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Perspective

Marine and Freshwater Research, 2014, 65, 667–673 http://dx.doi.org/10.1071/MF13192

Environmental flow management using transparency and translucency rules Ivor Growns A,C and Ivars Reinfelds B A

New South Wales Department of Primary Industry, Office of Water, PO Box U245, Armidale, NSW 2351, Australia. B New South Wales Department of Primary Industry, Office of Water, PO Box 53, Wollongong, NSW 2500, Australia. C Corresponding author. Email: [email protected]

Abstract. River flow regimes and their variability are considered by many authors to be the most important factor structuring their physical and ecological environment. In regulated rivers, environmental or instream flows are the main management technique used to ameliorate the ecological effects of flow alteration. We highlight two concepts that are not commonly used in a managed flow regime but help return natural flow variability to a managed river, namely, transparent and translucent flow rules. Transparency flows target lower flows up to a defined threshold so that all inflows are released from a dam or are protected from abstraction. Translucency flows form a percentage of inflows greater than the transparency threshold that are released to maintain a proportion of flow pulses in the river system. The main ecological concept underlying transparency and translucency flows is that riverine biota are adapted to the historical flow regime. Although the loss of small to moderate flood events may arise from implementation of translucency and/or transparency flow regimes, we advocate that these rule types would, nonetheless, be beneficial in many managed flow regimes and present two case studies where they have been defined and implemented. Additional keywords: flow variability, river management, river regulation. Received 17 July 2013, accepted 1 January 2014, published online 4 July 2014 Introduction There is an increasing need to provide reliable and potable water supplies to growing human populations, posing a world-wide challenge to water-service providers (Poff et al. 2010). That challenge, however, is substantial as local communities and environment agencies frequently require that water development maintains, or increases, ecosystem health and opportunities for cultural and recreational activities (Arthington 2012). However, communities and agencies also require minimal disruption to ecosystem services, such as water supply, security and quality (Naiman et al. 2002; Postel and Richter 2003; Dyson, Bergkamp and Scanlon 2008). The provision of water for human requirements is mainly carried out by damming rivers, frequently causing regulation of natural flows. River regulation is implicated in the decline of aquatic ecosystems worldwide (e.g. Ward and Stanford 1979; Perry and Perry 1986; Petts et al. 1993; Lloyd et al. 2003; Bernez et al. 2004; Poff and Zimmerman 2010), with impacts on aquatic ecosystems varying according to the nature and degree of changes to the flow regime. The hydrological impacts of river regulation can be broadly classed into the following three types: rivers that are regulated principally to deliver bulk irrigation water; rivers downstream of reservoirs that form a major source of urban water supplies; and, rivers that are regulated for hydroelectric power generation (Gehrke and Harris 2001). The broad Journal compilation Ó CSIRO 2014

nature of downstream hydrological impacts varies considerably among these three types of regulated flow regimes. Rivers downstream of irrigation dams can exhibit shifts in seasonal timing of flows and commonly show increases in the frequency and duration of lower discharges and reductions in the magnitude and frequency of small floods (Walker and Thoms 1993; Maheshwari et al. 1995; Thoms and Sheldon 2000). Flow regimes in rivers downstream of major water-supply reservoirs are commonly dominated by relatively invariant, low-volume downstream releases when compared with the natural flow regime (e.g. Growns and Growns 2001; Reinfelds et al. 2010). Rivers downstream of hydro-electricity dams often exhibit rapidly fluctuating flow regimes related to the switching on and off of turbines for power generation (Ward and Stanford 1979). One of the main river-rehabilitation techniques used to ameliorate the ecological impacts of river regulation is to limit abstraction timing and volumes and to actively manage water releases from dams as ‘environmental’ or ‘instream’ flows (Arthington 2012). ‘Environmental flows’ are defined as ‘ythe quantity, timing and quality of water flows required to sustain freshwater and estuarine ecosystems and the human livelihood and well-being that depend on these ecosystems’ (Poff 2009). Since regulation of flow can occur as a result of direct infrastructure impacts from dams and weirs, as well as through other forms of diversion such as run-of-river pumping, there are www.publish.csiro.au/journals/mfr

