Chapter 3.1 Stream Morphology, Drainage and Flood Management
Port Phillip and Westernport Regional Catchment StrategyRelevant Objectives:RCS-WO2 Protect and improve the environmental health and social and economic values of waterways and wetlands. RCS-WO5 Ensure the management of water resources minimises risks to natural ecosystems, public land, private assets and public safety Relevant Regional Targets:RCS-WT6 Improve the condition of the regions waterways so that: RCS-WT7 Progressive improvement in the condition of waterways across the region, as measured by the Index of Stream Condition, including the bed and banks, streamside zone and aquatic life. RCS-WT9 No loss of hydraulic capacity and environmental values of floodplains. RCS-WT10 Timely flood warnings provided for all major waterways and risks to infrastructure minimised. RCS-WT15 No net loss in the extent and health of wetlands of each existing type. RCS-WT16 Progressively improve the overall health and social value of natural wetlands, including those that are nationally and internationally recognised. Note that in these objectives and targets, RCS stands for Regional Catchment Strategy, and W stands for water. |
Stream morphology
The alignment and geomorphological form of Merri Creek and its tributaries originates from the uplift of the underlying Silurian siltstones and mudstones creating a fall towards the south. More recently (geologically speaking) the spread of lava southwards from twelve separate eruption points including Mount Fraser, Green Hill, Bald Hill and Hayes Hill in the area between Wallan and Craigieburn formed the basalt plains characteristic of the catchment. The youngest of these lava flows was from either Mount Fraser or Hayes Hill some 800,000 years ago. With the lava surface sloping down towards the east and south from these eruption points, the creeks course came to be determined by natural depressions and ridges in the lava surface and by the major fractures and joints in the volcanic rock (Rosengren, 1993). Today parts of the beds of Merri Creek and its tributaries have cut through the basalt to the underlying Silurian rock.
Merri Creeks natural geomorphology has been extensively disturbed since European settlement. Opportunities for preservation of the stream geomorphology and the protection of some of the natural interactive processes between the stream and its floodplain have been largely lost. PIRG (1975) and Rosengren (1993, p. 8-9) have detailed the nature of human activities over the past 150 years which have significantly altered Merri Creek and identified those sites most affected. Rosengren concludes that a highly modified environment has been produced over long sectors of Merri Creek, especially at Wallan East, south of Bald Hill, Donnybrook and Craigieburn East, and virtually continuously from Campbellfield to the Yarra River. Merri Creeks tributaries have suffered similar fates. Known sites of geological significance are discussed in chapter 2.2 of this report, and Melbourne Waters Merri Creek Waterway Management Activity Plan indicates a number of sections which are less disturbed and of value because of their relatively intact stream morphology.
In the upper reaches the most significant changes to stream and floodplain morphology have come about because of works to improve drainage and mitigate flooding. These involved straightening and channelisation of significant segments of the Merri Creek and tributaries Wallan, Mittagong, Taylors and Kalkallo Creeks, and drainage of extensive swamps in the early to mid 1900s.
Prior to being drained in the 1940s Hernes Swamp, on the main stem of the Merri, covered about 300 ha when full and held water for up to six months. It supported thousands of waterfowl and ibis. Together with the former 600 ha Inverlochy Swamp to the west of the Hume Highway in the Kalkallo Creek sub-catchment and the 100 ha Camoola Swamp to the south-east on the Merri these swamps formed a shallow freshwater marsh system of over 1000 ha.[216] Although drainage channels have been constructed through the swamps, some of the areas are still prone to flooding and retain significant amounts of water after heavy rains.
Apart from their important nature conservation role, these swamps had a vital hydrological role in retaining flood waters, a role which although diminished is still partly present (see section on Flooding).
Floodplains and wetlands such as at Sumner Estate, Northcote, and Northcote Golf Course were reclaimed in the early 20th century.
Extensive channel modification work by the Melbourne Metropolitan Board of Works (MMBW) over many decades up to the mid 1980s was principally responsible for the changes to the urban reaches of the creek and its tributaries. Bends were straightened in many places, the channel form was deepened and modified to a trapezoidal shape, Edgars Creek in Reservoir and Thomastown was converted to a concreted channel, and Merlynston Creek in Coburg was piped underground. In-stream woody debris was removed to the extent that in many reaches, none is now present.
