SUDS-based approaches are increasingly being relied upon for stormwater management in new developments. However, there is very little indication of the extent to which they represent a viable rehabilitation option for existing catchments that experience flooding and/or pollution problems arising from ineffective stormwater management. Excess discharges from combined sewer overflows (CSOs) or flooding associated with the rapid conveyance of stormwater into urban rivers might both be feasibly addressed by transferring the stormwater from the piped system into a SUDS system. This disconnection-based approach is referred to as retrofit SUDS.
This briefing sheet brings together evidence from UK implementation and desk-based case studies that illustrate the potential applications of retrofit SUDS.
This study was undertaken by Leeds City Council for Yorkshire Water, with collaborative input from the University of Sheffield. Further details may be found here.
Meanwood is situated 4 km to the north west of Leeds city centre, has a catchment area of 55.8 ha and is served by a combined sewer system. During extreme storm events (approximately 1 year return period) the trunk sewer surcharges and produces flooding (at the location shown in Figure 1a). The catchment largely comprises 20th Century housing, retail premises and a small number of institutional buildings (schools, library and churches). None of these institutional buildings is connected to Meanwood's combined sewerage system. Much of the housing is semi-detached. Figure 1 b) shows a typical housing and street layout, which was judged to be typical of many areas in the UK's older cities. The catchment contains significant amounts of grassed and wooded areas, with many of the residential areas including grassed verges that were judged suitable for the retrofitting of swales.
|(a) Location of flooding||(b) Typical street|
Figure 1 The Meanwood Catchment
The use of infiltration devices was considered to be viable within the central sections of the Meanwood catchment, since much of this region is underlain by sandstone deposits. However, the soil conditions of the remainder of the catchment (clay and mudstone) are less suitable for infiltration. The Meanwood catchment is not part of a Groundwater Protection Zone.
The proposed retrofit SUDS solution incorporates infiltration-based schemes (at-source and off-site) applied to residential roofs, some storage associated with residential roofs and some offsite controls associated with road runoff. In total 46% of the roofed area and 31% of the paved area was proposed for disconnection to infiltration-based SUDS, with scope to disconnect the remaining roofed areas into storage-based SUDS.
The effectiveness of the SUDS proposals has been evaluated in terms of both hydraulic performance and construction cost. Hydraulic performance was modelled using an adapted HydroWorks model of the catchment. If implemented it is anticipated that this scenario would result in a 68% reduction in the ten year design storm flood volume. The SUDS proposal would need to be coupled with a reduced level of conventional sewer rehabilitation in order to entirely eliminate the flooding problem.
Costs for a range of conventional and hybrid rehabilitation options suggested that the hybrid options were cheaper than each of the three conventional proposals. The most costly hybrid proposal was 11.8% cheaper, and the least expensive hybrid proposal was 23.5% cheaper, than the cheapest conventional solution.
In a second Leeds catchment it was found that a SUDS-based solution could provide the necessary hydraulic control (in terms of reduced spill volumes) at 50% of the construction cost of a conventional storage-based solution. In order to achieve a comparable reduction in the number of spills, however, a SUDS-based scheme would cost 23% more than a conventional storage-based solution.
Ashley et al., 2000, desk study - Forfar: In a study of the options for alleviating inner-city sewer flooding, and reviewing urban expansion needs, it was found that a mixed SUDS/conventional solution would be more cost-effective and less disruptive during construction, than a simple conventional approach (Ashley, R M, Blackwood, D J, McKissock, K, and Wotherspoon D J, 2000. Cost Effective Source Control In Urban Stormwater Drainage, Hydro International Conference, Oxford. May).