No abstract

SUMMARYRegression equations with the form y =f(q), where y is a channel characteristic and q is a discharge of a particular frequency are widely used in fluvial geomorphology. They imply that channel geometry is related to a dominant discharge. For this to be so, the river must be 'in regime' i.e. its characteristics must fluctuate about a mean condition which is effectively stationary at the time scale considered. The static regression-dominant discharge approach does not fully describe the relationship between channel characteristics and discharge and an alternative model is proposed with the form y ( t ) = l ; h ( u ) . q ( t -u ) d u . This model states that channel form is a product of the whole series of discharges experienced by the channel. The dominant discharge-static regression approach provides a working approximation of the relationship between channel characteristics and discharge only when temporal variations in q are small or when y is insensitive to variations in q.

Abstract:Description of the terrain surface through digital elevation models (DEMs) strongly depends on data collection methods and DEM data structures. For eciency and availability reasons regular point distributions are most common, which yield artefacts such as depressions and preferential¯ow directions. These facts need to be considered when natural phenomena are modelled, as is shown for handling depressions and for estimation of ow paths and upslope contributing areas. Analysis of the main reasons for the occurrence of depressions shows that they usually better re¯ect the terrain than their surroundings. Thus, the most common remedial method of raising depressions is rejected. Algorithms that`cut' a¯ow path from the depression through its bounding barrier are favoured instead. Several¯ow routing algorithms are evaluated for their behaviour in regular grids. It is shown that the multiple¯ow direction (mfd) algorithm that distributes water from a grid cell to the lower of its eight neighbours proportionally to their elevation dierences (slope) exhibits correct¯ow directions and the best rotation invariance. It is suggested that the estimation of upslope contributing areas (TCAs) is undertaken in two steps: ®rst, a high quality¯ow direction data set is derived by a well-behaved mfd algorithm or by subgrid modelling of¯ow paths; secondly, the upslope contributing areas are obtained by counting the upslope elements. #

Suspended sediment concentrations were determined during storm events for five small catchments in the Wallagaraugh River in southeast New South Wales between July 1977 and June 1978. A total of thirty‐nine storm responses were analysed and both storm hydrographs and hysteresis loops were produced. The behaviour of suspended sediment concentration was extremely variable and seven different types of response were recognised. In these catchments there is no simple single constant relationship between discharge and suspended sediment concentration. The free availability of suspendable material appears to be an important limiting control on sediment concentration and this appears to be related to the soil moisture conditions

There is a general agreement on the impact of beaver dams regarding the increasing diversity of habitats and the improvement of the water quality, whereas the retention effect during flood events is still being discussed. In this study, we modeled 12 beaver dam cascade scenarios in two catchments for eight flood events with a two-dimensional (2D) hydrodynamic model. The implementation of the potential cascades in the model is based on the developed three-stage model for predicting location-dependent dam cascades in Bavaria. A Bavaria-wide questionnaire regarding dam occurrences and characteristics in combination with a detailed survey of 51 dams was used to set up a prediction scheme. It was observed that beaver dams are most likely built in rivers with riparian forest, with widths from 2 to 11 m and depths smaller than 1 m. The hydraulic model results showed larger inundation areas (>+300%) for the beaver dam scenarios. There is a noticeable peak attenuation and translation for elevated peak discharges (five times the annual mean discharge: up to ≤13.1% and 2.75 h), but no remarkable effect could be observed for flood events with return periods of more than 2 years. We conclude from the results that beaver dam cascades can have an impact on runoff characteristics, but do not lead to relevant peak reductions during flood events and therefore cannot be counted as flood mitigation measure.