Barnaby P.G. Smith

An approach to hydrograph classification

The stream hydrograph is an integration of spatial and temporal variations in water input, storage and transfer processes within a catchment. For glacier basins in particular, inferences concerning catchment-scale processes have been developed from the varying form and magnitude of the diurnal hydrograph in the proglacial river. To date, however, such classifications of proglacial diurnal hydrographs have developed in a relatively subjective manner. This paper develops an objective approach to the classification of diurnal discharge hydrograph ‘shape’ and ‘magnitude’ using a combination of principal components analysis and cluster analysis applied to proglacial discharge time-series and to diurnal bulk flow indices. The procedure is applied to discharge time-series from two different glacier basins and four separate ablation seasons representing a gradient of increasing hydrological perturbation as a result of (i) variable water inputs generated by rainstorm activity and (ii) variable location and response of hydrological stores through a systematic decrease in catchment glacierized area. The potential of the technique for application in non-glacial hydrological contexts is discussed. Copyright

Longitudinal variations in exposed riverine sediments: a context for the ecology of the Fiume Tagliamento, Italy

1. A key component of physical habitat along braided river systems is the exposed riverine sediment within the active zone. The relatively unmanaged, gravel-bed Fiume Tagliamento, Italy, provides the focus for exploring two ecologically important properties of exposed riverine sediments: their within-patch and between-patch variability in calibre. 2. To characterize between-patch variation in exposed riverine sediments, replicate (within-patch) samples were obtained from three geomorphologically distinct locations along 130 km of the river: bar heads along the margin of the low-flow channel, the heads of major bars across the exposed surface of the active zone, and floodplain surfaces. A photographic technique enabled rapid and consistent field sampling of the coarse sediments at bar heads along the low-flow channel margin and on major bars across the dry bed. 3. A downstream decrease in particle size and an increase in within-patch heterogeneity in sediment size were observed within bar head sediments along the margin of the low-flow channel. Comparisons between major bar and low-flow channel samples revealed greatest within-patch variability in individual sediment size indices (D50, A- and B-axes of the larger particles) at headwater sites, greatest between-patch variability in the three measured indices in the central reaches, and lowest between-patch variability at downstream sites. However, there was a distinct increase in the overall heterogeneity in particle size, which was sustained across all patches, in a downstream direction. 4. There was a clear downstream decrease in the size of floodplain sediments in the headwaters, but thereafter there was no distinct downstream trend in any of the calculated particle size indices. 5. The geomorphological controls on the observed patterns and the potential ecological significance of the patterns, particularly for plant establishment, are discussed in relation to the relative relief of the active zone, and the highly variable hydrological and climatic regime along the river.

The temporal and spatial variability of sediment transport and yields within the Bradford Beck catchment, West Yorkshire.

Recent EU directives have emphasised the need to understand and limit potential water quality problems within urban river systems. Under certain conditions sediments and contaminants derived from industrial and domestic waste and the urban surface may be expelled into urban rivers via the sewer and drainage system. These discharges may lead to water quality problems within urban catchments. One aspect of water quality is the suspended sediment. This can be directly detrimental to water quality by affecting the habitat for fish and other biota but it is also closely associated with pollutants, such as heavy metals, which may be adsorbed onto the sediments surface. This paper presents a comparative analysis of sediment yields for a small mixed rural/urban catchment, the Bradford Beck in West Yorkshire, over a number of precipitation events. Flow and water quality parameters were monitored at a high temporal resolution at strategic sites within the urban watercourse over a 2-year period. Rainfall was measured at six locations within the catchment. Analysis of discrete rainfall events allowed an understanding of the temporal and spatial variability of sediment transport within the catchment to be developed. The results demonstrated that for individual storms the sediment yields from the urban sub-catchment were generally higher than those from the rural system although the annual yields were comparable. Sediment transport within the urban area, for large events, was dominated by the impact of the Combined Sewer Overflows discharging. Within these events peak suspended sediment concentration, SSC, were generally higher than the rural system. Within smaller events the main sediment source within the urban area was the surface runoff discharging directly to the urban river. Analysis of SSC and discharge relationships illustrated the different sources of sediment for contrasting events. Within the rural system clockwise hysteresis, indicating exhaustion of sediment supply, was noted for larger storms. The high temporal resolution monitoring has enabled a better understanding of sediment dynamics within the Bradford Beck system to be developed. The general conclusions drawn can provide guidance for addressing sediment related water quality issued in other urban systems.

The influence of storm events on fine sediment transport, erosion and deposition within a reach of the River Swale, Yorkshire, UK.

The spatial and temporal dynamics of suspended sediment fluxes through a 55 km reach of the lowland River Swale, Yorkshire, UK are investigated for the period July 1994-June 1996. Seventeen storm events were monitored, each of which produced significant suspended sediment transport. Variations in sediment flux dynamics and reach sediment storage through events, representing a combination of channel bed erosion and/or deposition, bank erosion and floodplain deposition, were observed. Some, but not all, of this variation can be explained in terms of event size, precipitation distribution and intensity, and antecedent sediment storage conditions. In particular, marked variations in the size of the reach sediment store over the extremely wet 4-month winter period from December 1994 to April 1995 are demonstrated. Net sediment loss from the reach took place over during this winter period. These event-scale and seasonal patterns form a background to annual variations in the catchment sediment budget.