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Is Your Project Site Prepared for Flooding?

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While Townsville now begins the vast task of flood recovery, many construction projects around Australia are constantly facing a degree of flood risk from either storm-related flash flooding or widescale riverine flooding.

Founding principal of Molino Stewart consulting, Steven Molino, tells Jobsite that understanding the risks of flooding at the early project stages is essential. It might appear we are having more floods more often considering the number of times both flash flooding and riverine flooding hit the headlines.

“The problem hasn’t changed. What has changed is the value of assets we have in areas prone to flood,” said Molino.

When Overflow Is Inevitable

We have only been building in many areas for 200 years or less, which may also mean there is a lack of experience to inform the risk picture. Couple this with Australia’s highly variable rainfall, older stormwater systems in urban areas designed for lower volume UK rainfall patterns, and the tendency to build near waterways for their many benefits, such as water supply and transport—and you end up with a risk-laden scenario.

Molino says it is not possible to design every stormwater system to have the capacity to take any possible rainfall event. Therefore, we need to accept they are likely to overflow at some point.

Flood Resilience Strategies

There are some guidelines available to developers, planners, designers and builders; they may serve as a reference for best practice flood resilience strategies. These guidelines include the National Best Practice Guidelines from the Australian Institute for Disaster Resilience and the NSW state government guidelines. Brisbane City Council also created extensive flood risk assessment and management resources.

The rules about whether a development must adhere to guidelines or not vary from state to state. In Western Australia, South Australia and Tasmania, for instance, adherence to the national guidelines is not required, Molino says.

What builders, developers and designers should be looking for is information specific to a site’s degree of flood risks—what type of flooding is possible, the likely frequency, and the predicted water levels and flows.

In Victoria, Molino says, the local Catchment Management Authorities are the best place to start with gathering that intelligence.

In other states, local councils are the key. However, he warns the comprehensiveness of information can vary from council to council. Moreton Bay Regional Council in Queensland makes that information very accessible, with an on-line flood risk mapping database that can be freely accessed, but not everyone is as accommodating. In other states, the council may tell you nothing more than that there are planning controls that have been put in place due to a known flood risk.

Managing the Consequences

From understanding the risks, the next step is to think about what the consequences of flooding could be.

Firstly, Molino says, you should consider the risks to the finished project. You need to think about the impacts on the building itself and any plant and equipment exposed to flood waters. The building may also cause water to flow towards neighbouring buildings or infrastructure and damage them.

“Designers are good at finding ways to manage these risks according to best practice.”

Then there are the risks to people occupying or visiting finished development. Molino says this means taking into account people’s possible behaviours.

Considerations include “what if the street becomes a river?” and “what if people come into the building and get trapped?”

Where projects often come to grief is these things haven’t been considered. Then, the Development Application is rejected by consent authority until the risks are managed.

Molino says more and more project proponents are finding they have to consider flooding but often do so once the design is extremely advanced and making modifications is costly. Flood risk to projects during construction he says is “rarely considered,” partly because the construction timeframe is relatively short.

In fact, Molino has consulted on projects where the project design, construction timeframe and location combined together created a real risk that needed managing.

As an example he gives is a site next to a stormwater canal which was likely to overflow in a one in five-year storm event. While the design had considered and adapted to managing the risks of a 100-year flood event, part of the solution was a four-storey basement car park that would be accessible via a car lift and would be sealed to prevent water ingress.

Molino pointed out that having a four-storey basement excavation next to a canal that has a 50/50 chance of overflowing during the two-year basement construction period was a risk that needed to be addressed.

Assessing High and Low Risks

In general, there are a number of practical things projects at risk of storm-related flash flooding, tidal flooding, or riverine flooding need to consider.

The first is gaining a thorough understanding of the likelihood and consequences of the risks and make decisions about how acceptable the consequences might be. For example, if the worst-case scenario is some silt fences might be flattened, that might be considered an acceptable risk, Molino explains. “But if you have earth moving equipment that might be damaged, you might think about how to protect that.”

Recently, he consulted a project where there were high risks at part of the site. In that case, the most sensible thing to do was to change the planned site layout so that the plant and materials stockpile were located at the highest part of the site.

As well as the potential magnitude of damage to assets, materials and equipment, considering the risk to the workforce also matters. Even at the tender stage, it would be sensible for builders to identify and understand flood risks and build a margin for any mitigation or protective equipment into the budget, Molino says.

“You can’t change the probability of flooding—but you can change the likely consequences.”

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