Melbourne Water Flood Risk Assessment: How flood impacts are assessed in the Port Phillip and Westernport region INTRODUCTION Melbourne Water is the regional drainage We have developed a Flood Risk Assessment and floodplain management authority for Framework to help us assess flood risks the Port Phillip and Westernport Region. and identify the benefits of flood risk We manage and maintain a number of management measures over time. retarding basins, water quality treatment This framework helps us deliver on the wetlands, levee banks, pump stations, commitments we made in our Flood flood gates and underground drains. Management and Drainage Strategy, and Spanning some 12,800 kilometres and moves us closer to achieving the targets covering major catchments including outlined in the Waterways Operating Charter Werribee, Maribyrnong, Yarra, Dandenong and Waterways Water Plan. and Westernport, the flood management This document explains how the framework and drainage challenges in the region was developed, outlines its key features, are significant. and details how it’s used to priortise the By working with a wide range of stakeholders management of flood risk across the region. we are constantly striving to reduce the flood risk to the community, property and infrastructure, while increasing people’s understanding of and preparedness for floods. Floods are natural and we can’t stop them from happening but we work to minimise the level of damage they cause to people, places and communities. Floods are natural and we can’t stop them from happening but we work to minimise the level of damage they cause to people, places and communities. Yarra River near Kew and Balwyn 2005 MELBOURNE WATER: MANAGING MELBOURNE’S WATER TODAY, TOMORROW AND BEYOND Melbourne Water is owned by the Victorian Our roles and responsibilities at Melbourne Government and responsible for $9.4 billion Water are both diverse and comprehensive. of key water-related assets. Specifically, we: We’re dedicated to working to ensure a • Manage water resources sustainably sustainable water future for all Melburnians, and secure supplies for a range of uses and pride ourselves on building strong in the context of population growth and relationships and partnerships to achieve climate change this common goal. • Protect public health by providing safe We consider social, environmental and water, sewerage and drainage services financial effects and short term and long-term • Protect, conserve and improve natural assets implications in all our business decisions. and use natural resources sustainably Our key stakeholders are customers, • Plan, build and manage assets efficiently by government, regulators, other water adopting innovative solutions and whole of businesses, land developers, the community life approaches to meet customer service, and suppliers. These stakeholders and our community, and environmental objectives other strategic partners, including alliance • Are recognised as a reliable and trustworthy colleagues and research organisations, help organisation, willing to listen, work us achieve our objectives. collaboratively and deliver on our promises. Dry reservoir 2007 FLOODS AND FLOODING: A NATURAL PART OF MELBOURNE As the waterway manager for the Port Phillip and Westernport drainage pumping stations region, Melbourne Water looks 22 after a diverse asset base: 213 retarding basins kilometres of rivers and creeks that drain into Port kilometres of levee banks 8400 Phillip and Western Port that restrict floodwaters 174 in storm events 1463 kilometres of drains As the drainage and floodplain constructed waterway management authority for treatment systems and wetlands the region one of our key 305 responsibilities is to protect monitoring stations on the community from the many 277 waterways and drains impacts of flooding. Port Phillip and Westernport region Primary drainage catchments Major waterways Maribyrnong Werribee Yarra Dandenong Westernport Port Phillip Bay and Westernport region FLOODS AND FLOODING: A NATURAL PART OF MELBOURNE WHAT IS FLOODING AND WHAT WHAT SORT OF FLOODING DOES CAUSES IT? THE PORT PHILLIP AND WESTERNPORT REGION EXPERIENCE? Floods occur when a surge of water caused by high rainfalls breaches the banks of a river or Historically speaking, the Port Phillip and creek, or when the amount of runoff exceeds Westernport region predominantly faces the normal capacity of underground drainage three main types of flooding: systems and causes them to overflow. 1. Riverine flooding, which is caused when While flooding occurs naturally, some floods rivers and creeks breach their banks and are considered extreme and require measures flow onto the surrounding floodplain. that reduce both their risk and impact. 