Adapted from a paper by Heather Peachey, Ben Dyer, Paul Sureda, Jack Smart and Hugh Christie
The Murray-Darling Basin Authority has completed an ambitious project to artificially replicate the now scarce natural flooding events on which the Hattah Kulkyne National Park’s ecosystem relies (See ‘Flooding the Forest’, Pump Industry February 2014 edition). In this article we take a more detailed and technical look at the project and the design challenges and opportunities this novel pumping application presented.
About the Hattah Lakes project
Hattah Lakes, located in the Hattah Kulkyne National Park located south of Mildura in Victoria, consists of 20 perennial and intermittent freshwater lakes. These lakes naturally receive inflows from the River Murray through the Chalka Creek system during periods of high flow. Regulation and dry weather conditions substantially reduced the frequency and duration of flows to the Hattah Lakes. However, the disruption of the natural flooding cycle was determined to have a detrimental effect on the Hattah Lakes ecosystem. To reduce the impact of regulation on flows to Hattah Lakes, a series of works including the construction of a permanent pump station were undertaken to deliver water directly to Chalka Creek.
One of the challenges in designing the works was defining an appropriate pump size that would allow sufficient water to be pumped to the lakes to provide acceptable inundation duration for the various vegetation present at the site. Detailed hydrologic modelling was undertaken to determine the temporal and spatial patterns of lake filling for a range of pump sizes, for comparison with target inundation times. This identified a large range of inundation durations that occurred naturally and showed that the target inundation times originally estimated when the project was conceived were too simplistic.
While the investigation identified the pump capacity required to deliver ideal flows, this needed to be balanced against cost and construction issues through the detailed design phase. This work allowed the development of a project that was both affordable and ensured that the desired ecological outcomes were achieved.
The Living Murray project
The Living Murray (TLM) Environmental Works and Measures Program is one of the largest ecological restoration programs to be undertaken on an Australian river system. It involves approximately $290 million of expenditure on environmental infrastructure works and measures to allow water to be managed for the benefit of the riverine and floodplain environments within an operationally constrained river.
The Hattah Lakes restoration project was one of six Icon Sites identified under the Murray Darling Basin Ministerial Council’s ‘The Living Murray (TLM) Initiative’ in 2002. The Icon sites were created in response to widespread concerns about the environmental and economic health of the River Murray system. Nine TLM ecological objectives were created for the Hattah Lakes Icon Site. Two of those objectives are to; restore a mosaic of hydrological regimes, which represent preregulation conditions; and maintain, and where practical, restore, the ecological character of the Ramsar site with respect to the Strategic Management Plan (DSE 2003). The other ecological objectives were dependent upon the hydrology and refer to increases in bird, fish and macrophyte numbers, and providing refuge habitat for local and international birds. These ecological objectives also provided a mechanism for evaluation and monitoring to help determine the success of the restoration project.
The frequency of flooding in the Hattah Lakes system was reduced over the past decade due to river regulation, with no natural flooding as a result the extended dry climatic conditions experienced over this time period. The aim of the project was to enhance biodiversity and ecological values of the Hattah Lakes waterway systems through the construction of strategically positioned flow control structures within the National Park. Investigations determined the works as; regulators, blocking banks, sill lowering and a pump station, at an estimated cost of $29 million. In developing a major works program it was essential the desired ecological outcomes could be achieved in a cost effective manner with minimal negative consequences.
About the ecosystem
The Hattah Kulkyne National Park wetlands are of varying areas and depths and are located at different elevations. This produces a unique mosaic of ecological vegetation classes creating different habitats. Lakes Hattah and Mournpall, which are deepest lakes in the system, are known to hold water for up to 3 and 7 years, respectively.
The site hydrogeology consists of sand and clay sediments which isolate the shallow local groundwater systems from the regional groundwater system in the sands below. Connection exists between these groundwater systems, however the available data suggests that this connection is quite restricted in the region of the Hattah Lakes system.
Under natural flooding conditions a flow of 152,000 ML/day at Euston (Lock 15) is required to water all wetlands (excluding the Dry Lakes and Lake Boolca in the most north western section, which requires a flow of over 300,000 ML/day at Euston. However, under regulated conditions river flows are rarely operated above 20,000 ML/day at Euston unless the operating rules require additional flows to fill Lake Victoria or if high unregulated flows pass through the river system. In addition to this, the 2030 climate change model predicts flows in the River Murray will be reduced significantly (around 700 GL/yr on average less at Euston) leading to less probability of flooding into the Hattah Lakes system.
