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SUDS Retrofits in Scotland

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Presentation on theme: "SUDS Retrofits in Scotland"— Presentation transcript:

1 SUDS Retrofits in Scotland
Barbara Barbarito Environmental Quality Advisor Scottish Water

2 Summary SUDS retrofits in Dunfermline
SUDS retrofits for Bathing Waters improvements Caw Burn wetland

3 Retrofitting Sustainable Urban Drainage Systems- Dunfermline Case Study
Scottish Executive Scottish Water SEPA Hyder Consulting

4 Desk based study investigating Dunfermline and Iron Mill Bay
Lyne Burn Tower Burn Calais Burn Dunfemline Town & Iron Mill Bay Iron Mill Bay Dunfermline was chosen as the study area because of the amount of data on the sewerage system already gathered and the poor quality of three burns (Lyne, Calais and Tower) in the past. These were addressed by the construction of storm tanks and upgrading of the sewerage system but the water quality is not yet as good as it could Dunfermline and the surrounding area is divided into two catchment: Dunfermline-including main city itself and outlying areas to east and south, (60% combined) Iron Mill catchment – north west of the city and villages to the north and west (55% combined)

5 Objectives Identify and rank in order of importance areas contributing to the flow in the sewer system Establish runoff generated by the subcatchment targeted for investigation Identify areas where SUDS could be retrofitted Assess costs and benefits of SUDS vs conventional approaches Quantify water quality improvements that can be provided by introducing SUDS The project looked at both catchments but looked at Iron mill bay as the SUDS area for SUDS implementation.

6 Results Road and roof area contributes the highest percentage of runoff to the combined system. Suds retrofits in isolation will not solve problems with the sewerage system-need strategic approach.

7 SUDS Retrofit Research Project – Ayrshire study
Scottish Executive Scottish water SEPA WS Atkins

8 Aim Study area: Ayrshire
To pilot SUDS techniques to minimise CSO spills impacting onto identified bathing waters by limiting the entry of surface water into the combined sewer system. Study area: Ayrshire

9 Objectives Identify sites suitable for retrofitting SUDS facilities
Design and construct SUD schemes Undertake pre and post construction monitoring Compare the cost and benefit of the constructed scheme to conventional alternatives Develop methodologies for the adoption of SUDS retrofitting, to assist the incorporation of retrofitting into Water Industry capital investment programmes

10 Method Phase 1 – Site identification, feasibility and selection of sites for SUDS implementation Identification of sites suitable for SUDS retrofitting was undertaken using Hydraulic Models of catchment areas within the study envelope. In summary the methodology involved thematically mapping impervious areas which were modelled as being connected to the foul/combined sewer. “Hot spots” of impervious area could then be highlighted, in which retrofitting of SUDS could be undertaken to reduce flow contribution from runoff. Identified areas could then be mapped against aerial photographs to pinpoint potential areas suitable to the installation of SUDS devices, and their proximity to downstream CSOs (fig 2, 3). Once a short list as drawn up including all those areas meeting the identified criteria, site investigation would take place.

11 Method Phase 2 – Detailed design, construction, monitoring, cost comparison, reporting Irvine industrial estate - combination of conventional drainage network and feature pond Girvan residential area - filter drains to intercept land and roads drainage, with underground infiltration device. A number of sites had to be dropped after site investigation. An industrial site in Irvine was selected for detail design. The site was selected due to its proximity to bathing waters. Additionally, a pumping station emergency overflow was situated immediately downstream of the site, therefore it was anticipated that some benefits to system performance and potentially bathing water quality could be achieved by retrofitting a SUD scheme into the site. The proposed scheme consisted of a conventional drainage network conveying intercepted storm runoff to a feature pond system via silt traps and petrol and oil interceptors. The second site progressed to detailed design was situated in Girvan, where Scottish Water operations had advised the project team that a separate storm system was connected to the foul network downstream of a development site. The storm sewer was also known to have illegal connections from roads and land drainage, and had been overloaded in the past causing property flooding. The proposed SUDS solution intercepted land drainage and severed the connection between the foul and storm sewerage systems. No construction took place

