7
My Point of View It could be argued that this urban civil construction imbroglio is somehow unique to SEQ, where house construction on newly developed land is such an important contributor to the regional economy – especially the coastal shires in the south east of the state. However, similar anecdotal evidence has been “reported” for other Australian states, albeit that independent, quantitative evidence is missing.
Photo: Ted Gardner
More Research Needed
Catastrophic creek sedimentation during construction of a small subdivision in SEQ. I argue that the reality is far removed from this ideal, for not only is field testing of newly installed WSUD installations (mainly bio-retention basins and wetlands) relatively infrequent – perhaps only three or four well-documented case studies in Australia – but also there has essentially been nil reporting of their performance after the age of three years. Indeed, there are frequent anecdotal observations that ageing WSUD systems have lost much of their planted vegetation, as well as having sedimentation occurring at much higher rates than initially budgeted for.
My own conservative measurements in a small SEQ subdivision a few years ago returned a figure of 285 tonnes/ha. Subsequent modelling by others (e.g. Hoban, 2012) using the RUSLE model suggests a figure of 250t/ha is more appropriate as a regional estimate. So, assuming a fully functional, postoccupation WSUD is installed, about 260 years will be required to prevent the same sediment loss as that which occurs during, say, 18 months of subdivision construction. One has to wonder if current stormwater legislation and WSUD practice is looking at the wrong end of the (sediment) stick. There are technologies and practices available that can significantly reduce this enormous figure of 250t/ha. These include: high efficiency sedimentation basins (pioneered by Auckland Regional Council); staged development at the larger sites; no civil works during the peak of the wet season; and soil conservation measures that maintain soil cover. But if regulatory enforcement is weak, and fines for lack of compliance are far less than the cost of implementing compliance methods, then it is an uphill battle to implement change towards sustainable practices.
It’s also important to get a sense of perspective on the magnitude of contaminant sources affecting the water quality of receiving bays and estuaries. In Melbourne and Adelaide, for example, contaminant export into nutrient-sensitive Port Phillip Bay and Gulf of St Vincent is dominated by urban runoff – hence the sensible focus on WSUD or end-of-catchment wetlands to reduce nutrients, especially nitrogen. However, in Moreton Bay in SEQ, excessive sediment input is particularly important in limiting seagrass vigour to perhaps even a terminal endpoint. Healthy Waterways estimates that average diffuse urban sediment export into the Bay is 132,000t/year (see www.ehmp.org/TheStrategy/LinkstoOtherPlans.aspx), while rural diffuse is 318,000 t/yr, which in turn spiked at c. 16,000,000 t during the floods of 2011 and 2013 (TERN: www.tern.org.au/Newsletter 2014). Hence, to protect the ecosystem function of Moreton Bay, sediment reduction measures need to be particularly focused on rural catchments as well as those urban civil construction sites that add an estimated further 295,000 t/year (Hoban, 2012).
Photo: Alan Hoban
So, painting the picture with a fairly broad brush, admittedly, there is serious concern that over the medium to longer term even standard WSUD features are not delivering the modest reduction in contaminant export required of them. And by modest, I am comparing untreated, household occupancy stage sediment losses (about 1.2 tonnes/ha/year) with the sediment loss that occurs during the relatively short-lived civil construction stage.
So do we need more enforcement of the existing Erosion & Sediment Control (ESC) rules? Or more research into methods that prevent or contain erosion on site? I would suggest both, as recent opportunistic surveys by the independent not-for-profit advisory organisation, Healthy Waterways (healthywaterways. org), in SEQ subdivisions reported suspended solids concentrations in the range of 1,000s to 10,000s mg/L, when the discharge limit is 50mg/L. And, of course, suspended solids constitute only the tip of the sediment export iceberg.
An industrial-scale bioretention basin tacked onto a subdivision in SEQ.
June 2015 water