Sediment Erosion Rates Research Articles

Quantifying sand provenance and erosion (Marsyandi River, Nepal Himalaya)

We use petrographic and mineralogical data on modern sediments to investigate erosion patterns in the Marsyandi basin of the central Himalaya, a privileged natural laboratory in which a series of multidisciplinary geomorphological, sedimentological, geochemical and geochronological studies have been recently carried out to unravel the interrelationships between tectonic, climatic and sedimentary processes in high-relief orogenic belts. Although relative erosion patterns are effectively constrained by analyses of replicate samples along six successive tracts of the Marsyandi River, uncertainties are caused by potential compositional variation between the monsoon and post-monsoon season. Estimates of erosion rates are significantly affected by poor knowledge of total sediment flux through the basin. Our results support focused erosion of the southern, tectonically-lower part of the Greater Himalaya in the hangingwall of the MCT Zone, where the summer monsoon reaches its peak intensity (up to 5 m/a), and sediment yields and erosion rates reach 14,100 ± 3400 t/km 2 and 5.1 ± 1.2 mm/a. Erosion rates sharply decrease southward in low-relief Lesser Himalayan units (1.6 ± 0.6 mm/a), and also progressively decrease northwards in the high-altitude, tectonically-upper part of the Greater Himalaya, where rainfall decreases rapidly to < 2 m/a. Even areas of extreme topography such as the Manaslu Granite are characterized by relatively low erosion rates (2.4 ±0.9 mm/a), because precipitations become too scarce to feed significant ice flux and glacial activity. Monsoonal rainfall decreases further to < 0.5 m/a in the Tethys Himalayan zone farther north, where erosion rates are ∼ 1 mm/a. Coupling between erosion and peak monsoonal rainfall along the southern front of the Greater Himalaya is consistent with both channel-flow models of tectonic extrusion and tectonic uplift above a mid-crustal ramp. Altitude and relief are not the principal factors controlling erosion, and the central Nepal eight-thousanders may be viewed as topographic anomalies in cold desert climate at the southern edge of the Tibetan rain shadow.

Inter-comparison between five devices for determining erodability of intertidal sediments

Five erosion devices were compared using five intertidal estuarine sites covering a range of sediment stability from newly settled mud to very cohesive mud at the margins of a saltmarsh. The erosion devices use different methods of fluid shearing from horizontal currents/bed shear stresses to vertical water jets, and have different ‘footprint’ areas. The devices included: (1) the annular flumes (AFs—diameter 64 cm; footprint area 0.17 m 2) of the Plymouth Marine Laboratory (PML); (2) PML's mini-annular flume (MAF—diameter 19 cm; area 0.026 m 2); (3) the annular mini-flume (AMF—diameter 30.5 cm; area 0.032 m 2) of the National Oceanography Centre Southampton (NOC); (4) NOC's Cohesive Strength Meter (CSM—diameter 3 cm; area 0.0007 m 2); (5) NOC's EROMES (ER—diameter 10 cm; area 0.0079 m 2). The quantification of threshold shear stress for bed erosion ( τ e) and sediment erosion rate was complemented by the measurement of physical, chemical and biological properties of the sediment (grain size, bulk density, water content, organic content, chlorophyll a, carbohydrates, macrofauna). The results demonstrated a significant correlation ( r 2=0.98) between the PML AF (laboratory measurement of undisturbed cored sediment) and PML MAF ( in situ) for measurement of erosion thresholds for bed sediment. However, there were no significant correlations between AFs, the CSM and EROMES. There were no consistent correlations with physical or biological sediment properties due to the spatially unrelated sites and the marked differences in benthic assemblages. The sources of differences and the lack of correlations between erosion devices were due to several factors, including operational procedures (e.g., sediment resuspension during filling with water), definition of erosion threshold, the nature of the force applied to the bed, and method of calibration. In contrast to the CSM and EROMES, both types of AFs were able to record significant differences in the erodability of soft sediments from four sites. This indicates that the CSM and EROMES may not be very effective at measuring the differences in erosion thresholds of soft estuarine sediments.

EXPERIMENTAL STUDY ON EROSION RATE OF COHESIVE SEDIMENT BY USING A MOVABLE APPARATUS OF EROSION TEST

Erosion rate of soft cohesive sediment was investigated experimentally in the present paper. In this study, we developed a movable apparatus of erosion test, which was designed to conduct a field measurement of erosion rate. In order to check the validity and utility of this apparatus, the several series of measurements were carried out outside in large-scale experimental flume. It was verified that the results by this new measuring system has almost same order of accuracy as that by our standard test in laboratory. Effect of water temperature on the erosion rate were also evaluated quantitatively.

