St Marys Flood Risk Managment Project

Background

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 28 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 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.  The rivulet passes through the middle of the township over public and private land.  All these factors and more need to be taken into account. 

Objectives

The St Marys Flood Risk Management project aims to examine the flooding problem and identify actions that will reduce the risks to the community.  The project will include putting in place flood management actions identified by the project.

The project has two parts.

A ‘flood management study’

Creating a computer model from land and weather data to simulate how floods work in St Marys, map flood risks and test some flood management options to find the best ones.  

Involving the community and relevant organisations

A Project Reference group includes representatives from emergency management and government agencies, the community of St Marys including residents and businesses, to provide their guidance and input.  .

The broader community has a role too; to receive the flood simulation results, help identify the best flood management options and to get flood plans of their own. 

Once the project is complete, organisations like the SES as well as businesses and community members can make use of the findings to make their own ‘flood plans’ for households, businesses and the community.  

 

Project schedule

September 2017

Project launch, call for flood photos, meetings for community and Reference Group

 

Flood simulation modelling and mapping

December 2017

Community flood management workshop, flood mapping and identify flood management options to test.

 

Floodplain management study, evaluate flood risks and test management options

February 2018

Community flood management workshop, Floodplain management study results, recommendations and community feedback.  Start Community & Household Flood Planning.

 

Priority flood risk management actions identified.

March 2018

Community and Household Flood Plans, community members supported to prepare their plans.

 

Undertake flood mitigation actions.

June 2018

Project finished

 

Project area 

Click HERE to see the intial January 2016 flood map.

Click HERE to see the Project area map.


Funding

The project is funded by the Natural Disaster Resilience Grants Program (NDRGP) through the State Emergency Service. 

Water Technology www.watertech.com.au is the consultancy firm hired to undertake the flood modelling work and their Project Manager is Julian Skipworth.  

Flood management 

The computer modelling for the St Marys Flood Risk Management project will identify the likelihood and average recurrence of a range of different size flow events – from those that remain within the banks right up to major flood events that break out of the bank and across the floodplain. 

St Marys Flood Risk Management project will map a range of different floods, from the relatively common small events to bigger rare events. The flood modelling will also calculate the ‘flood hazard’ (velocity times depth) of floodwaters where they overflow the channel and in the channel. 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 modelling will generate this sort of information for a range of flood probabilities and some different catchment and weather scenarios, mapping it across the study area to greatly improve the understanding of flood risk from St Marys rivulet.  With this information people with property homes, businesses, and assets close to the stream can make better decisions. 

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.