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