STUDY ON POLLUTANT LOAD INTO A EUTROPHIC LAKE WITH ATTENSION TO SEDIMENT TRANSPORT IN A LOWER REACH OF RIVER

Monitoring of pollutant load in an influent river flowing into a eutrophic lake is not generally conducted at the mouth of the river but at stations just upstream from the river mouth. To compare the pollutant load at the upstream station and the river mouth, we have conducted continuous measurements and numerical computations of pollutant load with special attention to the sediment transport in the lower reach of river. The observed data indicate that the appreciable differences of SS fluxes at the upstream stations and river mouth appear under rainy conditions while the yearly-averaged SS flux at the upstream station is almost comparable to that at the river mouth. It should be also noted that the numerical accuracy of sediment transport may be strongly dependent on the treatment of spatial variations of an erosion rate of sediment and evaluations of bottom friction.

Comparison of Two Techniques to Measure Sediment Erodibility in the Fox River, Wisconsin

This paper compares the results of two sediment erodibility test methods that have been applied on surficial sediments at a number of locations on the Fox River in Wisconsin. The methods include a straight flume that is deployed in situ (the FLUME) and a straight laboratory flume (the SEDFLUME). The sediment erosion rates measured near the surface (in the top four centimeters) as a function of bottom stress were compared. On average, the erodibility measured by the SEDFLUME was about 5.5 times greater than that measured by the FLUME. A possible reason for the difference is the relatively short test section of the SEDFLUME.

Petrology of Nile River sands (Ethiopia and Sudan): Sediment budgets and erosion patterns

Detrital modes of modern Nile sands, together with estimates of sediment volumes trapped in Sudanese reservoirs, allow us to calculate sediment loads of major tributaries (Blue Nile, White Nile, Atbara) and erosion rates in the Nile catchment. A tridimensional array of high-resolution bulk-petrography and heavy-mineral data was obtained on both levee (suspended load) and bar (bedload) deposits, analysed separately for each grain-size subclass at 0.5 Φ intervals. From available information on sediments stored in the Roseires, Khashm el Girba and Lake Nasser reservoirs between 1964 and 1990, the total Nile load is reassessed at 230 ± 20 10 6 t/a, an estimate two to four times higher than figures reported so far, on which previous estimates of sediment yields and erosion rates were based. Of such huge amount of detritus, 82 ± 10 10 6 t/a are contributed by River Atbara, which carries more volcanic rock fragments, brown augite and olivine from basaltic rocks, and 140 ± 20 10 6 t/a by the Blue Nile, which carries more K-feldspar and hornblende from amphibolite-facies basement rocks. The additional ≤ 10 7 t/a of almost purely quartzose sediments supplied by the rest of the Nile catchment, corresponding to insignificant average yields and erosion rates, represent the stable residue which survived extreme subequatorial weathering in southern Sudan swamps (White Nile, Bahr ez Zeraf, and Sobat sands) or fluvial and eolian recycling of ancient quartzarenites in hyperarid climates (Nubian sands). Sediment production is thus markedly focused on Ethiopian rift highlands, where rainfall is concentrated in a single July–August peak. High average yields and erosion rates (800 ± 150 t/km 2 a, 0.29 ± 0.05 mm/a) partly reflect anthropically-accelerated erosion caused by deforestation and intensive land use, and cannot be extrapolated far in the past. Erosion patterns may have changed repeatedly during Quaternary climatic oscillations, and possibly also in the longer term during the multistage rift-related events which, since impingement of the Afar plume and eruption of flood basalts in the Oligocene, caused elevated topography and monsoonal climate in Ethiopia.

COMPARATIVE ANALYSIS OF WATER QUALITY ENVIRONMENTS UNDER RAINY CONDITIONS IN SEVERAL URBAN RIVERS

For understanding water environments in urban rivers under rainy conditions, the intensive field measurements of water quality and hydrodynamic environments were carried out at several urban rivers and the Edo River in which the scale and environments of the watershed are different. The observed results indicate that the differences of the water quality environments between urban rivers under rainy conditions are closely related with those of sedimentary environments. The contribution of the sedimentary environments to water quality environments in the Edo River is much smaller than that in the urban rivers due to the property of erosion rate of sediments. The origin of the particulate matters in urban rivers is quantitatively discussed with the observed data for the erosion rate of sediments.

