By Kelly McKendry -Director of Water & Environment
We often have clients approach us seeking guidance around stormwater management. Quite often the key focus is on the size of stormwater basins. Unfortunately, we see many examples of stormwater reports that simply aim to comply with the standard and comprise of pages and pages of technical jargon that only stormwater engineers can make sense of. So how do you know if it’s the right solution and the basin is the right size?
While the technical jargon in reports may be unavoidable to obtain approval from Authorities, we prefer to see solutions broken down as simply as possible. We have implemented the following ideas to great effect on recent projects to reduce basin sizes and in some instances remove them altogether. Given these great outcomes, we thought that it is worthwhile sharing them with you.
1. Controlling Flows
In many instances we see developments where new roads crossings are proposed across waterway corridors. Under the right conditions culverts can be designed in such a way to hold back flows in lieu of detention basins. By taking advantage of this, you may be able to avoid having detention basins and regain developable land within your site.
2. Regional Solutions
Regional solutions are those that consider the wider catchment and not just your site. In some instances, these may already exist or in other cases it might be appropriate to negotiate a regional solution with the authority. These solutions are generally located within overland flow paths or waterway corridors or associated with major road crossing locations. While a larger body of work may be required to facilitate this, the benefits are again focused on locating the basin outside of the developable area to regain valuable land. Don’t assume that everyone is aware of an existing regional solution.
3. Stomwater Contributions
Some authorities have an option to pay a contribution towards a regional solution rather than having a basin located on your land. It is worthwhile checking the authority policies to see if this is an option as the contribution may end up being cost neutral. This is also likely to avoid land take and ongoing maintenance costs.
4. Co-incidental Peak Flows
Co-incidental peak flows occur when a larger regional event coincides with a local catchment event. Imagine having a dam in a paddock that releases water to a river. You wait till the river is at its peak flood level and then let the open the dam gates and allow it to flow into the river. You make the flooding worse. While detention basins aren’t exactly the same as a dam, a similar theory applies. If your development happens to be located near an existing waterway that is already impacted by regional flooding, then co-incidental peak flows may occur. Therefore, it may be more appropriate to demonstrate this to the authorities so that the solution involves no detention. Not every site needs detention and blindly following a position from an Authority that it does can actually make downstream flooding worse.
5. Utilising Existing Stormwater Assets.
In some instances, we have come across sites where the authorities have requested stormwater quality basins but opportunities have been found to negotiate the removal of these basins. We recently had a site where an existing downstream basin happened to be oversized and went above and beyond the requirements for the catchment. With this in mind it was possible to direct flows from our site to the existing basin and utilize the “spare” capacity. Instances like this can occur when there has been a change to the authority standards or there may have been some conservatism built into a previous stormwater model. Either way, if there is an existing basin that looks a little large, it is worthwhile checking the sizing to see if it has spare capacity.
In summary, it pays to step back and have a look at the site specifics when it comes to stormwater management. In many cases we have been able to optimize basin sizes by applying some stormwater smarts. In other cases, we have been able to remove basins altogether from the developable areas of sites.