Tanks join wider mix
This 6,000m3 underground storm water storage tank was recently built on the site of the former Guisborough STW

Reducing the risk of flooding has often relied heavily upon the ability of water and sewerage companies (WASCs) to increase the capacity of their network of storage tanks and pipes. Indeed, in an era where sustainable drainage systems (SuDS) are playing a greater role in the battle against flooding, tanks and pipes still provide a regularly-utilised option for engineers who want to deliver projects that reduce the flood risk to a community.

However, where previously tanks were always the go-to option, they now fall behind the removal of surface water, retaining surface water at a property level, and diverting it to a water course. Tanks are also now often used alongside other measures. Their continued regular use reflects the fact that the last resort is often the only suitable option available for water companies to do the best possible job.

Sunderland solution

 One project where tanks are a part of the wider mix is in the city of Sunderland, where Northumbrian Water is investing £8 million in upgrades to its sewer network in Roker, Seaburn, Cleadon and St Peter’s.

The year-long construction, which includes installing a mixture of new sewer pipes, new tanks and sustainable drainage systems, will reduce both the flood risk and the number of times waste water is discharged into the sea at Whitburn during heavy rainfall.

The first underground tank, being built in farmland in Seaburn, will have a capacity of 3,500m³. It will have a 70m by 20m footprint and be 2.5m deep.

The second, which will be situated between the University of Sunderland and the River Wear, will measure 23m by 19m, but with a 4m depth and a 1,800m³ capacity.

Design options

When it comes to design and construction choices for underground tanks, there are a large number of options, including concrete chambers, cast in situ or pre-cast, oversized pipes, box culvert sections and storm cell solutions.

On projects such as the one at Sunderland, where high-volume storage is the prime function, we often look to concrete tanks, whether cast in situ or pre-cast, choosing the solution to reflect the best value in that particular instance.

Concrete tanks have good structural integrity and it is a form that we use most regularly, meaning that we have a full understanding of the methods of maintenance.

Visual inspections and jetting are also much easier, especially as we can build in as many manholes as we feel necessary. In catchments where siltation is a known issue, we can also install measures to ensure that the silt remains in suspension, removing this maintenance liability.

With a concrete solution, it is also easy for us to make the structure as large or as small as is required and our network has tanks ranging from 50m³ to 11,000m³.

Challenges

As with any engineering project, many of the challenges associated with concrete underground tanks relate to health and safety and the protection of those involved in their construction and maintenance.

During construction, we need to dig out vast volumes of earth, which then needs to be held back away from the building site. Once the tank is built and covered, there is a duty to ensure that the chamber can be entered safely in order to remove any foreign objects, or to inspect the structure. This is a consideration that must be taken into account with the design of all tanks, in accordance with Construction Design and Management (CDM) Regulations.

However, while we build our tanks to meet the requirements to contain the water from heavy storm events, such storage chambers have a finite capacity. Additionally, they are expensive to build, difficult to fit into urban areas, and they only provide a flood risk management benefit, without addressing the cause.

Including SuDS

Now that there is an increased focus upon preventing as much surface water as possible from entering the system, means moving away from tanks being the sole solution, to being part of the wider overall plan for reducing flood risk.

In partnership with local authorities and the Environment Agency, as well as other partners, including business and developers, we are looking to ensure sustainable drainage systems (SuDS) are utilised wherever a suitable opportunity arises.

The benefits of SuDS extend beyond flood risk management. They are cost effective to construct, have a greater perceived public value, address the causes of flooding and are sustainable. There are also multiple options available to suit each scenario.

Supporting role

One of the alternatives to concrete tanks, storm cell constructs, are sometimes used to support sustainable options where it is necessary to add storage capacity and keep surface water away from the system, rather than storing it as part of the combined sewer network. Suitable for lower-volume, surface-water-only projects, rather than combined networks, storm cell tanks are made by linking multiple smaller crates together to create a larger capacity and sealing them by wrapping them in polyethylene.

Such storm cell tanks are often used by private developers looking to store surface water in their sites. However, due to the difficulties around maintenance and access, this is not an option we select unless there is no alternative.

The disadvantages of storm cell tanks include this limited access for maintenance and CCTV coverage, as well as problems with the removal of silt.

One project where we have chosen this solution, to run alongside a future SuDS projects is at Ovingham in Northumberland, where a circa 100m³ storm cell tank is being installed in local authority-owned land.

With concrete tanks, storm cell and wider SuDS solutions, they require space in which to work. While water companies have statutory powers to construct in relation to all of these options, and landowners are largely supportive of our work, it is often the case that our local authority partners provide the space required to deliver a SuDS solution to flood risk reduction, or we purchase the necessary land.

Flexibility

The use of concrete chambers, the more flexible tank option, is ideal for projects such as that at Sunderland. However, as with much in engineering, the mix of concrete and storm cell tanks, and wider SuDS schemes, is a case of the right solution for each application, all with their own advantages and disadvantages.

We are moving towards a future where the continued implementation of SuDS means a broader mix in the choice of applications – tanks or otherwise – when it comes to flood risk management.

ww.nwl.co.uk