Fluvial sediment transport and deposition following the 1991 eruption of Mount Pinatubo

The 1991 eruption of Mount Pinatubo generated extreme sediment yields from watersheds heavily impacted by pyroclastic flows. Bedload sampling in the Pasig–Potrero River, one of the most heavily impacted rivers, revealed negligible critical shear stress and very high transport rates that reflected an essentially unlimited sediment supply and the enhanced mobility of particles moving over a smooth, fine-grained bed. Dimensionless bedload transport rates in the Pasig–Potrero River differed substantially from those previously reported for rivers in temperate regions for the same dimensionless shear stress, but were similar to rates identified in rivers on other volcanoes and ephemeral streams in arid environments. The similarity between volcanically disturbed and arid rivers appears to arise from the lack of an armored bed surface due to very high relative sediment supply; in arid rivers, this is attributed to a flashy hydrograph, whereas volcanically disturbed rivers lack armoring due to sustained high rates of sediment delivery. This work suggests that the increases in sediment supply accompanying massive disturbance induce morphologic and hydrologic changes that temporarily enhance transport efficiency until the watershed recovers and sediment supply is reduced.     

The response of subaqueous dunes to floods in sand and gravel bed reaches of the Dutch Rhine

Abstract The branches of the River Rhine in the Netherlands, characterized by a sand–gravel bed in the upstream part and a sand bed in the downstream part of the river system, show migrating dunes, especially during floods. In the last 20 years, these dunes have been studied extensively. High-resolution echo-sounding measurements of these dunes, made with single and multibeam equipment, were analysed for three different sections of the Rhine river system during several floods. This analysis was done to quantify the growth, decay and migration rates of the dunes during floods. In addition, the migrating dunes were used to calculate bedload transport rates with dune tracking. The results of dune growth and decay and migration rate are shown to be very different for the various sections during the various floods, and these differences are related to differences in grain size of the bed and to differences in the distribution of discharge over the main channel and the floodplain. The relations are used to show that the growth and migration rate of dunes, and the calculated bedload transport rates during the rising stage of a flood wave can be predicted from the mobility of the bed material with simple power relations.     

Gravel transport and morphological change in braided sunwapta river,Alberta, Canada

The relation between morphological change and patterns of variation in bedload transport rate in braided streams was observed by repeated, daily topographic surveys over a 25 day study period in a 60 m reach of the proglacial Sunwapta River, Alberta, Canada. There are two major periods of morphological change, each lasting several days and each involving the complete destruction and reconstruction of bar complexes. Bar complex destruction was caused by redirection of the flow and by downstream extension of the confluence scour zone upstream. Reconstruction involved accretion of unit bars on bar head, flank and tail and in one case was initiated by disection of a large, lobate unit bar. High rates of sediment movement, measured from net scour and fill of the cross-sections, coincided with these morphological changes. Sediment was supplied from both bed and bank erosion, and patterns and distances of transfer were highly variable. Rates of transport estimated by matching upstream erosional volumes with downstream deposition were much greater than those estimated from either a step-length approach or a sediment budget. Measurements of scour and fill and observations of morphological change indicate that step lengths (virtual transport distances) were typically 40–100m during a diurnal discharge cycle. Shorter step lengths occurred when transfer was confined to a single anabranch and longer steps involved channel changes at the scale of the entire reach. Sediment budgeting was used to describe the spatial patterns of sediment transport associated with the morphological changes and to estimate minimum daily reach-averaged transport rates. Mean bedload transport rates correlate with discharge, but with considerable scatter. The largest deviations from the mean relation can be tied to phases of channel incision, bank erosion, scour hole migration, bar deposition and channel filling apparently controlled by changes and fluctuations in sediment supply from upstream, independent of discharge. These are interpreted as field evidence of ‘autopulses’ or ‘macropulses’ in bedload transport, previously observed only in laboratory models of braided streams.     

Temporal variations in bedload transport rates associated with the migration of bedforms

Temporal variations in bedload transport rates that occur at a variety of timescales, even under steady flow conditions, are accepted as an inherent component of the bedload transport process. Rarely, however, has the cause of such variations been explained clearly. We consider three data sets, obtained from laboratory experiments, that refer to measurements of bedload transport made with continuously recording bedload traps. Each data set is characterized by a predominant low-frequency oscillation, on which additional higher-frequency oscillations generally are superimposed. The period of these oscillations, as isolated through the use of spectral analysts, ranged between 0·47 and 168 minutes, and was associated unequivocally with the migration of bedforms such as ripples, dunes, and bars. The extent to which such oscillatory behaviour may be recognized in a data set depends on the duration of sampling and the length of the sampling time, with respect to the period of a given bedform. Several theoretical probability distribution functions have been developed to describe the frequency distributions of (relative) bedload transport rates that are associated with the migration of bedforms (Einstein, 1937b; Hamamori, 1962; Carey and Hubbell, 1986). These distribution functions were derived without reference to a sampling interval. We present a modification of Hamamori's (1962) probability distribution function, generated by Monte Carlo simulation, which permits one to specify the sampling interval, in relation to the length of a bedform. Comparisons between the simulated and observed frequency distributions, that were undertaken on the basis of the data described herein, are good (significant at the 90 per cent confidence level). Finally, the implications that temporal variability, which is associated with the migration of bedforms, have for the accurate determination of bedload transport rates are considered.     

