Paper template for HIC2004
11th ISE 2016, Melbourne, Australia
Full Paper
APPLICATION OF 2D HYDRAULIC MODELS TO HELP PREDICT ECOSYSTEM RESPONSES TO IN-CHANNEL ENVIRONMENTAL FLOWS ROBYN J. WATTS Institute for Land, Water and Society, Charles Sturt University Albury, NSW 2640, Australia MICHAEL R. GRACE Water Studies Centre, Monash University Clayton, Victoria 3800, Australia NICOLE MCCASKER Institute for Land, Water and Society, Charles Sturt University Albury, NSW 2640, Australia SUSANNE C. WATKINS Institute for Land, Water and Society, Charles Sturt University Albury, NSW 2640, Australia Environmental flows are being implemented to restore river systems affected by the impacts of river regulation. Environmental flows that are wholly contained within the channel have the potential to inundate riverbanks and other in-channel geomorphological features. This can increase the wetted benthic surface, which is important for primary productivity, and change the velocity profile of the river, creating slow water habitat that is important for many aquatic organisms. We used two-dimensional hydraulic models to estimate the change in wetted benthic surface area and the change in area of slackwater ( 0.3 ms-1) in 2-4 km reaches of four rivers the Edward-Wakool system in south-eastern Australia. Models were constructed for six discharge scenarios ranging from low flows to bankfull flows, including discharge scenarios that were similar to environmental flows delivered to Colligen Creek and Yallakool Creek in 2011-12. The modelled environmental flow in Colligen Creek (800 MLd-1) resulted in an estimated 14% increase in wetted benthic surface area compared to the operational flow (200 MLd-1). The modelled environmental flow in Yallakool Creek (560 MLd-1) resulted in an estimated 22% increase in wetted benthic surface area compared to the operational flow (170 MLd-1). The modelled inundation of an unregulated flow that occurred in August 2012 resulted in an increase in wetted benthic area relative to the operational flow; 47.8% increase in Colligen Creek and 58.9% in Yallakool Creek. The relationship between discharge, wetted surface area and area of slackwater was not linear. These results highlight the importance of considering the interaction of hydrology with geomorphology when considering potential ecosystem responses. It may be more appropriate to use area of inundation instead of daily discharge to better understand the relationship between inchannel flows and ecosystem responses. Future research could focus on examining the nature of this relationship in other river systems to improve models for predicting the responses of in-channel flows on slackwater dependent biota and ecosystem functions. 1
INTRODUCTION
Growing awareness of the impacts of river regulation has led to increased interest in the delivery of environmental water to restore the ecological function of regulated river systems [e.g. 1, 2] Two major types of environmental watering are overbank flows that inundate wetlands and floodplains and instream flows that are contained within the channel. Internationally, there have been a few high profile examples of in-stream environmental flows, such as the experimental flood downstream of the USA‘s Glen Canyon Dam in the mid1990s [3, 4] and more recently in 2009 [5], and an experimental flood in the Spöl River in Switzerland [6]. In Australia in-stream flows have historically been used to disperse algal blooms and dilute contaminants [e.g. 7, 8]. Prior to 2010 there were few examples in Australia where environmental water was used to create in-stream pulsed flows [9]. In-channel flows have recently been implemented as part of the national water reforms introduced by the Australian government to protect and restore the rivers, wetlands and floodplains of the
Murray-Darling Basin. The ecosystem benefits of in-channel environmental flows are being assessed through the Australian government’s Long-Term Intervention Monitoring Program [10]. Environmental flows that are wholly contained within the channel have the potential to inundate riverbanks and other in-channel geomorphological features and can create low flow zones (slackwaters) having low velocities and shallow depths. These habitats are important for river productivity and can facilitate the survival of organisms such as larval fish and larval shrimp through provision of habitat and food. Modelling can be used to compare estimates of the extent of inundation under different discharge scenarios and provides results more cheaply and efficiently than ground based survey methods. In Australia, previous studies have modelled river flow and floodplain inundation for wetlands on the Darling River [11], and floodplains on the Murrumbidgee River [12] and the River Murray [13, 14]. These studies have focused on estimating floodplain inundation during overbank flows. However, in systems where environmental flows are contained within the channel, there are few examples where models have been used to compare the extent of inchannel riverbank inundation under different discharge scenarios. Vietz et al. [15] used two dimensional hydraulic modelling to examine the impacts of flow regulation on slackwaters in the Broken River, south-eastern Australia. They demonstrated that the area of slackwaters decreased with increasing discharge until inundation of higher-elevation bars and benches [15]. The aim of this project was to undertake two-dimensional hydraulic modelling for four river reaches in the Edward-Wakool system in south-eastern Australia to estimate the extent of wetted benthic surface area, area of slackwater ( 0.3 ms-1) for six discharge scenarios, ranging from low flows up to bankfull flows. Comparison of these parameters at discharges expected during operational flows (no environmental water) and during environmental flows were undertaken to assess the effect of environmental water on benthic wetted area and area of slackwater. The modelling will assist the interpretation of ecological responses to flows by providing a more useful metric than discharge alone, and contribute to decision making regarding the magnitude of environmental watering and will assist the communication of likely outcomes of planned watering events with landholders.
