Sediment Deposition Model Research Articles

Modeling dependence of moraine deposition on climate history: the effect of seasonality

A simple shallow-ice flowline glacier model coupled to a model of sediment transport and deposition is used to simulate formation and preservation of moraines. The number, positions, and volume of moraines formed all are sensitive to the climate history assumed. We drive the model with the GISP2 central-Greenland temperature record, and with reduced-millennial-amplitude versions of that record, to test the hypothesis that the Younger Dryas and other millennial oscillations were primarily wintertime events and thus had less influence on glacier behavior than did the Last Glacial Maximum with its strong summertime as well as wintertime signal. We find that forcing the model by GISP2, with dampened strength of the millennial-scale signal, provides modeled moraine configurations that reflect observed moraine records in the Northern Hemisphere.

Erosion and delivery of deposited peat sediment

Erosion of deposited organic peat sediments from peat harvesting ditches was studied in both laboratory and field conditions. Peat sediments were placed in a recirculation flume at controlled pH levels and allowed to stabilize from 15 min to 10 d. Afterward, different flow velocities were applied and the peat erosion was measured. The erosion rate was related to the physical properties of peat: the particle size and form, settling velocity, peat type and degree of decomposition (von Post). The mean average settling velocity for the peat was measured to be 0.87 m h−1. The median sediment particle size was 42 μm, which indicates that the peat sediment is partly cohesive. The deposited sediment formed a loose layer overlaid by more stabilized layers. Critical shear stress for the surface of the loose peat layer was 0.01 N m−2 and for the more consolidated layer was 0.059 N m−2 after a stabilization of three days. The erosion coefficients determined during laboratory study varied between 3.7·10−4 and 4.1·10−6 kg m−2 s−1. The determined erosion and sedimentation rates were used to calibrate sediment erosion and deposition models. A new approach was then developed for the uses of these models to simulate peat sediment delivery from field ditches. The model was tested against measured values which were in accordance to observed sediment transport rates.

The impact of improved landuse cover on the range of modelled sediment yield from two sub-catchments of the Mae Chaem, Thailand

A sediment source, transport, and deposition model known as SedNet has been applied to two sub-catchments of the Mae Chaem River in Thailand, the Mae Suk (95 km 2) and the Mae Kong Kha (91 km 2). The applied models were analysed to determine the dominant sources and sinks of suspended sediment in these catchments, and to examine the impact of an improved landuse coverage on the range of modelled sediment loads. SedNet model scenarios were run using landuse grids from 1995 to 2003. The 1995 landuse classification was derived from satellite imagery and contained a relatively undifferentiated landuse classification (lumping all forest types into one category); the other in 2003 was derived from a mix of satellite imagery, ground truthing, and mapping, and better differentiated between landuse types (dividing forest types into evergreen, deciduous, and pine plantations). Results indicate significant differences in predicted sediment export in 2003 compared to 1995. It is difficult to say whether these changes were due to actual changes in landuse between 1995 and 2003, or due to the improved landuse classification in 2003. The source areas of suspended sediment also changed significantly between 1995 and 2003, and these changes in source area can clearly be linked to the improved mapping of landuse in 2003. The improved landuse classification in 2003 also led to significant reductions in the range of possible suspended sediment exports from both catchments. Despite these improvements in landuse classification, significant uncertainty in predicted suspended sediment yield still exists for both sub-catchments. Further improvements in identifying the total volume and source areas of suspended sediment will best be achieved through an improved landuse coverage which identifies the type of crop being grown.

Coastal sedimentation associated with the December 26, 2004 tsunami in Lhok Nga, west Banda Aceh (Sumatra, Indonesia)

With wave heights of 30 m and runups reaching 50 m a.s.l. and 6 km inland, the December 26, 2004 tsunami in Sumatra was one of the largest tsunamis in recorded human history. In this paper, we present a description of the event in the Lhok Nga Bay (west coast of Banda Aceh) and an interpretation of the tsunami sand deposits, mostly based upon grain-size analysis. The 3.5 km long transect of Lampuuk displays two landward-fining, thinning and sorting sequences. The thickest cross-sections of the first sequence of deposits (0–1.5 km inland) suggest a deposition by three consecutive runups (inflows) and a final backwash (return flow or outflow). From 1.5 to 3.5 km inland, the record is dominated by the second and highest wave (15–30 m). Normally-graded couplets or triplets of layers were used to identify the runup of each wave. The topmost layers, interpreted as the backwash deposition, describe a seaward sequence of increasing mean grain-size and decreasing degree of sorting. The local effects of the topography could be identified: thickest deposits in the topographic lows (50–80 cm), great spatial variations in thickness and upper laminated texture when the sedimentation was limited by steep slopes, landward coarsening and very poor sorting at the wave breaking point, bimodal grain-size distributions reflecting different sources of sediments. Finally, by coupling the longitudinal and vertical trends of the 26 cross-sections, we propose a model of sediment transport and deposition in a large tsunami wave.

