Management of Farm Dams using Low Flow Bypasses to Improve Stream Health

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1 Counterpoints 2003 Celebrating Diversity in Research Management of Farm Dams using Low Flow Bypasses to Improve Stream Health Susan Lee School of Geography, Population and Environmental Management Abstract Diversion of streamflows into dams and storages interrupts the natural flow regime of a stream. The effect of this interrupted flow regime is most noticeable during times of low and medium flow. This project aims to determine the effectiveness of low flow bypass structures to prevent diversion of low and medium streamflows into farm dams, thus retaining a natural flow regime in the stream. The research aims to answer the key question: Does using low flow bypasses to retain a natural flow regime have an effect on stream health? Three pairs of study sites have been carefully chosen for this project, such that within each pair the dams are as similar as possible to each other. As this is not a laboratory study it would be impossible to eliminate all potential dissimilarities between the dams. This paper discusses the comprehensive methodology that has been adopted to minimise these potential sources of error. Introduction Australian rivers are among the most variable in the world (Walker 2002). A natural stream has a natural flow pattern or regime. In the southern part of Australia this pattern usually involves high flows in winter and early spring to coincide with the period of highest rainfall. The lower rainfall of summer and autumn is accompanied by lower flows. Native flora and fauna have evolved under these conditions and hence are dependent upon them. The native flora and fauna thus depend upon the flow regime triggers of high and low flows to trigger feeding and breeding patterns (Thoms et al. 2000). Abstracting or removing water from the stream environment alters the natural flow regime and thus has deleterious effects on the health of that stream. Water is generally removed during critical times of the season and is hence not available when required for key environmental processes. In the Marne catchment (Figure 1) more than 75% of the water in the catchment is being retained in farm dams (BC Tonkin & Associates Consulting Engineers 1998) and a decline in stream health has been noted. 72

2 Counterpoints 2003 vol 3 no 1!"#$$%&'( The aim of the project is to evaluate the use of low flow bypass structures to prevent abstraction of low flows and not allow farm dams to abstract water until the flow has reached a predetermined minimum level. The low flows that are depended upon for key ecological processes are hence allowed to remain in the stream and the dams will be allowed to fill once the minimum flow to satisfy ecological processes has been provided. Low flow bypasses A concept design of a low flow bypass structure for use with an on-stream dam is shown in Figure 2. The structure acts to prevent low flows from being removed from the stream and captured in the dam (Department for Water Resources and Clare Valley Water Resources Planning Committee 2002). This retains a natural flow regime in the stream as the dam will only be allowed to fill when there is sufficient flow in the stream that the flow pattern will not be interrupted. The diversion pipe is designed so that its maximum flow is the predetermined minimum flow for the stream. During low flow events the water will enter the PVC pipe and be diverted around the dam and discharged downstream of the dam. The mesh covering over the bypass serves to prevent debris from clogging the diversion pipe. When the flow increases to greater than the predetermined minimum flow the diversion pipe will flow at maximum capacity and the bypass will act as a weir allowing water to pass over the top of the structure and into the dam. The minimum flow will continue to flow around the dam. During high flow events the dam will overtop and the overflow will flow downstream. 73

3 Counterpoints 2003 Celebrating Diversity in Research ' ))*+,),( Study area The study takes place in the Marne catchment (Figure 1), part of the Eastern Mount Lofty Ranges catchment, itself a subcatchment of the River Murray Catchment. The main towns in the upper catchment are Springton, Eden Valley and Keyneton. Springton is approximately 110km north east of Adelaide. The two main tributaries in the catchment are the Somme or North Rhine River and the Marne River. The Somme River joins the Marne River just upstream of the Marne Gorge and the Marne River then flows into the River Murray near Wongulla. The catchment is divided by the Palmer Fault. Upstream of the fault (often referred to as the Hills Zone ) the area is characterised by hard rock aquifers and downstream of the fault ( Plains Zone ) is characterised by sedimentary aquifers. The study area for this project is the Hills Zone. The most common land uses in this area are viticulture and agriculture (Environment Protection Authority 2002). The volume of water stored in dams in the catchment has increased threefold between 1974 and This increase has been particularly significant in the Western Slopes Region of the Hills Zone (Figure 3) during the latter part of the 1990s and the most recently installed dams are mainly over 10ML with many greater than 100ML (BC Tonkin & Associates Consulting Engineers 1998). Monitoring procedure In order to assess the effects of the low flow bypass structures on the health of the streams a sampling procedure has been developed to assess changes in stream condition before and after the installation of the structures. The sampling procedure encompasses physical, chemical and biological aspects of the health of the stream in order to present an holistic appraisal of the stream system. The sampling procedure is based on other standard methodologies to enable the results to be analysed using standard techniques (Environment Protection Agency c.2001) 74

