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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
Location of Caw Burn SUDS
SUDS
SEPA routine water chemistry monitoring site
SEPA routine biological monitoring site
Houston Industrial Estate
Head waters of Caw Burn emerge from culvert
To River Almond
Edinburgh 20 km
Acknowledgement: Digimap
1 km
N
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
Caw Burn SUDS design
Settlement pond891 m2 area; 600 mm max depthMin. retention time 24 minutes
Coir booms
Overland flow zone4060 m2 area; 800 mm max depthMin. retention time 12 minutes
Gabion baffle wall (crushed rocks/concrete kerbstones
Caw Burn SUDS design: inlet
5 x 250 mm diameter pipes divert 85-425 l s-1
Caw Burn SUDS design: outlet
Caw Burn main
channel
Outlet swale from SUDS
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
Comparison of Caw Burn SUDS design with CIRIA guidelines
Storage volume(m3)
Retention time(days)
Caw Burnsettlement pond
610 0.017
CIRIA guidelines forretention ponds
34536 14-21
Caw Burnsettlement pond as% of guidelines
1.8 0.1
Caw Burn wetland 3248 0.008
CIRIA guidelines forwetlands
25902 14
Caw Burn wetlandas % of guidelines
12.5 0.06
Dye tracer tests to determine current residence times
10 minutes 15 minutes
50 minutes
Current water retention times
Retention times Settlementpond
Swale
Travel time of dye frominlet (mins)
96 99
Mean water retentiontime (mins)
295 363
Design retention time athigh flows (mins)
24 36
CIRIA guidelines (days) 14-21 14
Overflow from settlement pond
Flow channelised in swale
Preferential flow paths and overflows
Short-circuiting from wetland into Caw Burn
Cawburn SUDS: water chemistry performance
• Meets conditions of discharge consent
• Mean water chemistry at SUDS outlet mainly class A2/B (Good/Fair)
Improvement in BOD in Caw Burn SUDS
0
10
20
30
40
50
60
70
80
90
Mar-97 Mar-98 Mar-99 Mar-00 Mar-01 Mar-02
BO
D (
mg
l-1
)
InletOutletClass B maximum valueClass C maximum value
Class B
Class C
Class D
Improvement in biological quality of Caw Burn downstream of SUDS
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Mar-87 Mar-89 Mar-91 Mar-93 Mar-95 Mar-97 Mar-99 Mar-01 Mar-03
Ave
rag
e sc
ore
per
tax
on
MeasuredClass B minimum valueClass C minimum value
SUDS constructed
Class B
Class C
Class D
Sedimentation and vegetation growth
July 1998 May 2004
May 1998Jan 2004
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
Remedial work to the Caw Burn SUDS
Option Costs:benefits Risks
i. Repair gaps in earth bank low:low Low
ii. Block preferential flow pathways
low:low Low
iii. Remove sediment from settlement pond
low/moderate:low Moderate
iv. Reduce vegetation in overland flow zone
low/moderate:low Moderate
v. Increase bank height around SUDS
moderate:low Low
vi. Flood valley floor high:moderate/high High
vii. Offline SUDS on other side of Caw Burn
high:high Moderate
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
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
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