HIGH-RESOLUTION ADCP MEASUREMENTS ON VERTICAL STRUCTURES OF CURRENT AND SUSPENDED SOLID IN AN URBAN RIVER

To realize more accurate measurements for pollutant loads in an urban river, in the present study, a high-resolution, acoustic Doppler current profiler (HR-ADCP) was used to measure the vertical distributions of current and SS in an urban river under rainy conditions. The field measurements with the bottom-mounted HR-ADCP were carried out at the downstream point of the Oohori River, one of main inflow rivers in Lake Teganuma. The observed results show that characteristic vertical structures of the current and SS appear in hydrologic events and then the vertical distribution of SS flux in the streamwise direction vary considerably in rising and falling stages. With the HR-ADCP measurements, we grasp the temporal variation for the dependence of the erosion rate of sediments on flow conditions.

Long-term erosional responses after fire in the Central Spanish Pyrenees: 1. Water and sediment yield

This paper reports the results of a study on how fire effects water and sediment losses in the Central Spanish Pyrenees where land abandonment results in an increase of the scrubland and fire-affected surfaces. In 1991, two plots (control and burnt) were installed to collect runoff, suspended sediments and solutes. One of the plots was burnt (burnt-1) and the other was covered by shrubs and herbs (control). During 1993, another burnt plot was installed (burnt-2). Data was collected from the three plots until 1999 and is used as the basis for assessing the soil erosion changes after the fire. Runoff, solute and suspended sediment concentration and erosion rates were slightly greater during the first 2 years following fire on burnt-1 compared to the control plot. The average runoff coefficients during the 8-year study period were 4.2% and 7%, the total sediment concentrations were 189 and 181 mg l −1, and the erosion rates were 89 and 143 kg ha −1 year −1 for the control and burnt-1 plots, respectively. In contrast, burnt-2 plot responded differently than burnt-1 plot to fire, although these two plots were designed to be replicated treatments. The average runoff coefficient for burnt-2 plot was two times greater than that for the control plot. Likewise, total sediment concentration was 649 mg l −1 and erosion rate was 566 kg ha −1 year −1, which are 3.6 and 4.5 times greater than the control plot. Differences between burnt and control plots were analysed at hourly, daily, monthly and yearly temporal scale. Both burned plots began to recover after the fire. However, burnt-1 plot had a recovery time of 2 years compared to burnt-2 plot; while 7 years after the fire, the runoff sediment concentration and erosion rates from burnt-2 were higher than those for the control plot. Water and sediment losses from the two burnt plots were markedly different.

Erosion Measurements in Linear, Oscillatory, and Combined Oscillatory and Linear Flow Regimes

Many contaminated sediments and dredged material mixtures of cohesive and non-cohesive sediments occur in wave-dominated environments. In-situ analysis is imperative in understanding the erosion and transport of these sediments. Recent research efforts have developed a flume with unidirectional flow that can measure in-situ sediment erosion with depth (SEDflume). However, the flow regime for the SEDflume has limited applicability to wave-dominated environments. Therefore, a unique device, called the SEAWOLF flume, was developed and used by Sandia National Laboratories to simulate high-shear stress erosion processes experienced in coastal waters where wave forcing dominates the system. The SEAWOLF is capable of testing in-situ or laboratory prepared cores. Erosion rates of cohesive and non-cohesive sediments prepared in the laboratory were determined in oscillatory and combined oscillatory and linear flow regimes. Results of these tests were compared to results from the unidirectional SEDflume. Although maximum shear stresses for oscillatory flows were as high as 7 Pa for the tests, the associated erosion rate for specific sediment over the entire wave cycle were comparable to much lower shear stresses found for constant, linear flows. For example, sediment exposed to a maximum of 7 Pa over a 15 s period resulted in erosion rates similar to results for a constant linear shear stress of 3.4 Pa. Analysis of results for all sediments tested led to a determination of values for an effective shear stress that relates wave-induced erosion to linear flow induced erosion.