Numerical modelling of floodplain hydraulics and suspended sediment transport and deposition at the event scale in the middle river Elbe, Germany

AbstractThe objective of this case study was to calibrate and verify detailed transport model of sediment in a 4‐kilometre stretch of the middle Elbe floodplains in Germany. The hydraulic RMA‐2 model and the SED2d‐WES sediment transport model were used. These models were calibrated and validated by detailed measurement of the surface water elevations, the velocities at six profiles, and the suspended sediment concentration and deposition (by means of 10 sediment traps). The flow was modelled for three steady‐state discharges. The surface water elevations were calculated to an accuracy of less than 5 cm compared to measurements. The differences between the calculated and measured velocities were with one exception smaller than 0.2 m/s (measured range 0.1…︁1.0 m/s). An average sediment input of 35 g/(m2 d) was calculated for the flood event studied. The highest calculated sedimentation rates of 700 g/(m2 d) (dry density 90 kg/m3) occurred in quiescent zones and abandoned channels. Twenty‐five percent of the deposited sediment settled in the quiescent zones (which only account for 13% of the area). The most sensitive parameters of the sediment transport model were the settling velocity and critical shear stress. The modelling techniques used allowed sediment deposition on the floodplains of the Elbe to be realistically depicted.

Climatology of Large Sediment Resuspension Events in Southern Lake Michigan

Lake Michigan, particularly the southern basin, is subject to recurrent episodes of massive sediment resuspension by storm-induced waves and currents. The purpose of this paper is to investigate the climatology of these events for Lake Michigan, including an analysis of associated meteorological conditions. This paper begins by examining turbidity records from two water treatment plants (Chicago, IL and St. Joseph, MI) for which long-term records are available. The turbidity records from the two plants show significant differences indicating that turbidity measurements from a single location would probably not be representative of a basin-wide climatology. A one-dimensional sediment resuspension and deposition model for fine-grained sediments is then developed and calibrated with data from the water treatment plants. The one-dimensional model is applied at 15 points around the southern basin for a 45-year period for which Lake Michigan wave climatology is available and the results are averaged to obtain a basin-wide turbidity index (Southern Lake Michigan Turbidity Index, SLMTI). A frequency distribution of the turbidity index is presented and meteorological conditions associated with the largest events are examined. Our analysis indicates that significant resuspension events in southern Lake Michigan are usually caused by a strong cyclone passing to the east of the lake. The most likely time of the year for this to occur is October to April. There is an average of 1 event per year with SLMTI above 25 mg/L and each event typically lasts about 3 days. Our analysis indicates that events have occurred more frequently since the late 1980s as the number of winter storms has increased and ice cover has decreased.

Numerical prediction of suspended sediment concentrations in the Ariake Sea, Japan, using a time-dependent sediment resuspension and deposition model

The Ariake Sea, a typical semi-closed bay located in Kyushu Island, western Japan, has a maximum tidal range of 6 m in spring tide, and its vast tidal flats are composed of both sandy bottom areas and muddy bottom areas. In this study, two-dimensional depth-averaged finite difference numerical models have been developed for predicting the tidal flow velocity and suspended sediment concentration in the Ariake Sea. In the suspended sediment transportation model, a time-dependent sediment resuspension and deposition process was modeled as a response to the tidal flow by considering the presence of cohesive bottom sediment. The shifting-particles method, known as an operator-splitting technique, was used for time integration of the convective-dispersion equation. The model results were compared with in situ measurements of suspended sediment concentration and tidal flow velocity at several points in Isahaya Bay—located at the western part of the Ariake Sea—and also with the estimated suspended sediment concentration distribution via a Landsat Thematic Mapper image. The results indicated that the models were able to predict the measured values for suspended sediment concentration and tidal flow velocity, as well as the spatial distribution pattern of the suspended sediment concentration as estimated by the Landsat Thematic Mapper image.

