Will your house flood? Floodplain management in Perth

When people think of Perth they tend not to think of flooding like that which happened in Brisbane in January 2011. However, many Perthites do live in and around the city’s iconic Swan River and its multiple tributaries.

Rosalie, Brisbane – before (left) and after (right) the January 2011 Brisbane River floods (Source: https://www.abc.net.au/news/specials/qld-floods/)

The risk of flooding from our river system around the city is very much real, as demonstrated by the record breaking rain and floods in February 2017 (114 m in 24 hours, 9-10 Feb) which destroyed crops and damaged infrastructure across WA, including the Swan Valley, where it has had a dramatic impact on grape growers. The flooding was estimated to have caused hundreds of millions of dollars in damage with roads, bridges and farms affected.

The 2017 river floods had a huge impact on grape crops in the Swan Valley, Perth. (Source: Office of Premier & Cabinet, WA)

People have used flood plains for thousands of years. Highly fertile soils resulting from sediments brought by past floods and proximity to rivers provides water as well as transport and energy. As a result, many of our farms and towns are on floodplains. The farms make sense, but cities and towns do not, at least to the extent that buildings and infrastructure is located in places we know will be regularly flooded. Assuming we know!

Physical engineering options such as dams can protect floodplains to some degree. But they are limited in that they can never protect against all floods and are often expensive. They can also have impacts on the environment and river system.

The crew from Perth’s Dunno Engineering have famously come up with solutions for making sure beers can still be enjoyed while seated, even in potential floods! (Source: Facebook)

Assessing flood risk is important for helping the community to manage and minimise the impact of such flood. It requires the ability to accurately model and predict flood events.

Determining what areas are likely to flood is rather complex and based on a number of factors including: surrounding landuse (% vegetation cover versus hard impermeable surfaces), soil type, soil moisture, rate of runoff, proximity to waterways, river and catchment conditions, duration and intensity of a storm……all this information can be input into hydraulic models which allow us to predict where flooding might occur, under what conditions, and with what probability. This is incredibly useful in helping us make decisions on how to manage potential floods.

To quote: Planning is the cheapest way of managing floods!

The latest UWA Environment Seminar had Simon Rodgers, DWER Supervising Engineer – Floodplain Management, provide a brief history of floodplain management in WA and a framework for on-going flood risk management.

Simon’s group has been working on the “Swan and Helena Rivers Flood Risk Management Strategy” project with a focus on updating floodplain mapping that informs land use planning, and preparing mitigation options to reduce the risk of flooding. Outputs from their Swan and Helena Rivers flood study include:

  • Statistical models and data files
  • Mapping
  • Risk and vulnerability information
  • Strategies, plans and policies to manage flood risk
Simon Rodgers, UWA Environment Seminar, 15th August 2019
(Source: Yingyuan Shi, Urbaqua)

One of the major outcomes of this project was the creation of the Swan and Helena Rivers Floodplain Development Strategy Story Map to provide a summary of the flood risk assessment undertaken for the Swan and Helena Rivers floodplain, including the regional Floodplain Development Strategy. The Story Map supports technical reports and other digital materials, including figures, maps and GIS data.

A floodplain mapping web tool was also launched to make it easier for the community to understand flood risk in their local area. Users can view floodplain maps and flood levels, as well as floodplain development strategies. This information can definitely be useful when:

  • Buying a new property;
  • Building a new house;
  • Planning a new subdivision;
  • Developing a personal response plan to minimise the impact of flooding on their property;
  • Purchasing flood insurance; and
  • Performing property valuations.
A snapshot of Heirisson Island in Perth’s metro area in DWER’s new floodplain mapping tool

Definitely worth checking out to see whether yours or your family/friend’s homes could be flooded! Understanding risk and knowing what to do when a flood warning is issued is critical in the rare, but potentially huge storm events that Perth sometimes experiences.

Having good, accurate tools and flood models is important in making sure we can effectively protect ourselves and our homes and infrastructure. It also serves as a reminder of the power of nature and the need to make sensible use of floodplains!

Read more:

Manging the floodplain: a guide to best practice in flood risk management in Australia developed by the National Flood Risk Advisory Group

Flood hazard develpoed for NFRAG by the Water Research Laboratory of University of NSW