In the upper catchment, until recently outside the area of Melbourne Water responsibility, parts of Wallan Creek and Taylors Creek in Wallan have been piped underground. Many smaller tributaries throughout urban parts of the catchment have likewise been piped.
Many of these works were designed to increase the capacity of the waterways to carry flows from flood events. While these works may have solved short-term local concerns, they have also reduced opportunities to manage the streams as a natural resource due to loss of natural morphology. Loss of the streams natural floodplains through filling and development is a further element in the degradation of these natural systems.
Typically, streams across the basaltic soils of Melbournes north and west display a pool and run form where they successively pond at regular points before entering a run or riffle sequence. Where modification of the creek channel has caused the complete loss of natural form (such as concreted sections of Edgars Creek) only comparatively minor improvement works in the form of retrofitting riffles can be undertaken to alleviate some of this loss and create a more natural stream form. Some sections of the waterways which were channelised but not concreted are beginning to re-form a more meandering pattern with occasional pools and riffles more closely resembling the natural morphology. Other sections of the creeks (e.g. parts of Edgars Creek in Coburg North or Merri Creek through Galada Tamboore) retain good natural morphology. Opportunities are present in some reaches of the creeks to modify the trapezoidal channel form through earthworks to achieve a more natural bank profile.
Besides protection of the creeks remaining morphology, a related key element of waterway management is protection of the existing channel and bank stability. Creek banks which are subject to erosive forces can, if left untreated, contribute massive amounts of sediment to the creek, the Yarra and Port Phillip Bay. Excessive sediment levels are a major contributor to loss of stream life and its diversity.
The mid to lower reaches of the Merri Creek and tributaries on the whole do not suffer significant bank erosion problems. In some parts this may be due to the restriction on valley widening brought about by the toughness of the lava flows defining the stream. Rosengren (1993) describes Merri Creek as having, for the most part, well-defined flows within a narrow valley bounded by steep slopes and bluffs with rocky cliffs and gorge sectors.
The upper reaches of the Merri Creek are described as suffering moderate to extensive bank instability (Amenta 2002) due mainly to stock access. Some parts of the upper reaches also display active gully erosion and slumping of banks. Kalkallo Creek is described as experiencing active gully erosion, extensive scour on bends and undercutting of banks. This is particularly the case for its western most tributary which rises on sedimentary rock.
Edgars Creek where it flows through the base of an ancient swamp at North Coburg is one of the few sites within the urban section of the Merri system where significant erosion of the stream bank has taken place in recent times. Bank reinforcement works involving the placement of large basalt boulders were undertaken by Melbourne Water in 1996/7.
The Port Phillip and Westernport Regional River Health Strategy (2007) has a target of improving the condition of stream habitat and stability for the Merri Creek in urban reaches, from moderate to good. In rural reaches it aims to prevent any further damage to habitat and stability which is rated as being in moderate condition.
Stream Flow and Hydrology
Stream flows and their effects on the waterway ecosystem have been inadequately studied along the Merri waterways. In pre-European settlement times flows were probably minimal during late summer/autumn with even major creeks such as the Merri reduced to a series of pools. Smaller tributaries probably dried up entirely and wetlands seem to have been seasonal. Today the effects of summer drying are exacerbated by diversions, including farm dams for stock and domestic purposes in rural areas and for commercial or irrigation purposes.
Urbanisation also exacerbates summer drying (see Box below) although water main leakage may offset this somewhat at present. Yarra Valley Water has a program in place to actively identify points of leakage in the water main system and eliminate them. This work is possible following the installation of zone metering, which enables us to more accurately pinpoint areas in which unaccounted-for water use is high.
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URBANISATION AND HYDROLOGICAL CHANGES Typically, urbanisation leads to an increase in impervious area and decreases in infiltration, use by vegetation, and natural valley storage. This results in well documented hydrological changes including increased storm peak flows, higher storm runoff volumes, more frequent higher-flow events and bank-full flows in the receiving waters, and less base flow. Where the stormwater drainage system is directly connected to local waterways (i.e. without end of pipe wetlands), even quite small rain events produce increased stream flow compared to non-urban catchments. Use of water sensitive urban design features which provide for reuse of stormwater (e.g. rainwater tanks) or greater infiltration of stormwater (e.g. vegetated swale street drainage, raingardens) can greatly reduce the extent of these hydrological changes, reducing the frequency and intensity of higher flow events, and improving base flow. |
Extension of the metropolitan sewerage system to the outer northern suburbs of Melbourne has led to the redundancy of the Craigieburn Sewage Treatment Plant (STP), which discharges treated sewage to Merri Creek. In order to meet State Environment Protection Policy requirements the level of treatment of the effluent must be improved or discharge cease. The base flow provided by the STP may be beneficial in some ways to the Creek; in other respects it may be having detrimental effects[217].