Spatial evaluation of a multi‐class sediment transport and deposition model

AbstractThis paper presents a model for the description of sediment fluxes and deposition processes in a two‐dimensional spatial context using the multiple‐class Hairsine et al. sediment deposition equations. The model describes size‐selective net deposition as a result of simultaneous deposition by settling and non‐selective re‐entrainment of previously deposited sediment. A critical comparison of field observations of net erosion and deposition in a watershed with complex topography with this physically based model is presented. The evaluation shows that the deposition model is capable of simulating reasonably both the observed size selectivity and spatial patterns of the deposition process. In this field study, the zone where a net deposit of sediment is re‐entrained by overland flow was predicted to be quite limited spatially due to low sediment concentrations and the absence of aggregates in the eroded soil. Copyright © 2004 John Wiley & Sons, Ltd.

Subglacial erosion and englacial sediment transport modelled for North American ice sheets

Reconstructions of North American glacial history have either been based on interpretation of the geologic record or on numerical models of continental ice dynamics. Because such models make no explicit reference to the geological products of glaciation, the full potential of a combined approach has not been realized. We present the results of a process-based model of sediment production, entrainment, transport and deposition that represents a first step toward uniting these two lines of investigation. In this model basal-ice processes are dependent on ice sheet geometry, hydrology and dynamics. A three-dimensional thermomechanical ice sheet model yields the ice geometry, flow and thermal conditions that are used by a model of subglacial hydrology; together these drive the basal process model. Comparisons of simulation results with mapped debris distributions of the Hudson Bay Paleozoic carbonates and of the distinctive Dubawnt Group are used to test the validity of the transport model. Dispersion of debris radially out of Hudson Bay is greatly overestimated by the model, and dispersion toward the bay equally underestimated. This is because the modelled ice sheet has a single dome over Hudson Bay at glacial maxima, including Last Glacial Maximum, rather than a saddle separating ice domes over Quebec-Labrador and Keewatin. Estimated rates of glacial erosion for the Cordilleran and Laurentide ice sheets are compared against those predicted by the model. The model underestimates these rates because it underestimates the vigour of the quarrying process.

Numerical modelling of depositional sequences in half‐graben rift basins

ABSTRACT A three‐dimensional numerical model of sediment transport and deposition in coarse‐grained deltas is used to investigate the controls on depositional sequence variability in marine half‐graben extensional basins subject to eustatic sea‐level change. Using rates of sea‐level change, sediment supply and fault slip reported from active rift basins, the evolution of deltas located in three contrasting structural settings is documented: (1) footwall‐sourced deltas in high‐subsidence locations near the centre of a fault segment; (2) deltas fed by large drainage catchments at fault tips; and (3) deltas sourced from drainage catchments on the hangingwall dip slope. Differences in the three‐dimensional form and internal stratigraphy of the deltas result from variations in tilting of the hangingwall and the impact of border fault slip rates on accommodation development. Because subsidence rates near the centre of fault segments are greater than all but the fastest eustatic falls, footwall‐sourced deltas lack sequence boundaries and are characterized by stacked highstand systems tracts. High subsidence and steep bathymetry adjacent to the fault result in limited progradation. In contrast, the lower subsidence rate settings of the fault‐tip and hangingwall dip‐slope deltas mean that they are subject to relative sea‐level fall and associated fluvial incision and forced regression. Low gradients and tectonic tilting of the hangingwall influence the geometry of these deltas, with fault‐tip deltas preferentially prograding axially along the fault, creating elongate delta lobes. In contrast, broad, sheet‐like delta lobes characterize the hangingwall dip‐slope deltas. The model results suggest that different systems tracts may be coeval over length scales of several kilometres and that key stratal surfaces defining and subdividing depositional sequences may only be of local extent. Furthermore, the results highlight pitfalls in sequence‐stratigraphic interpretation and problems in interpreting controlling processes from the preserved stratigraphic product.

Late Quaternary stratigraphy of Laurentian Fan: a record of events off the eastern Canadian continental margin during the last deglacial period