Hydraulics of runoff and loess loam deposition

On many more or less loamy soils, rill erosion is reported to start on slopes that are equal to or steeper than 2–3°; critical Froude numbers for the start of rill wash on these slopes vary between 2·0 and 3·0. This explains why colluvial deposition often occurs on slopes below 2–3°, when water spreads out at the downslope extremities of the rills. The critical hydraulic conditions for loess loam deposition were tested in the laboratory for slopes of 0·5° and 2°, applying unit-discharges (q) up to 10 cm²/s. It appeared from these experiments that for afterflow, without raindrop impact, deposition starts for critical load concentrations (c_cr) varying between several g/1 and about 60 g/l. Under rain c_cr amounts to a minimum value of 100–125 g/l and it increases when the runoff film becomes thinner. Nevertheless, deposition in pluvial runoff is also possible, as was the case during the Weichselian, according to data from quarries in Belgium and in The Netherlands. A modified Kalinske equation is proposed for c_cr prediction, with the introduction of a typical empirical coefficient C_r and considering such factors as shear stress and mean particle size. Massive sedimentation may occur when it stops raining and afterflow starts, since c_cr values are then much lower. It is shown from the Shields' diagram that loamy suspensions are more sensitive to sedimentation than sands in clear water.     

Structure,composition and bulk properties of upland stream gravels

Upland gravel-bedded streams in the U.K. have received only scant attention from both hydrologists and sedimentologists, but are worthy of further investigation. The sedimentology of three small streams in Teesdale in the Pennines has been examined in detail. Grain-size characteristics, bedforms, structure, composition and packing characteristics of these deposits are described, and compared where appropriate with published information. It is argued that a fuller appreciation of gravel bed composition and morphology should eventually contribute to an improved understanding of sediment transport and deposition mechanisms, and, hence, to improved accuracy in sediment transport and deposition estimates.     

Numerical simulation of hydrodynamic characteristics and bedload transport in cross sections of two gravel-bed rivers based on one-dimensional lateral distribution method

Accurate evaluation and prediction of bedload transport are crucial in studies of fluvial hydrodynamic characteristics and river morphology.This paper presents a one-dimensional numerical model based on the one-dimensional lateral distribution method(1 D-LDM) and six classic bedload transport formulae that can be used to simulate hydrodynamic characteristics and bedload transport discharge in cross sections.Two gravel-bed rivers,i.e.the Danube River located approximately 70 km downstream from Bratislava in Slovakia and the Tolten River in south of Chile are used as examples.In the 1 D-LDM,gravity,bed shear stress,turbulent diffusion,and secondary flow are included to allow for accurate predictions of flow velocity and consequently bed shear stress in the cross sections.Six classic formulae were applied to evaluate the non-dimensional bedload transport rate,and the bedload transport discharge through a river cross section is obtained by integrating the bedload transport rate over the width of the cross section.The results show that the root mean square error(RMSE) and mean absolute error(MAE) of velocity and water discharge were less than 8% of the observed magnitude,while the correlation coefficient between model predictions and observations was close to unity.The formulae proposed by Ashida and Michiue(1972),in which particle collision with the bed is taken into account,and by Camenen and Larson(2005),which allows for yielding a non-zero bedload transport rate even when the bed shear stress is smaller than the critical bed shear stress value,appeared to be more appropriate for predicting the observed bedload transport rate in the studied cross sections of two gravel-bed rivers.If non-uniform sediment mixtures were considered,the bedload transport discharge through a cross-section could change considerably by up to 22.5% of the observed magnitude.The relations proposed by Ashida and Michiue(1972) and Egiazaroff(1965) for parameterizing the hiding factor yielded more realistic model predictions in comparison with observations for the measured data set collected for the Tolten River,while the one proposed by Wilcock and Crowe(2003) performs the best for the data set measured for the Danube River.     

The influence of organic debris on channel morphology and bedload transport in a New Zealand forest stream

The behaviour and form of, and bedload sediment transport through, a 3.5 m wide forest stream have been monitored for nearly three years. Bedload transport is highly episodic and spatially variable, and is controlled less by water discharge than by sediment availability. Organic debris in the channel creates temporary base levels and sites at which coarse sediment may remain stored for long periods; collapse or disruption of log and debris jams makes sediment available for transport in only a small proportion of the runoff events that are actually competent to move the material. Even then, sediment travels only a short distance before being redeposited, frequently behind debris accumulations further downstream. Rates of sediment transport during a given runoff event can vary markedly over short distances along the stream, again depending on whether sediment was made available for transport by log jam collapse upstream. Organic debris is therefore a major constraint on the application of physical laws and theories to explaining sediment movement in, and the morphology of, this stream.     

Problems in the calibration of an acoustic device for the observation of bedload transport

Developments in theoretical and empirical modelling of bedload transport processes are hindered by the lack of an adequate data base for testing or establishing the models. Conventional methods of measuring bedload transport rates fail to provide the necessary continuous or frequent record of variations at a single section. Acoustic techniques have the potential to overcome this deficiency, but their application has been very limited. Some of the problems of calibrating an acoustic device in the field and in a laboratory flume are discussed, and a possible circuit design described which might minimize calibration difficulties by automatically subtracting the noise generated by flow turbulence.