Monthly Archives: May 2024

SuDS design & water quantity: back to basics – Drain Trader May 2024

SuDS design and water quantity: back to basics

 The purpose of SuDS – sustainable drainage systems – is to mimic nature. But should that automatically mean that we only consider nature-based solutions, asks Stuart Crisp.

The weather so far this year has provided a stark reminder of the challenges we face due to heavy and intense rainfall. In January, Storm Henk brought a month’s rainfall in four days to some areas, leading to flooding, the worst of it in the Midlands. And in April, the impacts of Storm Kathleen and Pierrick caused flooding across the country, especially on parts of the south coast.

Much of this flooding is exacerbated by urban development. By replacing green fields and vegetation with hard surfaces – roads, roofs and driveways – we change the way and rate at which surface water flows out of an area or catchment.

A good SuDS design for water quantity aims to mimic the flows of water from developed site so that they are as close as possible to what would have happened, had there still been a greenfield site there. That means throttling the flow of water – in other words providing storage and releasing it later, more slowly – so that it is not rushing so quickly into sewers and water courses and overwhelming them, leading to flooding.

Many policy documents from organisations that currently adopt SuDS – typically Scottish Water in Scotland, the county or unitary authority in Wales or a water company in England mandate natural or green SuDS, often prohibiting the use of proprietary or grey SuDS.

While green SuDS, such as swales and ponds are the right solution for some developments, a blinkered approach may not best mimic nature, could ramp up capital costs and potentially lead to longer-term problems and higher maintenance and operation costs.

Although designing for water quantity and the urgent need to reduce the risk of flooding is vital, there are the other pillars of SuDS to think about too: water quality, amenity and biodiversity. Previous articles in Drain Trader’s June 2023 and February 2024 issues, looked at water quality issues and what type of management trains were best suited to different types of development, depending on pollutant loads.

Operational and maintenance costs for SuDS schemes are often overlooked and ignored, but the promised implementation of Schedule 3 of the Flood and Water Management Act 2010 in England will shed a harsh light on these. SuDS Approval Bodies (SABs), likely to sit within local authorities, will require robust information about inspection and maintenance regimes and expected costs over the lifetime of a development. 

An article in Drain Trader’s March 2023 edition looked at why poorly thought-out SuDS can lead to shorter service lives and higher operational costs than those expected from the design.

Mimicking nature
On a greenfield site, the first 5mm of rain would not typically generate surface runoff, it would infiltrate into the ground, evaporate or transpire through the leaves of plants. Then, when water flows from the surface of a catchment and exceeds the capacity of the receiving water body, the additional volume of water would spill over onto the floodplain.

When we develop on that catchment to add impermeable surfaces, such as roofs, roads and hardstandings, the amount of surface water runoff generated can increase dramatically. In a dense, urban environment, 95% of the rainfall could flow off the catchment as surface runoff, increasing the volume of water that reaches water courses or sewers. It is also likely that the peak flow will be higher and come sooner, than had it remained a greenfield site.

CIRIA C753 The Suds Manual gives a graphical illustration of this. Figure 3.1, in Chapter 3 which deals with design for water quantity, shows hydrographs for the discharge rates of surface water for an area in three situations: when it is greenfield; when developed without flow attenuation; and when developed with flow attenuation. (Figure 1 shows a similar graph, courtesy of SUSDRAIN).

Figure 1: Urban development increases the volume and rate at which surface water flows from a catchment area and the peak flow. Credit: Susdrain

The amount of water that a SuDS system will need to attenuate or store, and the rate at which the water should be discharged is the subject of the hydraulic design for that system. For those that want to go back to first principles, these are set out in BS EN 16933-2 Drain and sewer systems outside buildings – Design – Part 2: Hydraulic design.