Any assessment of the situation is complicated by the fact that areas upstream of the STP are rapidly urbanising bringing predictable hydrological changes. Of key concern is the downstream population of Growling Grass Frogs. It is not possible to determine with any certainty whether this population would be adversely effected by closure of the STP and it has been suggested that the most prudent approach would be to maintain a flow[218]. In 2003 Yarra Valley Water proposed closing the plant, and replacing its discharge with a discharge from the Aurora Treatment Facility being built to recycle water for the new suburb of Aurora to the east.
The possible removal of the STP flow raised the question of what is the desired flow regime for the Creek. This matter is complicated by the fact that in many respects Merri Creek is highly modified, and by the unknown impacts of such things as rural dams, and the extent of new urban areas. The latter will undoubtedly lead to changes in stream hydrology, but the extent of this depends on the degree to which water-sensitive urban design is implemented and the extent to which stormwater use or reuse occurs.
In established urban areas stormwater runoff has led to increases in both peak flows and peak velocities. Further, the removal of stream meanders has meant that quiet areas of water for fish breeding and habitat have been virtually lost in the lower catchment (see Deep pool refugia below).
Melbourne Water has undertaken limited analysis of flows in Merri Creek as part of their Stream Flow Deviation Project. However, an understanding has not been achieved of what the Merri Creeks natural flow regime is. Such an analysis would be part of a Stream Flow Management Plan, which Melbourne Water has undertaken for a number of waterways, but is not planning to do for Merri Creek.
Yarra Valley Water has recently revised its strategy for the provision of sewerage services to the Craigieburn area. This has resulted in the continued operations of the Craigieburn STP. In consultation with Melbourne Water, EPA and other relevant stakeholders such as MCMC, it is also undertaking a FLOWS study to determine an ideal flow regime to sustain the habitat for key values of the Merri Creek downstream of the Craigieburn STP discharge point such as the Growling Grass Frog. The results of this study will contribute to the long term operating strategy of Craigieburn STP and may result in a change to the EPA operating licence in terms of minimum daily flow to be discharged to the Merri Creek (which is currently not specified).
YVWs Aurora STP, although located in close proximity to Merri Creek, will have no discharge to the Merri Creek as long as Craigieburn STP remains in operation. Effluent treated at the Aurora STP is stored in a 280 ML open storage dam and is then used to produce Class A recycled water from their Aurora Recycled Water Treatment Plant (RWTP). This recycled water is supplied to customers via a third pipe system. The Aurora RWTP is currently being commissioned and is expected to be operational in early 2009.
According to the Port Phillip and Westernport Catchment Condition Report 2004/5, environmental flows are in place for Merri Creek; however it is not clear how this assessment has been made given the lack of understanding of what natural or desirable flows are.
Diversions from Merri Creek are controlled by Melbourne Waters Local Management Rules. The rules aim to limit diversions to a sustainable diversion limit for the Creek (derived from the Stream Flow Deviation Project), and to stop diversions when the flow in the Creek drops to an identified minimum level. The minimum flow is based on the flow that was exceeded at least 95% of the time since 1986. Farm dam licences issued before July 2004 are exempt from the flow requirements.
Deep pool refugia
Areas with permanent water are more likely to contain the highest aquatic biodiversity and have greater significance as refuge sites from where biota can recolonise areas in favourable seasons, particularly where these are located in reaches which regularly dry out. Such refugia can be particularly important for fish species as they can be long lived and disperse widely.
Protection of these deep pools and their immediate environs should be a high priority.
Flood Management
Types of Flooding
Flooding can be classified into two main types:
· riverine flooding, which occurs when water spreads out from a creek or river when it overtops its banks; and
· overland flooding, which occurs when localised stormwater flows from intense rainfall events exceed the capacity of underground drains, and travel overland finding the lowest path to the nearest waterway.
