Waking up to modular cells

It is important that the benefits and flexibilities of modular cell systems are taken on board by designers, writes Dr Jason Shingleton, of Polypipe Civils. And modular systems combined with silt traps meet varying site constraints, as well as health and safety and installation benefits.

There is an urgent and definite need to readdress how surface water run-off is controlled and managed.

Modern building regulations and environmental legislation place further pressure on designers and engineers to adopt a sustainable approach to managing water and reduce the risk of flooding and pollution to the wider environment.

It is essential, though, that the tools to facilitate this approach are fully understood so that a designer can have a range of solutions to meet ever changing environment and legislation requirements.

Traditional drainage applications were not generally designed with the wider environment at the forefront of the agenda. Drainage systems were chosen for strength and capacity to facilitate the carriage of high water levels at a time when it was desirable to discharge water run-off to the nearest drain run as quickly as possible.

These products, while suitable for certain applications, do not guarantee the most effective and sustainable method of controlling excess water run-off from a built-up area, where water control and quality are now the major considerations.

A major development within the water engineering and design industries is the introduction of modular cells. These are relatively new to the UK market, but have been used on a global scale for some time.

While the use of modular cells is widespread throughout the construction industry, the use by water companies is still some way behind. The common consensus among design engineers is that the capacity within the modular cells will reduce over time as siltation builds within the structure. This is true to a point but there are many design solutions that address this issue.

The most effective way of eliminating siltation is to implement some form of silt trap application upstream of the attenuation or soakaway structure. This does not necessarily restrict designers to the use of catchpits. While these provide effective silt prevention, they can also suffer from limitations in terms of providing true silt removal from water run-off.

There are now multiple opportunities available to designers and engineers to remove siltation and provide solutions to meet various site constraints and requirements.

One of the most efficient silt traps on the market is the vortex hydrodynamic separator. This provides a 95-98% removal of silt within the surface water run-off before it enters the attenuation or soakaway structure.

Not only does this offer exceptional efficiency, but it also has many health and safety benefits, as maintenance access can be gained easily without the need for a person to climb inside the silt trap.

The use of a large GRP silt trap placed in front of the system is also a very effective way of removing silt. These can be designed bespoke to the site, and allow for conventional techniques of silt removal to be implemented.

Designing an offline solution using a large GRP silt trap can provide an effective method of silt removal. And it will have been tried and tested across a wide range of applications, and can easily be incorporated into a modular cell system.

Design is integral to the effectiveness of a solution. And eliminating the issue of silt can be achieved through a variety of methods.

An alternative design approach is the use of multiple installations, which enable several smaller tank structures to be used rather than one single cell structure. This ensures that a number of systems can still ensure soakaway or attenuation should one tank begin to silt up.

While this may be a more expensive option than installing one single tank, it could still be cost-effective in high silt-loaded areas when compared with culverts.

Holistic approach

Design engineers state a concern regarding performance of modular cells once silt loading occurs within the structure. To address the issue of performance, it is imperative for designers to understand that a holistic approach using various systems can create a solution which overcomes siltation worries.

For example, the Environmental Protection Group has stated that by simply designing a further 10% capacity in the modular cell systems, the system should achieve a design life of 59 years, allowing for the excess storage of silt.

Using the basis of over-designing tanks by 10%, silt can be allowed to build up within the cells to minimise environmental impact from downstream flooding and pollution.

Good design to effect siltation in a controlled manner should be promoted, providing regular maintenance and easy access can be undertaken.

The Water Framework Directive, a key piece of legislation due to come into effect in 2010, takes the sustainable urban drainage philosophy one step further by stating water run-off should not only be controlled at source, but also treated to improve water quality.

Current practices of flushing the existing pipe network to remove silt and other debris only causes pollutants and toxins, particularly from industrial or road areas which contaminate the silt, to enter watercourses further downstream, leading to water quality and flooding issues.

Water companies will also suffer further siltation issues when silt enters a treatment works as the settlement is unavoidable. To guarantee improved water quality it is a far more effective solution to capture silt at the source of run-off where it can easily be maintained and therefore reduce the risk of environmental impact further downstream.

Is it fair to assume that, with the introduction of the Water Framework Directive, silt control at source will need to be used more in the future? With the correct holistic design using the correct tools available, allowing for siltation within a cell structure may assist the designers to go a step further in reaching the objectives set out by the Water Framework Directive.

To alleviate issues surrounding water control in an attenuation application, a simple flow control device can be used to allow for the discharge of water run-off at a controlled rate set by the Environment Agency (EA). This dramatically reduces the impact of downstream flooding, and enables the structure to control run-off at source in-line with sustainable drainage philosophy.

The key to achieving a sustainable drainage application is to combine the correct product solution with bespoke design and engineering. It is therefore imperative that a better understanding of the benefits and flexibilities of the modular systems are taken on board by designers, so that they can harness the real benefits.

For a designer, modular cell systems in conjunction with silt traps opens up a range of solutions to meet varying site constraints, as well as health and safety and installation benefits. Siltation is something that can be designed for and should be captured on a site rather than allowed to cause problems downstream. But it is also essential that there is clear guidance from government and the EA on who is responsible for the adoption of the SUDs systems. Defra is now believed to be looking at this issue.

Forward thinking

There are water companies already looking at modular systems with a view to implementing cell structures in the future. And Polypipe WMS recognises its duty as a responsible manufacturer to assist engineers in finding the most suitable option available. While this is progress, there needs to be more forward thinking from the water industry, and a move away from traditional drainage methods to avoid issues of flooding and water contamination.

Fluctuating peak flows and water-borne pollutants are a major risk to the wider environment. And long-term sustainable solutions are the most suitable method for water companies to minimise environmental impact. Both of these can be achieved through modular soakaway and attenuation solutions.

Dr Jason Shingleton is marketing and technical director at Polypipe Civils, Polypipe Water Management Solutions.

T: 01509 615100.

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