2
METHODS
2.1 Study area The Edward-Wakool system is a major anabranch and floodplain of the Murray River in the Murray-Darling Basin in southern-eastern Australia. It is a complex network of interconnected streams, ephemeral creeks, flood runners and wetlands intersected by irrigation channels. The system begins upstream of Deniliquin in the Barmah-Millewa Forest, and travels northwest before discharging back into the Murray River. This system has a high native species richness and diversity including threatened and endangered fish, frogs, mammals, and riparian plants. It is listed as an endangered ecosystem, as part of the ‘aquatic ecological community in the natural drainage system of the lower Murray River catchment’ in New South Wales (NSW Fisheries Management Act 1994). Like many rivers of the Murray-Darling Basin, the flow regimes of the Edward-Wakool system has been significantly altered by river regulation [16, 17]. Watts et al. [18] compared long-term pre-development and post-development modelled flow data for two sites on the Edward River, at Deniliquin and downstream of Stevens Weir. Natural flows were shown to be strongly seasonal, with high flows occurring typically from July to November. Flow regulation has resulted in a marked reduction in winter/spring high flows, including both extreme high flow events and average daily flows during this period [18]. In addition, there has been an increase in daily flows during the low-flow months (January to March). These flow changes reflect the typical effects of flow-regime reversal observed in systems used to deliver dry-season irrigation flows [19, 20]. Environmental flows in this system since 2011 have been delivered during spring/early summer [18, 21, 22].
2.2 Hydraulic modelling Discharge scenarios were modelled for four 2-4 km river reaches in Colligen Creek, Yallakool Creek, Wakool River and Little Merran Creek. Each reach was represented within the hydraulic model using a digital elevation model (DEM). Several significant artefacts were removed from the Little Merran Creek and Colligen Creek DEMs to ensure normal stream flow was not impeded. Artefacts were removed by identifying erroneous
elevation values and integrating corrected values directly into the elevation surface using a process of data fusion. To account for vegetation in each reach the surface friction coefficient (Manning’s n) within the model was set to a value of 0.05 with the exception of the Yallakool Creek site where a value of 0.04 was deemed more appropriate. Six discharge scenarios were selected to be modelled for each river ranging from low flow to estimated bank-full flows (Table 1). Estimates of daily discharge (MLd-1) at low flow, operational flow and bankfull scenarios were provided by D. Green (MDBA). An environmental watering scenario was modelled for Yallakool Creek and Colligen Creek. In Colligen Creek the estimated half bankfull discharge (800 MLd -1) was approximately the same as an environmental watering action undertaken in 2011-12. Only three of the original scenarios could be successfully modelled for Little Merran Creek because the Light Detection and Ranging (LiDAR) survey of the terrain surface was undertaken when discharge in this system was approximately 200 MLd-1, so low flow and operational flow scenarios could not be modelled in this system. An additional scenario of 430MLd-1 was modelled for this system (Table 1). Table 1. Summary of discharge scenarios modelled for the four river reaches in the Edward-Wakool system; Wakool River, Yallakool Creek, Colligen Creek and Little Merran Creek. N/A = not applicable Scenario Discharge MLd-1 Wakool R Yallakool Ck Colligen Ck Little Merran Ck Low flow 25 30 30 Not modelled Operational flow 50 170 200 Not modelled Median flow (2011-2013) 110 271 314 230 Additional scenario Not modelled Not modelled Not modelled 430 Environmental flow N/A 560 800 N/A Half Bankful flow 500 800 800 500 Max daily discharge 2011-2013 1442 1913 2808 1062 Bankfull 3000 4000 4000 Not modelled Discharge values were converted from MLd-1to m3s-1 and supplied to the model as static flow values. Each scenario was modelled assuming an initial dry starting condition with no residual water in the system with the exception of Little Merran Creek reach where a base flow of 200 MLd-1was present when the DEM was captured. All scenarios were run until stable state flow was achieved whereby the instantaneous flow rate at the downstream boundary condition stabilised and matched the upstream inflow value. The exception was the bankfull scenario for the Wakool River reach where a steady state flow could not be achieved without a loss from the system into the Edward River. Discharge scenarios were modelled using the 2D grid implementation of Eonfusion Flood (Myriax Software) with model outputs post-processed using the GIS functionality of Eonfusion (Myriax Software). Post-processing, including surface area calculations, was achieved using Eonfusion (Myriax Software), Quantum GIS and made distributable using Google Earth. Post processing was also undertaken to quantify the spatial configuration of three velocity categories : • velocity zone 1: < 0.02 ms-1 (slack water) • velocity zone 2: 0.02 – 0.3 ms-1 (slow water) • velocity zone 3: >0.3 ms-1 (fast water) Stable state data frames captured during the modelling were used as the starting point for determining the benthic surface area exposed to the three water velocity zones. Stable state data frames were converted into multiband raster data frames containing water depth, velocity and absolute elevation attributes. Multiband rasters were then converted into vector format, triangulated and trimmed to produce discrete spatial features for each velocity zone. Calculation of wetted benthic surface area, and the depth range within each velocity zone, was undertaken on the 3D surface to take into account the vertical relief of the river bed. Summary statistics of the depth range (minimum, maximum, mean, standard deviation) were calculated for each velocity zone under each discharge scenario. Post-processing, including surface area and depth calculations, was achieved using Eonfusion Quantum GIS and Excel and made distributable using Google Earth.