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different ways in which environmental flows can be provided (Dyson et al. 2008). Environmental flows can typically be delivered in the following six main ways (Chessman and Jones 2001): (1) embargos and diversion limits, which preclude increases in the total volume of water extracted, leaving greater volumes of water in rivers for environmental needs; (2) pumping thresholds, which prohibit or limit water extraction when river flows are below a particular level; (3) end-of-system rules, which require a certain minimum flow to be retained downstream of areas where major extraction occurs; (4) ‘off-allocation’, and other high-flow access rules, which limit pumping volumes and times to when reservoirs spill or high flows enter regulated river systems from unregulated tributaries; (5) environmental allocations, which create a ‘bank’ of reservoir water that can be released for specific environmental purposes, such as flushing blue-green algal blooms, reducing salinity or flooding wetlands to promote bird breeding events; and (6) transparent and translucent flow rules, which require some proportion of reservoir inflows to be passed immediately downstream. Despite the concept of transparency and translucency flow rules being present in the literature for almost two decades, they are not commonly incorporated into managed flow regimes. In this perspective, we highlight the concept of these flows rules, identify their benefits, provide a review of their historical application and demonstrate their implementation in two case studies in Australia. Definitions and theory The behaviour of light through glass provides an analogy for understanding the transparency and translucency (TT) components of an environmental flow regime (Boyes 2006). The transparency component is used to set operational rules, to ensure that inflows up to a defined threshold are passed transparently through a dam as though it did not exist. All inflows less than the transparency threshold are allowed to pass downstream in a manner similar to light passing through a transparent piece of glass. The translucency component of a TT environmental flow regime means that the dam acts similarly to an opaque piece of glass, where some of the light (or inflows) can pass through. The proportion of light (or inflows) that passes through is expressed as a percentage. For example, 20% translucency in the context of TT environmental flow regimes means that, of inflows to a dam, 20% are passed through to the river downstream and 80% are reserved for consumptive use and management (Fig. 1a, b). The purpose of applying TT rules in the development of an environmental flow regime is to mimic natural flows as much as is possible within the societal and operational constraints of a dam (HNRMF 2004). The two concepts of ‘transparency’ and ‘translucency’ are used in combination to create environmental flow regimes that seek to protect or restore the natural range of low flows, protect or restore a portion of flow pulses and moderate flows, and protect or restore natural flow variability at seasonal, monthly and daily time scales within the lower flow