Today, however, many drainage engineers rely on software such as InfoDrainage and MicroDrainage (AutoDesk/Innovyze), Flow (Causeway) and Site3D. But this ‘black box’ approach to calculations can mean that designers don’t have the opportunity to properly understand the assumptions and coefficients that have been used – and the impact on water quality based on the SuDS components selected to satisfy the hydraulic design – which may lead to a suboptimal design.

In a SUSDRAIN factsheet from March 2014, Assessing attenuation storage volumes for SuDS – another useful resource for designers – author Anthony McCloy explains the risks of this approach:

“Don’t expect exact answers from the calculation process, it is a usable approximation that can provide acceptable solutions for design. Most of the inputs are based on statistics and calibration factors; therefore we can only ever achieve an approximation of how the system will behave in reality. The results of calculations and modelling need to be used alongside professional judgement to provide the design.”

Figure 2: This pond is one element of an extensive SuDS management train at a housing development in Hamilton, Leicestershire. Credit: Susdrain

Ten years later, this point is just as relevant as ever, perhaps even more so as design software is more widely used, with newer generations of engineers never called on to design from first principles. McCloy also advises that those assessing potential SuDS, such as SABs, must also have a basic understanding of first principles of storage volumes and hydraulic design so that they can carry out their statutory duties.

Water storage toolbox
There are many ways that we can create storage for excess surface water. Storage can be online, meaning that flow enters the element, passes through and out the other side or offline where flow enters and exits via the same point. A design could include both online and offline storage where, for example, flow above the 1-in-30-year return period is directed offline to accommodate the 1-in-100-year event.

Vegetative or surface-based solutions include ponds, detention basins which are dry until excess water needs to be accommodated and swales which can be used to accommodate volume, as well as to communicate flow between SuDS elements and for infiltration.

Ponds can be an attractive choice, potentially ticking boxes for all the four pillars of SuDS. From a water quantity perspective, they must have sufficient capacity to cope with rising water levels during higher rainfall events throughout their design lives. Any sediment entering the pond will settle out quickly, since the sediment particles soon reach terminal velocity allowing them to settle to the bottom of the pond.

As an asset owner or adopter, it is important to anticipate the amount of sediment that will settle out over time, because the accumulated sediment must be somehow removed at intervals to allow the pond to provide sufficient storage volume. One issue that is sometimes overlooked in the design of ponds is providing safe and cost-effective means of accessing the pond to remove sediment when required.

A recent story from Gloucestershire illustrates the costs involved. Local residents petitioned Gloucester City Council in January this year to desilt Saintbridge pond in Abbeydale because it produces a foul smell in the summer and the silt is negatively impacting on water quality and habitats. The council refused, saying that it was only 15 years since it last desilted the pond and that the operation would cost £700,000.

Commenting on the story, Jo Bradley, director of operations at Stormwater Shepherds, pointed out the error of not having installed an upstream sediment separator at the same time as the pond. “If a manufactured sediment separator had been included, it could have been emptied every year, costing maybe £700 – £1000. That would have cost up to £20,000 over the 20-year cycle and avoided the £700,000 cost that they are now facing.”

Bradley pointed out that by bringing in heavy plant every 20 years, removing vegetation and sediment, the pond’s habitats and inhabitants are disturbed, negatively impacting on nature. And she added that the sediment in the pond could well be contaminated with toxic, bio-accumulative pollutants and tyre-wear particles.

Figure 3: Small detention basin at Woodberry Down, Hackney, part of an extensive SuDS scheme in a regeneration project of a post-war housing estate. Credit: Susdrain

CIRIA’s manual says that a sediment separator or sediment forebay should be installed upstream of every pond. And, depending on the pollutants likely to be washed into the pond with the surface water, other pre-treatment could be needed. Without this, there is a danger that wildlife will be attracted to the pond, only to suffer damage.

This point was illustrated by researchers at  Glasgow University, who compared SuDS ponds with natural ponds and found that pollutants were higher in some of the SuDS ones, negatively affecting amphibian breeding and development. A paper published on the research issues this warning: “The function of SuDS and other urban drainage systems to sequester pollutants increases their potential to be ecological traps by advertising false cues of suitable habitat.”