Effects of bed material properties on cohesive sediment erosion

A straight benthic in situ flume was used to measure the erosion rate of cohesive sediments in freshwater and saltwater environments in New Zealand. The data show an exponential decay of erosion rate with time, which is indicative of depth-limited erosion. Two approaches for the calibration of an erosion formula are used, both leading to similar results. The data show the dependency of erosion rate on bed material properties, such as dry bulk density, water content, organic content and sand content. Both methodologies also highlight differences between data from freshwater and saltwater environments. Also shown is a requirement for direct measurements of bed density profiles and bed load transport during in situ investigations.

Role of physical and biological processes in sediment dynamics of a tidal flat in Westerschelde Estuary, SW Netherlands

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 274:41-56 (2004) - doi:10.3354/meps274041 Role of physical and biological processes in sediment dynamics of a tidal flat in Westerschelde Estuary, SW Netherlands J. Widdows1,*, A. Blauw2, C. H. R. Heip3, P. M. J. Herman3, C. H. Lucas4, J. J. Middelburg3, S. Schmidt5, M. D. Brinsley1, F. Twisk6, H. Verbeek6 1Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK 2WL | Delft Hydraulics, PO Box 177, 2600 MH Delft, The Netherlands 3Netherlands Institute of Ecology, Centre for Estuarine and Marine Ecology (NIOO-KNAW), Korringaweg 7, PO Box 140, 4401 AC Yerseke, The Netherlands 4School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK 5Laboratoire des Sciences du Climat et de l¹Environnement (CEA-CNRS), Domaine du CNRS, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France 6National Institute for Coastal and Marine Management, Grenadresweg 31, PO Box 8039, 4330 EA Middelburg, The Netherlands *Email: j.widdows@pml.ac.uk ABSTRACT: This article synthesises a series of studies concerned with physical, chemical and biological processes involved in sediment dynamics (sedimentation, erosion and mixing) of the Molenplaat tidal flat in the Westerschelde (SW Netherlands). Total sediment accretion rate on the flat (sand to muddy sand) was estimated to be ~2 cm yr-1, based on 210Pb and 137Cs profiles. 7Be showed maximum activity in the surface sediments during summer, reflecting accretion of fine silt at this time of year, and total vertical mixing of sediment to be in the order of 50 cm2 yr-1. The extent to which different physical and biological processes (tidal currents, air exposure, bio-stabilisation, biodeposition and bioturbation) contributed towards sediment dynamics was estimated. A sediment transport model based on physical factors estimated sedimentation rates of 1.2 cm yr-1, but did not account for tidal or seasonal variation in suspended particulate matter (SPM), wind or effects of spring-neap tidal cycles. When the model was run with an increased critical bed shear stress due to the microphytobenthos, net sedimentation rates increased 2-fold. These higher rates were in closer agreement with the rates derived from the depth profiles of radionuclides for the central region of the tidal flat (2.0 to 2.4 cm yr-1). Therefore a significant part of the sedimentation rate (~50%) may be explained by spatial-temporal changes in biological processes, including Œbio-stabilisation¹ by microphytobenthos, together with the enhanced biodeposition of silt by suspension feeders, and offset by processes of Œbio-destabilisation¹ by grazers and bioturbators. In the centre of the tidal flat there was a shift from high sediment stability in spring-summer 1996 to low sediment stability in autumn 1997, quantified by a significant reduction in critical erosion velocity of 0.12 to 0.15 m s-1, and accompanied by a 30- to 50-fold increase in sediment erosion rate. The change was associated with a shift from a tidal flat dominated by benthic diatoms and a low biomass of bioturbating clams (Macoma balthica), to a more erodable sediment with a lower microphytobenthos density and a higher biomass of M. balthica. Vertical mixing of sediment and organic matter, studied using a variety of tracers, was rapid and enhanced by advective water flow at sandy sites and by burrowing polychaetes and bivalves at silty sites. KEY WORDS: Sediment dynamics · Erosion · Sedimentation · Mixing · Microphytobenthos · Biostabilisation · Bioturbation · Modelling Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 274. Online publication date: June 24, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Suspended Sediment Transport from the Dokriani Glacier in the Garhwal Himalayas

