of the contractor in terms of buildability.
“The scope of works included building a flood storage structure to control the flow of water through the town of Louth and providing a flood storage area where storm water could be retained and released over time.”
Control Structure
The Control Structure is a precast concrete shaft made up of seven 1m rings containing fourteen segments with an internal diameter of 7.5m. Inside the shaft is a diaphragm wall and flow control device. The device is known as a Hydroslide and is basically a float valve with a blade that shuts the watercourse down and throttles the flow to a rate at which the downstream networks can manage without flooding.
The Hydroslide is a new technology in flood defence and this is the first time one has been used in a flood control environment in this country. The main benefit to using this new technology is the control this device offers. Using the Hydroslide has meant that the flood storage level has been reduced by approximately 1.0m. This has avoided installing an additional 5000 tonnes of clay embankment.
The dams
The height of the dams will be around 6m, retaining a combined volume of 213,000m3 of storm water during a flood event. Both storage areas fall under the requirements of the Reservoirs Act 1975.
The northern and southern storage areas are both located immediately upstream of Louth’s bypass. The design is for two stand-alone impounding embankments. The fill material for the sites is clay from borrow pits within the flood storage area.
Construction process – Louth North and South
Construction commences with the installation of a reinforced concrete base, inlet and outlet box culverts, and a precast concrete shaft with cover slab. The shaft is divided by a reinforced concrete wall and the flow is controlled through an opening in the wall by a Hydroslide. Upon completion of the structure the earthworks will commence, taking clay from borrow pits within the site and placing this in layers to construct the dam across the valley. Further to these works, the site will have service and access roads for the client and landowners, landscaping, and an automated weed screen cleaner.
Here are some examples of best practice initiatives carried out by VolkerStevin:
- From the outset the project team were in contact with the local flood group. The group visited the site on a number of occasions for presentations and a tour of the works, and this was extended to Monks’ Dyke Tennyson College, the Rotary Club and the Women’s Institute.
- Monks’ Dyke Tennyson College brought their A-Level Geography class to the site for a presentation of the works and the project team brought in a Lego flood model of Oxford Town Centre to enable the pupils to take part in a class challenge. The students were tasked with constructing model flood defences for the town, with points scored for various areas and buildings in order of priority. This was a successful day with some really positive feedback.
- The valleys to both schemes required cutting off in order to establish the flood water storage areas. Part of the planning consent stipulated that no imported material was permitted for the construction of the embankment. With the embankments being subject to the Reservoirs Act 1975, the team had a very tight specification to work towards. This was achieved by excavating and testing several areas of site material in order to establish the areas from where the clay could be excavated. This brought an obvious cost saving to the VBA team by using site-won clay, but the reduction of impact on the local community by not having thousands of deliveries of clay was a huge relief to the town.
- Information boards were placed around the site perimeter, most notably around the public footpath that runs around the south site boundary. These show details of the project, with cross sectional drawings of the structures and the hydroslides, as well as a 3D image of the completed schemes.
- Environmental enabling works were conducted before work commenced to establish endangered and protected species within the site boundary. Catchment areas were then established around the works areas, with water vole trapping carried out to enable relocation of the animals into suitable upstream habitats. In support of this, additional areas of habitat creation were established upstream, with mink monitoring and controls established to protect the water vole population.
- Ecological monitoring of protected species is ongoing, with consideration given to ensuring safe passage through the structures. The implementation of mammal ledges, access routes and strict dimensions of the constructed river channel have all been taken into consideration to maintain flow rates for migratory fish.
- The eco-survey revealed the presence of species such as voles, mink and crayfish, whose habitats needed to be considered in addition to the river protection issues.
- Compound generators were unavoidable but the project team used hybrid versions to mitigate inefficiency.
- The watercourse was diverted through the structure using 3.0m x 3.0m box culverts, which extended to the inlet and outlet structures at the foot of the embankment. By constructing the structure in this manner, the project team provided the control structure with minimal future maintenance and safe access for operational personnel.
- The vast majority of the structural concrete involved was precast to minimise the impact on the local environment. This also drastically reduced the impact on the local community, with far fewer deliveries of materials such as concrete, steel reinforcement and plant. The use of precast concrete also minimised the need for large concrete pours within close proximity to the watercourse – a very sensitive and diverse ecosystem.
- Accidents were analysed and safety alerts were posted, as well as near miss reporting arrangements in place, incentivised by a prize draw.
- Current safety information was included on the site hazard board, provided in daily briefings.
- Drug and alcohol policy was enforced by random testing.
- Appropriate attention was given to Leptospirosis, with information cards issued to workers.
- Men’s health issues are addressed by posters and Mencap mental health posters were also evident.
- Health checks were used for specific occupations and included general health surveillance.
- Worker fatigue due to travelling was monitored.
- Considering the remote location, the quality of the welfare facilities was notable, including a quiet prayer room and a spacious smoking shelter with seats and smoking cessation advice.
- Wi-Fi was available for operatives.
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