The Quantitative assessment of river reach morphology

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 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 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 by Ad0.9. 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 recurrence interval of two years was proposed as a proxy for bankfull discharge. Attempts to accurately measure bankfull discharge at the study sites using field techniques were unsuccessful, as was the use of commonly used flow resistance equations. Large variances were found between estimates from different flow resistance equations, particularly at sites with deeper channels and high hydraulic radius. Estimates of peak discharge of small floods developed previously were used to develop suitable methods to estimate stream-power values. Substantial variability in downstream trends in stream-power was found in the Pipers, Ringarooma and Scamander rivers. These rivers were found to have different longitudinal trends and to deviate from the general downstream stream power trends found elsewhere. There was some evidence of an association between channel morphology and stream power, with high WD values/ low R values occurring at locations with high stream power. The river and catchment metrics developed in earlier chapters were then used in conjunction with univariate and multivariate statistical analysis in an attempt to develop meaningful hydromorphological groupings of north-eastern Tasmanian rivers. using objective and quantitative methods. A range of univariate and multivariate statistical techniques was applied to a dataset consisting of 164 channel cross-sections from thirteen sites, and the results were assessed against a separate dataset containing 58 cross-sections from 15 sites. A strong source of underlying variability in the channel morphdataset was found to occur along an orthogonal axis which had high values of width-to-depth ratio at one end and high hydraulic radius values at the other. The variability of bankfull channel morphology both along a reach and between sites was examined, and Principal Components Analysis and agglomerative clustering used to examine the underlying structure in the data and identify the best low-dimensional representation of the variability in channel morphology. The river and catchment metrics developed in earlier chapters were then used in conjunction with multivariate statistical analysis of channel cross-sectional data from field surveys to investigate variability in channel morphology and develop a quantitative morphological typology. A strong source of underlying variability in the dataset was found to occur along an orthogonal axis which had high values of width-to-depth ratio at one end and high hydraulic radius values at the other. While analysis was able to identify two groupings with membership based on either high width-to-depth ratio or high hydraulic radius, methods to derive the groupings of sites based on channel parameters using remote parameters were generally unsuccessful; combinations of the remote parameter values were unable to reproduce natural grouping identified using channel parameters. However drainage density was identified as a strong remotely sensed predictor variable. The results suggest that localised and reach scale factors have more influence on channel morphology than catchment scale controls in north-eastern Tasmanian rivers. The overall study results suggest that north-eastern Tasmanian rivers are highly variable and have characteristics different from those found elsewhere. This has implications for the use of remotely sensed data and GIS tools in the study of regional hydromorphological characteristics