As the climate changes, and significant rain events become more frequent, the challenge of managing surface water is increasingly difficult. Unfortunately, tackling this issue effectively can be a significant headache that slows down builds and holds back profitability.
And yet, there are simple-to-use, cost-effective modular solutions available that use optimise the space available to provide the required level of attenuation or infiltration.
Our handy guide to using modular attentuation crates as either attenuation tanks or soakaways, brings everything you need to know together in one place plus explains the best methods of attenuation tank design.
Reducing the risk of flooding has been a central part of planning new developments since the Flood and Water Management Act 2010, when Schedule 3 of the Act proposed far-reaching changes regarding sustainable drainage systems (SuDS).
Although Schedule 3 hasn’t been adopted yet in England, the spirit of the Flood and Water Management Act flowed into amendments the government chose to make to the National Planning Policy Framework instead. These changes stipulated that all planning applications for developments of ten dwellings or more must include sustainable drainage systems – unless the developer can show they are inappropriate. The Framework amends also empowered the Local Planning Authority, in conjunction with the Lead Local Flood Authority, to decide whether to approve sustainable drainage systems and to oversee their appropriate maintenance.
Since these regulatory shifts, climate change has increased the intensity of rainfall across the UK, triggering more localised flooding. At the same time, high-density living has led to a loss of greenspace and natural water management capabilities: neighbourhoods dominated by the most recent generation of housebuilding have up to 40% less green space provision than older neighbourhoods. This shrinkage of green spaces means more water is diverted into sewage systems rather than naturally filtering into the ground.
Sustainable drainage systems are now the developer’s first line of defence when dealing with surface water runoff, supplementing natural environmental drainage with manufactured capabilities. Within a new development, there’ll be opportunities to create spaces for natural infiltration such as ponds, swales and wetlands, however, it’s unlikely most developments could support enough green space for natural drainage alone to manage all surface water runoff.
The core idea of sustainable drainage is to manage as much of the excess water flow at its source through infiltration, rather than overwhelming stormwater drains and risking flooding. Specialist sustainable drainage systems collect the excess water and release it into the environment via infiltration or, where that’s not possible, into sewers and watercourses at a manageable rate.
The main regulations to note here are the Building Regulations 2010, Section H Drainage and Waste Disposal, that outline a clear hierarchy of methods to manage surface water runoff, with infiltration via soakaways at the top. We’ll talk you through the key points in our ‘how to’ section, later, as we focus in on attenuation tanks and soakaways.
The broad principle of capturing excess surface water in an underground tank that works as a reservoir divides into two key forms: attenuation tanks and soakaways crates (also known as soakaway tanks). They often look very similar and can be constructed from the same materials.
The fundamental difference lies in how the tank disperses the water it’s stored. A soakaway is wrapped in a permeable geotextile that allows the water to soak into the ground via infiltration at the natural rate of the surrounding environment. The infiltration takes place as and when saturation allows, so is dictated by the environment.
An attenuation tank uses a flow control chamber to manage the amount of water discharged, and the water is either discharged through a gravity stormwater pipe system or pumped via a pumping chamber. Rather than soaking into the ground immediately around the attenuation tank or crate, the water travels via the main drainage system into a nearby water course, such as a river or reservoir, or into a storm drain. This dramatically cuts the possibility of localised flooding because the surface water runoff is slowed down to match whatever is appropriate for the surrounding environment.
In many cases, the same technology can be used to meet both soakaway and attenuation requirements. For example, Wavin’s AquaCell NG can be used as a temporary storage or as a soakaway. Simply by installing AquaCell NG tanks in suitable pervious soils and wrapping the units in a geotextile, what could have been an attenuation tank becomes a soakaway crate, allowing stormwater infiltration into the surrounding ground.
Geocellular construction is the leading form of building attenuation tanks and soakaway crates, ticking all of the boxes that modern developments need. They’re such a popular choice because they’re modular (with a honeycomb structure) and so can fit into small spaces, offering flexibility over length, width and depth that allows engineers to easily construct configurations to match a specific site.
Geocellular attenuation tanks or attenuation crates are also extremely space efficient, with a typical surface water holding capacity of 95% or above. Geocellular attenuation or soakaway crates are also extremely versatile, which means they can be configured around site constraints
AquaCell NG is designed primarily for landscaped and light traffic applications but can configured to an extra strong version that is suitable for heavy traffic situations by interlocking two units.
This efficiency stretches further into their installation. Geocellular attenuation tanks are lightweight and have been specifically designed to be easy to install. The Wavin AquaCell NG, for example, features lightweight units with hand grips to make them easy to carry, as well as integrated, push-fit connectors that click into place without pegs, clips or tools, three times quicker than it takes to install comparable units.