Figure 4: Swale in a housing scheme at Upton, Northampton. Credit: Susdrain

Proprietary attenuation
The most commonly used proprietary solutions include permeable paving, where the water passes through the gaps between blocks to be stored in unbound granular material below them; geocellular crates which are buried below ground; and large-diameter pipes below ground. There are also innovative systems, new to the UK, such as arch-shaped chambers which combine both filtration and attenuation without the need for additional upstream devices.

Figure 5: Permeable concrete block paving at a housing scheme in Ipswich, showing detail of joints to allow water to pass into granular fill below. Credit: Susdrain

Permeable block paving can be useful because it requires a limited volume of excavation. In terms of water quality, block paving provides two stages of water treatment: water is filtered as it passes through the gaps between blocks and pollutants can then be broken-down by bacteria naturally present as it passes through the granular fill beneath it and then either infiltrates the ground or flows into another part of the system.

The capacity of permeable paving to accommodate surface runoff decreases over time. And regular maintenance is needed to clean out the joints between the paving blocks so that they do not become clogged up, making sure that the wash water does not enter the drainage system served by the permeable pavement. Also, note that water companies do not adopt permeable paving as it is not only a SuDS component, but also a structural pavement.

Geocellular crates can be a good choice if there is a limited plan area for water storage, since they provide a large volume for a limited footprint. They are typically manufactured from polypropylene (PP) or PVC by injection molding, or extrusion of joining thermoformed sheets.

Crate-based systems are relatively simple to design and due to their lightweight and modular construction, they are easy to install. There are a variety of types on the market, from shallow sub-base replacement systems to be used beneath car parks to crates which can be used at a greater depth for higher volumes of storage.

However, crates are likely to decrease in storage capacity over time. Installation of catchpits, silt separators and other sediment pre-treatment measures upstream are required – but often under-designed, which can lead to issues as maintenance to remove sediment build up in crate-based systems can be difficult or impossible. There are also question marks over their long-term durability, as some crates do not comply with the latest industry specifications for structural integrity.

Clause 21.5.3 and Table 21.2 of the SuDS Manual suggests adding an additional 10% to the storage volume of crates to account for sediment build up. Asset owners and adopters of SuDS systems should determine if a crate-based system has been appropriately up-sized by 10% and if not, that the SuDS designer or manufacturer has provided evidence to demonstrate that all the sediment can be removed from the crate-based system. Not all adopting bodies will accept crates.

Large diameter pipes, laid as a single run or in parallel, with manifolds are an established solution, one of the forerunners of proprietary underground water attenuation which can be created from existing products. They can be a cost-effective choice with concrete and plastic the most commonly used materials; thin steel pipes and a hybrid plastic and steel product are other choices.

Figure 6: Major problems with flooding this year demonstrate the urgent need to attenuate flow from developed areas. Credit: iStock

A downside to large-diameter pipes is that they are larger and can be heavier to transport and install, requiring more transport movements for delivery, more storage space on site and a larger excavation footprint. Depending on the type of pipe material, they may not be adoptable by some asset owners.

Innovative underground water storage systems such as arch-shaped chambers are simple to design and install and can be configured to fit irregular areas or to fit around existing infrastructure or obstacles. Their arch shape means that the embedment material around them is shaped into ‘stone columns’ allowing them to be installed at shallower depths, while taking heavier loadings. They are lightweight and stackable which means they are easy to transport, store and install.

Figure 7: Buckshaw swale inlet. Credit: Stormwater Shepherds

One proprietary brand of arches, StormTech, can be installed with an integrated pre-treatment element, Isolator Row, which takes out sediments and other surface water pollutants from the first flush of a rainfall event. The inclusion of Isolator Row can remove the need to install upstream pre-treatment devices such as sediment traps and bypass separators, resulting in considerable cost savings. StormTech has been designed for ease of maintenance, since only Isolator Row needs to be cleaned, accessed from the manhole using standard sewer cleaning equipment.