Examination and correlation of 10 piston cores from Laurentian Fan, off the eastern Canadian continental margin revealed a high-resolution stratigraphic record of events since the Last Glacial Maximum (LGM). The five lithofacies identified in the cores are: (1) bioturbated, olive-grey, silty mud with varying amounts of biogenic carbonate; (2) olive-grey and reddish-brown silt-laminated mud with rare fine to very fine sand beds; (3) poorly sorted, light grey and brick-red gravelly, sandy mud; (4) colour-banded silty mud with colours alternating at a centimetre-scale between olive-grey and reddish-brown; and (5) brown silty mud, commonly massive but occasionally silt-laminated. The dominant depositional processes include hemipelagic sedimentation, turbidity currents, and ice rafting. In these sediments, colour is diagnostic of the sediment source area, with reddish sediment being delivered to the fan from the Gulf of St. Lawrence and Laurentian Channel whereas greenish sediment arrives on the fan from St. Pierre Bank through Grand Banks Valley. Turbidites are the most volumetrically significant facies. Conversely, ice-rafted beds are volumetrically minor, but they are the most chronostratigraphically significant facies, and include Heinrich layers H0, H1, and H2. The chronological control provided by radiocarbon dating and stable oxygen isotopes suggests that sedimentation rates for the reddish-brown turbidites deposited between the LGM and 14 ka exceeded 10 m kyr −1. Mapping of the units, together with a simple sediment deposition model, suggest that these beds resulted from deposition by turbidity currents over 1 km thick that lasted on the order of weeks. Less information can be ascertained from the olive-grey turbidites that interbed with H2 and directly overlie H1 because of a lack of sampling, except that they had a lower sedimentation rate and include sediment derived from Newfoundland and the Grand Banks. Comparison of the stratigraphy on Laurentian Fan with that of other deep-water areas further north (southwest Grand Banks Margin, Flemish Pass, and the Labrador Sea) shows striking similarities over the late Quaternary, with the deglacial period characterized by rapid sediment accumulation. However, only on Laurentian Fan does sediment gravity flow deposition occur during the LGM. Reddish-brown turbidites deposited on Laurentian Fan during the deglacial period have no equivalent facies further north. These unique features suggest important glaciological differences between the ice stream occupying Laurentian Channel and ice streams farther north. The onset of widespread hemipelagic deposition occurred after ca. 12 ka on Laurentian Fan and in Flemish Pass, but not until ca. 8.2 ka in the Labrador Sea, consistent with the earlier retreat of glacial ice out of Laurentian Channel as compared to Hudson Strait.

Evaluating a multiclass net deposition equation in overland flow conditions

The movement of sediment and related pollutants in overland flow from the point of erosion to a stream normally occurs through a series of deposition zones. Here we present the evaluation of a new multiclass model of sediment transport through these zones. This model simulates size‐selective net deposition as a result of simultaneous deposition by settling and nonselective reentrainment of previously deposited sediment [Hairsine et al., 2002; Sander et al., 2002]. Multiclass sediment deposition models provide a useful basis for simulating off‐site effects of soil erosion, but information on the grain‐size composition of deposited and exported sediment is crucial for calibrating and evaluating such models. Therefore the multiclass sediment deposition model was tested using detailed experimental data. Information on the amount of sediment deposition, on the grain‐size distribution of the deposited and exported sediment, and on the spatial pattern of the deposits was collected during sediment deposition experiments. Calibration of the multiclass model, using one part of these data, was performed by adjusting the fraction of stream power available for sediment reentrainment by minimizing errors between both simulated and observed total outflow sediment concentration and grain‐size distribution of the exported sediment. Evaluation was conducted, using the other part of the data, over a wide range of conditions. The evaluation shows that the deposition equation is capable of reasonably simulating the observed size selectivity of the deposition process. However, the model is not capable of simulating correctly the spatial pattern of the deposited sediment because of simplifications used in the hydraulic model.

Application of an analytical model of critically stratified gravity-driven sediment transport and deposition to observations from the Eel River continental shelf, Northern California

An analytical model of down-slope sediment transport and deposition by wave-supported gravity-driven flows is applied to examine the formation of the mid-shelf flood deposit on the continental margin off of the Eel River in northern California. The model reproduces observed time series of near-bed velocity and deposition following flood events when sufficient fine sediment is available to critically stratify the wave boundary layer. Comparative estimates of down-slope gravity-driven flux and along-shelf sediment delivery suggest that critical stratification will dominate the near-bed dynamics when greater amounts of sediment are delivered to the inner shelf by river floods than can be removed by gravity-driven processes. If insufficient sediment is delivered to cause critical stratification in the wave boundary layer, energetic waves can enhance drag, retard down-slope transport, and limit deposition. Analytic predictions of deposition suggest that the magnitude of wave energy is more important than the magnitude of the flood event in controlling the local thickness of mid-shelf gravity-driven deposition following floods of the Eel River. Higher wave energy increases the capacity for critically stratified gravity flows to transport sediment to the mid-shelf and results in greater gradients in flux and hence deposition. Flood magnitude determines how close to shore the flood deposit begins and how far along-shelf it extends. The bathymetry of the Eel margin plays a critical role controlling wave-supported gravity flows. Analytic predictions indicate that gravity-driven deposition on the mid-shelf begins roughly 7–8 km north of the river mouth. Closer to the river mouth, the seaward increasing mid-shelf slope associated with the concave downward subaqueous delta causes gravity-driven flux divergence, preventing significant mid-shelf deposition by wave-supported gravity flows and favoring sediment bypassing. A seaward decrease in shelf slope in the vicinity of the observed flood depo-center leads to greater flux convergence by gravity-driven flows, and hence greater deposition. Farther north, the supply of sediment diminishes sufficiently to prevent significant gravity-driven deposition. The analytic predictions of mid-shelf mud deposition are spatially and temporally consistent with field observations and provide strong evidence that wave-supported gravity flows control the emplacement and location of the Eel margin flood deposit.