I. Growns and I. Reinfelds

portion of the hydrograph (Boyes 2006). Low flows are passed transparently (up to a limit), and are thus fully protected. All remaining (higher) flows are passed translucently, which protects a portion of these higher flows and can help restore moderate flow pulses. The portion that is not passed downstream as environmental flows is available for storage, and for subsequent transfer or release downstream for consumptive uses. The main ecological concept underlying transparency and translucency flows is that riverine biota are adapted to the historical flow regime, including the range and patterns of flows that existed before any anthropogenic modifications were made (Blu¨hdorn and Arthington 1994). By providing or restoring components of the natural variability of flows in an environmental flow regime, the principal premise is that the effects of flow regulation may be minimised or ameliorated. Flow rules that help maintain natural flows are useful in that they preserve the natural discharge variability with other biological or ecological triggers such as temperatures and daylength variability (Bunn and Arthington 2002). As a general principle, it is beneficial to maintain natural variations in the pattern and timing of flows, because many ecological processes are cued by flow variations in conjunction with other environmental triggers through linkages that are not always understood but are best preserved by maintaining or restoring a natural flow pattern (Bunn and Arthington 2002; Boyes 2006). A direct translucency approach (i.e. scaling-down flows) has been criticised (e.g. Poff et al. 1997) because it may fail to provide threshold events that are necessary for normal ecological and geomorphological functioning. This is particularly the case for small- to moderate-size flood events in systems with large dams where the frequency of such events is often substantially reduced (e.g. Maheshwari et al. 1995; Thoms and Sheldon 2000). The greatest trade-off in the implementation of TT flow regimes in systems with large dams is the loss of small- to moderate-size floods events (from 1.1 to 5–10 years annualrecurrence interval). The loss of such events from river systems downstream of major reservoirs commonly leads to excessive growth of aquatic plants, especially those preferring lowervelocity habitats, siltation, encroachment of riparian vegetation and stabilisation of seasonally active river-bed sediments (Williams and Wolman 1984; Erskine et al. 1999; Batalla and Vericat 2009). Another potential weakness of the flowtranslucency approach is that although it simply scales down the flows, the channel in which the flows are passed is not necessarily scaled down (Gippel 2001). Thus, for low flows, for example, a given reduction in flow could produce a disproportionately large loss of hydraulically suitable habitat area. However, a combined approach encompassing both transparency and translucency components in a flow regime is able to address particular ecologically based flow thresholds. For example, the combined TT approach has demonstrable benefits to improving fish passage across natural low-flow barriers (Reinfelds et al. 2010) and attaining flow thresholds known to stimulate migratory responses in fish (Reinfelds et al. 2013). Historical recommendations and current implementation The majority of recommendations and/or implementation of TT flow rules have been in Australia. Transparency-type flow rules were first suggested by Blu¨hdorn and Arthington (1994) for a proposed environmental flow regime for the Bjelke-Petersen

Transparency and translucency environmental flows


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(a) 400

Dam outflow (ML day⫺1)

350 300

Translucent component

250 200 150

Transparent component

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Discharge (ML day⫺1)

(b) 100 000

10 000

1000

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10

1 0

100

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500

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700

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Day Fig. 1. Application of an 80th percentile transparent-flow (200 ML day 1) and 20th percentile translucent-flow rule regime to a theoretical river. (a) Outflows released from a dam calculated on flow rules applied to dam inflows. (b) Application of these flow rules to a time series, with dam inflows as the solid line and dam outflows as dotted line.

Dam in Queensland (Table 1). However, the transparency component suggested by Blu¨hdorn and Arthington (1994) was actually a translucency flow because the natural flow regime was suggested to be scaled down in magnitude (using various functions), while maintaining similar levels of flow variability (Gippel 2001). Translucency flow rules were also recommended for the Wivenhoe Dam in Queensland (Arthington et al. 2000). Arthington et al. (2000) demonstrated how the operation of Lake Wivenhoe as a ‘translucent dam’ could restore not only ecologically important flow magnitudes but also flood pulses and flow variability at various temporal scales (daily, flood events, monthly, and from year to year). However, to the authors’ knowledge, neither of these recommended flow regimes were implemented. The first implementation of a transparency flow rule was during the Campaspe Flow Manipulation Project, which aimed to assess the effectiveness of a ‘translucent dam’ approach to environmental flow allocation (Growns 1998; Humphries and Cook 2004). The transparency component was 25% of input flows once a dam-volume trigger level of 64% of storage capacity was reached outside the normal irrigation season (Humphries and Cook 2004). However, the trigger level for releasing the translucent environmental flow was reached only a

short time before the end of the flow-manipulation project and only one week of experimental flows actually occurred (P. Humphries, pers. comm.). Although environmental flows were released, they were considered too small to warrant an effective comparison (King et al. 2003). Transparency and translucency (TT) flow rules were trialled in the Murrumbidgee River from 1998 onward and were formally recognised in an environmental flow regime as a part of the New South Wales Water Management Act (2000). Of seven water-sharing plans developed under the Water Management Act, two included the use of transparency or translucency flow rules. In the Lachlan River system, only translucency flow rules were implemented and were designed to deliver flows to the wetlands in the lower river reaches (LRWG 2013). In the Murrumbidgee River, transparency rules were developed to protect low flows and require the release of up to 615 ML day 1 from Burrinjuck Dam and 560 ML day 1 from Blowering Dam (NSW Government 2003; Hardwick et al. 2012). However, it is difficult to assess any benefits of the TT flow rules developed for both of these river systems because of the suspension of the water-sharing plans in 2004 as a result of drought (plans reinstated in 2011). In addition, the benefits attributable to these flows are potentially masked by effects from concurrent