12 Results Retrofitting in isolated areas across a given catchment is unlikely to significantly reduce the frequency and volume of sewage discharges to receiving waters SUDS may be retrofitted for similar costs to those estimated for conventional solutions. Retrofit SUDS can offer longer term benefits in comparison to conventional practice, through reduced maintenance and operational expenditure. It was not possible to quantify the benefits which can be achieved by undertaking retrofitting on the basis of pre and post construction field measurement because no schemes were taken forward to construction. However, modelling results indicate that retrofitting in isolated areas across a given catchment area is unlikely to achieve flow reductions in the sewer of sufficient magnitude to significantly reduce the frequency and volume of sewage discharges to receiving waters, and is therefore unlikely to result in any appreciable improvement to receiving water quality. The benefits of SUDS retrofitting in isolated properties across a given catchment area are therefore related to improving the overall performance of the sewerage network; reducing pumping and treatment costs, minimising flood risk, and providing additional capacity for future development. In addition, the normal benefits of SUDS can be realised, such as provision of amenity and potential educational resource, water re-use and enhancement of habitat. Depending on the number of sites retrofitted in a particular catchment, water quality improvements may be realised over time. Further research would be required in order to quantify the benefits which can be expected by retrofitting SUD systems on this basis. What has been seen from the work conducted is that SUDS may be retrofitted for similar costs to those estimated for conventional solutions. Where the capital costs associated with SUDS have been estimated as being more expensive than conventional solutions, it has been demonstrated that the SUDS solution has offered benefits in addition to the potential for water quality improvement, which have not been provided by conventional engineering options. These benefits include reduction in operational costs and enhanced flood protection in localised drainage networks, where new conveyance systems have been required in SUDS installation Overall, comparison of estimated cost for maintenance of SUDS and conventional options suggests that retrofit SUDS can offer longer term benefits in comparison to conventional practice, through reduced maintenance and operational expenditure. To give confidence to the Water Industry that retrofitting offers a viable cost benefit ratio, improvements in water quality improvements must be demonstrated. The economic value of water quality improvements must also be determined, in order to undertake proper cost benefit analyses. Most importantly, a wider data set must be examined in order to draw reliable conclusions from cost analyses.

13 Caw Burn Wetland & Catchment Improvements
(Stage 1) SEPA Scottish Water University of Edinburgh (Dr Kate Heal & Dr Miklas Scholtz) Stirling University (Dr Nigel Willby)

14 Caw Burn Wetland Constructed in 1996 to treat diffuse pollution entering the Caw Burn (West Lothian) from the Houston Industrial Estate, Livingston Overflow to SUDS The wetland was constructed by East of Scotland Water in 1996 to treat diffuse pollution entering the catchment from the Houston industrial estate and residential areas to the Caw Burn. The industrial estate is served by a separate sewerage system, therefore the majority of the pollution with foul flows directed to WwTW and surface water flows directed to the Caw Burn. The Majority of the pollution thus derives from surface water runoff from the impervious loading areas, car parks and highways. The industrial estate contains an iron works, a number of chemical and food manufacturers and therefore is served by a large number of heavy vehicles This contains high concentrations of oils, detergents, BOD, and ammoniacal nitrogen. In addition the occasional presence of cross connections of wastewater to the surface water system meant some sewerage contamination as well. Industrial estate surveys have been carried out on a number of occasions to identify the source of these cross connections. This pollution caused the Caw Burn to be classified by SEPA as a "Seriously Polluted " (Class D) watercourse and the SUDs System was design to address the Caw Burn water Quality problems and upgrade it to, at least, class C ("poor") could you put a circle on plant to show location of pond, map doesn’t show whole catchment