3
RESULTS
There was considerable variation in wetted benthic area under operational flows among reaches (Table 2) ranging from 38,169 m2 in Yallakool Creek study reach at 170 MLd-1 to 48,292 m2 in the Colligen creek study reach at 200 MLd-1. The wetted benthic area under low flows, operational flows and bankfull flows for Little Merran Creek could not be modelled. In Colligen Creek the environmental flow of 800 MLd-1 resulted in a 14% increase in wetted benthic surface area relative to the operational flow of 200 MLd -1. Similarly, in Yallakool Creek an environmental flow of 560 MLd-1 resulted in a 22% increase in wetted benthic surface area relative to the base flow of 170 MLd-1 (Table 2). The estimate of wetted benthic surface area during the maximum daily discharge scenario experienced in 2011-2012 during unregulated flows was considerably higher than during the modelled operational flow or environmental flow in these rivers. On the peak of this unregulated flow event the modeled wetted benthic surface area relative to the base flow 200 MLd -1 scenario increased by 47.8% in Colligen Creek and 59.7% in Yallakool Creek (Table 2). In contrast, in Little Merran Creek the estimated half bankful and maximum discharge 2011-13 scenarios resulted in considerable lateral connection and inundation of low lying geomorphic features (Table 2). The models estimate there would be a considerable further increase in wetted benthic surface area during a bankfull flow, however this type of flow event has not occurred since 2010 and could not be modelled. Table 2. Estimates of wetted benthic surface area under a range of discharge scenarios in four river reaches in the Edward-Wakool system. Note: In Colligen Creek the environmental flow was approximately the same discharge as the estimated half bankfull discharge (800 MLd-1). N/A = not applicable Scenario Modelled estimate of wetted benthic surface area (m2) Wakool R Yallakool Ck Colligen Ck Little Merran Ck Low flow 42,196 33,159 43,257 Not modelled Operational flow 43,587 38,169 48,292 Not modelled Median flow (2011-2013) 46,222 41,510 49,982 160,908 Additional scenario Not modelled Not modelled Not modelled 218,529 Environmental flow N/A 46,432 55,110 N/A Half Bankful 58,547 49,924 55,110 242,902 Max daily discharge (2011-2013) 200,455 60,989 71,337 542,584 Bankfull 264,109 86,404 84,820 Not modelled The representation of the spatial coverage of the benthic surface extent for each velocity zone for Colligen Creek shows that there was a large area of slackwater available in the low flow scenario (63% of the total available benthic surface area, 27,391 m2), but very little slackwater available during the operational flow (9.8%, 4,712 m2), median flow (6.4%, 3184 m2) and half bankfull/environmental flow scenarios (3.1 %, 1708 m2)(Figure 1). Slackwater was created along the margins and particularly in low lying geomorphic in-channel features during the maximum discharge scenario (7.4 %, 5,308 m2) and bankfull scenario (13.6%, 11,575 m2)(Figure 1). The results for Yallakool Creek and the Wakool River are not presented here, but were similar to those for Colligen Creek. In contrast to the results for Colligen Creek, in Little Merran Creek there was a large area of slackwater available under the median flow scenario (70% of the total available water surface area). Slackwater was also abundant in the half bankfull scenario (25.5%) and maximum discharge scenario (41.5%)(Figure 2). Only a very small percent of the area was classed as fast water in each of these scenarios. The relationship between discharge and wetted surface area in these four river reaches was not linear (Figure 3). If there were a similar increase in discharge in two rivers, it would not produce in the same increase in wetted benthic surface area. For example, an increase from 200 MLd-1 to 800 MLd-1 in Colligen Creek resulted in a modelled 14% increase in wetted benthic area, whereas a similar increase from 170 MLd-1 to 800 MLd-1 in Yallakool Creek resulted in a modelled 30% increase in wetted benthic area (Table 2). There was a very large increase in estimated wetted surface area with increasing discharge in Little Merran Creek.
Figure 1. Maps showing the extent of wetted benthic surface area for each velocity zone in Colligen Creek under six discharge scenarios. The half-bankfull discharge scenario (800 MLd-1) was approximately the same as the environmental flow delivered in this river system in 2011-12. Velocity zone 1 = slack water; velocity zone 2 = slow water; velocity zone 3 = fast water.
Figure 2. Maps showing the extent of wetted benthic surface area for each velocity zone in Little Merran Creek under three discharge scenarios. Velocity zone 1 = slack water; velocity zone 2 = slow water; velocity zone 3 = fast water. 600,000
Estimated wetted surface area (m2)
Colligen Yallakool
500,000
Wakool Little Merran
400,000
300,000
200,000
100,000
0
500
1000
1500
2000
2500
3000
3500
4000
Discharge (MLd-1)
Figure 3. Relationship between discharge (MLd-1) and modelled wetted surface area (m2) for reaches in Colligen Creek, Yallakool Creek, Wakool River and Little Merran Creek from the Edward-Wakool system.