Under natural conditions the Hattah Lakes system would, on average, receive a small flood every two years and a large flood every five years. Prior to the 2010-11 floods many of the further outlying lakes had not received water since 1996, Lake Kramen since 1993 and the Dry Lakes area since 1975.
The lack of connectivity to the river and the complete drying of the system had detrimental effects on the condition of the lakes’ ecosystem and its ability to act as a refuge for flora and fauna during prolonged drought periods. To address this issue, TLM water entitlements were accessed for emergency watering four times between 2005 and 2010 via temporary pumps. This provided water to stressed River Red Gums and understorey and provided drought refuge for fauna.
The watering resulted in an improved condition of River Red Gums, germination and growth of aquatic vegetation in the lakes and promoted waterbird habitat numbers which exceeded 16,000 individuals. Native fish were found in the lakes, all of which entered via the pumps, presumably as eggs or larvae. Species present included Murray Cod, Silver Perch, Golden Perch and Australian Smelt and in low numbers, common Carp.
Although the emergency watering provided a temporary refuge for flora and fauna only vegetation at the lower elevations were watered. Water was also retained within the Chalka Creek and the wetlands. Without permanent works in place the vegetation on the higher elevations (Black Box and Lignum communities) was unlikely to receive water unless the River Murray flows reach above the 152,000 ML/day at Euston. The higher elevated watering was also necessary to connect the floodplain with Chalka Creek and the River Murray, providing opportunity for exchanging essential nutrients.
It became clear that unless something was done the Hattah Lakes system would change from a floodplain and wetland system to a dryland arid zone landscape. This would result in the loss of the ecological processes and associated cultural, commercial and environmental values that Hattah Lakes represent, with the site recognised as an internationally important Ramsar site and an Icon Site under The Living Murray.
Determining the right works
The River Murray is a ‘working river’ with operational constraints so water at Euston could not just be allowed to flow freely to solve the problem.
Therefore, a more innovative solution was required to find the perfect compromise.
Various works as part of large scale restoration project would be required.
Hattah Lakes’ initial investigations proposed approximately 20 small regulators, strategically placed to provide flexibility of water movement within the system, and a pump station. However, budget constraints reduced this to 4 regulators, a pump station, 3 stop banks, and refurbishment of an existing regulator (Figure 1).
The package of works to deliver water to the central lakes region consisted of:
- Sill lowering in the southern arm of Chalka Creek (which carries water from the river to the lakes) to increase the frequency of inflows,
- Construction of four new regulators and three stop banks and refurbishment of an existing regulator to retain water in the targeted lakes; and
- Construction of a pumping station to deliver water to the lakes during extended low‐flow periods and top up small to medium natural floods to water vegetation at higher elevations.
The works aimed to water the central lakes using natural connectivity for small and medium flood events in the River Murray and use the pumps to top up the lakes to the desired level. The desired level for each watering event was determined using the history of watering from previous environmental flows and historical actual and modelled natural events, the ecological objectives at that point in time and the volume of environmental water entitlements available. After watering for the appropriate period water drains back to the River Murray via the natural waterway of the southern and northern Chalka Creek.
The works enabled watering of nearly 6,000 ha of wetlands and floodplain communities, including all 12 Ramsar‐listed wetlands at the site. This provides suitable conditions for large waterbird breeding events at least one year in eight; and most other years will support small breeding events.
Key design criteria
The key design criteria that were considered to be fundamental in determining the type of works required to achieve the TLM ecological objectives were;
- the maximum elevation to which the lakes will be filled in a managed watering event,
- the maximum rate at which water is to be pumped into the lakes from the River Murray, and
- the maximum rate that water is to be released from the lakes back to the Murray via the southern and northern Chalka creek arms.
The maximum elevation chosen for the works to achieve was 45mAHD (Australian Height Datum). This was determined by examining a hydraulic model that showed elevations above 45mAHD increased the number of breakout points and that the scale of works required to achieve the ecological objectives above 45mAHD increased disproportionally to the environmental benefit.
Pump rate and drainage rate
The rate at which water levels rise and fall depends on the scale of pumps and regulator works. A hydraulic model of the lakes was developed and calibrated and was used to parameterise a detailed hydrologic model of the lakes. The hydrologic model had the advantage of faster run times and was integrated into the River Murray system model. This hydrologic model could simulate natural conditions or scenarios over a 117 year period at a daily timestep.