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ARI: Average Recurrence Interval The average interval in years between the occurrence of a flow, discharge or rainfall greater than or equal to a specified amount. It is implicit in this definition that the periods between exceedances are generally random. |
Underground drainage infrastructure is generally designed to cater for up to 5-year Average Recurrence Interval (ARI) events although there are examples in older parts of the Merri catchment where capacity is exceeded more frequently than this.
In more recent urban design larger events are accommodated through overland flow paths designed into the road network or through reserves set aside as floodways. Storms can therefore result in localised flooding when the capacity of the drainage system is exceeded, the overland flow path is poorly defined or non-existent, becomes blocked for some reason, or is inadequate for a larger storm event.
In older parts of the Merri catchment, developed prior to the 1970s, numerous properties are affected by overland flows because dedicated paths to carry excess stormwater along roads and reserves were not adequately created when the areas were subdivided. This results in flows following natural drainage paths through valley lines and properties. Properties in Thornbury in the subcatchment of the Preston Main Drain (a tributary of the Merri), which were flooded following extreme rain events in December 2003 are in this category.
In all of Melbourne there are about 80,000 properties exposed to risk of flooding from overland flows.
Major floods on Merri Creek in 1963 and 1974 affected numerous properties through riverine flooding. Subsequent works have reduced this flooding risk (see below).
Flood Protection
After the major flood along Merri Creek in May 1974 the then Melbourne and Metropolitan Board of Works acquired several parcels of land for construction of retarding basins to manage such events. A major retarding basin was constructed on Kalkallo Creek in Mickleham, below Gums Gully Road, as well as retarding basins on Campbellfield Creek in Fawkner and Merlynston Creek, Dallas. Land was also purchased at Galada Tamboore, in Campbellfield/Thomastown for the proposed Campbellfield retarding basin on Merri Creek itself. Due to the completion of the other retarding basins and levee works at Thornbury, Northcote and Brunswick (see below), this retarding basin is unlikely to be required within the next 20 years.[219]
As mentioned earlier the swamps of the northern catchment still perform some flood control function. In a 1974 evaluation of the severe flood on Merri Creek in Brunswick and Northcote that year, MMBWs Chief Engineer of Main Drainage, C.T. Earl said these swamps are of importance in the natural regime of the Merri Creek and it is considered these formations, with the associated barriers of rock across the valley control peak flooding along the downstream sections of the creek. At times of high rainfall in the Merri catchment &.several thousand acre feet of water is stored temporarily on these lands during major flooding and as a consequence potential peak flows from the upper catchment are retarded by in-catchment storage. 2006 correspondence from Melbourne Water indicates that the Hernes Swamp area provides valuable reduction in flows to the downstream creek system, reducing flows to about 25% of inflows[220].
In the 1980s numerous flood mitigation works were also carried out by the MMBW in the lower catchment. Examples are earthen levee projects integrated into parkland at Strettle Reserve/Anderson Road in Thornbury, Winifred Street in Northcote and Alister Street in East Brunswick. These sites demonstrate that flood mitigation works can be designed in a manner which is environmentally sensitive and which meet multiple-objective waterway management requirements.
Retarding basins can be designed in such a way as to enhance waterway habitat values. The Merri Park wetland in Northcote is located within a small retarding basin and provides a range of ephemeral wetlands and indigenous vegetation. The Campbellfield Creek Retarding Basin has also been redesigned to provide a full water quality treatment wetland within the base of the retarding basin. Other retarding basins have potential to be similarly transformed.
Despite these works a number of urban properties are still located within areas prone to flooding. The presence of floodplains is generally indicated in the planning scheme through the Urban Flood Zone, the Land Subject to Inundation Overlay and the Rural Flood Overlay. Information for these overlays is provided to Councils by Melbourne Water, with the Councils being responsible for the appropriate amendments to incorporate the overlays into the planning scheme.
The susceptibility of properties to overland flow flooding is usually indicated in local planning schemes through designation of a Special Building Overlay. This overlay triggers a requirement for a planning permit and typically requires that floor levels be raised a specified height above the 100 year ARI when new building works are being undertaken.