4
DISCUSSION
The models of in-channel flow scenarios demonstrate that the relationship between discharge and wetted surface area and the area of slackwater in these four study reaches in the Edward-Wakool system is not linear and is most likely strongly influenced by geomorphology. Similar to the findings of Vietz et al. [15], we found that in some river reaches (e.g. Colligen Creek) the area of slackwaters decreased with increasing discharge until inundation of higher-elevation bars and in-channel benches. However, in Little Merran Creek a different pattern was observed with large area of slackwater available at a range of different discharges. As the data presented
here are for only relatively short 2-4 km reaches of each river, it is important to consider that the reported relationships may be reach specific and that these relationships need to be examined over longer river distances in the study rivers. These results have important implications for studies of ecology-flow relationships for in-channel flows. They suggest it may be more appropriate to examine the relationship between inundation area and ecosystem responses to in-channel flows rather than focusing on relationships solely with daily discharge, as has commonly been the practice. In-channel 2D hydraulic modelling under different flow scenarios can be used to i) better understand the relationship between in-channel flows and ecosystem responses, ii) predict the consequences of in-channel flows on slackwater dependent biota and ecosystem functions, and iii) facilitate better planning and management of the future in-channel environmental flows. In particular, 2D hydraulic modelling may assist managers examine trade-offs between ecosystem benefits and third party impacts, such as inundation of private land, prior to the delivery of environmental water. Modelling of in-channel flow scenarios can also help managers determine the optimum discharge at which there can be an increase in riverbank inundation and creation of slackwater to produce ecosystem responses to environmental flows, but with minimal third party impacts.
5
ACKNOWLEDGMENTS
Thanks to landholders in the Edward-Wakool river system for providing access to their properties for fieldwork. Maps were prepared by Simon McDonald and Deanna Duffy (Charles Sturt University Spatial Analysis Unit). The modelling was undertaken by Dr Hugh Pederson from Marine Solutions. The modelling presented in this paper was partly funded by the Ecological Responses to Altered Flow Regimes Research Cluster which was a collaboration between the CSIRO Water for a Healthy Country Flagship, Griffith University, the University of New South Wales, Monash University, Charles Sturt University, La Trobe University and the Arthur Rylah Institute of the Victorian Department of Environment and Primary Industries. The project was also partly funded through additional cash contributions from Charles Sturt University (Institute for Land, Water and Society), Murray Catchment Management Authority (thanks to Patricia Bowen and John Conallin) and Monash University (Water Studies Centre).
6
REFERENCES
[1] Poff N.L., “Managing for Variability to Sustain Freshwater Ecosystems”. Journal of Water Resources Planning and Management, 135(1), (2009), pp1-4. [2] Arthington A.H., Naiman R.J., McClain M.E. and Nilsson C., “Preserving the biodiversity and ecological services of rivers: new challenges and research opportunities”, Freshwater Biology, 55(1), (2010), pp 1-16. [3] Shannon J.P., Blinn D.W., McKinney T., Benenati E.P., Wilson K.P. and O'Brien C., “Aquatic food base response to the 1996 test flood below Glen Canyon Dam, Colorado River, Arizona”, Ecological Applications, 11(3), (2001), pp 672–685. [4] Valdez R.A., Hoffnagle T.L., McIvor C.C., McKinney T. and Leibfried W.C., “Effects of a test flood on fishes of the Colorado River in Grand Canyon, Arizona”, Ecological Applications, 11(3), (2001), pp 686– 700. [5] Cross W.F., Baxter C.V., Donner K.C., Rosi-Marshall E.J., Kennedy T.A., Hall R.O., Wellard Kelly H.A. and Rogers R.S., “Ecosystem ecology meets adaptive management: food web response to a controlled flood on the Colorado River, Glen Canyon”, Ecological Applications, 21, (2011), pp 2016-2033. [6] Robinson C.T., Uehlinger U. and Monaghan M.T., “Stream ecosystem response to multiple experimental floods from a reservoir”, River Research and Applications, 20, (2004), pp 359–377. [7] Maier H.G., Kingston G.B., Clark T., Frazer T. and Sanderson A., “Risk-based approach for assessing the effectiveness of flow management in controlling cyanobacterial blooms in rivers”, River Research and Applications, 20, (2004), pp 459–471. [8] Mitrovic S.M., Oliver R.L., Rees G., Bowling L.C. and Buckney R.T., “Critical flow velocities for the growth and dominance of Anabaena circinalis in some turbid freshwater rivers”, Freshwater Biology 48, (2003), pp 164–174.
