ADS UK manager on how engineers are getting sediment management wrong

A historical focus on water quantity over water quality in SuDS design means that engineers are getting sediment management wrong, says Stuart Crisp, UK manager of Advanced Drainage Systems (ADS).Stuart Crisp is UK manager at Advanced Drainage Systems (ADS) Given the record amounts of rainfall experienced in many parts of the UK over recent months, now seems a good time to consider the role of sediment management in sustainable drainage systems (SuDS). It’s a subject that isn’t well understood by every drainage engineer. The most common mistakes we see are: the incorrect positioning of treatment devices in a SuDS management train; no means to divert flow once a treatment device has reached its maximum treatment flow rate; failure to consider sediment management at all; and unrealistic expectations on maintenance. The result is systems that don’t operate as the designer intended – in terms of hydraulic performance, water quality or both. Sediment management must be considered for both nature-based and proprietary infrastructure, and is covered extensively in The SuDS Manual, Ciria C753. The manual explains that loss of storage volume due to sediment deposition should always be taken into consideration in designing for water quantity (para 3.2.5). And it warns of the potential harm of sediments entering water bodies, which can lead to a decline in biodiversity (para 4.1). First line of defence Sediment removal is a vital first line of defence for nature-based attenuation and treatment solutions such as ponds and swales or where surface water is flowing directly into streams and rivers. They may also be necessary for below-ground attenuation devices such as crates, which are difficult to inspect and clean. In designing and specifying treatment elements for SuDS, it is first important to understand sediment distribution; the proportions of different particle sizes that are likely to be present in surface water. Next, mean and maximum treatment flow rates should be calculated. British Water’s How to Guide on applying C753’s water treatment principles to proprietary systems gives guidance on this. Proprietary sediment management components – such as gravity sedimentation tanks, gully pots or catchpits – deploy settlement to remove sediment. However, a major downside for all of these components is that they can fill up quickly, and more intense rainfall events can remobilise sediment particles, washing them and the attached pollutants downstream. Other treatment mechanisms include flotation, filtration and adsorption. The How to Guide sets out where devices deploying these mechanisms should go in a SuDS management train. Some proprietary SuDS components combine sediment management methods. For example, ADS’ StormTech below-ground arch-shaped attenuation chambers have an integrated system, Isolator Row, which uses both settlement and filtration to remove sediment and associated pollutants. Where pollution or sediment loads are higher, requiring a higher level of assured performance, hydrodynamic separators (HDS) may be a preferred solution. In the UK, vortex separators are most commonly used, but there are other types of HDS can be more effective, even providing treatment at higher flow rates within the same footprint. For instance, the ADS Arcadia HDS forces water to flow through a series of angled baffles, which reduces turbulence, traps pollutants and helps prevent the resuspension of captured contaminants. HDS tend to come with a built-in bypass, so that water is redirected once the flow rate exceeds the device’s maximum treatment flow rate. Published in association with Like what you've read? To receive New Civil Engineer's daily and weekly newsletters click here.