ABSTRACT: Three-Dimensional Numerical Modelling of Eustatic vs Local Controls on Depositional Sequences

A three-dimensional numerical model of sediment transport and deposition in coarsegrained deltas is used to investigate the controls on depositional sequence variability. We consider the stratigraphic response to eustatic sea-level (amplitude and rates) and local controls, such as physiography, sediment supply and local subsidence. The development of key stratal surfaces, stratal geometry and facies stacking patterns show a systematic development with respect to stages in a eustatic sea-level cycle. However, even with constant sediment supply and a simple sinusoidal eustatic sea-level variation many of the features developed in the models are, by their nature, three-dimensional and twodimensional analysis of the model results can lead to erroneous interpretations of the causes of along-strike variability. Local controls generate significant variability in depositional sequence development, including the timing and amount of fluvial incision during relative sea-level fall, and the timing and nature of transgression during rapid phases of relative sea-level rise. The model results suggest that different systems tracts may be coeval and that key stratal surfaces defining and subdividing depositional sequences may be of local extent. Furthermore, the results highlight pitfalls in sequence stratigraphic interpretation and problems in interpreting controlling processes from the preserved stratigraphic product.

Water discharges determined from sediment distributions: a palaeohydrological method

The article presents a simple sediment deposition model, based on current knowledge of the processes of transport and deposition of suspended material. It indicates possibilities of interpreting flow conditions from grain‐size distributions at two localities, situated along the water flowline. The model reveals new information on ancient hydrological and sedimentological processes and events. Previously the model has been applied to forecast trap efficiency of reservoirs and predict reservoir sedimentation. In this study the model is used as a palaeohydrological method in studies of annual varves.In varve geochronology a major attention has been paid to varve correlations and varve thickness variations, whereas other sedimentological data have been more or less disregarded. In this article new information on the spatial distribution of particle sizes demonstrate that further knowledge can be gained by careful studies of the patterns of grain‐size distributions.Annual variations in water discharges during late glacial time were studied in the Indalsälven and Dalälven valleys in Sweden. Water discharges during the deglaciation seem to have been of the same order of magnitude as in present‐day rivers during normal years. During some years catastrophic events occurred, resulting in exceptional water discharges. The magnitudes of these discharges were estimated, and drainage directions were established. It appears from the studies that there are promising prospects of finding out both the magnitude and frequency of processes that until now have been described only in qualitative terms.

Modelling input of fine granular sediment into drainage systems via gully-pots

An existing model of sediment wash-off from paved surfaces has been extended to predict fine granular sediment inputs into drainage systems via roadside gully-pots. The model comprises three separate components: (1) a model of sediment accumulation on paved surfaces; (2) a model of sediment wash-off from these surfaces; and (3) a model of sediment deposition and erosion in gully-pots. The first two model components have been previously successfully tested, whereas the third component of the model is presented in this paper for the first time. The model was applied to two small catchments located in Dundee, Scotland, where data on flow rates and sediment concentrations were collected at the inlets into the main drainage systems from the road gullies. It was found necessary to calibrate only the build-up component of the model to achieve good agreement between the simulated and measured data. It was concluded that only two calibration parameters are required to model the input of fine granular sediment into storm drainage systems via gully-pots.