[9] Watts R.J., Allan C., Bowmer K.H., Page K.J., Ryder D.S. and Wilson A.L., “Pulsed flows: a review of environmental costs and benefits and Best practice”, Waterlines report, National Water Commission, Canberra, (2009). [10] Gawne B., Brooks S., Butcher R. , Cottingham P., Everingham P., Hale J., Nielson D., Stewardson M. and Stoffels R., “Long Term Intervention Monitoring Logic and Rationale Document”, Final Report prepared for the Commonwealth Environmental Water Office by The Murray-Darling Freshwater Research Centre, MDFRC Publication 01/2013, May, 109pp (2013). [11] Shaikh M., Green D. and Cross H., “A Remote Sensing Approach to Determine the Environmental Flows for Wetlands of the Lower Darling River, New South Wales, Australia”, International Journal of Remote Sensing, 22, (2001), pp 1737-1751. [12] Frazier P.S., Page K.J., Louis J., Briggs S. and Robertson A., “Relating Wetland Inundation to River Flow Using Landsat TM Data”, International Journal of Remote Sensing, 24 (19), (2003), pp 3755-3770. [13] Overton I.C., "Modelling Floodplain Inundation on a Regulated River: Integrating GIS, Remote Sensing and Hydrological Models”, River Research and Applications, 21 (9), (2005), pp 991-1001. [14] Overton I.C., McEwan K., and Sherrah J.R., “The River Murray Floodplain Inundation Model – Hume Dam to Lower Lakes” CSIRO Water for a Healthy Country Technical Report, CSIRO: Canberra, (2006). [15] Vietz G.J., Sammonds M.J. and Stewardson M.J., “Impacts of flow regulation on slackwaters in river channels”, Water Resources Research, 49, (2013), pp 1797–1811. [16] Green D., “The Edward-Wakool System: River Regulation and Environmental Flows”, Department of Land and Water Conservation. Unpublished Report, (2001). [17] Hale and SKM, “Environmental Water Delivery: Edward-Wakool”, Prepared for Commonwealth Environmental Water, Department of Sustainability, Environment, Water, Population and Communities, (2011). [18] Watts, R.J., McCasker, N., Thiem, J., Howitt, J.A., Grace, M., Kopf, R.K., Healy, S., Bond, N. “Monitoring the ecosystem responses to Commonwealth environmental water delivered to the Edward-Wakool river system, 2014-15”, Institute for Land, Water and Society, Charles Sturt University, Final Report. Prepared for Commonwealth Environmental Water, (2015). [19] Maheshwari B.L., Walker K.F., and McMahon T.A., "Effects of regulation on the flow regime of the river Murray, Australia", Regulated Rivers Research and Management, 10, (1995), pp 15–38. [20] McMahon T.A., “Droughts and anti-droughts: the low flow hydrology of Australian rivers”, Freshwater Biology, 48, (2003), pp 1147-1160. [21] Watts, RJ, Kopf, R.K., Hladyz, S., Grace, M., Thompson, R., McCasker, N., Wassens, S., Howitt, J.A., A. Burns, and Conallin, J, “Monitoring of ecosystem responses to the delivery of environmental water in the Edward-Wakool system, 2011-2012”. Institute for Land, Water and Society, Charles Sturt University, Report 2. A report to the Commonwealth Environmental Water Office, Canberra, Australia, (2013). http://www.environment.gov.au/water/cewo/publications/monitoring-ecosystem-responses-deliveryenvironmental-water-edward-wakool-river-system-2011-report-2 [22] Watts, R.J., McCasker, N., Thiem, J., Howitt, J.A., Grace, M. Healy, S., Kopf, R.K., Dyer, J.G., Conallin, A., Wooden I., Baumgartner L., and Bowen P, “Monitoring the ecosystem responses to Commonwealth environmental water delivered to the Edward-Wakool river system, 2013-14”. Institute for Land, Water and Society, Charles Sturt University, Final Report. Prepared for Commonwealth Environmental Water Office, (2014). http://www.environment.gov.au/water/cewo/publications/monitoring-ecosystem-responses-cewedward-wakool-river-system-2013-14-final-report
Full Paper
APPLICATION OF 2D HYDRAULIC MODELS TO HELP PREDICT ECOSYSTEM RESPONSES TO IN-CHANNEL ENVIRONMENTAL FLOWS ROBYN J. WATTS Institute for Land, Water and Society, Charles Sturt University Albury, NSW 2640, Australia MICHAEL R. GRACE Water Studies Centre, Monash University Clayton, Victoria 3800, Australia NICOLE MCCASKER Institute for Land, Water and Society, Charles Sturt University Albury, NSW 2640, Australia SUSANNE C. WATKINS Institute for Land, Water and Society, Charles Sturt University Albury, NSW 2640, Australia Environmental flows are being implemented to restore river systems affected by the impacts of river regulation. Environmental flows that are wholly contained within the channel have the potential to inundate riverbanks and other in-channel geomorphological features. This can increase the wetted benthic surface, which is important for primary productivity, and change the velocity profile of the river, creating slow water habitat that is important for many aquatic organisms. We used two-dimensional hydraulic models to estimate the change in wetted benthic surface area and the change in area of slackwater ( 0.3 ms-1) in 2-4 km reaches of four rivers the Edward-Wakool system in south-eastern Australia. Models were constructed for six discharge scenarios ranging from low flows to bankfull flows, including discharge scenarios that were similar to environmental flows delivered to Colligen Creek and Yallakool Creek in 2011-12. The modelled environmental flow in Colligen Creek (800 MLd-1) resulted in an estimated 14% increase in wetted benthic surface area compared to the operational flow (200 MLd-1). The modelled environmental flow in Yallakool Creek (560 MLd-1) resulted in an estimated 22% increase in wetted benthic surface area compared to the operational flow (170 MLd-1). The modelled inundation of an unregulated flow that occurred in August 2012 resulted in an increase in wetted benthic area relative to the operational flow; 47.8% increase in Colligen Creek and 58.9% in Yallakool Creek. The relationship between discharge, wetted surface area and area of slackwater was not linear. These results highlight the importance of considering the interaction of hydrology with geomorphology when considering potential ecosystem responses. It may be more appropriate to use area of inundation instead of daily discharge to better understand the relationship between inchannel flows and ecosystem responses. Future research could focus on examining the nature of this relationship in other river systems to improve models for predicting the responses of in-channel flows on slackwater dependent biota and ecosystem functions. 1
