The following is the copy from an article that I wrote for the January 2016 edition of the Stormwater Australia Bulletin.

Cheers, Jack Mullaly


I am so passionate about making this water sensitive urban design thing work. Seriously, it’s not funny how much I want this to work.

When I say water sensitive urban design, I mean the big picture dream. The environmental protection. The green leafy streets. The creeks you can play in, with the play equipment right there. The trees in the street for kids and adults alike to climb.

If we’re going to get this to work, we need to cherish that dream and share it widely, but we also need to get the fine details right. Today I’m going to talk fine details. But if you ever want to talk big picture then you know where to find me.

Recently I’ve been chatting with a few people about how to prioritise which stormwater treatment systems to maintain when you’re short on cash and you have many different types of system. Sounds like your local Council right?

In the past I have talked heaps about how to prioritise which bioretention systems to maintain. Simply put, you prioritise keeping the good ones in good condition before you spend money fixing the poor ones. What I haven’t talked about though is how this differs from grey stormwater treatment assets like GPTs, proprietary products, trash racks etc.

At the moment, many Councils have experience maintaining GPTs and are applying that experience to vegetated stormwater assets like bioretention systems and wetlands. That is a great idea. I applaud the Councils who are doing that. Keep up the good work. However, I want to bring a subtle difference to your attention so that we don’t accidentally trip ourselves up along the way.

Imagine a continuum that describes the condition of stormwater treatment assets. It runs:

  • Good condition
  • Routine maintenance required, but if you don’t maintain it, it will still treat the water
  • Routine maintenance required and it’s not treating water any more
  • Rectification required

Imagine a GPT. You construct it. It’s in good condition. It rains and it collects some sediment. You could do some maintenance now and give it a clean but it’s not full yet so there’s no point. It rains some more and at some point it fills up. You don’t notice so you don’t maintain it. That’s fine. It doesn’t collect any more sediment but it doesn’t suffer any damage. Eventually you give it a quick clean and it’s good to go.

Now imagine a bioretention system. You construct it. It’s in good condition. You leave it for a bit. Some weeds start to come in, but it still treats the water perfectly (weeds are plants too right). You leave it a bit longer. There are more weeds but it’s still treating the water. One day you realise that the weeds have taken over. A resident complains and you need to fix it. It’s no longer maintenance. You’re spending capital money on new plants.

There’s the difference. The GPT declines fairly rapidly in condition from good to moderate, but is unlikely to fail outright unless there is a freak event that causes structural damage. The GPT just fills up and starts bypassing. The bioretention system on the other hand shows some resilience early on. It can handle not being maintained for a bit and function 100%, but at some point it hits a threshold and suffers damage that needs a fair bit of money to fix.

Now this isn’t an argument for GPTs over bioretention or vice versa. There are pros and cons for either, but what it does highlight is that when you’re prioritising which assets to spend your money on, you can’t treat GPTs and vegetated assets the same. Imagine you have four assets:

  • 1 good bioretention basin
  • 1 failed bioretention basin
  • 1 good GPT
  • 1 failed GPT

Imagine you only have money to maintain the two good systems, but you receive a public complaint about the failed GPT so you need to spend money on it instead of one of the good assets. Which do you keep funding and which do you stop maintaining?

Take a minute to think about it.

You stop maintaining the good GPT, and keep maintaining the bioretention system. Correct. If you do it this way, the GPT will fill up and stop working, but it won’t suffer any damage. You can just clean it out at some point down the track. You invest a little in the bioretention basin and it rewards you with good performance.

Do it the other way round and the bioretention system stays resilient for a little but might eventually reach a threshold and suffer serious damage. This costs money to fix. You don’t want that.

Keep up the good work. I love hearing about people who are getting out there and making WSUD and stormwater management work. Keep using our experience maintaining GPTs and apply it to vegetated stormwater assets, but remember, there are some subtle differences. Make the adjustments as necessary and everything will work out perfectly. We will be able to have all our stormwater assets functioning properly regardless of whether they are GPTs, bioretention basins, wetlands, street trees or something else.

Did you like this article but wish there were some visuals to go with it? Check out the accompanying video.