The implementation of water management regulations for new developments in Schedule 3 has prompted increased scrutiny from designers, developers, and local authorities regarding the effective handling of water. In drainage design, the emphasis frequently lies in capturing run-off and swiftly directing it offsite to mitigate flooding, addressing regulatory requirements and catering to the needs of those using the space. However, the discussion often overlooks the quality of the run-off water and its environmental repercussions. In this blog I will shed light on drainage design methodologies that not only excel in their primary function but also contribute to enhancing the quality of run-off.
In conventional drainage systems, run-off is typically captured in a combined sewer, making its journey to a water treatment facility. However, the challenge arises with new developments – these expansions mean additional contributing areas funnelling into a fixed capacity pipe network. The consequence? More frequent and severe flooding episodes. Often these networks include Combined Sewer Overflows (CSOs) which discharge water from the network into the environment when the network is overwhelmed by a storm event, posing a threat to local water courses. To reduce run-off into a sewer network, Sustainable Urban Drainage Systems (SuDs) encourage infiltration into the ground or direct surface water run-off into a receiving watercourse.
How important is the quality of surface water run-off?
Allowing large volumes of poor quality water to run-off into watercourses can have a detrimental effect in any area. Surface water run-off can contain pollutants harmful to groundwater and surface water systems and habitats. Different pollutants pose different threats. Some heavy metals, for instance, don’t break down naturally and can accumulate in habitats and become toxic. Other pollutants, such as excessive nitrogen and phosphorous, can lead to eutrophication, where algae forms on the surface of a watercourse, blocking light penetration and causing underwater aquatic life to die out.
Introducing the ‘treatment train’
SuDs philosophy introduces the idea of a ‘treatment train’ to resolve this. The intention is to use a .series of SuDs features, such as raingardens, swales, and permeable paving, to provide treatment at several stages. The advantage lies in the fact that each of these systems removes a portion of certain pollutants, and some features excel in removing specific pollutants more effectively than others. Therefore, by strategically designing the system to channel captured run-off multiple SuDs features, a more extensive range of pollutants can be captured and removed from the run-off.
Different SuDs features remove pollutants through various mechanisms. For example, when the flow velocity is reduced through an attenuation feature such as a detention basin, suspended solids within the run-off can settle down and collect on the bottom of the basin. Permeable paving can remove pollutants via a sieving effect of passing through the gaps between the individual aggregate granules, and some pollutants may naturally adsorb to the aggregate of the subbase. In other SuDs features, natural vegetation may uptake pollutants through their roots, preventing them from discharging downstream.
A drainage design method to improve water quality
The CIRIA SuDs manual includes a simple index approach design method, which is used when designing for water quality for areas with low to medium pollution hazard levels, including domestic driveways, residential roads, low-traffic roads, non-residential parking, and commercial yard delivery area. Please note that in some cases, risk screening and consultation with an environmental regulator is necessary when discharging to groundwater.
Below is an example of how to design drainage to improve water quality, including a practical example. This demonstrates one of the methods we use at Marshalls to support our clients and customers on their developments. Scroll to the bottom of this blog for more details about how you could tap into our in-house expertise.
Stage 1 - A suitable pollution hazard indices is selected from the table below appropriate for the site for each contaminant type.
| Pollution hazard indices for different land uses | ||||
|
Land use |
Pollution Hazard Level |
Total Suspended Solids |
Metals |
Hydrocarbons |
|
Residential roofs |
Very low |
0.2 |
0.2 |
0.005 |
|
Other roofs (typical commercial/industrial roofs) |
Low |
0.3 |
0.23 |
0.005 |
|
Individual property driveways, residential car parks, low traffic roads (e.g. cul de sacs, homezones and general access roads) and non-residential car parking with infrequent change (e.g. schools, offices), ie <300 traffic movements/day |
Low |
0.5 |
0.4 |
0.4 |
|
Commercial yard and delivery areas, non-residential car parking with frequent change (e.g. hospitals, retail), all roads except low traffic roads and trunk roads/motorways1 |
Medium |
0.7 |
0.6 |
0.7 |
|
Sites with heavy pollution (e.g. haulage yards, lorry parks, highly frequented lorry approaches to industrial estates, waste sites), sites where chemicals and fuels (other than domestic fuel oil) are to be delivered, handled, stored, used or manufactured; industrial sites; trunk roads and motorways2 |
High |
0.8 |
0.8 |
0.9 |
1 Motorways and trunk roads should follow the guidance and risk assessment process set out by Highways Agency (2009).
2 These should only be used if considered appropriate as part of a detailed risk assessment. When dealing with high hazard sites, the environmental regulator should be consulted for pre-permitting advice first. This helps determine the most appropriate approach to developing a design solution.
