Water requirements for the rehabilitation of Bolin Bolin Billabong Remnant standing water in Bolin Bolin Billabong, February 2010 Report to Parks Victoria Dodo Environmental 15 Yawla Street, McKinnon VIC 3204 [email protected] (03) 9557 3342 25 April 2010 Document control sheet Report title Water requirements for the rehabilitation of Bolin Bolin Billabong Version Final report Date 25 April 2010 Author Dr Paul Boon Client Parks Victoria (Garry French, Parks Victoria – Westerfolds Park) Contact details Dr Paul I Boon Dodo Environmental 15 Yawla Street McKinnon VIC 3204 AUSTRALIA Phone/fax: (03) 9557 3342 e-mail: [email protected] ABN: 12 365 734 616 Disclaimer This report has been prepared on behalf of and for Parks Victoria. Dodo Environmental accepts no liability or responsibility for or in respect of any use of or reliance upon this report by any third party. The report was prepared in accordance with the scope of work and purposes outlined in the proposal. It is based on generally accepted practices, knowledge and standards at the time of preparation. No other warranty, expressed or implied, is made as to the professional advice included in this report. The report shows the approach taken and sources of information used by Dodo Environmental; I have not necessarily made an independent verification of the information used to prepare the report. The report is based on information available and conditions encountered at the time of preparation; Dodo Environmental disclaims any responsibility for any changes that may have occurred since then. Dodo Environmental does not warrant this document is definitive nor free from error and does not accept liability for any loss caused, or arising from, reliance upon the information provided herein. Contents Executive summary 4 1. Scope of the project 7 2. General approach, information sources and terminology 2.1 General approach 8 2.2 Information sources 10 2.3 Some notes on terminology 12 3. Historical perspective, current ecological condition, and threatening processes 3.1 Landscape context and neighbouring billabongs 14 3.2 Social significance 15 3.3 Ecological value 16 3.4 Hydrology and wetland type 16 3.5 Conceptual models of billabong structure and function 16 3.6 Ecological condition – findings of the literature review 18 3.7 Changes to Bolin Bolin Billabong since European colonization 24 3.8 Hydrological regimes at Bolin Bolin Billabong 31 3.9 Vegetation responses to chronic desiccation 33 3.10 Main threatening processes 38 3.11 Synthesis 41 4. Management objectives 43 5. Hydrological requirements for rehabilitation 5.1 Flows in the Yarra River required for billabong inundation 45 5.2 Hydrological regime required for rehabilitation – broad recommendations 49 5.3 Fine-tuning the water regime to maximise biodiversity 51 5.4 Sources and volumes of water required 54 5.5 Potential hazards with use of treated storm-water 62 5.6 Synthesis and conclusions 65 5.7 Assessment of likely rehabilitation success 65 6. Implementation and on-going management 6.1 Importance of adaptive management approach 68 6.2 Need for long-term monitoring program 69 7. References 72 3 Executive summary Dodo Environmental was contracted by Parks Victoria in February 2010 to undertake an analysis of the water requirements of Bolin Bolin Billabong, a high-value ox-bow lake on the floodplain of the Yarra River in metropolitan north-east Melbourne. The billabong is located in the Yarra Flats Parkland, which is managed by Parks Victoria. A number of agencies, however, have an interest in the rehabilitation and on-going management of the site, including Parks Victoria, Manningham Shire Council and Melbourne Water. Like many wetlands on the Yarra floodplain, Bolin Bolin Billabong has been subject to significant land-use changes in the catchment and, particularly over the recent ~12+ years of drought, marked changes to its wetting and drying cycles. Bolin Bolin Billabong and its surrounding floodplain is one of the few wetlands in the metropolitan area of north-eastern Melbourne whose geomorphology has not been modified since European colonization. Moreover, with other nearby billabongs such Banyule Billabong, it has the potential to form a landscape mosaic of potentially very high ecological and social value. Given these factors, an appropriate management objective is that Bolin Bolin Billabong should be rehabilitated as far as possible to reflect its ecological condition before European colonization. Complete restoration is an unreasonable aim: the presence of weeds in the floodplain flora, the likely continuous infestation with exotic fish, and on-going presence of feral animals such as deer, cats and dogs, for example, make an aim of complete restoration unrealistic. The guiding principle behind rehabilitation should be to maximise the ecological resilience of the site. Resilience is this context means the ability of the billabong and floodplain to withstand external threats to their ecological condition and to return to the original condition when such threats pass. A process was developed to identify a hydrological regime suitable