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Late Holocene Floodplain Processes and Post-European Channel

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Late Holocene Floodplain Processes and Post-European Channel University of Wollongong Thesis Collections University of Wollongong Thesis Collection University of Wollongong Year 2003 Late holocene floodplain processes and post-European channel dynamics in a partly confined valley of New South Wales Australia Timothy J. Cohen University of Wollongong Cohen, Timothy J., Late holocene floodplain processes and post-European channel dy- namics in a partly confined valley of New South Wales Australia, Doctor of Philosophy thesis, School of Earth and Environmental Sciences, University of Wollongong, 2003. http://ro.uow.edu.au/theses/1931 This paper is posted at Research Online. 5. Post-European channel dynamics in the Bellinger valley CHAPTER 5 - POST-EUROPEAN CHANNEL DYNAMICS IN THE BELLINGER VALLEY 5.1 INTRODUCTION The Bellinger River, like many other systems in south-eastern Australia (see review by Rutherford, 2000), has undergone a series of planform and dimensional changes since European settlement. This chapter investigates the nature and timing of these changes and relates them to both the pre-European state of the channel and floodplain, along witii the rainfall and discharge patterns established in Chapter 3. The chapter consists of four main sections. Section 5.2 briefly outiines the methodological techniques used in this chapter, while Section 5.3 provides a historical assessment of channel and floodplain character in the Bellinger catchment. Sections 5.4 - 5.5 quantify the magnitude of change and present a summary of the evidence for cyclic channel changes. 5.2 METHODOLOGICAL APPROACH 5.2.1 Historical and anecdotal evidence for the BeUinger valley In order to determine the character of the Bellinger River and the surrounding floodplain environment at the time of settlement a number of historical data sources have been utilised. Eariy surveyor's notes between 1841 and 1842 provide the first written description of the Bellinger valley. Parish maps, along with portion plans for individual properties within each of the three reaches were used to construct the position of the channel at the time of settlement. Interviews with some of the older residents provide additional anecdotal evidence for the early to mid 20* century, but also includes oral descriptions from their parents. Historical photos supplement both the historical and anecdotal data, while aggregate extraction records supply details of the mid-late 20* century gravel extraction operations that occurred on the Bellinger River. 5.2.2 Palaeochannel analysis Planform data from parish maps and portion plans revealed evidence for the timing of planform changes within the study reaches. The two lower valley reaches. Wills and Gordonville Reach contain palaeochannels, while additional palaeochannel data has been obtained from an abandoned cutoff upstream of Orama Reach (Darkwood Palaeochannel) 206 5. Post-European channel dynamics in the Bellinger valley and downstream of Gordonville Reach, at Bellingen township (Bellingen Paleochannel). Unlike palaeochannel analysis in previous studies (Erskine et al., 1992; Cohen and Brieriey, 2000; Brooks et al., 2002), only two of the five palaeochannels are completely abandoned, with the other three remaining as secondary channels. Cross-sections were undertaken at cutoff apices and inflection points, with the estimation of prior channel depths obtained from auger holes and historical evidence. The five palaeochannels provide a crude estimate of the dimensions of the channel prior to the major post-European dimensional changes documented later in the chapter. The limitations of such a data set are outiined in Section 5.4.1. 5.2.3 Spatial analysis of channel characteristics Aerial photographs from a 1943, 1956, 1960, 1973, 1982, 1994 and 2000 time series allowed assessments of the magnitude and timing of planform and dimensional changes in the lati:er half of the 20* century (see Appendix 5.1 for aerial photograph details). Ground control points were collected using real time kinetic (RTK) differential GPS tied to die Australian Height Datum (AHD), with an X,Y accuracy of ± 2 m and a Z accuracy of ± 0.1 m. In order to test the notion that channel geometry adjusts in a cyclical manner associated with the inferred cyclical changes in discharge regime (FDRs and DDRs), four time intervals were chosen for channel geometry analysis. These include the 1943, 1956, 1973 and 2000 aerial photographs, with the 1943 representing the last DDR (1900 - 1947), the 1956 and 1973 representing the early and later stages of the previous FDR (1948 - 1977), with the 2000 aerial photograph/field data representing the current DDR (1978 - present). All historical data sets have been collected using the same map grid co­ ordinates (based on the differentially derived GPS ground control points) as the current field data, allowing for the most accurate comparison of past channel geometries, with those of today. In order to maintain consistency within the four time intervals (1943, 1956, 1973 and 2000) the ground control points were used to rectify the 2000 aerial photographs, while all previous time series were rectified using the nearest neighbour polynomial image-image warping method within ENVI 3.4. Both current and historical cross-sections occur approximately one channel width apart (125 - 150 m) and include at least one cross-section in a representative pool or riffle location. Using the (RTK) ground control points photogrammetrically derived cross-sections were obtained from the 1943 and 1973 data series, with each of these data sets providing accurate cross-sectional geometry, but of necessity limited to the channel above the Mow- flow' water level. For direct comparison current field-surveyed cross-sectional geometry 207 5. Post-European channel dynamics in the Bellinger valley has therefore been adjusted to exclude the small areas (i.e. < 5 %) below low-flow. The 1956 rectified aerial photographs provide an additional width data set alone. Each of the photogrammetrically derived data sets has an error associated with observational error, determined by the scale, quality and resolution of the aerial photographs. The 2000 field data provide the most accurate representation of channel geometry, while the 1943 photogrammetrically derived data (from 1:18,500 photography) provide the second most accurate information. The 1973 1:41,000 photography is of poorer resolution thereby resulting in both observational error and errors associated with the setting up of the photogrammetry model. The 1973 photography, however, has more ground control points per photogrammetry model than the 1943 photography due to the fact that it covers a greater length of the valley. Thus, the discrepancy between differences in scale is offset by an increased level of ground control (P.Tuckerman, pers.comm., 2002). Residuals on ground control for the 1943 and 1974 aerial photography range from 0.2 - 0.6 m. However, given the level of accuracy of the ground control points all photogrammetry and field survey measurements are presented to the nearest 0.1 m. The 1956 photography at 1:40,000 is also of poor resolution and is subject to error within the rectification process. In all rectified images root mean squared errors (RMS) were kept below 3 m. This data are therefore subject to a ± 3 m spatial error, in addition to pixel resolution error of ± 1 m, coupled with observational ertor associated with defining on-screen bank tops. All data sets are small (n = < 30) and therefore normal distributions are not assumed and were tested using Shapiro-Wilk's test for normality. Differences in variances have been assessed using Levene's test for equality of variance, while differences in means have been assessed using paired t-tests with a pooled variance estimate. Where populations displayed a divergence from a normal distribution, non-parametric analysis utilising the Wilcoxon signed-rank sum test has been used. Riparian and hillslope vegetation changes since 1943 have also been quantified using the rectified data sets, expressing riparian vegetation changes along each of the reaches as an area/unit length index. All vegetation within a 150 m buffer from the mid-point of the channel was digitised for the four selected time intervals. The chosen buffer width represents the maximum of the mean channel widths for the three reaches and provides a consistent approach to assessing temporal changes to riparian vegetation within one channel width's distance from the centre line of the channel. This method, however, sometimes includes vegetation beyond the immediate riparian zone such as where the channel impinges against the bedrock valley margin. This vegetation index is therefore quantifying spatial changes to both riparian communities and hillslope communities within the 150 m buffer. 208 5. Post-European channel dynamics in the Bellinger valley 5.2.4 Reach analysis, channel processes and an assessment of prior bed levels Cross-sectional and longitudinal profile data from individual channel units was obtained witiiin each reach as part of a broader reach analysis (Chapter 2). While pool and riffle locations were systematically surveyed, additional channel cross-sections were obtained in areas of bedrock constriction and expansion. Riffles and pools were identified based on morphological criteria but compared to the polynomial regression method of Richards (1976). Each cross-sectional data set includes some sections benchmarked in 1992 and resurveyed in 2000. A number of sections were re-surveyed following the 2001 flood which modified channel geometry in
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