When inflows are 560 ML day 1 for Blowering Dam and 615 ML day 1 for Burrinjuck Dam, depending on catchment conditions Not applicable

80th percentile

95th percentile

Brisbane

Campaspe

Murrumbidgee

Lachlan

Nepean and Hawkesbury

Hawkesbury

Shoalhaven

Wivenhoe

Lake Eppalock

Burrinjuck and Blowering

Wyangala

Upper Nepean Dams, Broughtans Pass weir, Pheasants Nest weir, irrigation weirs on the Hawkesbury River Warragamba

Tallowa

80th percentile

None actually recommended but a series of transparency flows greater than 500 ML day 1 were modelled Not applicable

Not applicable

Barker Creek

Bjelke-Petersen

Transparency component

River

Dam

20th percentile

20th percentile

Variable depending on waterstorage levels from 15 May to 15 November 20th percentile

25% of input flows once a dam volume trigger level of 64% of storage capacity is reached outside the normal irrigation season Not applicable

Not applicable

Variable depending on inflows

Translucency component

Recommendation is being reassessed 2011

2010

2004, but suspended until 2011

2004, but suspended in 2006 and recommenced in 2011

1998

Not implemented

Not implemented

Date of implementation

Under assessment

Not applicable

Under assessment

Not yet assessed

Not yet assessed

Not assessed because the trigger level was reached a week before the end of the project

Not applicable

Not applicable

Assessment

Table 1. History of recommended transparency and or translucency flow rules from Australia

NSW Office of Water (2010).

HNRMF (2004)

HNRMF (2004)

LRWG (2013)

DIPNR (2004)

Growns (1998); Humphries and Cook (2004)

Blu¨hdorn and Arthington (1994) Arthington et al. (2000)

Reference

670 Marine and Freshwater Research I. Growns and I. Reinfelds


implementation and release of environmental contingency allowances for specific environmental purposes such as flushing algal blooms and bulk irrigation-water releases. In 2004, the Hawkesbury–Nepean River Management Forum (HNRMF) recommended combined translucency and transparency flow rules for six of the major dams/reservoirs and two off-take weirs that make up the water supply network for the Sydney metropolitan area (HNRMF 2004). The TT flow rules were recommended following extensive ecological, water-yield and socio-economic analysis of a range of TT options (see Case study below). For five of the six dams and all weirs, inflows up to the 80th flow-duration percentile (lowest 20% of flows) were recommended as a transparency flow, and for these structures, 20% of the flows above the 80th flow percentile were to be released as a translucency flow. It was recommended that for the primary water storage for Sydney, Warragamba Dam, inflows up to the 95th flow-duration percentile should be released as a transparency rule, combined with a 20% translucency for higher flows (Warner 2013). The HNRMF (2004) also recommended that numerous irrigation and off-take weirs downstream of the major dams be modified to allow unimpeded passage of the 80 : 20 TT flow rules for the entire length of the Hawkesbury– Nepean River. Since implementation of those recommendations in 2010, all dams and weirs in the Nepean and Woronora watersupply systems have been modified to allow the passage or release of water using the 80 : 20 flow rules. Subsequent to the recommendation and implementation of those TT rules, the same rules have been adopted for Tallowa Dam on the Shoalhaven River, also part of the Sydney Water Supply network (NSW Office of Water 2010). Flow-regime scenarios for Warragamba Dam are currently being investigated by the NSW Government. Transparency flow rules to protect low flows can also be applied to unregulated rivers where run-of-river abstraction is the main form of flow alteration (Reinfelds et al. 2004). In New South Wales, water-sharing plans for unregulated rivers always include a provision where abstractors are required to cease pumping below a certain threshold, often the 95–98th flowduration percentile (the lowest 2–5% of daily flows). A combination of TT rules has also been applied to rivers without storages in Canada. Locke and Paul (2011) recommended that to maintain ecosystem health, the lowest 20% of flows should not be extracted and should be allowed to pass transparently through the river system as an ecosystem base flow. They also recommended that 85% of the volume of flows above the 20% threshold should be maintained to incorporate the spatial and temporal flow conditions necessary to ensure long-term protection of aquatic environments. The benefits of these TT rules in rivers without dams have yet to be assessed. Case studies Recommendation of specific transparency and translucency flow rules following system yield modelling In 2001, the New South Wales State Government commissioned the HNRMF to provide strategic advice to assist in the management of the water resources of Sydney, following an observed decline in river health including algal blooms, excessive growth of aquatic weeds, a decline in the numbers of native


