Floods in a changing climate
Australia has been hit hard by floods in the last decade. In 2011, the floods in Queensland claimed over 30 lives and reduced Australia’s GDP by an estimated $30 billion. The floods in New South Wales earlier this year were also deadly and lead to evacuation orders in several towns. Populated floodplains are common in Australia, so we are vulnerable, and we know that the risks are likely to increase in a warming world. So what can we do about it?
The good news is that there’s a whole industry dedicated to flood risk management in Australia. Every time we build a piece of infrastructure, approve a piece of land for development or review the risks faced by a community, we consider flooding. We can estimate the risks associated with flooding for a particular area using hydrologic models. These models aim to predict how much flow (and hence flooding) we will get in a certain catchment, given a certain rainfall event. If we work smartly, we can go a long way towards understanding the level of flood vulnerability and investing to reduce it (or, at the very least, ensure that we don’t exacerbate any issues with poorly-designed infrastructure on the floodplain).
There are a number of reasons why hydrologic models are helpful. Firstly, a particular amount of rainfall won’t necessarily generate the same amount of flow in different locations. The factors (other than rainfall) that contribute to flooding include:
- Land use; the natural or man-made features of the landscape. This is important for flooding because different land types respond differently to rainfall. For example, rain that falls on a paved carpark will quickly run off and generate flow. On the other hand, when rain falls on a forest, a lot of it will be intercepted by leaves, soak into the soil or pool around roots, so less flow will be generated downstream for the same amount of rainfall.
- Soil type. Some soils will soak up more rainfall than others, leading to reduced flow and hence less severe flooding.
- Antecedent conditions. This is a term hydrologists use to describe how wet the catchment is prior to the rainfall event. Dry soil will typically soak up more water than wet soil, so if it’s been raining in the weeks leading up to a storm, flooding may be more severe.
- Catchment size and shape. When rain falls on a catchment, it runs downhill to a watercourse such as a stream, which directs it to the catchment outlet. Intuitively, a larger catchment can generate more flow (as it will catch more rain), but some of the rain may take a long time to flow from the upper reaches of the catchment to the watercourse. This can lead to floods that last days or even weeks. ‘Flash flooding’ is typically associated with smaller catchments with a shorter response time.
These are just a few examples outlining why flooding is so complicated. For these reasons, increases in rainfall intensity (which are expected in a warming world) will not simply translate into proportional increases in flooding. In fact, we still know a lot less about future flooding than we do about future rainfall.
There is a lot of debate in the hydrology community around consideration of climate change in flood risk management. Our models are developed and set up based on past observations, so it’s difficult to know whether they can be considered equally reliable under changing climatic conditions. At the moment, hydrologists typically assume a nominal rainfall increase to account for climate change (often 20% based on our understanding of how moisture is stored in the atmosphere, but this is still an active area of research). This effectively ignores other potential impacts on flood generating processes, which could relate to changing antecedent conditions and/or vegetation dynamics. We know we need to do better to make sure we keep communities safe and optimise our water resource management into the future.
Hydrology is a dynamic and complex field. If you’re an aspiring engineer or scientist looking to use your technical skills to solve practical problems (and concrete blocks don’t excite you), hydrology or catchment science could give you the right opportunities. If you’ve been impacted by flooding or are just interested in understanding more about the world around you, I hope this post was able to shed a little light. More to come!