INTRODUCTION
Growing awareness of the impacts of river regulation has led to increased interest in the delivery of environmental water to restore the ecological function of regulated river systems [e.g. 1, 2] Two major types of environmental watering are overbank flows that inundate wetlands and floodplains and instream flows that are contained within the channel. Internationally, there have been a few high profile examples of in-stream environmental flows, such as the experimental flood downstream of the USA‘s Glen Canyon Dam in the mid1990s [3, 4] and more recently in 2009 [5], and an experimental flood in the Spöl River in Switzerland [6]. In Australia in-stream flows have historically been used to disperse algal blooms and dilute contaminants [e.g. 7, 8]. Prior to 2010 there were few examples in Australia where environmental water was used to create in-stream pulsed flows [9]. In-channel flows have recently been implemented as part of the national water reforms introduced by the Australian government to protect and restore the rivers, wetlands and floodplains of the
Murray-Darling Basin. The ecosystem benefits of in-channel environmental flows are being assessed through the Australian government’s Long-Term Intervention Monitoring Program [10]. Environmental flows that are wholly contained within the channel have the potential to inundate riverbanks and other in-channel geomorphological features and can create low flow zones (slackwaters) having low velocities and shallow depths. These habitats are important for river productivity and can facilitate the survival of organisms such as larval fish and larval shrimp through provision of habitat and food. Modelling can be used to compare estimates of the extent of inundation under different discharge scenarios and provides results more cheaply and efficiently than ground based survey methods. In Australia, previous studies have modelled river flow and floodplain inundation for wetlands on the Darling River [11], and floodplains on the Murrumbidgee River [12] and the River Murray [13, 14]. These studies have focused on estimating floodplain inundation during overbank flows. However, in systems where environmental flows are contained within the channel, there are few examples where models have been used to compare the extent of inchannel riverbank inundation under different discharge scenarios. Vietz et al. [15] used two dimensional hydraulic modelling to examine the impacts of flow regulation on slackwaters in the Broken River, south-eastern Australia. They demonstrated that the area of slackwaters decreased with increasing discharge until inundation of higher-elevation bars and benches [15]. The aim of this project was to undertake two-dimensional hydraulic modelling for four river reaches in the Edward-Wakool system in south-eastern Australia to estimate the extent of wetted benthic surface area, area of slackwater ( 0.3 ms-1) for six discharge scenarios, ranging from low flows up to bankfull flows. Comparison of these parameters at discharges expected during operational flows (no environmental water) and during environmental flows were undertaken to assess the effect of environmental water on benthic wetted area and area of slackwater. The modelling will assist the interpretation of ecological responses to flows by providing a more useful metric than discharge alone, and contribute to decision making regarding the magnitude of environmental watering and will assist the communication of likely outcomes of planned watering events with landholders.
2
METHODS
2.1 Study area The Edward-Wakool system is a major anabranch and floodplain of the Murray River in the Murray-Darling Basin in southern-eastern Australia. It is a complex network of interconnected streams, ephemeral creeks, flood runners and wetlands intersected by irrigation channels. The system begins upstream of Deniliquin in the Barmah-Millewa Forest, and travels northwest before discharging back into the Murray River. This system has a high native species richness and diversity including threatened and endangered fish, frogs, mammals, and riparian plants. It is listed as an endangered ecosystem, as part of the ‘aquatic ecological community in the natural drainage system of the lower Murray River catchment’ in New South Wales (NSW Fisheries Management Act 1994). Like many rivers of the Murray-Darling Basin, the flow regimes of the Edward-Wakool system has been significantly altered by river regulation [16, 17]. Watts et al. [18] compared long-term pre-development and post-development modelled flow data for two sites on the Edward River, at Deniliquin and downstream of Stevens Weir. Natural flows were shown to be strongly seasonal, with high flows occurring typically from July to November. Flow regulation has resulted in a marked reduction in winter/spring high flows, including both extreme high flow events and average daily flows during this period [18]. In addition, there has been an increase in daily flows during the low-flow months (January to March). These flow changes reflect the typical effects of flow-regime reversal observed in systems used to deliver dry-season irrigation flows [19, 20]. Environmental flows in this system since 2011 have been delivered during spring/early summer [18, 21, 22].