Although overland flooding often results in community demands for the building of new drainage infrastructure to give better protection to properties affected by flooding, this is often very expensive and can to be quite disruptive in existing urban areas.
Melbourne Water has developed a Flood Management and Drainage Strategy for the Port Phillip and Western Port region. It covers roles and responsibilities for urban drainage management, targets, service level standards and methods for prioritising mitigation works to reduce intolerable flood risks. Importantly it identifies the need to better understand how communities are affected by flooding and the need to increase community understanding and preparedness for floods. A key target is that all Councils have Flood Management Plans in place by 2013.
Urban Development and Flood Protection
Current standards require that Merri Creek, its tributaries and associated floodplains should be capable of storing and transmitting the 100 year ARI flood event generated from the catchment, without impact on life and property. Achieving and maintaining this capacity is primarily Melbourne Waters responsibility.
The conduct of this responsibility means that Melbourne Water and Councils implement appropriate drainage strategies and works to address the provision of that level of flood protection (see Drainage Strategies below). Critical to this is ensuring that no development is permitted that would reduce the hydraulic capacity of floodplains. In practice this does not mean no development at all in floodplains. The conditions under which such development can occur are described in Melbourne Waters Land Development Manual (Melbourne Water 2008).
Drainage strategies may be written with knowledge of environmental values, but do not necessarily seek to implement outcomes other than flood protection. For example, other relevant targets include WT9 No loss of &environmental values of floodplains and WT16 Progressively improve the overall health and social value of wetlands& from the Port Phillip and Western Port Regional Catchment Strategy.
Melbourne Waters area of responsibility was expanded in 2005 to include the upper Merri catchment (mainly in the Mitchell Shire). Flood modelling is being undertaken in the upper Merri catchment to determine floodplain capacity needs, including potentially, the re-establishment of lost capacity. This is particularly important given the extent of urban development occurring in the Wallan, Taylors and Kalkallo Creek sub-catchments.
All new urban developments, both residential and industrial/business are required to meet urban drainage standards (Standard C25 from clause 56.07 of the planning scheme) which importantly should not increase flood levels on upstream or downstream properties. This is an ongoing challenge as urban development extends northwards from Craigieburn and Epping, and as Wallan and Beveridge in the upper catchment expand. Generally land needs to be set aside for retardation purposes. This is because the increased proportion of hard surfaces associated with urban development leads to increased volumes and accelerated velocities of stormwater runoff. According to the Hume Growth Area Report approximately 10 per cent of the land area for each stormwater sub-catchment will be required for retardation purposes.
The lack of oversight of the upper catchment by a water authority prior to 2005 has meant that some inappropriate rezonings have been approved that now require fixing. For example the Mixed Use Zone and Development Plan Overlay southeast of Wallan encroach significantly on Hernes Swamp, which is useful as documented above for holding floodwater, and inappropriate for development because:
· Development within the flood zone of Hernes Swamp will affect its hydrology and capacity, and ultimately Merri Creek downstream
· The development will drain into Merri Creek and development should not occur unless water quality, storm water flows, and flood protection measures are in place
· Plains Grassy Wetlands are so rare they are considered extinct in the Victorian Volcanic Plain[221], and associated species are in decline, so these are the areas most in need of protection and restoration.
Developers seem keen to develop these areas despite the flood hazards and environmental values.
Willow and other woody weed control and drainage function
A key and sometimes controversial element of waterway management is willow control. Willows are a problem because of their capacity to spread rapidly from branch fragments, and their ability to change stream form due to their matting root form. In some reaches they can retard flood flows and in a severe flood, break off, be washed downstream, lodge against a bridge, capture large volumes of debris putting pressure on the bridge and ultimately cause the bridge to fail. Willows also diminish stream eco-systems through shading, and reduced food availability and habitat quality. In particular growth of macrophyte plants in the stream, which provides much of the in-stream habitat, is severely reduced by dense overhanging willows.
Nonetheless, sections of the community value mature willows (especially Weeping Willows) for aesthetic reasons and control works must be managed with these sensitivities in mind. Generally weeping willows are only removed if their health is declining and they are showing signs of breaking up. Willow control works along Merri Creek have been conducted for many years by Melbourne Water, Councils and the Merri Creek Management Committee (MCMC). Melbourne Water has developed a control procedure for willows which sets out the scope of the issues and the methods for control (Melbourne Water 1997).