Estimation of sediment load from glacierized basins is very important for planning, designing, installation and operation of hydro-power projects, including management of reservoirs. In the present study, an assessment of suspended sediment concentration, load, yield and erosion rate has been undertaken for the Dokriani Glacier drainage basin located in the Garhwal Himalayas. About 60% of the total drainage area of this basin is glacierized. Data were collected for four ablation seasons (1995-1998). The mean daily suspended sediment concentrations for June, July, August and September were 452, 933, 965 and 275 mg 1-1, respectively, indicating highest suspended sediment concentration in August, followed by July. Similar trends were also found for the sediment load and about 88% of the total suspended sediment load of the melt period was transported during the months of July and August. Sediment yield for the study basin was computed to be about 2,800 t km-2 yr-1, which is comparable with glacierized basins (10-30% glacierized) in the Pamir region. For the entire ablation period, the erosion from the Dokriani Glacier basin is estimated to be about 1.0 mm. There was a poor relationship between suspended sediment concentration and discharge. The average percentages of clay, silt and sand were found to be 1.4, 67.3 and 31.3%, respectively, which suggest maximum content of silt followed by sand. There was limited variation in the content of clay, silt and sand in the suspended sediment during the ablation period.

The intertidal erosion rate of cohesive sediment: a case study from Long Island Sound

Over the past decades, many attempts have been made to generate useful bottom erosion models for the study of cohesive sediment movement. This study addresses some of the key questions involved in determining the functional relationship between erosion rate and bottom shear stress. Current, wave, and turbidity data were collected from a bottom mounted instrument array in a moderately energetic estuarine environment. The bottom shear stress was calculated from a wave–current interaction model. The erosion rate was derived from the observed sediment concentration using a vertical mixing model. Examination of the relationship between erosion rate and bottom stress showed that the erosion rate varied at intertidal frequency. When averaged over the tidal fluctuation, the erosion rate remained approximately constant at low stress, but increased sharply when the shear stress rose above a critical value. This suggests two-stage erosion. The bed has a layered structure, in which a thin layer of loose, high water content material overlies a more consolidated bed. The top layer of high water content material (fluff) was easily disturbed and re-suspended by tidal currents, but the consolidated bottom layer was eroded only under conditions of high shear stress.

ASSESSING CHANGES IN WATERSHED FLOW REGIMES WITH SPATIALLY EXPLICIT HYDRAULIC MODELS1

ABSTRACT: Urbanization, farming, and other watershed activities can significantly alter storm hydrographs and sediment erosion rates within a watershed. These changes routinely cause severe economic and ecological problems manifested in the form of increased flooding and significant changes in channel morphology. As the activities within a watershed influence the hydrologic, hydraulic, and ecological conditions within a river, interdisciplinary approaches to predict and assess the impacts that different land uses have on streams need to be developed. An important component of this process is ascertaining how hydrologic changes induced by specific watershed activities will affect hydraulic conditions and the accompanying flood levels, sediment transport rates, and habitat conditions within a stream. A conceptual model for using spatially explicit (two‐dimensional) hydraulic models to help evaluate the impacts that changes in flow regime might have on a river is presented. This framework proposes that reproducing and quantifying flow complexity allows one to compare the hydraulic conditions within urban, urbanizing, and non‐urban streams in a more biologically and economically meaningful way. The justification, advantage, and need for such a method is argued through the results of one‐ and two‐dimensional hydraulic model studies. The implementation of this methodology in watershed urbanization studies is described.

Reconstructing the erosion history of glaciated passive margins: applications of in situ produced cosmogenic nuclide techniques

Abstract Offshore sediment accumulations provide an intriguing record of the net sediment output resulting from geomorphological evolution of the circum-Atlantic continental margin since the commencement of Neogene glaciation. However, the onshore record of the timing, pattern and amount of bedrock erosion that produced these sediments is comparatively poorly constrained and understood, although there are good general models of glaciation history. The geomorphology of circum-Atlantic continental margin mountains, as assessed from remote sensing data and field observations, includes palimpsest landforms and landscapes that reflect a complex pattern of spatial and temporal variations in the impact of glacial, fluvial and periglacial processes. Perhaps most surprising is that, despite having been repeatedly overridden by large ice sheets, parts of the landscape appear to be relict, with nonglacial morphology. This has important implications both for glaciological conditions under ice sheets, and for sediment source areas and erosion rates. Conventional dating and analysis have provided an excellent way to begin unravelling the timing and pattern of erosion, landform development, and possible landform preservation under ice. However, testing hypotheses developed from current models, and addressing critical unresolved questions, requires additional approaches. The use of in situ cosmogenic nuclide production in bedrock is a new approach for investigating landscape evolution in mountainous areas. With careful interpretation of geomorphological settings, cosmogenic nuclides can be used to determine apparent surface exposure age and landscape preservation, and constrain erosion depths and duration of burial by ice. Here we provide a framework for the interpretation of cosmogenic nuclide concentrations in bedrock surfaces of landscapes affected by glacial, fluvial and periglacial processes, illustrated with examples from the northern Swedish mountains. This demonstrates potential uses of cosmogenic nuclide techniques, and provides a foundation for attempts to improve geomorphologically based reconstructions of relict landscapes, to reconstruct and analyse the dynamics of landscape change in glacial times, and to define the consequences of different process regimes in terms of erosion patterns, sediment transport, and the supply of sediments that are deposited offshore.

