The Caw Burn SUDS: performance of a settlement pond/wetland SUDS retrofit

1. The Caw Burn SUDS: performance of a settlement pond/wetland SUDS retrofit Kate Heal & Miklas Scholz University of Edinburgh Nigel Willby, University of Stirling Bess Homer, Scottish Water

2. N 1 km Location of Caw Burn SUDS SEPA routine water chemistry monitoring site To River Almond Edinburgh 20 km SEPA routine biological monitoring site Houston Industrial Estate SUDS Head waters of Caw Burn emerge from culvert Acknowledgement: Digimap

3. History of Caw Burn SUDS • Early 1960s: work starts on Houston Industrial Estate • Caw Burn headwaters culverted • Diffuse urban pollution: oils, detergents, BOD, NH4-N • Caw Burn = Class D river (Seriously Polluted) • 1996: partnership agreement (Lothian Regional Council, East of Scotland Water, SEPA): • Caw Burn SUDS designed and constructed at cost of £50,000 • Sized to maximise land available • Discharge consent to East of Scotland Water • 1997: SUDS operational • Minimal maintenance

4. Caw Burn SUDS design Overland flow zone 4060 m2 area; 800 mm max depth Min. retention time 12 minutes Coir booms Settlement pond 891 m2 area; 600 mm max depth Min. retention time 24 minutes Gabion baffle wall (crushed rocks/concrete kerbstones

5. Caw Burn SUDS design: inlet 5 x 250 mm diameter pipes divert 85-425 l s-1

6. Caw Burn SUDS design: outlet Caw Burn main channel Outlet swale from SUDS

7. Caw Burn Wetland & Catchment Improvements Project 2004 • Stage 1: identify and cost structural & maintenance improvements in the Caw Burn SUDS likely to result in upgrading of water quality in Caw Burn to at least class B • Specific objectives: • Comment on original design and current situation in relation to good practice guidelines, including CIRIA manual C521 • Assess nature and costs of maintenance needs and structural changes to wetland • Cost/benefit ratios for any suggested improvements to the wetland

8. Comparison of Caw Burn SUDS design with CIRIA guidelines

9. 10 minutes 15 minutes 50 minutes Dye tracer tests to determine current residence times

10. Current water retention times

11. Overflow from settlement pond Flow channelised in swale Short-circuiting from wetland into Caw Burn Preferential flow paths and overflows

12. Cawburn SUDS: water chemistry performance • Meets conditions of discharge consent • Mean water chemistry at SUDS outlet mainly class A2/B (Good/Fair)

13. Improvement in BOD in Caw Burn SUDS Class D Class C Class B

14. SUDS constructed Improvement in biological quality of Caw Burn downstream of SUDS Class B ClassC Class D

15. May 2004 July 1998 May 1998 Jan 2004 Sedimentation and vegetation growth

16. Sedimentation and sediment quality • Sedimentation rate ~ 2 cm year-1 • 25% of settlement pond volume infilled with sediment since construction • SEPA persistent pollutants urban rivers survey (2002): • PAH ratios indicate pollution source is oil spillages • total hydrocarbons highest in survey (3382 mg kg-1 dry weight) • sediment classified as Special Waste if excavated

17. Remedial work to the Caw Burn SUDS

18. SUDS extension Use rest of available land on valley floor • New SUDS flooded to 1.5 m depth: • Storage volume: 45 → 71 % of CIRIA guidelines • Retention time: 3.1 → 5.0 % of CIRIA guidelines

19. Conclusions • Caw Burn case-study highlights advantages & disadvantages of retrofit SUDS: • undersized (land area, cost, ownership (?) constraints) • significant benefits (water chemistry, improved diversity) • Pragmatic, cost-effective measure • Retrofit SUDS only one tool for addressing diffuse urban pollution • Phase 2 of Caw Burn project: • reduce contaminant load to Caw Burn • Scottish Water has identified need for further investment