for rehabilitating the billabong and adjacent floodplain, based on a consideration of the geomorphological setting, present and likely prior vegetation types, current ecological condition, threatening factors, and management objectives. Factors that could limit rehabilitation success were identified, as well as a brief outline of future work needed to advance rehabilitation of the site. The quality of treated storm-water required to maintain biodiversity and the provision of ecosystem services was investigated, and interim trigger levels proposed for Total Nitrogen and Total Phosphorus. In addition to longer-term issues with eutrophication, a risk could be posed by the creation of ‘blackwater’ events when the billabong is initially filled after the recent but prolonged period of dryness. Nevertheless, re-instating a more natural hydrological regime is crucial to the success of other rehabilitation efforts. The recommended water regime should facilitate the ecological rehabilitation of Bolin Bolin Billabong. A deep pool in the eastern loop of the billabong should remain near-permanently inundated to a maximum depth of ~2 m. Water levels within this pool, however, should be allowed to fluctuate naturally with the seasons. The western ends of the northern and southern arms should be inundated to a depth of at least 30 cm for the first three years, in order to help drown out invading River Red Gum saplings and other terrestrial vegetation. After that time, a longitudinal and lateral gradient in wetting should be established so that a complex mosaic in vegetation (and animal habitats) is created along the east-west thalweg of the billabong. The surrounding floodplain should be inundated annually (to a depth of ~10 cm) in order to re- establish the pre-European pattern of wetting and drying. It may be necessary to implement a program of River Red Gum thinning to remove the abundant saplings on the billabong floor. 4 After initial removal of unwanted saplings, subsequent wetting and drying should be sufficient to prevent their re-colonizing this part of the billabong. There are three potential sources of water for the recommended water regime: Yarra River water, achieved through natural bank-full (via flood runners and the two channels) or larger but rarer over-bank flows Yarra River water, pumped from the river at appropriate times, and Treated storm-water from the surrounding urban catchment. Natural flooding of Bolin Bolin Billabong via bank-full or over-bank flows of the Yarra River is currently constrained by four factors: i) chronic extraction of water from the Yarra River for human use, which has lead to markedly depressed river levels; ii) historically low discharge as a result of dry conditions over the past ~12+ years; iii) presence of small raised areas at the western end of the billabong which alienate it and the floodplain from the river; and iv) dense growth of exotic plants on the floodplain and in the western channels, which increase hydrological roughness for over-bank and bank-full flows. Given these factors, it is recommended that the initial filling of the billabong should be done by pumping water from the Yarra River. This conclusion agrees with the earlier recommendation by Sinclair Knight Merz (2006). After the billabong has been filled and the subsoils thoroughly wetted, there are many advantages to then use treated storm-water to maintain more natural wetting and drying cycles in the billabong and floodplain. Sinclair Knight Merz (2006) estimated the volume of water to fill Bolin Bolin Billabong at 74 ML. A more sophisticated analysis was undertaken in the current study, using a conceptual hydrological model to identify the various components of the overall water requirements of the site: VTotal = VThalweg + VFloodplain + VSoil saturation + VEvaporation/transpiration – VRainfal/run-off The table below summarises the likely volumes of water required in first and subsequent years of the rehabilitation of Bolin Bolin Billabong. Estimates of water volumes required to rehabilitate Bolin Bolin Billabong. These estimates should be read in conjunction with the assumptions and caveats listed in the body of the text. Component Volume per year (ML) Year 1 Year 2 Year 3 Billabong thalweg inundation 40 * * (pool only) Billabong thalweg inundation 10 10 10 (wet-dry arms)** Floodplain inundation** 8 8 8 Soil saturation (billabong ~20–55 ~20–55** ~20–55** thalweg only) Soil saturation (combined ~50–150 ~50–150*** ~50–150*** billabong-floodplain complex) Evaporative losses (billabong 24–48 24–48 24–48 only) * nil if evaporative losses of 24-48 ML year-1 are made up annually ** likely to be at lower end of range in 2nd and 3rd years, due to prior saturation of thalweg during 1st wetting *** assumes one inundation per year: approximately double this value for two inundations annually 5 These volumes sum to a total of ~82–106 ML for the better-understood components of filling the billabong thalweg (pool and alternately wet-dry arms: 50 ML), inundating the floodplain (8 ML), and accounting for evaporative losses from the billabong only (24–48 ML).