Other exotic woody weeds such as Desert Ash can cause similar problems. Methods to optimize environmental benefits of woody weed removal are discussed further in chapter 3.3.
Various Melbourne Water woody weed and revegetation projects have seen severe infestations of woody weeds replaced with indigenous vegetation especially in the last five years. Works have occurred along reaches of Merri Creek in Brunswick/Northcote, and Queens Parade to Carr St in Fawkner-Coburg.
Native vegetation management and drainage function
Melbourne Water, Councils, MCMC, Parks Victoria and private landowners are involved in management of the remnant flora and fauna of the waterways and their riparian zones as well as revegetation and habitat enhancement (discussed further in Chapter 3.3 and section 2)
An important consideration in revegetation works is the need to maintain the essential drainage function of the waterway, especially its ability to handle major flood events. Excessive use of shrubby species in revegetation works, particularly immediately upstream and downstream of major road crossings or critical culverts, can slow flow during major flood events and raise flood levels. In some instances along Merri Creek this may endanger the structural integrity of crossings and even threaten the ability of nearby levees to hold back flood waters. As a result, Melbourne Waters bank revegetation projects limit the use of shrubby species. Details of this approach are described in the Merri Creek Waterway Management Activity Plan.
The Port Phillip and Westernport Regional River Health Strategy (2007) has a target of improving the condition of streamside vegetation for the Merri in urban reaches, from very poor to poor. In rural reaches it aims to prevent any further damage to streamside vegetation condition.
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Urbanisation and the protection of ephemeral streams the Edgars Creek example One of the impacts of the hydrological change which accompanies urban development is the change to local waterways, as described in the stream flow and hydrology section (above). These changes are perhaps seen most dramatically in ephemeral streams in which increased flows not only change their basic flow characteristics, but also lead to changes in bed and bank form and higher rates of erosion because of the greater flows. An innovative approach to this problem has been proposed for the new suburb of Aurora in North Epping. Aurora is located on the upper reaches of Edgars Creek, a major tributary of the Merri. In this location the Edgars Creek drains surrounding rural land and is ephemeral. Although in poor ecological condition, the creek is considered to have at least local geomorphological significance and to be at risk of increased bed and bank erosion after urbanisation (Prentice 2007). In order to retain the ephemeral nature of the creek and reduce the risk of erosion and minimise the impact on the existing geomorphology, the waterway concept for Aurora involves:
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Urbanisation and waterway hydrology
Implementation of water-sensitive urban design (WSUD) is likely to become an integral part of new development and has a positive benefit in reducing the impacts of hydrological changes associated with the increase in impervious surfaces in urbanised catchments.
As urban consolidation in the lower parts of the catchment proceeds, resulting in a reduction in permeable area of private gardens, retrofitting of WSUD features will play an important role in reducing (or even reversing) the impact on hydrology. The redevelopment of activity centres and large infill sites provide opportunities for large scale incorporation of these features. For individual houses, the installation of a reasonably large water tank or rain garden can make a measurable difference. Standard C25 in Clause 56.07-4 of the State section of the planning scheme specifies stormwater drainage standards. Melbourne Water has identified the need to review these to better reflect the need to manage physical impacts of increased urban runoff on waterways (Waterways Water Quality Strategy 2008).
The City of Whittlesea has some concerns with lot-scale rain gardens because of potential access and maintenance problems (assuming Council may be required to maintain this infrastructure, or that residents may not adequately maintain lot scale rain gardens themselves.
Development Services Drainage Schemes
Melbourne Waters Development Services Schemes provide the orderly provision of infrastructure in growth areas such as underground drains, overland flow paths, retarding basins, wetlands and stormwater treatment measures, and specify the appropriate treatment and protection of rivers and creeks.
Redevelopment Services Schemes ensure that redevelopment in existing urban areas is properly serviced and that higher density development does not reduce existing levels of flood protection
Drainage schemes enable appropriate planning to recover costs of infrastructure necessary to ensure urban development meets the current Melbourne Water standards for flood protection and environmental protection.
