12 months ago, the Queensland government gave birth to the latest edition of the State Planning Policy (SPP). The proud parents duly put a birth notice out, highlighting the joyous occasion and inviting the industry to embrace the newly born policy.
The SPP is a key instrument of the Planning Act 2016; it sets out policies for planning and/or development assessment.
A key state interest is water quality and so the SPP includes stormwater management design objectives for construction.
The SPP is, therefore, part of legislation. It’s the law and it’s the focus of all local government planning scheme. However, a year on and only a few people have shown up for the party and tried to implement treatment measures that meet these objectives.
Perhaps it’s a lack of knowledge of what the SPP is trying to achieve or perhaps it’s a lack of knowledge on how to meet the objectives in a cost effective way.
What are the SPP Construction Water Quality Objectives?
The SPP construction water quality objectives cover the usual suspects; erosion control, drainage control, sediment control, litter, hydrocarbons and other contaminants, waterway stability and flood-flow management.
Obviously erosion control is the most important control, as it prevents the problem in the first place by keeping soil covered, but typically constructors have little time and scope to stage earthworks as much as they should and so they tend to rely on sediment controls to clean up the mess at the end of the line.
The SPP has raised the bar on sediment control outcomes; it states:
“All exposed areas greater than 2,500m2 must be provided with sediment controls which are designed, implemented and maintained to a standard which would achieve at least 80% of the average annual runoff volume of the contributing catchment being treated (i.e. 80% hydrological effectiveness) to 50mg/L Total Suspended Solids (TSS) or less, and pH in the range 6.5-8.5.”
That doesn’t sound too hard, but what does it mean?
The best management sediment control practices over the last 10 years have been the Type D sediment basin which captures ALL runoff from a nominated 5 day rainfall depth. The water is temporarily stored before manually adding coagulants or flocculants to encourage settlement of fine colloidal particles.
Testing is done to ensure the treated water meets the nominated discharge criteria (50mg/L) before it is pumped out within 5 days of the rainfall event ceasing. Collected sediment is dewatered and removed on a regular basis.
Given it was best practice, it was generally believed this would provide a sustainable outcome, and would safely protect the downstream environment from turbid waters.
However, this system falls short when there are multiple rainfall events within days of each other, or if there is a larger rainfall event than the basin is designed for. Once the basin is full, any further incoming water leaves the basin with only minor treatment as the coarse heavier particles drop out but the fine dispersed particles enjoy the ride through the basin and out over the spillway – Yippee!
Papers by Rowlands and Leinster (2015) and Robson (2015) estimate that Type D basins can only effectively treat around 30-35% of the average annual runoff volume leaving more than 65% of runoff from a development site untreated. Note that this figure gets worse the further you move north.
This doesn’t sound like best practice and the state government agreed, stating that now 80% of the average annual runoff must be treated to comply with the SPP. But how do we do this?
Additional Guidance on how to meet the SPP Objectives
Well, the SPP doesn’t provide any guidance on how to achieve compliance and nor should it. It simply states what the goals are and leaves it to industry to decide on how best to achieve them. Perhaps this is why there has been a lack of uptake in alternative sediment controls.
Up until recently there was little guidance on how to meet this objective biut Healthy Land and Water (HLW) has produced a new guideline titled Sediment Management on Construction Sites – Complying with the SPP (July 2017) – Technical Note for Local Government Development Assessment & Compliance Officers. The document can be found here:
Technical Note For Local Government Complying With The SPP 2017 Sediment Controls.
It states that there are five ways to meet the SPP construction water quality sediment control objective:
- High Efficiency Sediment Basins;
- Large traditional Type D basins;
- Total capture of all dirty water on site;
- Alternative innovative methods; and
- Erosion Control.
The first option (and the one I believe is expected to be adopted on most sites) is a High Efficiency Sediment (HES) Basins.
HES basins were developed in New Zealand and have been perfected by the Kiwis over the last 20 years…..well they have to do something in the summer months when there is no rugby!
HES basins are continuous-flow basins where coagulants or flocculants are automatically added and mixed to the incoming sediment-laden stormwater which is allowed to settle over hours rather than days. The basins can treat a much larger volume of stormwater much more quickly and so can achieve the 80% average annual runoff volume requirement (although I haven’t found any studies that show what average annual runoff volume a HES basin will achieve). However, just adopting a basin will give you the green light to meet the SPP….. according to HLW anyway.
IECA has produced a DRAFT guideline on how to design two types of HES basins which they have labelled a Type A or Type B.
The process is relatively straightforward but involves the trial of preferred chemicals to select a dosing rate to provide an expected settlement rate.
Designers then need to select an appropriate automated dosing unit to feed in the preferred chemical coagulant and/or flocculent. This can be either rainfall activated or flow activated.
This part puts some people off, but IECA has prepared some guidance on the use of various chemical coagulants and flocculants, and there are a number of suppliers who can provide further advice on their use to ensure no eco-toxicty issues are caused by overdosing.
The guideline is still in DRAFT however it should be finalised soon.
The second option in the HLW document is a larger Type D basin. However,they need to much larger than normal to ensure that the 80% AARV criteria is met.
The document provides minimum storage volumes for different parts of the state. A traditional Type D basin in Brisbane would need to become around 3-4 times larger to meet the SPP requirement.
The third option is the total capture of all water onsite. It sounds like a really, really big basin and is not practical for most sites. I did a quick check for a site in Brisbane and such a basin would need to be around 30-40 times larger than a traditional Type D basin if it had to retain all stormwater on site!
The fourth option is an alternative innovative method. Of course, any innovative methods would need to be accompanied by a range of calculations/models or test results to confirm the required performance can be achieved.
Interestingly erosion control is listed as the fifth option…perhaps because it is the most obvious one? As always, the best sediment control is erosion control.
A Birthday Toast
The HLW document fills in “the how” to comply with the construction water quality objectives for the SPP and, based on above, the most practical option (apart from erosion control) revolves around the adoption of the High Efficiency Sediment Basins.
The finalisation of the IECA best practice document (including guidance on chemical coagulants and flocculants) will provide the tool to design, operate and maintain sediment control measures so there shouldn’t be any more excuses to not comply with the SPP.
There are still questions to be answered on how the new HES basins will perform within our region and how the design methodology will evolve, but one thing is for sure, their adoption will result in more stormwater being treated on construction sites.
Hopefully, by the time the SPP celebrates another birthday, the industry accepts the invitation and our waterways receive the gift of cleaner discharges from our developments – and we can all drink to that.
Author: Bill Johnson
Strategic Environmental and Engineering Consulting