4 Counterpoints 2003 vol 3 no 1 Sampling points are located both upstream and downstream of each dam. At each sampling point a water sample is collected and analysed in the laboratory for phosphate, nitrate, nitrite and alkalinity. Dissolved oxygen, temperature, electrical conductivity, turbidity and ph are measured on site using portable meters. Flow velocity and channel cross section are measured and the stream discharge calculated according to the area-velocity method (Chaudry 1993). Biological monitoring of the stream uses macroinvertebrates as bioindicators. Macroinvertebrates have been selected in preference over fish or other bioindicators due to the ease of the sampling and analysis techniques. Further, there have been other studies conducted in this catchment and other similar catchments using macroinvertebrates as bioindicators for which results are as yet unpublished but have been obtained from the Australian Water Quality Centre and the Environment Protection Agency. Methodology design The methodology has been designed to be similar to previous or similar studies to enable statistical analysis of results. The methodology was largely modelled on the AusRivAS (Australian River Assessment Scheme) standard methodology to enable the possible use of this model in analysing the results. The methodology was adapted with the aid of an ecologist however, as the AusRivAS models have been largely developed interstate on streams with different characteristics from those in the study area. The study uses a BACI (before-after-control-impact) methodology with some modifications to the design. Dams for examination are selected in pairs such that each pair is as similar as possible. The selection of sites is discussed more fully in section 4.1. Within each pair one of the dams is to be fitted with a low flow bypass structure ( impact site) and the other will be left untreated ( control site). The stream sections immediately upstream and downstream of both the control and impact dams will be monitored for three years; one year prior to the installation of the bypasses and two years following installation. The selection of appropriate sites for examination is crucial in the project design as the integrity of the results is dependent upon the careful selection of the study sites. Procedure for selecting individual sites The quality of the study results depends upon the selection of appropriate study sites. Due to the variable nature of the environment, any scientific study conducted in the field will be subject to more variables than a study conducted in controlled conditions in a laboratory. It is thus crucial that the sites selected for this study were carefully chosen to eliminate as many variables as possible in order to be able to attribute any differences observed in the results solely to the low flow bypass treatment of the dams. Study sites were thus chosen to be as similar as possible in all respects (Harrison and Harris 2002; Wang et al. 2002). Each pair of dams were selected to be similar hydrologically (size of dam, location with respect to the stream, stream order, aspect), geologically (soil and geological formations) and climatically (rainfall, temperature). The dam must also be abstracting from a stream rather than collecting runoff from the catchment above it directly as the latter type of dam has no defined inlet onto which a bypass device could be fitted. A desktop study was conducted which examined rainfall records and geology maps for the catchment area and field verification undertaken to confirm the final selection. Three pairs of dams have been selected for inclusion in this study. Figure 3 shows the subcatchments of the Hills Zone and the locations of the three pairs of dams. 75

5 Counterpoints 2003 Celebrating Diversity in Research Pair One This pair of dams is located on the same property, a vineyard which also raises sheep in the upper reaches of the Keyneton subcatchment (Figure 3). The dams are similar in size (20ML and 25ML) and are both located on ephemeral first order streams (Strahler 1964, in Allan 1995). There is a rain gauge located on the property and the rainfall records for the last three years are very similar to those obtained from Bureau of Meteorology gauges located in the area. Anecdotal data indicates that the flow rates in the two streams are similar and that the two dams fill at the same rate. The geology of the area is the Strangway Hill Formation (Department of Mines 1969) which consists of grey-green metamorphosed siltstone, greywacke and arkosic small-scale cross-bedded siltstone. In all respects except the size of the dams the two dams are practically identical. Due to the small number of sites available for inclusion in this study it was not possible to select pairs of dams which were identical in size as well as the other criteria and hence this small difference in size must be considered when analysing the results. Pairs Two and Three The layout of these two pairs of dams is more complex than for Pair One (Figure 4). Pair Two consists of two large dams on the same property with the overflow from the larger dam (2a, Figure 4 120ML) feeding into the smaller dam (2b, Figure 4, 90ML). The property on which these dams are located is a vineyard which also raises sheep. The channel between the two dams is approximately 800m long and on average between 1 and 1.5m wide. Although the two dams are located on the same stream, the distance between them is sufficiently long (greater than 100 channel widths) to consider the dams to act independently (Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand 2000). If the two dams were located too close together on the same stream then it is possible that the two dams would affect each other and observed changes could not be attributed solely to the installation of the low flow bypass structure. The upper dam (2a) will be bypassed and the lower dam (2b) will not be treated in any way. Pair Three consists of two smaller dams which are not physically adjacent. One of the dams (3a, Figure 4, 15ML) is located upstream of Pair Two and the other (3b, Figure 4, 8ML) downstream. Both properties are vineyards which also raise cattle, however on the property on which the lower dam is located the dam is located further from the vines than is the upper dam. The upper dam (3a) will be bypassed and the lower dam (3b) will not be treated in any way. 76

