SuDS The business case. Steve Wilson

Transcription

1 SuDS The business case Steve Wilson

2 Introduction Principles of design for cost effective SuDS Evidence that capital costs of SuDS with source control are usually lower than piped systems designed using traditional approach to attenuation Cost of providing treatment Maintenance costs a grey area we ll leave that for another day!

3 Quick straw poll Who thinks capital cost of SuDS is greater than normal piped attenuation system? Who thinks the opposite?

4 Efficient drainage design SuDS and Source control Total storage volume = 1000m 3 Treatment comes free within the system (eg permeable paving)

5 Inefficient drainage design Treatment is extra cost rather than included in source control Moving large volumes of water from source to storage larger pipes = extra cost Treatment costs extra (proprietary treatment Same storage but deeper extra cost system) Total storage volume = 1000m 3

6 Cost of a basin Excavation = 2900 Disposal of spoil (if needed most times it gets re-used) = 0 if re-used Topsoil/seeding/etc = 6000 Total for Basin = 8900

7 Is cheaper to put a plastic crate in the hole and backfill it? Additional excavation - protective cover over tank = 4100 Tank = 20,000 Backfill and compact= 9000 Proprietary device for treatment = 10,000 Total additional cost = 43,100

8 Costs study by Defra SuDS cost Traditional drainage cost Marlborough Road, Telford 966,119 1,074,528 Matchborough School, Redditch, Worcestershire 93, ,700 Redhill School, Worcester 51, ,700 Daniel s Cross, Newport 780, ,052 Caledonian Road, Islington 22,700 75,100 Railfreight Terminal, Telford 51, ,259

9 Latest study by DEFRA Three sizes of housing development Small, medium, large All designed with SuDS to meet original version of draft National SuDS Standards Comparison with designs providing only attenuation and in accordance with B Regs and SfA7

10 Principle conclusions Difference in costs is scale related greater benefits in using SuDS on larger sites (larger site also master planned for SuDS) The designs are based on one possible solution there may be others (this could reduce costs of SuDS even more) SuDS have very great advantages on permeable sites where infiltration can be used Study based on sites in SE England in other hydrological areas SuDS will have greater advantage (especially SW and NE) Italics are my additions to the published conclusions

11 Principle conclusions Where SuDS have to be lined to prevent any infiltration of water into the ground SuDS are more expensive on small and medium sites (additional components required for interception) master planning could probably design this out Infiltration trenches and rain gardens are the least cost approach to providing interception if permeable paving is lined The use of DBM as a construction layer in permeable paving can significantly reduce cost advantage

12 Principle conclusions There is a significant cost advantage using SuDS for flat sites but on sloping sites SuDS could be less cost effective (this could easily be designed out) Masterplanning SuDS clearly maximises cost effectiveness (based on Upton) The study found that the approach of the design team in developing a development layout and the SuDS strategy will have a large impact on the capital costs and only small changes in both will have a large impact on the cost and affordability of the SuDS scheme

13 How to make SuDS expensive No multifunctionality extra space needed for SuDS Lack of master planning Avoid source control Ask for excessive commuted sums for maintenance Expensive details (eg put a bioretention system in a large concrete tank)

14 So how do we achieve cost effective SuDS? Throw away the old rule book Ditch the drainage modelling software Put brain into gear and think Masterplan the development for SuDS as much as possible Keep things shallow Multifunctional spaces Use permeable surfaces and other source control Look for opportunities

15 Distribution Centre Amenity nature trail & swale Wildlife wetland basin Ornate balancing pond Shallow swale system Treatment comes free within the system Kerbdrain soilt/oil treatment channels outfall to swale system

16 Look for the opportunities sports pitches on a roof designed as part of SuDS

17 Another school Traditional drainage 116,700 SuDS 93,000 Treatment comes free within the system

18 KPI have I changed your view? Who thinks capital cost of SuDS is greater than normal piped attenuation system? Who thinks the opposite?

19 Summary Capital costs of well designed SuDS should usually be lower than traditional solutions Master planning helps reduce costs Do not discount engineered or proprietary solutions depends on space Where you have the space landscaped surface SuDS are a no brainer