Green and grey drainage infrastructure: costs and benefits of reducing surface water flood risk
Keywords: sustainable development
green infrastructure
urban drainage
drainage infrastructure
surface water flood risk
sustainable drainage systems (SuDS)

It is now estimated that in the UK alone 3.2 million properties are at risk of surface water flooding – an increase of almost half a million from ten years ago – and it is expected that this problem will increase further under current climatic changes and urbanisation. Sustainable Drainage Systems (SuDS) seek to reduce flooding from surface water without relying on conventional piped sewer networks by restoring the pre-development hydrological conditions of an area through mimicking natural drainage processes. As their behaviour is more complex compared to their traditional, greyer counterparts, there is still incomplete understanding of their performance during intense rainfall. Research to-date has focused on the optimisation of their design at an infrastructure-scale for achieving hydrological benefits, and a growing number of case studies into their inclusion in small, neighbourhood developments. However, an understanding of the influence of external factors on SuDS behaviours and the additional range of co-benefits SuDS may provide are also important for the design of effective systems, whilst an appreciation of their potential role at greater scales will allow a more informed consideration of drainage alternatives in larger-scale developments. Thus, this thesis investigated how built form influences SuDS’ performance and how the inclusion of SuDS in regional-scale developments may contribute to wider environmental goals.
To analyse the effect of urban built form, a range of 1 hectare urban tiles were developed to represent different housing typologies, urban densities and SuDS implementations under current design principles drawn from The SuDS Manual (CIRIA 2015). The rainfall-runoff model Stormwater Management Model (SWMM) was used to simulate storm events of varying magnitudes and the resultant hydrographs analysed. These tiles were then applied to a proposed regional development spanning five counties in south-east England, the Oxford-Cambridge Arc, under eight different scenarios of urban development, and the length of pipes required to connect such developments estimated. Finally, a methodology was developed to further assess these regional-scale urban development designs for their potential contributions to green infrastructure (GI) networks. The designs were assessed against four goals for GI provision: 1. Ecosystem Services; 2. Ecological Status; 3. Ecological Connectivity; 4. Proximity to the Population. Each of these goals was assessed using existing approaches which utilised readily available datasets to allow for widespread application of the methodology.
It was found that the differences in impermeable surface areas as a result of different built form designs influenced peak and total runoff volumes from a storm event, both with and without the inclusion of SuDS, although to what extent was dependent upon the SuDS infrastructure(s) employed and their overall implementation. Notably, in some urban designs, a lower proportional implementation of a SuDS infrastructure at a higher development density saw greater reductions in peak and total runoff volumes than a higher proportional implementation at a lower development density. These proportions were of available surface type for SuDS (e.g. roof area for green roofs). More dense urban configurations provide greater potential surface area for their construction. The spatial arrangement of these built form elements, however, also proved an important consideration due to the spatial variation of external landscape characteristics (such as soil type and slope) which also impact runoff dynamics.
The spatial arrangement of these built form elements, however, also proved an important consideration due to the spatial variation of external landscape characteristics (such as soil type and slope) which also impact runoff dynamics. In such a way, the developed approach proves particularly useful, as by combining a tile approach for designing urban developments with rainfall-runoff modelling, the methodology allowed for these landscape and built form elements to be readily varied and scrutinised at both local and regional scales.
Investigation of pipe requirements found that for all housing typologies the use of SuDS could reduce the minimum required pipe diameter, although not consistently for all SuDS designs. Different spatial development approaches also resulted in different required pipe network lengths. Given that current guidelines permit high cost as a justification for not constructing SuDS in developments, such findings suggest that financial savings could be found elsewhere with a well-designed SuDS system.
When considering co-benefit provision, the inclusion of SuDS consistently saw greater GI provision scores, although it is worth noting that urban spaces presented opportunities for GI provision even without. When considering individual GI elements, this is particularly clear. For ecosystem services, very few SuDS designs were able to score higher than the pre-developed state, and these occurred only where existing land cover was poorly-scoring.
Once again, the specific SuDS infrastructure(s) employed played a strong role in determining which co-benefits were provided, and to what extent. By their nature, infrastructure-based SUDS, for example, require less free space in a development, and as such can help minimise loss of undeveloped land in an urban area (or provide more room for compact development to help reduce overall sprawl). Whilst this indicates that SuDS choice is an important component in achieving the specific aims of a development project, interactions between different SuDS infrastructures and/or elements of a development design highlights the need for trade-offs to be understood and adequately balanced in resultant designs.
From this research, four key considerations for urban planners arise when designing new development involving SuDS infrastructure. First, the location and layout of the development; second, the choice of housing typology; third, the comparison of multiple SuDS infrastructure and combinations; fourth, the opportunities posed by urban space in providing GI (even without SuDS).