6 Counterpoints 2003 vol 3 no 1 Pair 1 Pair 2 & 3 -,).,'//'&/( The dams are each located more than 100 channel widths from the other dams and hence may be considered independent of them. In addition to comparing the dams within the pairs, these two pairs will be compared to each other to assess the effectiveness of low flow bypass structures on small dams compared with large dams. The geology of this area is similar for all four dams, part of the Inman Hill formation (Department of Mines 1969). This comprises greywacke, arkose and turbidite, large-scale cross-bedding and slump structures. Around the lower dam of Pair Three is located a section of older weathered rock of the same composition. Rainfall data is collected on all three properties, is similar and corresponds with the Bureau of Meteorology data available for the area. All of the streams to be monitored are ephemeral and first order (Strahler 1964, in Allan 1995). 77

7 Counterpoints 2003 Celebrating Diversity in Research Low flow bypass 3a Low flow bypass 2a 2b 3b 0 1 ) Conclusions As this study seeks to test the impact of low flow bypasses on downstream health, all other variables that may impact stream health need to be removed from the investigation. As this study is undertaken in the field the methodology had to be carefully designed to minimise potential additional variables arising from the differences between the study sites. Certain compromises had to be made due largely to the limited number of sites volunteered for the study, hence perfectly matched pairs of dams were not available. However it was possible to limit the differences between the sites, the main apparent difference is in the size of the dams in each and in all cases this difference has been kept small. Nonetheless this will need to be considered when analysing the results. Acknowledgements Funding from the River Murray Catchment Water Management Board is gratefully acknowledged, along with the cooperation of several private landholders in the study area for allowing access to streams and dams located on their property. The Eden Valley Grape Growers Group is also gratefully acknowledged for their help in contacting landholders in the study area. 78

8 Counterpoints 2003 vol 3 no 1 References Allan, J. D. 1995, Stream Ecology: Structure and Function of Running Waters, Chapman & Hall, New York Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand 2000, Australian Guidelines for Water Quality Monitoring and Reporting, Environment Australia, Canberra, Report Number 7 BC Tonkin & Associates Consulting Engineers 1998, Impact of Water Use in the Marne Catchment on Water Resources, Adelaide, Report Number R-1 Chaudry, M. F. 1993, Open Channel Flow, Prentice Hall, New Jersey Department for Water Resources and Clare Valley Water Resources Planning Committee 2002, Background to Water Allocation Planning in the Clare Valley Prescribed Water Resources Area, Adelaide Department of Mines 1969, Adelaide, 1: , Geological Survey of South Australia Environment Protection Agency c.2001, AusRivAS Sampling and Processing Manual; South Australia, Environment Protection Agency, Australian Water Quality Centre, Natural Heritage Trust and Environment Australia, Adelaide Environment Protection Authority 2002, Land Status Data Mapping for the Mount Lofty Ranges Watershed, CD Rom, Government of South Australia Harrison, S. S. C. & Harris, I. T. 2002, 'The effects of bankside management on chalk stream invertebrate communities', Freshwater Biology, vol. 47, pp Savadamuthu, K. 2002, Impact of farm dams on streamflow in the Upper Marne Catchment, Department for Water Resources, Adelaide, Report Number DWR 02/01/0003 Strahler, A. N. 1964, 'Quantitative geomorphology of drainage basins and channel networks', in Handbook of Applied Hydrology, ed. V. T. Chow, McGraw-Hill, New York, Section 4-2 Thoms, M., Suter, P., Roberts, J., Koehn, J., Jones, G., Hillman, T. & Close, A. 2000, Report of the River Murray Scientific Panel on Environmental Flows: River Murray - Dartmouth to Wellington and the Lower Darling River, Murray-Darling Basin Commission, Canberra Walker, K. 2002, 'Ecology and Hydrology', in The Science of Environmental Water Requirements in South Australia, Adelaide, South Australia, 24 September 2002, The Hydrological Society of South Australia, pp. 1-7 Wang, L., Lyons, J. & Kanehl, P. 2002, 'Effects of watershed Best Management Practices on habitat and fish in Wisconsin streams', Journal of the American Water Resources Association, vol. 38, no. 3, pp