2.2 Hydraulic modelling Discharge scenarios were modelled for four 2-4 km river reaches in Colligen Creek, Yallakool Creek, Wakool River and Little Merran Creek. Each reach was represented within the hydraulic model using a digital elevation model (DEM). Several significant artefacts were removed from the Little Merran Creek and Colligen Creek DEMs to ensure normal stream flow was not impeded. Artefacts were removed by identifying erroneous
elevation values and integrating corrected values directly into the elevation surface using a process of data fusion. To account for vegetation in each reach the surface friction coefficient (Manning’s n) within the model was set to a value of 0.05 with the exception of the Yallakool Creek site where a value of 0.04 was deemed more appropriate. Six discharge scenarios were selected to be modelled for each river ranging from low flow to estimated bank-full flows (Table 1). Estimates of daily discharge (MLd-1) at low flow, operational flow and bankfull scenarios were provided by D. Green (MDBA). An environmental watering scenario was modelled for Yallakool Creek and Colligen Creek. In Colligen Creek the estimated half bankfull discharge (800 MLd -1) was approximately the same as an environmental watering action undertaken in 2011-12. Only three of the original scenarios could be successfully modelled for Little Merran Creek because the Light Detection and Ranging (LiDAR) survey of the terrain surface was undertaken when discharge in this system was approximately 200 MLd-1, so low flow and operational flow scenarios could not be modelled in this system. An additional scenario of 430MLd-1 was modelled for this system (Table 1). Table 1. Summary of discharge scenarios modelled for the four river reaches in the Edward-Wakool system; Wakool River, Yallakool Creek, Colligen Creek and Little Merran Creek. N/A = not applicable Scenario Discharge MLd-1 Wakool R Yallakool Ck Colligen Ck Little Merran Ck Low flow 25 30 30 Not modelled Operational flow 50 170 200 Not modelled Median flow (2011-2013) 110 271 314 230 Additional scenario Not modelled Not modelled Not modelled 430 Environmental flow N/A 560 800 N/A Half Bankful flow 500 800 800 500 Max daily discharge 2011-2013 1442 1913 2808 1062 Bankfull 3000 4000 4000 Not modelled Discharge values were converted from MLd-1to m3s-1 and supplied to the model as static flow values. Each scenario was modelled assuming an initial dry starting condition with no residual water in the system with the exception of Little Merran Creek reach where a base flow of 200 MLd-1was present when the DEM was captured. All scenarios were run until stable state flow was achieved whereby the instantaneous flow rate at the downstream boundary condition stabilised and matched the upstream inflow value. The exception was the bankfull scenario for the Wakool River reach where a steady state flow could not be achieved without a loss from the system into the Edward River. Discharge scenarios were modelled using the 2D grid implementation of Eonfusion Flood (Myriax Software) with model outputs post-processed using the GIS functionality of Eonfusion (Myriax Software). Post-processing, including surface area calculations, was achieved using Eonfusion (Myriax Software), Quantum GIS and made distributable using Google Earth. Post processing was also undertaken to quantify the spatial configuration of three velocity categories : • velocity zone 1: < 0.02 ms-1 (slack water) • velocity zone 2: 0.02 – 0.3 ms-1 (slow water) • velocity zone 3: >0.3 ms-1 (fast water) Stable state data frames captured during the modelling were used as the starting point for determining the benthic surface area exposed to the three water velocity zones. Stable state data frames were converted into multiband raster data frames containing water depth, velocity and absolute elevation attributes. Multiband rasters were then converted into vector format, triangulated and trimmed to produce discrete spatial features for each velocity zone. Calculation of wetted benthic surface area, and the depth range within each velocity zone, was undertaken on the 3D surface to take into account the vertical relief of the river bed. Summary statistics of the depth range (minimum, maximum, mean, standard deviation) were calculated for each velocity zone under each discharge scenario. Post-processing, including surface area and depth calculations, was achieved using Eonfusion Quantum GIS and Excel and made distributable using Google Earth.
3
RESULTS
There was considerable variation in wetted benthic area under operational flows among reaches (Table 2) ranging from 38,169 m2 in Yallakool Creek study reach at 170 MLd-1 to 48,292 m2 in the Colligen creek study reach at 200 MLd-1. The wetted benthic area under low flows, operational flows and bankfull flows for Little Merran Creek could not be modelled. In Colligen Creek the environmental flow of 800 MLd-1 resulted in a 14% increase in wetted benthic surface area relative to the operational flow of 200 MLd -1. Similarly, in Yallakool Creek an environmental flow of 560 MLd-1 resulted in a 22% increase in wetted benthic surface area relative to the base flow of 170 MLd-1 (Table 2). The estimate of wetted benthic surface area during the maximum daily discharge scenario experienced in 2011-2012 during unregulated flows was considerably higher than during the modelled operational flow or environmental flow in these rivers. On the peak of this unregulated flow event the modeled wetted benthic surface area relative to the base flow 200 MLd -1 scenario increased by 47.8% in Colligen Creek and 59.7% in Yallakool Creek (Table 2). In contrast, in Little Merran Creek the estimated half bankful and maximum discharge 2011-13 scenarios resulted in considerable lateral connection and inundation of low lying geomorphic features (Table 2). The models estimate there would be a considerable further increase in wetted benthic surface area during a bankfull flow, however this type of flow event has not occurred since 2010 and could not be modelled. Table 2. Estimates of wetted benthic surface area under a range of discharge scenarios in four river reaches in the Edward-Wakool system. Note: In Colligen Creek the environmental flow was approximately the same discharge as the estimated half bankfull discharge (800 MLd-1). N/A = not applicable Scenario Modelled estimate of wetted benthic surface area (m2) Wakool R