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fish, contamination of oyster beds, eroded river banks and siltation (HNRMF 2004). The HNRMF recognised from the outset that the provision of environmental flows was an essential part of returning rivers to a condition able to support acceptable environmental, social, economic, cultural and heritage values and that the environmental flows should be based on transparency and translucency components to maximise restoration of natural patterns of flow variability. The water-supply network of Sydney consists of multiple dams and weirs, and for each structure, 16 potential flow options, comprised of varying degrees of translucency and transparency, were examined as potential new environmental flow regimes. The transparency options ranged between the 97.5–80th percentiles and a translucency between zero and 60th percentile. For each option and for each dam and weir, hydrological metrics and total system yield were modelled. The performance of each of the options with regard to mimicking natural river flows was assessed using flow statistics estimating magnitude, duration, frequency, seasonality and variation. The potential flow options were then ranked according to their similarity to the natural flow regime. Multi-criteria analysis was used by the Forum to identify three preferred flow options from the initial list of 16 options. In addition to the five flow metrics, 20 additional evaluation criteria, covering a mix of environmental, socioeconomic and cultural-heritage issues, were used to assess and rank the environmental flow options. The three preferred flow options were then examined in detail to assess which provided the best water-quality outcomes for the river. The final recommended flow regime from each of the dams was an 80 : 20 TT flow rule from the majority of the dams and a 95 : 20 from the main storage reservoir, Warragamba Dam. The implementation of 80 : 20 TT flow rules throughout the Sydney water-supply system (with the exception of Warragamba Dam) illustrates that TT flow rules can be implemented in major water-management systems delivering water to large urban populations. Recommendation of specific transparency and translucency flow rules following hydraulic modelling for fish passage The HNRMF also recommended 80 : 20 TT flow rules from another part of the Sydney water-supply network, the Shoalhaven Water Supply Scheme. However, the flow rules were not immediately implemented. The New South Wales Department of Natural Resources (DNR) initiated the development of a new environmental flow regime for the Shoalhaven River downstream of Tallowa Dam on the Shoalhaven Scheme through a holistic assessment of flow-dependent ecosystem processes (Boyes 2006; Reinfelds et al. 2010). This holistic suite of studies focussing on low-, moderate- and high-flow ecosystem processes (e.g. Growns et al. 2009; Reinfelds et al. 2010; Reinfelds and Williams 2012) assessed the suitability of the existing interim flow and alternative TT options for ecosystem needs and water-supply system yields. The second case study for the recommendation of a specific TT flow rule follows a detailed hydraulic-modelling study of flow rates required to facilitate passage by a large-bodied native migratory fish (Australian bass, Macquaria novemaculeata) across natural barriers in the Shoalhaven River (Reinfelds et al. 2010). During drought periods, when the Shoalhaven

