Basin wide environmental objectives have been developed within the context of being deliverable in a working river system that contains public and private storages and developed floodplains. To understand and assess the implications of key constraints on the ability to achieve flow indicators specified for the Wimmera River Terminal Wetlands, MDBA has drawn upon a combination of existing information (e.g. Water Sharing Plans, operating rules of water agencies, flood warning levels) and practical knowledge of river operators supported by testing using hydrological modelling.
As described in Section 4 the hydrology of Lake Hindmarsh and Lake Albacutya is driven by significant inflows and filling of the lakes may not be achieved by a single inflow event. Modelled flow data and historical observations show that a wet sequence of years is likely to be required to fill Lake Hindmarsh and subsequently spill via Outlet Creek to fill Lake Albacutya.
The MDBA has a vision of a healthy working Basin that has vibrant communities, productive and resilient industries, and healthy and diverse ecosystems. The delivery of environmental flows as a managed watering event within a healthy working Basin is highly dependent on existing system constraints, accordingly the site-specific flow indicators have been classified into three broad types (Table 9). Consistent with this rationale, within the hydrological modelling process used by the MDBA to assess the achievement of site-specific flow indicators orders for environmental flows have been limited to be within the constraints represented by the baseline model. This limits the delivery of regulated flows to the Wimmera River Terminal Lakes.
Given the episodic natural hydrology of Lake Hindmarsh and Lake Albacutya, the achievement of site-specific ecological targets and flow indicators will be heavily reliant on large inflow events from a number of tributaries and potential storage spills. Regulated releases from storage may be used to augment unregulated high flows, however the flow indicators are beyond the scope of an entirely managed watering event.
Table 9 Site-specific flow indicators for Wimmera River Terminal Wetlands and the effect of system constraints
Flow indicators highlighted in blue are considered deliverable as mostly regulated flows under current operating conditions.
Achievable under some conditions (constraints limit delivery at some times)
Flow indicators highlighted in yellow are considered achievable when delivered in combination with tributary inflows and/or unregulated flow events. They may not be achievable in every year or in some circumstances, and the duration of flows may be limited to the duration of tributary inflows.
Difficult to influence achievement under most conditions (constraints limit delivery at most times)
Flow indicators highlighted in brown require large flows that cannot be regulated by dams and it is not expected that these flows can currently be influenced by river operators due to the river operating constraints outlined above.
7.Summary and conclusion
The Wimmera River Terminal Wetlands system is a key environmental asset within the Basin and is an important site for the determination of the environmental water requirements of the Basin. MDBA has undertaken a detailed eco-hydrological assessment of the Wimmera River Terminal Wetlands system environmental water requirements. Specified flow indicators are indicative of a long-term flow regime required to enable the achievement of site-specific ecological targets at Lake Hindmarsh and Lake Albacutya and for the broader river valley and reach. Along with other site-specific flow indicators developed across the Basin at other hydrologic indicator sites, these environmental flow requirements were integrated within hydrological models to inform the ESLT. This process including consideration of a range of constraints such as those outlined in Section 6 is described in further detail within the companion report on the modelling process ‘Hydrologic modelling to inform the proposed Basin Plan: Methods and results’ (MDBA 2012).
The flow indicators in this report are used to assess potential Basin Plan scenarios. MDBA (2012) summarises how the proposed draft Basin Plan released in November 2011 performs against flow indicators for the Wimmera River Terminal Wetlands system.