The process for undertaking a life cycle costing analysis for swales is the same as that described in Life Cycle Costing - Constructed Wetlands.
The origin of all of the ‘expected’ values and algorithms in MUSIC’s costing module, as well as the statistical operations used to generate ‘upper’ and ‘lower’ estimates for vegetated swales are explained in Table 1.
Note that the CRC for Catchment Hydrology’s survey used to gather real costing information for stormwater treatment measures from around Australia and to develop ‘size / cost’ relationships captured relatively little high quality data for vegetated swales.
Table 1 Summary of cost-related relationships for vegetated swales.
Element of Life Cycle Costing Model | Default Option for Estimation in music | Alternative(s) | Notes |
Life cycle | 50 years (Expert judgement) | 25 years (From collected survey data, n = 6) | One could convincingly argue the life cycle is infinite for well-maintained and ‘re-graded’ swales, but we need to set the life cycle to a finite number to calculate a life cycle cost. Expected, upper and lower estimates based on expert judgement. |
Total acquisition cost (TAC) | TAC ($2004) = 387.4 x (A)0.7673 R2 = 0.59; p = 0.04; n = 7 Where: A = surface area of treatment zone in m2. | No alternative size / cost relationships in music. For literature values, see Taylor (2005b) - Included in Appendix H.* | Warning: This algorithm derives from a combined data set involving vegetated swales and bioretention systems, as there was insufficient data to analyse swales on their own. In addition, the data for swales includes examples of grassed and landscaped swales, swales with cross-overs, etc. Upper and lower estimates derived using a 68% (or 1 standard deviation) prediction interval for the regression. "Treatment zone" refers to the area defined by the swale’s length and top width (not the base width). |
Typical annual maintenance (TAM) cost | TAM ($2004) = 48.87 x (TAC)0.4407 R2 = 0.94; p = 0.03 ; n = 4 | No alternative size / cost relationships in music. For literature values, see Taylor (2005b).* | Warning: Size / cost relationships for TAC, TAM, and RC derive from a combined data set involving vegetated swales and bioretention systems, as there was insufficient data to analyse swales on their own. Upper and lower estimates derived using a 68% (or 1 standard deviation) prediction interval for the regression. In approximate terms, TAM ≈ 4.4% of TAC (for the combined swale / bioretention system data set). |
Annualised renewal / adaptation cost (RC) | RC ($2004) = 2.0% of TAC p.a. n = 3 | No alternative size / cost relationships in music. For literature values, see Taylor (2005b). | Upper and lower estimates derived using a 84th and 16th percentile, respectively. |
Renewal period | 25 years n = 6 | No alternative in music. | There is great uncertainty surrounding this period (and the associated RC), given the lack of experience in corrective maintenance associated with modern designs for vegetated swales in Australia. |
Decommissioning cost (DC) | DC ($2004) = 39% of TAC n = 1 | No alternative size / cost relationships in music. | Warning: Only one set of data was available. |
General caveats / notes for this type of device | * There are several estimates of capital and maintenance costs reported in the literature for vegetated swales in Australia (see Taylor, 2005b or Appendix H for a summary). The quality of these estimates is unknown but still represents a reasonable alternative, given the limitations of the swale-related costing data from which the CRCCH’s size / cost relationships are derived. The typical annual maintenance cost is an average over the swale’s life cycle, so MUSIC’s life cycle costing model does not simulate elevated maintenance costs in the first few years of the swale. There is however evidence that elevated maintenance costs typically occur in the first five years of swales (e.g. see Lloyd et al., 2002). | ||