Hillslope evolution by nonlinear, slope‐dependent transport: Steady state morphology and equilibrium adjustment timescales

Soil‐mantled hillslopes are typically convex near the crest and become increasingly planar downslope, consistent with nonlinear, slope‐dependent sediment transport models. In contrast to the widely used linear transport model (in which sediment flux is proportional to slope angle), nonlinear models imply that sediment flux should increase rapidly as hillslope gradient approaches a critical value. Here we explore how nonlinear transport influences hillslope evolution and introduce a dimensionless parameter ΨLto express the relative importance of nonlinear transport. For steady state hillslopes, with increasing ΨL(i.e., as slope angles approach the threshold angle and the relative magnitude of nonlinear transport increases), the zone of hillslope convexity becomes focused at the hilltop and side slopes become increasingly planar. On steep slopes, rapid increases in sediment flux near the critical gradient limit further steepening, such that hillslope relief and slope angle are not sensitive indicators of erosion rate. Using a one‐dimensional finite difference model, we quantify hillslope response to changes in baselevel lowering and/or climate‐related transport efficiency and use an exponential decay function to describe how rapidly sediment flux and erosion rate approach equilibrium. The exponential timescale for hillslope adjustment decreases rapidly with increasing ΨL. Our results demonstrate that the adjustment timescale for hillslopes characteristic of the Oregon Coast Range and similar steep, soil‐mantled landscapes is relatively rapid (≤50 kyr), less than one quarter of the timescale predicted by the linear transport model.

Effects of sediment bulk properties on erosion rates

Considerable work has been done recently on the effects of sediment bulk properties on erosion rates. From this it is known that erosion rates depend on at least the following parameters: bulk density, average particle size, particle size distribution, mineralogy, organic content, volume of gas in the sediment, salinity of the pore waters, and time after deposition. This work is reviewed and discussed here with the purpose of presenting a quantitative overview of the effects of each of these parameters. This information is then used to demonstrate a procedure for estimating erosion rates of sediments based on a knowledge of their bulk properties.

EXPERIMENTAL STUDY TOWARD THE PREDICTION OF EROSION RATE OF COHESIVE SEDIMENT

Effects of several governing parameters on the erosion rate of cohesive sediment were investigated in the present study as a result of a series of systematically arranged experiments. The exerted velocity on the surface of test sample, a clay content ratio, a water content ratio, water temperature, the size of cohesionless sediment and the compactness of the sample are the important parameters to be considered. The relations between the erosion rate and each parameter were evaluated respectively. Based on these fundamental imformation, an erosion rate formula was roughly derived even though it was effective under the restricted condition.

Experimental study on the entrainment of bed material into debris flow

The present study describes entraining characteristics of bed material into debris flow, based on flume tests, numerical and dimensional analyses. Flume tests are conducted to investigate influences of bed sediment size on erosion rate by supplying debris flows having unsaturated sediment concentration onto erodible beds. Experimental results show that the relative erosion rate, E/E0, decreases monotonically with increase of relative sediment size, d/d0, although E/E0 changes slightly with sediment concentration of debris flow. Herein, E is the erosion rate of bed sediment of size, d, E0 is the erosion rate when solid particle size, d0, of debris flow, are the same of the erodible bed material. According to the relation between E/E0 and d/d0, erosion rate, E, can be estimated by using Egashira's formula for E0. Therefore, the validity of erosion rate formula for E0 is tested by solving numerically for debris flow characteristics in terms of governing equations. In addition, critical size of bed sediment entrainment is discussed by introducing non-dimensional effective bed shear stress which is formulated by using fluid shear stress (total shear stress minus yield stress), bed sediment size, d, specific weight of sediment particle in water and acceleration due to gravity, and it is found that the critical non-dimensional effective shear stress takes a value similar to critical Shields parameter for bed load movement.