Norton-Fitzwarren Dam

79.8%

Design Award

Designer: Hyder Consulting (UK) Ltd

The dam, designed by Hyder Consulting Ltd, is a flood attenuation structure designed to prevent flooding in the village of Norton Fitzwarren near Taunton. With potential flooding under control, residential development will now be permitted on the redundant cider factory site in the centre of the village.

Client for the project is the Norton Fitzwarren Consortium, comprising developers Barratt, Bellway and Strongvox. Contractor is Jackson Civil Engineering Ltd and it is planned to seek a combined Design & Construction award on completion of construction.

The project comprises the construction of a 450-metre-long earth dam across the valley of the Halse Water. It is 5.25m high at its highest point and when full will impound 750,000 cubic metres, meaning it falls within the remit of the Reservoirs Act 1973. It has a clay core and sheet-pile cut-off, with all the bulk earthworks materials being won from site. The excavation resulting from this is being retained and turned into a valuable wetland feature.

A 2.5-wide by 2.0-metre-high culvert will allow normal flows to pass through the dam uninterrupted but will limit the amount of water that flows downstream during abnormally high flows and thus reduce flood risk. The dam will start to impound when flows exceed the capacity of the culvert but it is only expected to fill to the top when a flood event with a return period of one in a hundred years is experienced (with an allowance of an extra 20% for climate change).

The site is in a rural agricultural setting close to a small hamlet. The ecology of the site is varied, with a number of protected species, including great crested newts, reptiles, bats, otters and badgers being present. Advance works were carried out to minimise impacts on local wildlife and provide new habitats to compensate for the dam construction where these were required. Post-construction habitat creation works will result in the ecological value of the dam and surrounding area being higher than the pre-construction situation.

Extensive areas of woodland and hedge planting will be undertaken to mitigate disturbance to the existing vegetation/field pattern and for visual amenity. It has been designed to have immediate visual impact to soften and eventually screen views from Monty's Farm hamlet. Planting will be undertaken using locally grown native species where possible.

In gaining its award, key features of the project have included:

• Winning all the bulk earthworks from within the site so as to minimise impact on local roads. and communities and CO2 emissions in transportation.
• Creating a permanent wetland in the resulting excavation
• Advanced Works Contract to remove hedgerows and temporarily exclude newts from the site.
• Creation of other ecological mitigation measures including a new pond and otter ledges.
• Landscaping to screen the hamlet from the dam.