This brand of arches provides four stages of water treatment: Isolator Row provides the first two stages.  Sedimentation – gravity separation of the silt particles, settling out on the bed of Isolator Row – and filtration, as the water passes through a layer of woven geotextile fabric. 

The next two treatment stages are akin to that for permeable concrete block paving; that is adsorption as the water passes over the granular material surrounding the chambers and biodegredation from the action of bacteria breaking down pollutants into non-polluting material. StormTech meets the requirements for adoption by water companies as set out in the Design and Construction Guidance (DCG) for adoptable sewers.

Figure 8: Innovative arch-shaped chambers for below-ground attenuation combine both water storage and treatment. Credit: StormTech
Figure 9: North Stoneham Park. Credit: Stormwater Shepherds

Green and grey
Attenuation of surface water in vegetative SuDS can be a good solution for some developments, particularly where there is lots of space and low pollutant loads. But insisting that they are the only choice could lead to designs which do not best mimic nature or provide the best options.

A more flexible approach to SuDS would allow a combination of green and grey elements to be deployed. This could allow some water to be stored in green SuDS such as ponds and swales, with proprietary solutions such as below-ground pre-treatment devices and attenuation chambers, deployed elsewhere. Alternatively, crates or arched chambers could be installed as additional storage beneath a smaller pond or under a parking area to deliver multiple SuDS benefits within a smaller available footprint.

A good SuDS design also factors in the cost and ease of maintenance of the various SuDS elements. There is a misconception that natural SuDS can be left to nature, but this is not the case. Vegetation needs to be maintained, litter must be picked at frequent intervals and for ponds, sediment build-up needs to be removed, as explained above, to maintain the capacity of the pond.

Figure 10: North Stoneham Park. Credit: Stormwater Shepherds

Equally for proprietary SuDS elements, maintenance to preserve the hydraulic capacity of water storage and the functioning of water quality treatment devices is an important consideration. When products are substituted in ‘value-engineering’ exercises which are, in fact, capital cost reduction exercises, the impact on water quality is often overlooked. Although pollutant loads could look low enough that upstream treatment is not needed, there will be a need to consider the long-term performance of the SuDS system over the lifetime of the development.

Finally, there is the issue of climate change. Although SuDS designs do apply factors to account for increasing rainfall intensity and frequency, they do not consider the fact that more intense periods of rainfall increase the amount of sediment and pollutants that are swept along with surface water. Add an extended dry period before that rainfall event, and the problem will be worse.

To limit designers to vegetative SuDS is to limit the choice of tools they have available to them. With a bigger toolbox of grey and green SuDS elements, they have a greater opportunity to create solutions that are technically, environmentally and economically viable over their entire lifetimes.

For more information on Advanced Drainage Systems, visit

Crate Expectations – New Civil Engineer May 2024

With strengthening focus on flooding and water quality, CIRIA’s advice to oversize crate-based attenuation tanks by 10% has never been more relevant, says Stuart Crisp, UK manager at Advanced Drainage Systems (ADS).

Flooding river Ouse causing bridge and road to be shut off. If a system is claimed to be exempt from the 10% up-sizing guidance, evidence that the system can be completely and easily cleaned should be demanded.

There’s a reason why the CIRIA SuDS Manual C753 suggests that “difficult to clean” SuDS attenuation systems, such as below-ground crates, should be oversized by 10%: there is a risk that sediment will build up in them, reducing their storage capacity.

With increasing legislation to manage flood risk and to prevent pollution, and more intense rainfall events, this advice becomes even more pertinent. There is a higher volume of water for SuDS devices to deal with and, when these events follow dry spells, more particulate matter and pollutants are swept along with surface water.

