An ideal dataset would have included actual out-turn construction costs for the full suite of sustainable drainage systems, for a range of sizes, covering a range of conditions / criteria. This would have provided both the mean and the bounds for cost estimates to be included in the study and would have contributed towards robust whole life cost estimates. An alternative would have been that full cost breakdowns be available, so that an assessment could have been made of the variability in contribution of different cost components, and their impact on overall construction costs.
An extensive consultation exercise was undertaken in an attempt to secure actual construction costs for sustainable drainage schemes across the UK. Significant difficulties in collating such information were encountered. These could mainly be attributed to the following issues:
1. Construction of sustainable drainage components providing open, surface storage tends to be undertaken as part of the post-construction, landscaping activities. Thus the costs specifically associated with the drainage are hidden within large, lump-sum, landscaping fees and cannot be extracted as a separate item.
2. Construction of sustainable drainage components providing covered, subsurface storage (e.g. pervious pavements) tends to be undertaken as part of the highway infrastructure development and again, costs tend to be hidden within larger bill items.
3. There is currently no system through which capital costs are recorded or presented in a consistent way.
4. Projects, of which sustainable drainage infrastructure form a part, are generally costed by building contractors and then bid for using a lump-sum, fixed price. A full breakdown of costs for drainage components is unlikely to be included as part of a tender. Final costs may exceed or fall below initial estimates.
Costs (£/unit)
5. The final costs of the sustainable drainage infrastructure are likely to be very dependent on whether appropriate equipment and labour is available on the site at the time. If specialist teams have to be engaged, then costs could rise rapidly.
6. Confidentiality / competitive pricing issues.
It was anticipated at project definition stage, that the Project Steering Group would be responsible for supplying the main bulk of this data, however little information was forthcoming (primarily for the reasons given above). An external consultation exercise was, therefore, also undertaken.
The schemes from which costs were collated are summarised in Table 6.3.
Table 6.3 Summary of SUD scheme construction costs
SUD ID Region Treatment
Volume £ £/m3 Description
1 SE
England 2,022m3 32,000 16
Residential site.
A series of 3 reed beds. Submerged pipes and grass spillways connect the reed-beds. Price includes reeds and inlet and outlet structures.
2 Midlands 3,487m3 125,000 36
Mixed residential, industrial, commercial and highway. Total construction cost.
Wetland
3 SE
England 1,680 m3 45,000 27 Residential site.
A series of 3 reed beds. Weir overflows connect the reed beds.
1 SE
England 5,625 m3 96,567 17
Residential site.
The site is served by a series of interconnected swales and ponds.
This is the largest pond and maintained as formal landscape feature. There is a large concrete weir control structure on the outlet. The pond has amenity and attenuation functions.
2 SE
England 3,000 m3 24,719 8
3 SE
England 5,362 m3 67,979 13
4 SE
England 393 m3 7,209 18
5 SE
England 1,162 m3 19,157 16
6 SE
England 1,320 m3 36,255 27
7 SE
England 1,725 m3 15,820 9
8 SE
England 925 m3 6,633 7
9 SE
England 2,595 m3 14,406 6
Residential site.
The site is served by a series of interconnected swales and 8 natural ponds. The ponds’ main function is attenuation but they also have emergent wetland vegetation.
10 NW
England 4,531 m3 256,331 57
Public site.
There is a reed bed and stream as well as an open pond. The pond has formal landscape features including a
fountain and timber edging. The pond has attenuation, amenity and water quality functions.
Retention Pond
11 SE
England 13,000 m3 71,000 6
Residential site.
The storm water is pre-treated by a series of reed-beds. The main function of the pond is attenuation.
Table 6.3 Summary of SUD scheme construction costs (continued)
SUD ID Region Size £ £/unit Description
Infiltration
Basin 1 SE
England 40,540 m3 578,387 14
Residential site:
The storm water is conveyed through a series of filter drains, a petrol interceptor and a hydro-brake before discharging to the basin. The basin is drained by infiltration with imported gravels to improve soil permeability.
1 SW
England 1,500 m2 52,215 35
Retail carpark.
The ground works mentioned in the costing would only cover the installation of the Sub-base material including rolling but it does not cover the cost of excavation and
preparation.
2 Midlands 5,188 m2 75,890 15
Public site.
The price includes excavation and installation. The system is in a Park and Ride car park and is just compacted gravel, not permeable blocks. The system is drained by infiltration.
3 Midlands 6,572 m2 233,700 36
Retail carpark.
The roads are asphalt, and drain the permeable carpark spaces. The system is lined and discharges to a river by a 225mm pipe. The cost includes the total carpark costs.
Permeable Pavement
4 SW
England 20,495 m2 718,036 35
Retail carpark.
Some areas are asphalt, and drain via a bypass interceptor to the permeable areas. The system is lined and discharges to a river by a 110mm pipe. The cost includes all the drainage costs.
1 Midlands 115 m3 6700 58
2 Midlands 216 m3 12,594 58
Infiltration Trench
3 Midlands 60 m3 3412 58
1 Midlands 5.9 m3 544 92
Soakaways 2 Midlands 1.15 m3 144 125
Public carpark.
A council park and ride served by soakaways, infiltration trenches and a permeable carpark.
1 Midlands 170 m3 14,895 88
2 Midlands 33 m3 3,840 116
Filter Drain
3 Midlands 55 m3 8,685 158
Residential site:
The storm water is conveyed through a series of filter drains, a petrol interceptor and a hydro-brake before discharging to an infiltration basin.
Table 6.3 Summary of SUD scheme construction costs (continued)
SUD ID Region Surface
Area £ £/m2 Description
1 SE
England 1,120 m2 13,440 12
2 SE
England 975 m2 11,700 12
Swale
3 SE
England 1,020 m2 12,240 12
Residential site:
The site is served by a series of interconnected swales and ponds. The swales are not typical. They are relatively deep and wide with a constant base flow. The main function is attenuation and conveyance. The channels are filled with emergent wetland vegetation.