2. Overland flows (flash floods), which are caused by runoff from severe storms beyond the capacity of underground drainage systems, where water runs downhill along overland flow paths (e.g. valleys). 3. Coastal tidal and storm surge flooding, which is caused by extreme weather conditions or ocean tides above normal sea levels causing the coastline and nearby tidal rivers to flood. While flooding occurs naturally, some floods are considered extreme and require measures that reduce both their risk and impact. Diamond Creek Eltham North 2005 FLOODS AND FLOODING: A NATURAL PART OF MELBOURNE FLOOD IMPACTS: PEOPLE, PLACES Intangible damage, is about the indirect AND PROPERTY impact that flooding has on people and their lives. It is difficult to measure in monetary Flooding deeply affects individuals, terms but is often emotionally crippling and households, and the community long after long lasting and are the effects that most floodwaters have subsided or receded. flood victims have to endure. Therefore, the impacts of flooding and flood damage are generally classified astangible Examples of intangible damage include: and/or intangible. • Perceived loss of security at home Tangible damage is about the financial and • Emotional suffering practical impacts of flooding. It is easy to • Loss of irreplaceable memorabilia. measure in monetary terms and refers to things that have been affected or destroyed, including property, services, utilities, cars and infrastructure. Examples of tangible damage include: • Damage to buildings and contents • Clean-up costs for home owners • Loss of stock for businesses • Public clean-up costs • Loss of business or trade • Value of lost earnings because of the disruption. Lollypop Creek Werribee 2005 THE HISTORY OF FLOODING IN THE REGION The Great Flood – Riverside suburbs of Collingwood, 1891 Richmond and Prahran flooded, with approximately 3,000 people left homeless. One of the largest floods to occur on the 1906 Maribyrnong River - 5.18m. 1916 Another major flood on the Maribyrnong River. 350mm of rain fell in 48 hours over the 1934 Yarra catchment, with 140mm falling over metropolitan areas. 6,000 people left homeless and 18 people died. The second largest flood on the Maribyrnong River, 1974 occurring on 15 and 16 May when 110mm of rain fell on the catchment over a 48 hour period. The Maribyrnong Flood Warning System installed 1975 to continuously monitor catchment conditions, rainfall and flows. Widespread riverine flooding causing little property 2005 damage but did cause localised disruption to transport infrastructure. Flash flooding in CBD and metropolitan areas 2010 resulted in hundreds of thousands of dollars damage to property. FLOOD MANAGEMENT: MAKING GREATER MELBOURNE SAFER WHAT IS A FLOODPLAIN? Floodplains are divided into two main parts: A floodplain is an area of mostly flat land 1. The Floodway: the main area affected surrounding a river, stream or creek that by an overflow, where the majority of experiences flooding when one of these water gathers to create a new, stronger waterways overflows its banks. water path 2. The Flood Fringe: the outer area that lies beyond the floodway, where less water gathers and it moves much slower. As defined in the Water Act 1989, Melbourne Water is responsible for managing the floodplains in the Port Phillip and Westernport region of Victoria. Flood level Normal water level Stream channel Fringe Floodway Fringe Little River Werribee 2005 FLOOD MANAGEMENT: MAKING GREATER MELBOURNE SAFER MANAGING MELBOURNE’S Melbourne Water uses the 100-year ARI DRAINAGE SYSTEM standard to define the extent of the overland flooding along the drainage system or along The drainage system is designed to control waterways, and to specify flood levels for the flow of water by collecting and funnelling example in flood warnings and to assess surface water to waterways and the bay. development applications. We work closely with councils, developers and Of the 100,000 properties that are currently landowners to ensure that the drainage system at risk of flooding in the Port Phillip and continues to function properly as Melbourne Westernport Region 82,000 of these are at grows. We assess new developments and risk of flooding from overland flows from the redevelopments of land to make sure they are drainage system. adequately designed to protect people and property from floods and to ensure the health Street gutters and of local waterways is not adversely affected. street drains collect A 1 in 100 year drainage standard has been stormwater run-off adopted as it is considered an appropriate COUNCILS balance between the likelihood of and MELBOURNE WATER the consequences of flooding for most developments. This is a commonly used international standard. The standard, correctly referred to as the 100-year