Merri Catchment Drainage Schemes:
Redevelopment Schemes:
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The best practice approach to stormwater treatment utilising distributed Water Sensitive Urban Design (WSUD) requires significant resourcing at the local level. This is because distributed WSUD can occur right down to individual lot level (i.e. in catchments much smaller than 60ha and hence Councils responsibility, not Melbourne Waters). The switch from end-of-pipe wetlands to distributed WSUD treatment entails some change in management and maintenance responsibility from Melbourne Water to local government. Funding arrangements must therefore be developed to ensure that WSUD management and maintenance costs are born equitably across the community, in concert with a better understanding of the lifecycle costs of WSUD.
As an incentive, developers who implement WSUD on their properties are given a discount on the stormwater quality component of the drainage scheme charge.
Drainage schemes first require the development of a drainage strategy with mapping and designation of flood prone areas to ensure these areas are protected from filling and other potentially inappropriate impacts of development.
Climate change
The identified likely climate change trends for southern Victoria include increased average and summer temperatures, reduced rainfall, reduced stream flows, and more extreme events with more hot days, more dry days and increased rain intensity during storm events. More intense storms may cause more frequent overland floods in some urban areas.
Melbourne Water has identified climate change as a challenge for management of river health and flood protection. Current modelling is based on past pattern of rainfall, utilising Australian rainfall standards. Revision of modelling and flood extent mapping will need to take into account potential long-term pressures on the existing drainage system created by urban consolidation and climate change[222]. It is unclear when this will be done and what it means for the Merri Catchment.
Relevant Regional ActionsRCS-WA7 Implement the Port Phillip and Western-port Regional River Health Strategy (MW with local govt and community, etc) RCS-WA11 Reduce by 500 the number of properties vulnerable to a one in 100 years flood. (MW with local govt) RCS-WA12 All new developments constructed with floor levels at the required safety margin above one in 100 years flood levels (MW with local govt) RCS-WA13 Develop and incorporate into planning schemes, protocols that contribute to the protection of the environmental values of floodplains (DSE with MW, local govt, CMA) |
Key references
Amenta V. (2002) Summary of water quality and stream health monitoring data. Appendix 2 Part B, Merri Creek Waterway Management Activity Plan (Final Draft), Melbourne Water
Beardsell C. (1997) Sites of Faunal and Habitat Significance in the North East Melbourne, A report prepared for the North-East Region of Councils (NEROC) by Dunmoochin Biological Surveys, Melbourne.
Department of Natural Resources and Environment (2002e) Victorias Native Vegetation Management: A Framework for Action, DNRE Melbourne.
Department of Sustainability and Environment (2005c), Hume Growth Area: Towards Melbourne 2030. Final Report July 2005, DNRE Melbourne.
Earl, C.T. (1974) The Merri Creek Flood, 15-16th May 1974, Cities of Northcote, Brunswick. Main Drain Division, Melbourne and Metropolitan Board of Works.
Melbourne Water (2003) Merri Creek Waterway Management Activity Plan Final Draft 2003.
Melbourne Water & Port Phillip and Westernport Catchment Management Authority (2007) Port Phillip and Westernport Regional River Health Strategy, Melbourne Water, Melbourne.
Melbourne Water (2006) Waterways Operating Charter
Port Phillip and Westernport Catchment Management Authority (2004) Port Phillip and Western Port Regional Catchment Strategy 2004-2009.
Prentice, J. (2007) Stormwater management and Edgars Creek strategy evidence. Expert witness report provided to Planning Panels Victoria, Whittlesea Planning Scheme Amendment C41. Coomes Consulting Group.
Public Interest Research Group (PIRG) (1975) The Merri Creek Study, PIRG, Melbourne.
Issues
1. Sections of creek with relatively intact stream morphology are valuable and should be protected.
2. Swamps in the upper catchment still have a vital role in retaining floodwaters.
3. These swamps also provide opportunities for wetland rehabilitation.
4. Channel modifications have reduced opportunities to manage streams as a natural resource. Where the waterway is not re-establishing a pool and run form, opportunities are present in some reaches of the Creeks to re-form the channel to achieve a more natural profile, including retrofitting of rock riffles, in part to help re-create pools.
5. Some tributaries are suffering moderate to extensive bank instability (e.g. Kalkallo Ck). The resulting sedimentation can cause loss of stream life and diversity.
6. Desirable flow regimes for the Merri Waterways are not yet known, and catchment changes may be making the flow regime worse. A stream flow management plan would address this.