There’s also a wide range of choice of geocellular attenuation crates, making it easy to find one to suit traffic-bearing and non-traffic-bearing areas. The Wavin AquaCell range offers AquaCell NG for easy storage, transpot nd installation, the AquaCell Eco for shallow, non-trafficked areas such as domestic gardens and the AquaCell Core-R unit is designed for regular and heavy traffic loadings such as cars and HGVs. The Wavin Q-Bic Plus, on the other hand, is an all-round solution, suitable for parks, domestic gardens, car parks, roads and industrial areas.
Plus, it’s worth remembering that geocellular attenuation tanks can be retrofitted, sited above vulnerable groundwater and fitted on contaminated sites.
The starting point for your sustainable drainage system will be the Environment Agency’s decision on the rate that surface water from your site can be released into the water course. This can involve running a percolation test to measure the rate of infiltration for your site and ground type (full details of how to do this can be found in the NHBC Surface Water Soakaways guidelines). These results will then be combined with a drainage calculation that takes into account the surface area of the water source and the rate of runoff.
If you want to do a rough estimation of the size of the soakaway you’ll need to create with geocellular attenuation tanks/soakaway crates, there’s a quick calculation you can use. Assuming you’re working with non-clay soil, one cubic metre of crate volume will be able to cope with runoff from 50 square meters of roof or road surface area.
Once you have your official calculations, you can work out what water storage volume you’ll need to provide in your development, and where on your site the required number of attenuation tanks or soakaway crates will best fit. The key points to remember are that any soakaway crates must always be a minimum of 2.5 metres away from the boundary and a minimum of 5 metres from the wall of any building.
It’s also mandated that soakaway crates should never be sited in ground where the water table reaches the bottom of the crate at any time of the year, or in areas where the water could be contaminated. They must also be far enough away from other soakaway sites that the soakaway capacity of the ground isn’t overwhelmed. And for attenuation tanks, the bedrock shouldn’t be within one metre of the drainage piping.
Leading sustainable drainage systems body, Susdrain, has produced a handy checklist to help you plan your attenuation system.
The Wavin AquaCell range of units has been specifically designed to meet every potential requirement on your development. Based on a tried and tested modular structure, they can be configured as an attenuation tank or crate, or as an infiltration/soakaway crate.
AquaCell NG, our next generation crate solution, offers a space-saving, stackable design and a good resistance to traffic loads. It’s quicker and easier to install than any of its predecessors, and is made from 100% recycled plastic for a lower carbon footprint.
And if you’re looking to install an attenuation or infiltration system in a no-traffic area, our AquaCell Eco is ideal. Its design has been tailored for shallow positioning in areas like gardens and landscaped green spaces.
For an attenuation tank or infiltration crate system that can easily withstand regular and heavy traffic, including HGVs, our AquaCell Core-R fits the bill perfectly. It’s fully British Board of Agrement (BBA) approved as a long-lasting, robust solution.
If it’s important to be able to inspect your infiltration or attenuation crate system, our AquaCell Plus-R units are the right choice. It’s easy to blend the AquaCell Plus-R with Core-R and AquaCell Eco units to create a cost-effective solution that meets every requirement.
AquaCell recently provided national housebuilder, Redrow Homes, with a comprehensive sustainable drainage system to protect a development of premium housing in Cheshire against regular and extreme weather conditions. The lead contractor was extremely impressed, saying: “With strict project timings to adhere to, AquaCell allowed us to install the system quickly and effectively, while its flexibility meant we were able to develop a custom solution that met multiple project requirements.”
And where you need rapid and easy accessibility into your attenuation tank or infiltration crate, our Q-Bic Plus geocellular modular system is also a strong option. It has the strength to support a wide range of usage scenarios, from parks and gardens through to car parks and roads. Plus, its push-fit connectors halve the installation time compared to units that are standard in the industry.
Our StormForce service takes care of everything for you, and is perfect for developers who want to free up their time to focus on other core business activities. Our experts work swiftly to design your capture, attenuation and dispersal solution, fully backed by assured quality and compliance processes and a five-year warranty on the installation.
Our StormForce team will advise you on the most effective, value-driven system for your site, then manufacture and coordinate the component parts before working with accredited partners to install your sustainable drainage.
Right from launch, StormForce has delivered for developers, as our client on the Hatfield rail station development confirms: “From one source supply and surface water management are totally sorted, so we can get on with the build. It’s fast, compliant, sustainable and top-quality”.
Our StormForce experts are ready to talk you through the process of attenuation tank design and installation, just get in touch if you’d like to find out more.