Some flood dangers, however, can be far less visible – to planners, developers and home-buyers. Sometimes, the danger comes from groundwater beneath the surface.
Earlier this year, for example, residents of the New South Wales town of Stuarts Point were evacuated and decontaminated after sewage spilled into their streets, as septic tanks filled with shallow groundwater.
These problems are not inevitable. Our recent report shows how changes to urban planning, building design and construction practices could reduce groundwater risks. That means better outcomes for residents, developers, governments and the environment.
Urbanisation can cause the water table to rise
In many cities around the world, the water table is only a few metres or less below the ground. Groundwater levels tend to naturally rise and fall with winter rains and summer drying, as this UK video demonstrates:
However, urbanisation can cause the water table to rise above its “natural” level.
When we clear bush or farmland for housing, we reduce opportunities for plants to intercept and soak up rainwater. More rain then infiltrates through the soil and causes aquifers to fill up more quickly.
Balancing the short and long-term costs
The urban suburbs most affected by high groundwater are typically newer and cheaper. To attract buyers, developers must keep prices low, but mitigating the risk of groundwater flooding is expensive.
Developers sometimes import and install sand as “construction fill” to elevate the ground level. They may also install drainage to control groundwater levels. These sorts of measures can represent one-third of the price of developing a housing lot.
Such measures on their own, however, don’t solve the problem.
Our recent report for the Cooperative Research Centre for Water Sensitive Cities suggested ways to improve groundwater management on Perth’s Swan Coastal Plain. These findings may help people in other areas dealing with similar problems.