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drought-augmentation scheme is operating, small floods and flow-pulse events up to approximately the 1.1 years annualmaximum event (90% annual exceedance probability, AEP) are usually captured by Tallowa Dam. Under the 97 : 0 TT flow regime, the capture of flow pulses and small flood events produced extended periods of invariant flow releases of ,130 ML day 1. Extended-duration, invariant low-flow releases across all seasons were identified as the primary hydrological impact of consequence to ecosystem processes, with fish passage across natural barriers being identified as a particular problem (Reinfelds et al. 2010). Detailed twodimensional hydraulic modelling of depths and velocities of a series of riffles, identified as being ‘worst-case’ examples of wide-shallow and steep-turbulent morphologies, was used to assess flow rates that would facilitate upstream passage by Australian bass. Hydraulic modelling results demonstrated that the 97 : 0 TT flow regime provided suboptimal conditions for post-spawning upstream migration by Australian bass. Flow conditions considered to facilitate upstream fish migration across natural passage barriers were assessed as being re-established by a transparency threshold set at the 80th flow-duration percentile (flows equalled or exceeded for 80% of time) and varied according to the monthly pattern of natural flows. This substantially increased baseflows in the Shoalhaven River during the winter–spring bass-migration season. Monitoring of actual fish movements and behaviour with acoustic telemetry has shown that a proportion of the Shoalhaven River Australian bass population undertakes post-spawning upstream migrations during baseflow conditions in spring (Reinfelds et al. 2013), confirming the need for provision of baseflows at a sufficiently high transparency threshold to facilitate migrations by large-bodied fish. Moreover, the 20% transparency threshold for flows greater than the lowest 80% of flows helps generate more frequent flow pulses of a magnitude approximating the natural (in the absence of Tallowa Dam) median daily flow (50th flow-duration percentile). The median daily flow forms a flow-pulse threshold confirmed by acoustic telemetry to stimulate pre-spawning downstream migratory responses in a proportion of the Shoalhaven River Australian bass population (Reinfelds et al. 2013). Conclusions In conclusion, we advocate the implementation of translucency and transparency flow rules in many managed environmental flow regimes to protect a portion of the natural variability of flows downstream of impoundments and in unregulated systems. These rules provide the opportunity to manipulate flows and mimic natural patterns of flow variability across seasonal, monthly and daily time scales, if appropriate infrastructure is in place. TT flow rules can be designed for small and large impoundments requiring various yields for human consumption or instream flow needs. However, the balance between degrees of transparency versus translucency needs to be tested using water supply-system yield modelling and rigorous assessments of ecological, physico-chemical and geomorphic processes. Key considerations in this regard typically centre around setting higher transparency thresholds at the expense of lower translucency-component flows, or, lower

I. Growns and I. Reinfelds

transparency-component flows with higher translucency or pulse-flow components within an environmental flow regime. Transparency and translucency environmental flow rules work best in water-supply systems where the river system downstream of the dam is not used as a conduit for delivery of consumptive water. The greatest trade-off in the implementation of TT flow regimes in systems with large dams is the loss of small- to moderate-size floods events (from 1.1 to 5–10 years annual-recurrence interval). The loss of such events from river systems downstream of major reservoirs commonly leads to excessive growth of aquatic plants, especially those preferring lower-velocity habitats, siltation, encroachment of riparian vegetation and stabilisation of seasonally active river-bed sediments and loss of connectivity between riverine and floodplain habitats (Williams and Wolman 1984; Erskine et al. 1999; Batalla and Vericat 2009). Nonetheless, adequate ongoing monitoring of any modified flow regime is required to assess ecosystem responses in an adaptive-management framework. Acknowledgements This perspective article is a result of discussions with Max Finlayson and is part of a series in Marine and Freshwater Research. Two anonymous reviewers provided excellent comments on the initial submission.

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