St Marys Flood Management

 

St Marys Rivulet is a valued part of the town’s landscape, parks and identity, but St Marys has also been affected by many floods over the years as the town has grown up around the rivulet. 

A flood on 28th May 2010 impacted the town significantly and willows along the rivulet were cut down afterwards to clear the stream channel.  By 2016 these willows had started to regrow.  On the 29th January that year a major rainfall event led to the rivulet breaking its banks and flooding roads, backyards and homes and businesses in the town.  A community petition calling for action on the flood problem followed. Flooding has occurred in St Marys for generations, often followed works such as those after the 2010 flood.

Council initiated the St Marys Flood Risk Management project in 2017 to scientifically quantify and map floods and risk levels in St Marys.  This flood simulation modelling would then enable the best actions to be tested and provide a sound foundation for future flood risk management for the community. Read more about the St Marys Flood Risk Management project here.

Flooding is a natural thing for rivers and streams.  The catchment and weather largely determine the regime of flooding flows and the behaviour of streams.  Flood problems arise when people, their land uses, infrastructure and property intrude on where flood waters want to go. 

There are several tributary streams that come together at St Marys and this catchment can get very heavy rainfall, particularly from easterly storm events.  St Marys Rivulet passes through the middle of the township over public and private land.  All these factors and more need to be taken into account. 

Flood management 

It is well known that St Marys floods at times and people have experienced inundation, flood currents and the impacts on their properties elsewhere, especially after the 2016 floods.  St Marys has not however had information to quantify the extent and dangers of the range of floods that can occur. 

The flood on January 2016 is shown below, as depicted by the flood model developed for the St Marys Flood Risk Management project.

Households, businesses, community as a whole and Council and other government organisations cannot make reliable flood management decisions to avoid flooding and to prepare for, respond to and recover from floods when they occur without sound information about the flood risks across St Marys.

That flood risk information has now been produced by the St Marys Flood Risk Management project with its Flood Study Report – St Marys Flood Risk Investigation report. You can view the report by downloading it in sections (the full report is a large document) from the Resources list opposite.  You can also view many of the report's maps online, where you can zoom in to particular locations, at http://www.tinyurl.com/StMarysFloodStudy.

The report includes Flood maps (extent/depth) and Flood Hazard maps (depth X velocity), maps flooded areas in terms of Flood Function categories, and models possible increased risks with future climate change and fully developing urban land along the stream.  These are modelled for a range of flood events from 20% Annual Exceedance Probability (AEP) to 0.5% AEP and a ‘Probable Maximum Precipitation’ event.  A “1 in 100 year” flood is equivalent to 1% AEP. 

The (Design) Flood maps are useful for identifying flood prone areas.  Flood Hazard maps combine the velocity of floodwaters with water depth.  Larger faster flows are more dangerous and it is perhaps more important to know where the flood hazard is than simply the extent of a flood or how deep the water is.  The Flood Function maps help identify places with roles for storage of floodwaters and moving them downstream.  With this information householders, businesses, property owners close to the stream can make better decisions.

The St Marys Flood Risk Management project is also looking at the best strategies to manage flood risks.  These include planning for when floods occur in the community, from individual flood plans through to a community flood plan integrated with municipal and state emergency management planning. Early flood warning is another important strategy since the project has highlighted St Marys can be subjected to flash flooding, such as the January 2016 flood.

Works to reduce flood risks are also important and the project has worked with the community to identify a range options for flood mitigation works.  The flood simulation modelling is being used to show they will reduce flooding and it impacts and with further community input select some to be considered for implementation. Some of these will be started as part of the project.

During the project the intuitive strategy of simply cleaning out the rivulet was raised and considered.  This approach is being tested by some of options for flood mitigation works.  One issue with going down a ‘stormwater drain’ pathway however is the possible consequences for St Marys Rivulet as a valued natural feature and attraction of the township

Why can’t we just ‘clean out’ the rivulet

Cleaning out the rivulet was put forward at the community meeting marking the start of the project in September 2016 as an obvious solution to flooding.

Clearing the channel by removing fallen trees, stumps and vegetation and gravel and other sediments would increase in-channel flows through the town to a degree. A bigger, deeper St Marys rivulet channel with less features creating friction for water flows will carry more water away and faster, with less risk of flood waters breaking the bank during in-channel and relatively small flood events. However, it is essential that the broader consequences of such works be considered. 

 

For example, typically the faster water flows, the greater the size and amount of sediment the water will carry. Sand, gravel and rocks (and bigger things, such as logs) can all be picked up and carried by the flowing water. The increase in flow energy created by the removal of fallen trees, stumps and vegetation results in the water moving material out of the stream bed and banks that would otherwise have stayed put. As a consequence, it is likely that the St Mary’s Rivulet will erode its streambed and banks and that erosion that may take many years to re-stabilise. These unintended consequences of removing vegetation and debris have been demonstrated in numerous rivers and catchments across Tasmania and Australia since European settlement.

The question of reducing vegetation and debris as a flood mitigation strategy was asked in the 2012 Victorian Parliamentary inquiry into the devastating floods of 2011/2012 in Victoria. The inquiry concluded, based on submissions by numerous experts and scientific studies, that clearing vegetation generally has a very small impact on flood risk. The scientific evidence presented to the inquiry also showed that the impacts of reducing vegetation decreases for larger floods. In big flood events, which fully engage the floodplain, such as that which occurred in St Marys in January 2016, the impact of clearing the creek will have a minimal impact on peak flood levels.

Cleaning out the channel of the rivulet is a flood management option that the St Marys Rivulet Flood Management project can test.  If the community genuinely wants this option considered, then the broader consequences need to also be considered.  These considerations include:

  •   Long term channel stability, including:
    •   The threats to both private and public assets/land adjacent the creek.
    •   Required erosion control structures to manage long term channel stability.
    •   The initial and on-going costs associated with the construction and maintenance of the erosion control structures. 
  •   The aesthetic changes to the creek. The removal of the removal fallen trees, stumps and vegetation and construction of erosion control structures using materials such as rock, will result in a substantially different looking Rivulet.
  •   The impacts both upstream and downstream of the study area.
  •   The ecological function of the removed material. 

 

Some video demonstrations

Woody debris slows streams https://twitter.com/theriverstrust/status/913377849775706114

Why Do Rivers Curve? https://www.youtube.com/watch?v=8a3r-cG8Wic

Unintended effects of dredging https://www.youtube.com/watch?v=OAZ_BuyM41s

 

A stormwater drain case study - Moonee Ponds Creek, Melbourne

The story of why Moonee Ponds Creek was turned into concreted stormwater drain in the 1950s and 1960s is a relevant case study.  The hydrology of the Moonee Ponds Creek catchment is a bit different to St Marys rivulet and the scale of urban development and rate of stormwater run-off it generates is much greater.  Communities around Moonee Ponds Creek are currently revisiting how the stream is managed to return some of the values of the Creek that were lost under the concrete, while ensuring the risks from flood are managed well. 

For the Moonee Ponds Creek story view www.ourcreek.com.au/chronology - the fourth page in this poster series talks about the Creeks stormwater drainage period of development in the 1950s and 1960s.