7. Protection of deep pool refugia is a high priority.
8. Many older urban areas in the Merri catchment do not have adequate overland flow paths.
9. Construction of the Campbellfield Retarding Basin at Galada Tamboore, or restoration of the storage capacity and function of the Hernes, Inverloch, Inverlocky or Camoola Swamps may be required to avoid worse floods.
10. There is scope to transform additional retarding basins in the catchment to improve habitat values and provide full water quality treatment wetlands within the floor of the retarding basin.
11. Some existing urban areas are still prone to riverine flooding and many more to overland flow.
12. Urban consolidation in the lower catchment has the potential to increase the amount of stormwater runoff and worsen flooding. On-site management using water-sensitive urban design features can help manage this to some extent, as can Redevelopment Services Schemes.
13. Greenfield development on the urban fringe has the potential to increase runoff volumes and rates and worsen flooding downstream unless appropriate measures are taken to reduce runoff and retard flows back to pre-development conditions.
14. Completion or preparation of Melbourne Water drainage schemes is needed to ensure flood levels do not increase upstream or downstream.
15. Localised water-sensitive urban design features are helpful in controlling runoff increases for lower end rainfall events. Councils need to be alert to these opportunities and the requirements of the planning schemes in their administration of planning processes.
16. A better understanding of lifecycle costs of WSUD is needed so that equitable funding arrangements for WSUD can be developed between local governments and Melbourne Water.
17. On an individual household level, and where applied extensively, installation of water tanks of a reasonable size and/or rain gardens can contribute to reducing the rate and volume of runoff.
18. At the street level, bioretention systems, such as grassed swales and raingardens can reduce the contribution of road runoff and its pollutants to waterways.
19. Waterways and floodplains in the catchment have lost much of their native vegetation, particularly in the urban area. Major revegetation programs are needed.
20. It is unclear what impact climate change will have on the Merri catchment and when flood modelling will be able to take this into account.
21. Willows and other woody weeds can create flooding problems (as well as ecological ones), but exotic tree removal can be controversial and should not be undertaken without adequate community consultation processes.
22. Inappropriately located shrubby vegetation immediately upstream and downstream of critical culverts (road crossings), has the potential, in major flood events, to raise flood levels and threaten the functioning of the culverts and nearby levees. Revegetation works need to be designed to avoid making flooding worse.
Objectives
1. Protect and improve the environmental health and social and economic values of waterways and wetlands (RCS-WO2).
2. Management of water resources to minimise risks to natural ecosystems, public land, private assets and public safety (RCS-WO5).
Targets
1. No loss of hydraulic capacity and environmental values of floodplains (see RCS-WT9).
2. Risks to infrastructure and life and property by flooding are minimised (see RCS-WO5).
3. Urban runoff from all new developments meets Standard C25 in the planning scheme.
4. Flood control works are aesthetically pleasing and capable of supporting a range of other uses especially recreation and habitat.
5. Design of greenfields urban developments incorporates water-sensitive urban design features at allotment (e.g. rainwater tanks & raingardens), streetscape (local bioretention swale systems in road reserve) and neighbourhood (local wetlands and/or swales, raingardens) scales to achieve optimum infiltration and minimise hydrologic changes.
6. Greenfields development takes into consideration the ephemeral nature of many of the waterways of the upper Merri catchment and the desirability of retaining the geomorphological character of these ephemeral streams.
7. Infill urban development incorporates water-sensitive urban design measures to reduce runoff volumes and rates and improve water quality.
8. In existing residential areas measures are encouraged which reduce peak runoff, including water tanks, infiltration beds, porous surfaces etc.
Actions
See Section E page 199.
[216] Beardsell Vol. 2 p. 14
[217] see Egis, Frood, YVW
[218] Robertson & Heard
[219] Letter to Friends of Merri Creek from Melbourne Water 11 July 2006.
[220] Letter to Mr. Ray Radford from Grant Wilson, Melbourne Water, dated 20/7/06
[221] PPWCMA (2006) file Victorian Volcanic Plains.xls on enclosed CD, although small areas do persist in the Merri catchment.
[222] Draft Melbourne Water Waterways Operating Charter 2008/09 to 2012/13
Merri Creek Management Committee. 2 Lee St, East Brunswick, Victoria, Australia 3057 