Unfortunately, this need to upsize crate-based attenuation tanks by 10% is often overlooked, thanks to focus on originally calculated target storage volumes. Furthermore, if the development is ‘value-engineered, the capital cost saving also often focusses on the original target storage volume. Up-sizing is often disregarded and, if the design is substituted for another type of attenuation system, the implications for water quality and the effectiveness of the treatment train are ignored. 

The result is an increased risk of flooding and an increased risk to water quality.

Risk-based approach

The SuDS manual suggests a risk-based approach to designing SuDS management trains. Considering sediment, this means assessing: how much is likely to build up, depending on the location, how that load might change over time, and how often and easily devices will need to be cleaned. 

Clause 21.5.3 of the manual explains the rationale for upsizing below-ground attenuation by 10%, while Table 21.2 shows the potential loss of storage capacity. Typically, commercial developments suffer from the highest sediment loading, followed by car parks, high-density residential developments and highways.

Removing sediment is also necessary to protect water quality and to avoid the negative impacts of pollutants. The predicted type, quantity and concentration of pollutants governs the choice of natural or manufactured SuDS element that would be most suitable.

Some underground attenuation systems, such as ADS’s StormTech arched system with its Isolator Row, are designed with in-built treatment systems which can be easily cleaned using standard equipment. This can negate the need for – and cost of – upstream treatment devices, while still protecting attenuation capacity and water quality.

Pay now or pay later

With the anticipated implementation of Schedule 3 of the Flood and Water Management Act 2010, SuDS adopting bodies will be looking hard at maintenance issues.  Asset owners will need to be assured that the SuDS they are taking on will perform as designed throughout their whole lives.

Short-term savings in capital cost, which skimp on capacity and compromise water quality, can lead to long-term problems and greater whole-life costs, resulting in higher commuted sums for developers and an additional, avoidable cost burden on UK plc.  

If a system is claimed to be exempt from the 10% up-sizing guidance, evidence that the system can be completely and easily cleaned should be demanded.

For more information on Advanced Drainage Systems, visit

Water Qualitatively and Quantitively – Local Authority Building & Maintenance May-June 2024

Stuart Crisp, UK manager of Advanced Drainage Systems (ADS), looks at how ‘good’ SuDS addresses water quality as much as water quantity when it comes to managing water down a ‘treatment train’.

Historically, driven by legislation, there’s a tendency to focus on water quantity – how to use SuDS to reduce or delay release of stormwater into sewers and prevent flooding. However, new legislation requires the additional focus on water quality.  The SuDS treatment train considers water quality and prevention of pollution from stormwater runoff (alongside flood risk mitigation), with the level of treatment provided based on the anticipated levels and types of pollution.

A SuDS treatment (or management) train should start with prevention such as reducing impermeable surfaces, to reduce run-off and remove sources of pollution. Next comes source control, where rainfall is dealt with close to where it falls by infiltration-based elements such as soakaways, rain gardens and permeable pavements. Site control sees water managed within a local area, for instance directing water to a soakaway or infiltration device and then onto an attenuation pond or tank. Finally, regional control would see a system that could handle run-off from several sites, perhaps resulting in a detention basin or wetland.

The SuDS Manual, C753, published by CIRIA, prescribes a risk-based approach to designing SuDS for water quality. If pollution risks are deemed to be low, then SuDS designers can prioritise water quantity, amenity and biodiversity. If they are medium, then all considerations must be balanced. And if they are high, water quality evaluation must take precedence. It should however be noted that in all cases, including medium and low pollution risk sites, appropriate mitigation should be put in place to reduce the risk of pollution.

There is a range of water quality treatment measures that can be included in a SuDS train. Sometimes it is possible to use a completely natural SuDS train to deal with both water quantity and quality issues. At other times, the best solution combines natural and engineered SuDS elements, or may require proprietary manufactured elements only. This could be due to high levels of pollution loading or the space available.

