The first thing I would suggest is to understand/to model the standard life of your stormwater assets. The Council I worked before had CCTV footages collected from last two decades, and relatively high confidence against the creation date of individual asset. After reviewed and scored these footages based on WSAA code, a correlation between condition scores and ages for certain assets group (i.e. 150-450mm RCP) can be built. By undertaking a cumulative frequency analysis on this correlation, it should help you to form a degradation curve and to predict the standard life (i.e. 90% of condition 1 less than 80 years old, 75% of condition 2 less than 110 years old etc.).
Using creation date + standard life to predict renewal date would not reflect the stormwater renewal in the real world. It is envisaged that a huge peak will be showing in your forecast. I would imagine for lots of Councils, a standard life + creation date based renewal forecast will only tell that no money needs to be spent in the next 10 - 20 years as majority of the stormwater assets are built post WWII.
Given stormwater asset life distributions is normally following a bell curve with the standard life as the peak, the Monti Carlo formula would be useful to smooth the peak. Excel can easily achieve that and generates a new forecast diagram with more flat or evenly distributed renewal forecast. Forecast the LTFD using this newly generated diagram is more close to the reality.
In terms of renewal cost, applying factors such as depths, locations, soil types, road hierarchy to reflect the costs of working at different locations would be more close to the actual spend, and this will also help to estimate the capital works for the following years.
Elements of Cost
16 The cost of an item of property, plant and equipment comprises:
(a) its purchase price, including import duties and non-refundable purchase taxes, after deducting trade discounts and rebates;
(b) any costs directly attributable to bringing the asset to the location and condition necessary for it to be capable of operating in the manner intended by management; and
(c) the initial estimate of the costs of dismantling and removing the item and restoring the site on which it is located, the obligation for which an entity incurs either when the item is acquired or as a consequence of having used the item during a particular period for purposes other than to produce inventories during that period.
17 Examples of directly attributable costs are:
(a) costs of employee benefits (as defined in AASB 119 Employee Benefits) arising directly from the construction or acquisition of the item of property, plant and equipment;
(b) costs of site preparation;
(c) initial delivery and handling costs;
(d) installation and assembly costs;
(e) costs of testing whether the asset is functioning properly, after deducting the net proceeds from selling any items produced while bringing the asset to that location and condition (such as samples produced when testing equipment); and
(f) professional fees.
The fully revised 2nd edition of the Australian Infrastructure Financial Management Guidelines is currently being finalised for publication by IPWEA (keep an eye out for it). It provides up-to-date and more comprehensive information regarding valuation and revaluation of long-lived infrastructure assets than the 1st edition did. It emphasises three factors relevant to the revaluation of stormwater and other long-lived infrastructure assets that don't have a market value.
Firstly valuation should be based on the cost of a modern equivalent replacement asset. If replacement involves incurring additional traffic management and other costs associated with the asset's site and circumstances then such costs should be taken into account in the valuation.
Secondly assets should be componentised for accounting purposes where appropriate based on local asset management practices. This means for example that if a stormwater pipe is expected to be relined then materiality considerations may warrant it being accounted for as two distinct assets, a shorter lived inner component and an outer component with a longer life.
Finally the Australian Accounting Standards Board has recently explicitly confirmed that residual values should only be applied to reflect expected net receipts from disposal of an asset to another party at the end of its useful life, i.e. sale to an external party. Expected savings to an organisation from renewal rather than replacement of an asset should not be recognised as a residual value. Such 'savings' are appropriately and fully recognised instead by componentising assets into shorter and longer-lived components, valuation as modern equivalent assets and depreciating the components over their expected useful lives.
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