The hydrologic model incorporated the proposed structures and a 2030 climate change scenario and was used as a tool to allow comparisons between different scenarios of inflow and outflow rates at three elevation levels (43mAHD, 44mAHD and 45mAHD) and three pump capacities (500ML/day, 750ML/day and 1000ML/day). This provided critical information on the vegetation tolerances under a natural watering regime. The results showed the drainage rate (outflow capacity) of the regulators was generally not the limiting factor, with the conveyance of the creeks limiting the outflows far more than the outlet structure itself (Figure 2a). As such the study focused on the pump capacity rates.
Pumping rates and duration of inundation
Cooling (2009) reviewed exceedence tolerances of three vegetation types; fringing Red Gum, Red Gum with flood tolerant understorey, and Black Box. It was found that the low threat (75th percentile) duration of floods at different elevation levels for Fringing Red Gum was 150 days, Red Gum with flood tolerant understorey was 90 days and Black Box was 60 days. However, Cooling (2009) also noted that it would be problematic to assign flood durations at Hattah as there are many local variations in the water regime and vegetation which would undermine any generalisations. Furthermore, there was no local data on flood tolerances to draw upon.
The simplicity of these numbers and the uncertainty regarding how sensitive the vegetation was to exceeding these durations raised questions. For example ‘did watering for a shorter period fail to meet the ecological requirement of the vegetation’? Further issues of pump cost were raised as it was essential the pump size was justified by ecological needs as operations looked to enhance ‘natural events’. This meant the whole situation was subject to a further layer of complexity. To address these concerns the hydrologic model was used to assess the durations and frequency of inundation for a range of pumping scenarios.
The analysis presented in Figure 2b and 2c determined the following findings;
- Although the larger pumping capacity did not meet the frequency of inundation at higher elevations. The operations would be flexible and pumping could be used to ‘top up’ natural floods to increase the extent of inundation and hence reach the higher elevations. Therefore the larger the pump the greater the flexibility to operate and match the natural water level.
- The smaller pump capacity resulted in the duration of inundation at lower elevations exceeding natural duration, which may lead to either killing the vegetation or stressing it excessively.
- The duration of pumping was bimodal with a small number of events requiring long durations of pumping and a large number of events requiring substantially shorter durations of pumping (Figure 2d). The long pumping duration relates to forced watering of the lakes from a low level, only by using the pumps. The shorter duration of pumping relates to topping up natural events – or only filling the lakes to the lower level.
This means that the chosen pumping capacity would incur a small environmental cost (ie. not match the natural hydrological regime at every wetland at each elevation).
Social and economic implications of pumping duration
The difference in pumping duration ranges from about 3 months (1000ML/day) to 8 months (500ML/day) (Figure 2d). There are a number of issues associated with the duration and timing of pumping.
- Under natural conditions Hattah Lakes would have received high flows during the Winter/Spring period, with a small number of high flows during Summer.
- Winter/Spring watering at a longer duration (ie leading into Summer) may be deemed unacceptable because pumping during the summer periods may not follow the natural pattern, compete with irrigator demands and may be an inefficient use of environmental water.
- Low pumping rates will require less expensive works but would increase the time required to reach target water levels in any given event.
To address these issues it was decided that water would be delivered between August to November to minimise conflicts with irrigation supply, maximise environmental benefits (ensuring natural cues for local flora and fauna as well as international migrating birds are met) and avoid pumping during the hottest period of the year. The pumps would allow water managers and irrigators to take advantage of high passing flows, maximising efficiency and reducing pumping costs due to reduced lift. Conversely, during periods of extreme low flow the delivery rate of water will be restricted.
After much consideration it was concluded that the largest possible pumping capacity (1,000 ML/day) would best meet the ecological objectives while avoiding pumping during the hottest period of the year, thus reducing impacts on irrigation demands. The works therefore allow the manager of the site to provide a watering regime at the appropriate seasonality, duration and frequency at the various elevations to meet requirements of the associated ecological communities.
For more information on the works themselves and the contractors involved in the project, see Flooding the forest, in the February 2014 issues of Pump Industry.
Cooling, M. (2009). Hattah Lakes Flooding Enhancement Risks Associated with Filling and Draining Rates.
Project AL033. Ecological Associates Pty Ltd. June 2009 (not published).
[DSE] Department of Sustainability and Environment (2010). Hattah Lakes Environmental Flows Project -- Investment Proposal. Report by the Mallee Catchment Management Authority, for the Murray Darling Basin Authority.
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[MDBC] Murray Darling Basin Commission (2008). The Living Murray Outcomes Evaluation Framework -- The living Murray Icon Site Condition Report October (2007a).
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[SKM] Sinclair Knight Mertz (2006a). Hydraulic Modelling of the Hattah Lakes -- Final Report — 15 May 2006. Report prepared for Mallee Catchment Management Authority, Mildura.