There are four main types of pollutant that can be found in stormwater run-off: sediments, metals, hydrocarbons and nutrients. Sediments, often referred to in pollution mitigation as total suspended solids (TSS), is particulate matter. It includes tiny particles of soil, such as silt and clay, which have been dislodged by rainwater as it passes over the run-off surfaces.

Metals and metal compounds can be dissolved in run-off or attached to silts and sediments in the water. Copper and zinc are most commonly found in surface water in the UK but there can be cadmium and other toxic metals too. Although plants require very small amounts of copper and zinc to grow, higher concentrations can be damaging to them.

Hydrocarbon pollution comes with run-off from roads, car parks and areas where machines operate and are maintained, due to oil and fuel spills, tyre and brake wear. Changing climate means that rainfall events can be further apart but more intense, which can lead to higher concentrations of pollutants from roads, as they build up for longer before being washed away.

Nutrient pollution, usually nitrogen or phosphorous based, can come from sources such as run-off from agricultural land where fertiliser has been used or combined sewer overflows (CSOs), where sewage and surface water are mixed and discharged into bodies of water. They can lead to algal blooms, which reduces oxygen levels in the water and can negatively impact on aquatic habitats.

Capturing TSS pollution should generally be the focus of water treatment strategies in SuDS design since this removes both the solid particles and any pollution clinging to them. This can be done using a natural SuDS feature, such as planting for bioretention or a swale. Manufactured components such as filtration devices, silt traps or vortex separators can also be used upstream to remove solids before water is discharged into ponds, for instance.

Removal of sediment and solids is also important from a water quantity perspective. Build-ups reduce the capacity of a water storage element, whether natural or manufactured. How and when to remove sediment should be considered at the design stage and should be part of a planned maintenance regime.

Without a means of reducing suspended solids upstream of an attenuation device, sediment build-up within the device can reduce its efficiency over time. Some recently introduced underground arch-shaped SuDS attenuation devices have their own built-in systems for intercepting solids which are fast and simple to maintain.

Mitigation indices

The SuDS Manual sets out a simple method for dealing with water pollution risks, requiring the determination of pollution hazard indices for the area under consideration and then matching a SuDS device with matching mitigation indices.

In Table 26.2, the manual provides pollution hazard indices for a range of applications and for three types of pollution: TSS, metals and hydrocarbons. So, for example, for a busy public car park such as a supermarket or hospital, the index for TSS is 0.7, for metals is 0.6 and for hydrocarbons 0.7.

The next step is to identify a form of SuDS treatment that can provide the necessary mitigation indices, either as a single treatment stage or using a combination of components. The Manual has a table for that too, Table 26.3. However, this table only provides mitigation indices for natural SuDS components.  The mitigation indices for the natural SuDS components were compiled by a team of experts, drawing information from a selection of published papers.

Mitigation indices for proprietary manufactured treatment systems must be provided by the manufacturer using recognised test methods and 3rd party verified data.

British Water has published a Code of Practice for the Assessment of Manufactured Treatment Devices Designed to Treat Surface Run-off. It uses rainfall time-series data for the UK to determine an appropriate treatment flow rates based on first-flush principles and uses a combination of two established test protocols – the German Deutsches Institut für Bautechnik (DIBt) and the New Jersey Corporation for Advanced Technology (NJCAT) from the US – to create the British Water test methods.

In 2022 British Water published a ‘how to’ guide, Applying The CIRIA SuDS Manual (C753) Simple Index Approach To Proprietary/Manufactured Stormwater Treatment Devices, which provides a calculation methodology to derive mitigation indices for TSS, metals and hydrocarbons based on the British Water Code of Practice, or the DIBt or the NJCAT, test results. This enables manufacturers of  proprietary treatment products to have their mitigation indices published so that they can be considered as part of a SuDS management train.

British Water publishes a List of Assessed Surface Water Treatment Devices. More details on the British Water web site.

For more information on Advanced Drainage Systems, visit