15 SUD system The system includes a main settlement pond and a vegetated overland flow zone (wetland) The flow enters the pond through a culvert (A= 891 m2 d=60 mm) The system includes a main settlement pond and a vegetated overland flow zone. Surface water runoff is drained through a large culvert. The pond, designed to trap sediments, is entered through 5 pipes the pond has an average retention time of 58 min at an inflow of 170 l/s and 24 minutes at an inflow of 425 l/s. From the main settlement pond the flow passes to the wetland through a gabion baffle wall designed to trap hydrocarbons. The wetland was designed to fine particulates and associate metals and hydrocarbons by filtration through the plant litter layer and remove BOD through biological filtration. It was created by constructing a heath bund to raise water level in an existing area of impeded drainage. The only constraints on the system and the size of the pond and wetland was determined by land availability. However it was designed to take the first flush of a rainfall event and the pond was sized to reduce water velocity and give adequate settlement to the flow. From the overland flow the water is directed to rejoin the Caw Burn channel through a planted swale and an outlet ditch. No maintenance was recorded apart from remedial work after an oil spillage. ARE culvert figures on the slide correct? Are these the best pictures to show? Is a swale included? Is then directed to the wetland (A= 4060 m2 d=800 mm)

16 Benefits Effective in removing sediments, oil, detergents and other pollutants Increase in diversity of aquatic life in 2Km of Caw Burn downstream Caw Burn class improved from River Quality Class D (seriously polluted) to C (poor) Benefits: the SUDS have worked reasonably well decreasing pollutants concentration from the Ind Estate. Discharge is meeting the consent conditions set by SEPA. Although the macro invertebrate fauna was found to be poor in recent surveying, probably due to high pollutant loadings, vegetation in the wetland appeared to be healthy and supported a variety of sauna such as tadpoles water snails, dragonflies etc..). There also appears to be no need for management or alteration of the vegetation.

17 System is considerably undersized
Problems System is considerably undersized Comparison of design treatment volume and retention time of the different components of the Caw Burn SUDS with the CIRIA manual guidelines (from; Heal K and Sholz M, Caw Burn wetland and Catchment Improvements Stage 1) Storage volume (m3) Retention times (days) All Caw Burn SUDS 3858 0.025 Average CIRIA guidelines for detention basins/wetlands 30219 14 All Caw Burn SUDS as % of CIRIA guidelines 12.8 0.2 Neither the pond or the wetland meet the guidelines set in the CIRIA manual (although this was not there at the time). The size and residence time are much smaller that what is needed to treat the flows and pollutant concentration to a suitable standard. The estimated treatment volume for the headwater considerably exceeds the design volume for both the pond and the wetland.

18 Problems ..cont 25% of pond volume has been in filled by sediments since construction = reduction in water retention time Preferential flow and short circuiting Evidence of increased erosion over time Algae also appear to increase Within the SUDS preferential flow and short circuiting of the SUDS and Caw Burn is occuring even at low flow

19 Proposed options Likely to raise water quality to class B
Flooding of entire valley floor Construction of additional offline SUDS Repair gaps in hearth bank where short circuits occur Block preferential flow pathways Remove sediments from settlement pond Flooding of entire valley floor: increase storage volume, increase retention time but high cost lack of control over Caw Burn Channel (online suds), loss of trees in flooded areas, initial increase in BOD output due to rotting vegetation. Construction of additional offline SUDS on opposite side of Caw Burn: increase storage volume, increase retention time but high cost, require major hearthworks to excavate additional SUDS, loss of trees in new SUDS area Repair gaps in hearth bank around the overland flow where short circuits occur low cost, increase storage volume but temporary solution requires frequent maintenance Block preferential flow pathways within the overland flow zone low cost temporary solution, requires frequent maintenance without increasing storage volume Remove sediments from settlement pond to increase storage volume low cost but risk of contaminant release from sediments, need to disposed of sediments, requires frequent maintenance will not increase storage volume significantly

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