The Quantitative Assessment of River Reach Morphology
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The Quantitative Assessment of River Reach Morphology By David A. Keast, BA, BSc A thesis submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy at the School of Land and Food, University of Tasmania, September, 2014. 1 Declaration of Originality This thesis contains no material which has been accepted for a degree or diploma by the University or any other institution, except by way of background information and duly acknowledged in the thesis, and to the best of my knowledge and belief no material previously published or written by another person except where due acknowledgement is made in the text of the thesis, nor does the thesis contain any material that infringes copyright. David Keast 2 Statement of authority of access This thesis may be made available for loan and limited copying and communication in accordance with the Copyright Act 1968. David Keast 3 Statement regarding published work contained in thesis The publishers of the paper comprising Chapter 2 hold the copyright for that content, and access to the material should be sought from the journal Water (doi:10.3390/w5041816). The remaining non published content of the thesis may be made available for loan and limited copying and communication in accordance with the Copyright Act 1968. 4 Statement of Co-Authorship The following people and institutions contributed to the publication of work undertaken as part of this thesis: Name and School = David Keast, School of Land and Food Name and institution = Joanna Ellison, School of Land and Food Author details and their roles: Paper 1, Magnitude Frequency Analysis of Small Floods Using the Annual and Partial Series Located in chapter 2, and published in Keast D, Ellison J (2013) Water 5(4), 1816-1829 doi:10.3390/w5041816 Candidate was the primary author and Joanna Ellison assisted with refinement and presentation We the undersigned agree with the above stated “proportion of work undertaken” for each of the above published (or submitted) peer-reviewed manuscripts contributing to this thesis: Signed: Joanna Ellison Jon Osborn Supervisor A/Head of School School Of Land and Food School of Land and Food University of Tasmania University of Tasmania Date 22 September 2014 5 Abstract Rivers are an integral part of the earth’s ecological, hydrological and physical systems, and also provide an indispensible range of services to humans. However, despite our strong dependence on rivers, human activities, and particularly the increased pressure on water resources, have resulted in a general degradation of river health world-wide. The long-term sustainability of many river systems is dependent on their successful management, and a key component of river management is to simply and meaningfully order streams into natural or arbitrary groups based on common characteristics. Such classification can assist in the management of rivers by increasing understanding of river form and process amongst the general complexity found in rivers. River classification has been central to developing an understanding of the links between hydrology, geomorphology and ecology and also allows knowledge from a particular river type at one location to be extrapolated to other locations of the same type, thus reducing resourcing requirements. Although possessing a relative abundance of water in comparison to many other regions, north-eastern Tasmanian faces pressures on water resources similar to those experienced elsewhere. In recent years, considerable effort has been directed towards achieving a balance between abstraction and environmental flows in north-eastern Tasmanian rivers, but this process has been complicated by the wide variety of riverine environments and channel and floodplain forms found in the region, and by the relatively few studies that have investigated the region’s geomorphology, hydrology, and ecology. Two approaches to classification have been adopted in the management of Tasmanian rivers. A nested hierarchical river classification system has been developed for Tasmania, and has formed the basis of a comprehensive river health assessment methodology. However this approach is relatively resource intensive, requiring data to be collected and expert analysis to be applied at different scales. In addition the large number of groups (river types) that this model produces means that developing relationships between flow alteration and physical and ecological response for each river type is impractical. A second approach has been the development of a broad two-classed model that classifies rivers on the basis of hydrological variability. Although this is an objective and quantitative model requiring few resources, the wide variety of river types included within each of the two classes mean that only the most generalised models linking hydrology, geomorphology and ecology can be developed. While each of these classification systems could be considered to fulfil the purpose for which they 6 were designed, the lack of a simple and objective method to classify rivers into a reasonable number of meaningful classes on the basis of channel morphology has limited the development of predictive ecosystem models to link the physical and ecological responses of north-eastern Tasmanian rivers to altered flows. The aim of this study was to develop an objective and quantitative method to group north- eastern Tasmanian rivers and streams into hydromorphologically meaningful groups. This was achieved through several sub-studies; estimation of the magnitude and frequency of small floods, investigation of bankfull channel morphometry, analysis of the first order estimates of peak discharge of small floods, and assessment of the hydromorphological characteristics of different river basins. The magnitude-frequency of small floods (average recurrence interval < 5 years) was estimated by analysis of data from 13 north-eastern Tasmanian stream-gauging stations. Empirical comparisons were made between flood frequency estimates based on the annual series data set, those based on the partial series, and the Langbein method of converting annual series average recurrence intervals to partial series intervals. Annual series estimates were found to be one third the magnitude of partial series estimates at low average recurrence intervals, but converged with partial series estimates at around 5 years. The methods developed in this chapter are used in a later sub-study to develop relationships between discharge and catchment area for north-eastern Tasmanian Rivers. Two quantitative methods for determining bankfull stage from plotted channel cross-sections, the minimum width-to-depth ratio and the first maximum of the bench-index were evaluated against qualitative estimates of bankfull stage on 89 cross-sectional surveys undertaken at nine river reaches in north-eastern Tasmania. Results indicated that while neither method offered a suitable stand-alone means for estimating bankfull stage, they may in combination provide a means to approximate the range of bankfull stage and serve as a useful adjunct to other methods. The results also highlight the large variability in channel morphology along a reach. The results from this sub-study were used to assist in the identification of bankfull channel morphology parameters in a later assessment of hydromorphological characteristics of rivers and development of groupings. First order estimates of the peak discharge of small floods at ungauged sites in north-eastern Tasmania were investigated through the development of power-law equations relating the 7 peak discharge of floods with average recurrence intervals ranging from 1.1 to 10 years to catchment area (Ad). Using data from 13 stream gauging stations, the analysis suggested that the discharge associated with a flood with two year average recurrence interval was estimated 0.9 by Ad . Intra-regional variation in the relationship was investigated and differences were found between those rivers which drain to the north coast and those which drain internally or to the east coast. The uncertainty and error associated with the study was also identified and discussed. Intra-regional variation in the relationship between discharge and catchment area identified a group of northward draining rivers in north-eastern Tasmania that plotted as negative residuals and a group of internally draining sites which plotted as positive residuals. Three hydromorphological characteristics of north-eastern Tasmanian rivers, drainage density, bankfull frequency and stream-power, were investigated using the results from the first order estimates of peak discharge of small floods sub-study. The range and variation in drainage density values was examined and found to broadly reflect variations in precipitation, elevation and geology. Variation in drainage density was found to be correlated to changes in the density of higher order streams, with little variation occurring in the density of first and second order streams. The range and variation in drainage density values was examined and found to broadly reflect variation in precipitation, elevation and geology. The two groups of rivers identified previously were found to have different drainage density ranges. Catchments draining northwards or eastwards to the coast were found to have drainage densities > 2 km km-1, while internally draining catchments were found to have drainage density < 2 km km-1. The estimation of bankfull discharge in north-eastern Tasmanian rivers was investigated, and peak discharge with an average