The spatial variability of overbank sedimentation on river floodplains

River floodplains are important landforms, and information on rates and patterns of overbank sediment deposition is essential for understanding their development. Such information is also needed for validating existing overbank floodplain flow and sediment transport and deposition models. The use of bomb-derived 137Cs for documenting rates of overbank sediment deposition on river floodplains offers the potential for assembling data for a large number of points on a floodplain and therefore for investigating the spatial patterns involved. This approach also overcomes some of the operational and sampling problems encountered in traditional monitoring techniques. The study reported uses 137Cs measurements on sediment cores to investigate the spatial variability of sediment deposition on selected floodplain reaches of five British lowland rivers. Attention is given both to the overall reach and to more detailed investigations of small areas. The observed spatial patterns of sediment deposition are interpreted in terms of the major controls involved, which include the floodplain microtopography and morphology. The relationship between the grain size composition of deposited sediment and sedimentation rate is also considered.

Numerical modelling of floodplain hydraulics and suspended sediment transport and deposition

A numerical model is presented for the prediction of floodplain inundation extents, flow depths and velocities, and patterns of suspended sediment dispersion and deposition. The model employs a simplified and novel treatment of fluid flow and a conventional convection–diffusion mass balance relationship for suspended sediment transport and deposition. These relationships are solved over a finite difference grid with a relatively fine nodal spacing, which enables a detailed representation of complex floodplain topography. The model is applied to a 600 m reach of the River Culm, Devon, UK. Continuous records of river stage and suspended sediment concentration monitored at the upstream boundary of the study reach provide the data needed to operate the model. These data are supplemented by measurements of the in situ settling characteristics of the suspended sediment load. Model predictions of overbank flow depths and velocities and patterns of suspended sediment concentrations and deposition amounts are presented. Predicted patterns of overbank deposition are described and compared with field measurements obtained with the aid of astroturf sedimentation traps. Grain size distributions obtained for samples of deposited sediment retrieved from the sedimentation traps are also examined and compared with model predictions. The results of both the field measurement programme and modelling work are shown to have significant implications for the representation of floodplain topography and suspended sediment particle size and settling properties in numerical simulations of overbank processes. © 1998 John Wiley & Sons, Ltd.

High resolution flow modelling in hydrology and geomorphology

Introduction THEORETICAL ADVANCES 1. Hydraulic modelling in hydrology and geomorphology: a review of high resolution approaches 2. Modelling wetting and drying effects over complex topography 3. The 2-D transport module SUBIEF: applications to sediment transport and water quality processes 4. Using lagrangian particle saltation observations for bedload sediment transport modelling HYDRAULIC MODELLING 5. High resolution two-dimensional spatial modelling of flow processes in a multi-thread channel 6. Numerical modelling of separated flow in river bends: model testing and experimental investigation of geometric controls on the extent of flow separation at the concave bank 7. Investigation of controls on secondary circulation in a simple confluence geometry using a three-dimensional model 8. Flood routing based on diffusion wave equation using mixing cell method 9. Investigating two-dimensional, finite element predictions of flood plain inundation using fractal generated topography 10. Integrating remote sensing observations of flood hydrology and hydraulic modelling HYDROLOGICAL MODELLING 11. An integrated physically-based model for arid region flash flood prediction capable of simulating dynamic transmission loss 12. Modelling the spatial variability in floodplain soil contamination during flood events to improve chemical mass balance estimates 13. Kinematic wave modelling of vertical movement of snowmelt water through a snowpack 14. Kinematic wave modelling of saturated basal flow in a snowpack 15. The introduction of runoff routing into large-scale hydrological models GEOMORPHOLOGICAL MODELLING 16. Numerical Modelling of floodplain hydraulics and suspended sediment transport and deposition 17. Modelling with adjustment in straight alluvial channels 18. Morphological modelling of rivers with erodible banks .

Filtration enhances suspended sediment deposition from surface water to granular permeable beds

We present results from an experimental study of suspended particle (4.5-36.5 µm silicon carbide powder) deposition from surface water to 'clean' equi-granular permeable beds in a small 12.5x12.5x15cm box and a re-circulating flume. Enhanced deposition rates of up to 5 times the accepted sediment deposition model (e.g., Einstein, 1968) are explained by filtration of particles in the bed. Compared to this model deposition increases with increasing surface fluid speed, decreasing suspended particle size and increasing bed particle size. These results can be explained by an increased ability of particles to penetrate into the bed with the pore water which increases the effective filter thickness. The predominant deposition mechanism within the bed pores appears to be settling. Enhanced deposition, evident in Einstein's (1968) experimental data, was previously attributed to flocculation but may be explained better by filtration. These enhanced deposition rates drop off to close to the accepted model predicted rate after a certain volume of sediment has entered the bed, which may be due to the slumping of deposits from the top of bed elements. This reduction in deposition rate occurs long before the bed is filled with fine sediment.