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Whole Life Costs And Benefits Of Sustainable Urban Drainage Systems In Dunfermline, Scotland

Congress: 2015
Author(s): Kate Heal, Daniel Wolf, Alison Duffy
University of Edinburgh, School of GeoSciences1, Urban Water Technology Centre, Abertay University2

Keyword(s): Sub-theme 14: Valuing water: monetary and non-monetary dimensions,
Abstract

Introduction Flooding, impaired water quality, and biodiversity loss are common consequences of traditional urban stormwater management practices (Marlow et al., 2013). Sustainable Urban Drainage Systems (SUDS) are increasing in popularity as a solution to many of these problems. Also referred to as Best Management Practices (BMPs), Low-Impact Development (LID), Stormwater Control Measures (SCM), or Water-Sensitive Urban Design (WSUD), the approach seeks to mimic natural drainage regimes using source control, permeable paving, stormwater detention and infiltration, and evapotranspiration (e.g. green roofs) in order to mitigate flooding, improve water quality, and augment the value of recreational amenities and other ecosystem services.

SUDS offer different costs and benefits than traditional drainage systems. Questions remain regarding the values of maintenance costs and environmental benefits delivered throughout the useful lives of SUDS. Sources of recommendations abound for maintenance activities and frequencies at which they should be carried out (CIRIA, 2007; Erickson et al., 2013), but there is a lack of accurate information on the operating and maintenance costs of SUDS. Perception among contractors, planners, and engineers about maintenance costs present a barrier to the implementation of SUDS, as well as uncertainty as to the multiple benefits they can provide (Moore and Hunt, 2012). Using the Dunfermline East Expansion area in Scotland as a case study, this study represents a novel approach in that it collects actual maintenance data, assesses ecosystem services, and carries out whole life cost analyses.

Methods Dunfermline Eastern Expansion (DEX) is a 5.9 km2 development comprised of residential, retail, industrial, and public recreation land uses located in Fife, central Scotland. To prevent downstream flooding and meet water quality targets, a variety of SUDS features were planned, including ponds, swales, wetlands, and permeable paving. Construction began in 1996 with completion expected by 2020. In comparison to many SUDS features elsewhere, the maturity of those in DEX make the site ideal for studying long term maintenance activities and ecosystem service provision.

Fifteen SUDS - five each of ponds, basins, and swale - were chosen for analysis to represent features of varying age, size, and maintenance quality. SUDS maintenance data were collected from the lead developer, maintenance organisations, field observations and residents to input into WLC methodologies. Three ecosystem services (ES) provided by each SUDS were evaluated: water quality, hazard management (flooding), and amenity. These particular ES were chosen for assessment based on author consultation with users and other stakeholders interested in quantifying ES benefits. Whole life costs for each SUDS were calculated using two methodologies: Water Environment Research Foundation (WERF) - BMP and LID WLC models (2009) and SUDS for Roads -- Whole Life Cost and Whole Life Carbon Tool (Scottish SUDS Working Party, 2009). The same construction costs were used in both WLC tools but site specific information on maintenance costs could only be incorporated into the WERF tool. The results of all monetary calculations are expressed as pounds sterling (£) valued at the times costs were incurred.

Results and Discussion The two WLC methodologies used produced different results that varied by £37,086 ± £27,810 (mean ± 1 standard deviation, n = 15). Because the same construction cost values were used in both methodologies, the differences in WLC cost estimates between the two models is likely due to the assumptions about maintenance cost and activities that are built into the SUDS for Roads tool and are not changeable by users. Of the three SUDS features studied, ponds had the highest WLC and detention basins the lowest WLC. The main reason for the differences between SUDS features is the variation in construction cost since maintenance activities varied little between SUDS features.

Neither of the two WLC methodologies used in this study accounted for ES provided by SUDS. To evaluate the impact of ES on net present value (NPV), WLC was subtracted from total ES values. Including ES did not completely offset the WLC for most SUDS sites, but reduced WLC by about a third for both WLC methodologies at 35 ± 37% and 33 ± 39% (mean ± 1 standard deviation, n = 15) for the WERF and SUDS for Roads methodologies, respectively. Two SUDS features yielded a positive NPV, meaning the value of the ES it provides are greater than that of its WLC. Both of these features lie within close proximity to a number of residences, which resulted in high amenity values.

Conclusion Findings suggest that SUDS may be accounted as a net asset, although assumptions inherent to ecosystem services assessment methodologies produce a wide range of uncertainty. Furthermore, evidence suggests SUDS provide a variety of ES beyond those assessed in this study, but so far no robust methodologies exist for investigating them. 1. CIRIA (2007) The SUDS Manual, London, UK.
2. Erickson, A.J., Weiss, P.T., and Gulliver, J.S. (2013) Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance. New York, NY, Springer Science+Business Media.
3. Marlow, D.R., Moglia, M., Cook, S., and Beale, D.J. (2013) Towards sustainable urban water management: A critical reassessment. Wat. Res., 47(20), 7150--7161.
4. Moore, T.L.C. and Hunt, W.F. (2012) Ecosystem service provision by stormwater wetlands and ponds -- a means for evaluation? Wat. Res., 46(20), 6811--6823.
5. Scottish SUDS Working Party (2009) SUDS for Roads Guidance Manual, Edinburgh, UK, SUDS Working Party.
6. Water Environment Research Foundation (2009) User's Guide to the BMP and LID Whole Life Cost Models Version 2.0, Alexandria, VA.

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