3 Up to 0.8 where there is potential for metals to leach from the roof
Stage 2 - SuDs features are then selected such that the total pollution mitigation index equals exceeds the pollution hazard indices for each contaminant type depending on the receiving water course.
| SuDS mitigation indices for discharge to surface waters | |||
| Type of SuDS component |
TSS |
Metals |
Hydrocarbons |
|
Filter Strip |
0.4 |
0.4 |
0.5 |
|
Filter drain |
0.4 |
0.4 |
0.4 |
|
Swale |
0.5 |
0.6 |
0.6 |
|
Bioretention system |
0.8 |
0.8 |
0.8 |
|
Permeable pavement |
0.7 |
0.6 |
0.7 |
|
Detention basin |
0.5 | 0.5 | 0.6 |
|
Pond |
0.7 | 0.7 | 0.5 |
|
Wetland |
0.8 | 0.8 | 0.8 |
Note that SuDs features used in series a factor of 0.5 is applied to the mitigation indices for each suds feature after the first.
| SuDS mitigation indices for discharge to surface waters | |||
| Characteristics of the material overlying the proposed infiltration surface through which the run-off percolates |
TSS |
Metals |
Hydrocarbons |
|
A layer of dense vegetation underlain by soil with good contaminant attenuation potential of at least 300mm in depth |
0.6 |
0.5 |
0.6 |
|
A soil with good contaminant attenuation potential of at least 300mm in depth |
0.4 |
0.3 |
0.3 |
|
Infiltration trench (where a suitable depth of filtration material is included that provides treatment, e.g. graded gravel with sufficient smaller particles but not single size coarse aggregate such as 20mm gravel) underlain by soil with good contaminant attenuation potential of at least 300mm in depth |
0.4 |
0.4 |
0.4 |
|
Constructed permeable pavement (where a suitable filtration layer is included that provides treatment, and including a geotextile at the base separating the foundation from the subgrade) underlain by soil with good contaminant attenuation potential of at least 300mm in depth |
0.7 |
0.6 |
0.7 |
|
Bioretention underlain by soil with good contaminant attenuation potential |
0.8 | 0.8 | 0.8 |
How we would design drainage to improve water quality
Imagine a development for a car park for a block of flats which are intending to be drained to a receiving surface water course. Using the simple design method above, a pollution hazard indices is assigned to the car park. Using table 1 this is classified as a residential car park with a low pollution hazard level, and has a pollution hazard indices of 0.5, 0.4, and 0.4 for Total Suspended Solids, Metals, and Hydrocarbons, respectively.
If we were to install a permeable paving car park for our site, permeable paving when drained to a surface water (from table 2) has a mitigation indices of 0.7, 0.6, and 0.7 for TSS, Metals, and hydrocarbons, respectively. Since the mitigation indices with just permeable paving is higher than the pollution hazard indices from the site use, then permeable paving alone would be sufficient. If the pollution hazard indices were greater than the mitigation indices from permeable paving, then additional SuDS features could be used (a factor of 0.5 is multiplied with the mitigation indices for successive SuDs features).
Benefit from Marshalls in-house expertise on your next project
If you want more in-depth written information to add detail to the above methods, see CIRIAs SuDs design manual (C753 SuDs Manual).
Marshalls offers bespoke design support, including pavement engineering, drainage design and digital visualisation. Please take a look at our services online, or email the team for details.
References
HA (2009a) “Road drainage and the water environment”. In: Design manual for roads and bridges (DMRB), vol 11, Section 3, Part 10, HD 45/09, Highways Agency, London, UK. Go to: http://tinyurl.com/p4yakc9 (accessed 22/09/2015)
HA (2009b) “Method C: Assessment of pollution impacts from routine runoff on groundwater”. In: Design manual for roads and bridges (DMRB), vol 11, Section 3, Part 10, HD 45/09, Annex 1, Highways Agency, London, UK. Go to: http://tinyurl.com/p4yakc9 (accessed 22/09/2015)