Yallakool Ck Colligen Ck Little Merran Ck Low flow 42,196 33,159 43,257 Not modelled Operational flow 43,587 38,169 48,292 Not modelled Median flow (2011-2013) 46,222 41,510 49,982 160,908 Additional scenario Not modelled Not modelled Not modelled 218,529 Environmental flow N/A 46,432 55,110 N/A Half Bankful 58,547 49,924 55,110 242,902 Max daily discharge (2011-2013) 200,455 60,989 71,337 542,584 Bankfull 264,109 86,404 84,820 Not modelled The representation of the spatial coverage of the benthic surface extent for each velocity zone for Colligen Creek shows that there was a large area of slackwater available in the low flow scenario (63% of the total available benthic surface area, 27,391 m2), but very little slackwater available during the operational flow (9.8%, 4,712 m2), median flow (6.4%, 3184 m2) and half bankfull/environmental flow scenarios (3.1 %, 1708 m2)(Figure 1). Slackwater was created along the margins and particularly in low lying geomorphic in-channel features during the maximum discharge scenario (7.4 %, 5,308 m2) and bankfull scenario (13.6%, 11,575 m2)(Figure 1). The results for Yallakool Creek and the Wakool River are not presented here, but were similar to those for Colligen Creek. In contrast to the results for Colligen Creek, in Little Merran Creek there was a large area of slackwater available under the median flow scenario (70% of the total available water surface area). Slackwater was also abundant in the half bankfull scenario (25.5%) and maximum discharge scenario (41.5%)(Figure 2). Only a very small percent of the area was classed as fast water in each of these scenarios. The relationship between discharge and wetted surface area in these four river reaches was not linear (Figure 3). If there were a similar increase in discharge in two rivers, it would not produce in the same increase in wetted benthic surface area. For example, an increase from 200 MLd-1 to 800 MLd-1 in Colligen Creek resulted in a modelled 14% increase in wetted benthic area, whereas a similar increase from 170 MLd-1 to 800 MLd-1 in Yallakool Creek resulted in a modelled 30% increase in wetted benthic area (Table 2). There was a very large increase in estimated wetted surface area with increasing discharge in Little Merran Creek.
Figure 1. Maps showing the extent of wetted benthic surface area for each velocity zone in Colligen Creek under six discharge scenarios. The half-bankfull discharge scenario (800 MLd-1) was approximately the same as the environmental flow delivered in this river system in 2011-12. Velocity zone 1 = slack water; velocity zone 2 = slow water; velocity zone 3 = fast water.
Figure 2. Maps showing the extent of wetted benthic surface area for each velocity zone in Little Merran Creek under three discharge scenarios. Velocity zone 1 = slack water; velocity zone 2 = slow water; velocity zone 3 = fast water. 600,000
Estimated wetted surface area (m2)
Colligen Yallakool
500,000
Wakool Little Merran
400,000
300,000
200,000
100,000
0
500
1000
1500
2000
2500
3000
3500
4000
Discharge (MLd-1)
Figure 3. Relationship between discharge (MLd-1) and modelled wetted surface area (m2) for reaches in Colligen Creek, Yallakool Creek, Wakool River and Little Merran Creek from the Edward-Wakool system.
4
DISCUSSION
The models of in-channel flow scenarios demonstrate that the relationship between discharge and wetted surface area and the area of slackwater in these four study reaches in the Edward-Wakool system is not linear and is most likely strongly influenced by geomorphology. Similar to the findings of Vietz et al. [15], we found that in some river reaches (e.g. Colligen Creek) the area of slackwaters decreased with increasing discharge until inundation of higher-elevation bars and in-channel benches. However, in Little Merran Creek a different pattern was observed with large area of slackwater available at a range of different discharges. As the data presented
here are for only relatively short 2-4 km reaches of each river, it is important to consider that the reported relationships may be reach specific and that these relationships need to be examined over longer river distances in the study rivers. These results have important implications for studies of ecology-flow relationships for in-channel flows. They suggest it may be more appropriate to examine the relationship between inundation area and ecosystem responses to in-channel flows rather than focusing on relationships solely with daily discharge, as has commonly been the practice. In-channel 2D hydraulic modelling under different flow scenarios can be used to i) better understand the relationship between in-channel flows and ecosystem responses, ii) predict the consequences of in-channel flows on slackwater dependent biota and ecosystem functions, and iii) facilitate better planning and management of the future in-channel environmental flows. In particular, 2D hydraulic modelling may assist managers examine trade-offs between ecosystem benefits and third party impacts, such as inundation of private land, prior to the delivery of environmental water. Modelling of in-channel flow scenarios can also help managers determine the optimum discharge at which there can be an increase in riverbank inundation and creation of slackwater to produce ecosystem responses to environmental flows, but with minimal third party impacts.
5
ACKNOWLEDGMENTS
Thanks to landholders in the Edward-Wakool river system for providing access to their properties for fieldwork. Maps were prepared by Simon McDonald and Deanna Duffy (Charles Sturt University Spatial Analysis Unit). The modelling was undertaken by Dr Hugh Pederson from Marine Solutions. The modelling presented in this paper was partly funded by the Ecological Responses to Altered Flow Regimes Research Cluster which was a collaboration between the CSIRO Water for a Healthy Country Flagship, Griffith University, the University of New South Wales, Monash University, Charles Sturt University, La Trobe University and the Arthur Rylah Institute of the Victorian Department of Environment and Primary Industries. The project was also partly funded through additional cash contributions from Charles Sturt University (Institute for Land, Water and Society), Murray Catchment Management Authority (thanks to Patricia Bowen and John Conallin) and Monash University (Water Studies Centre).
6
REFERENCES
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