Bed Level Changes Research Articles

Two-Dimensional Numerical Simulation Study on Bed-Load Transport in the Fluctuating Backwater Area: A Case-Study Reservoir in China

Numerical modeling of sedimentation and erosion in reservoirs is an active field of reservoir research. However, simulation of the bed-load transport phenomena has rarely been applied to other water bodies, in particular, the fluctuating backwater area. This is because the complex morphological processes interacting between hydrodynamics and sediment transport are generally challenging to accurately predict. Most researchers assert that the shape of a river channel is mainly determined by the upstream water and sediment, and the physical boundary conditions of the river channel, rather than random events. In this study, the refinement and application of a two-dimensional shallow-water and bed-load transport model to the fluctuating backwater area is described. The model employs the finite volume method of the Godunov scheme and equilibrium sediment transport equations. The model was verified using experimental data produced by a scaled physical model, and the results indicated that the numerical model is believable. The numerical model was then applied to actual reservoir operations, including reservoir storage, reservoir drawdown, and the continuous flood process, to predict the morphology of reservoir sedimentation and sediment transport rates, and the changes in bed level in the fluctuating backwater area. It was found that the location and morphology of sedimentation affected by the downstream water level result in random evolution of the river bed, and bed-load sedimentation is moved from upstream to downstream as the slope of the longitudinal section of the river bed is reduced. Moreover, the research shows that the river channel sedimentation morphology is changed by the change water level of the downstream reach, causing the dislocation of the beach and channel and random events that will affect the river, which is of certain reference value for waterway regulation.

Quantifying Bed Level Change at the Transition of Tidal Flat and Salt Marsh: Can We Understand the Lateral Location of the Marsh Edge?

AbstractBed level dynamics at the interface of the salt marsh and tidal flat have been highlighted as a key factor connecting the long‐term biogeomorphological development of the marsh to large‐scale physical forcing. Hence, we aim to obtain insight into the factors confining the location of the marsh edge (i.e., boundary between tidal flat and salt marsh). A unique data set was collected, containing measurements of daily bed level changes (i.e., integrative result of physical forcing and sediment properties) at six intertidal transects in the North Sea area. Moreover, various biophysical parameters were measured, such as sediment characteristics, waves, inundation time, and chlorophyll‐a levels. The data show that both bed level change and waves decreased from the lower intertidal flat toward the marsh edge and further diminished inside the marsh. However, no direct general relation was found between waves and bed level change. Bed level change inside the marsh was always small, regardless of wave energy. By combining the data sets, we demonstrate that the location of the lower marsh edge is restricted by two interacting factors: inundation time and bed level change. For vegetation establishment to withstand longer inundation stress, which slows down plant growth, more stable bed levels are required so that plants are not heavily disturbed. Conversely, to withstand more dynamic bed levels that disturbs plant growth, lower inundation stress is needed, so that plants grow fast enough to recover from the stress. The results suggest that bed level change is important in determining the position of the marsh edge.

Pathways of Water and Sediment in the Biesbosch Freshwater Tidal Wetland

Diversion of water and sediment into drowning delta wetlands aims at renewed sediment accumulation for ecosystem regeneration and compensation of sea-level rise and soil subsidence. The success of such measures requires sound understanding of flow pathways and deposition patterns within wetlands and their feeding channels. This study aimed to (1) identify such pathways in a tidal freshwater wetland in the Netherlands, (2) determine the relative contribution of the two feeding rivers (Rhine and Meuse) to the water and sediment budget and (3) explore whether moving boat campaigns can contribute to a comprehensive understanding of the system. Bed level change was highest in newly developed areas, where equilibrium sets in. Sedimentation takes mainly place in channels close to the feeding rivers, where sediment becomes replenished each tidal cycle. Distal tidal channels receive only water poor in sediment during flood tide and experience erosion. Short term changes in suspended sediment load, measured during moving boat campaigns were generally consistent with average long-term changes in bed level. This means that short-term field measurements can not only be used to explain the processes, but also to predict the longer-term development of the major part of the study area.

Morphological changes in the southern coasts of the Caspian Sea using remote sensing and GIS

Coastal areas are continuously under the regional changing and interaction of land-sea resulting in the short and long terms deformation. Study of beach morphology has been one of the most important issues in coastal engineering research projects. Managing and controlling the shoreline changes and behavior are essential for all marine projects and integrated coastal zone management policy in such environments. In the present study, Guilan Province region was selected and an analysis of the coastal zone behavior was carried out based on the available satellite images. According to results, we estimated the bed level changes and also submerged area, resulting from a rise in sea level at different locations in the study area (from Anzali Port to Boujagh National Park) and compared them with those obtained from the satellite images. The results were presented graphically and changes in shorelines were estimated using ETM+ sensors and OLI images from 2002 to 2013 which can be used for site selection and design of marine structures and establishing a data base for the coastal zone in the study region. Laplacian filter was applied to satellite images to establish the shoreline and to clarify the effect of ports and coastal structures constructed in the study region on beach and shoreline morphological changes. This study is a combination of remote sensing and GIS systems with field surveys on the ground. The innovation of this work is the application of Laplacian filter for shoreline detection and estimation of the sediment deposition area by calculating the distance between the observed shorelines. This will lead to the better understanding of the effect of coastal structures on the beach morphology using satellite images. The results show the remarkable changes occurred in the shoreline due to the environmental and human-based effects during the eleven years period. The present study can be considered as a contribution to the existing knowledge of the coastal process in the study area and referred to as a basis for the future coastal research projects.

Numerical Simulation and Large-Scale Physical Modelling of Coastal Sand Dune Erosion

Do, K.; Shin, S.; Cox, D., and Yoo, J., 2018. An international comparison of drowning in South Korea. In: Shim, J.- S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 196–200. Coconut Creek (Florida), ISSN 0749-0208.Coastal sand dune system is important in the nearshore environment for sand supply, ecosystem, and hazard mitigation. In this study, a process-based morphological model was performed and the results were compared with large-scale laboratory experimental data. Two-dimensional large-scale laboratory experiments were conducted with 1:6 geometric scale in the large wave flume (104m (L) × 3.7 m (W) × 4.6 m (D)) of the Hinsdale Wave Research Laboratory at Oregon State University (Maddux et al, 2006). Several different wave conditions were used with different water levels in this experiment including pre-storm, storm, and post storm based on random wave time series by using TMA spectrum. The data set included cross-shore Wave heights, fluid velocities, and the profile changes of the beach and dune. The process based model, XBeach (Roelvink et al., 2009) was used to simulate the nearshore hydrodynamics and bed level change during storm wave condition. Several semi-empirical parameters were used in the XBeach model to predict morphodynamic process (Roelvink et al., 2009) and recently the updated parameters (WTI settings) were suggested based on the results of field observations. The present study ran the XBeach model by using both default and WTI settings and compared with the results from the experiments. The results showed that the model results with WTI settings showed good agreement with the measured beach profile while the model results with default settings over-predicted the offshore sediment transport and dune erosion. Especially, the wave skewness (facSk) and asymmetry (facAs) gave the highest contribution to predict dune erosion.

Dynamic equilibrium behaviour observed on two contrasting tidal flats from daily monitoring of bed-level changes

Dynamic equilibrium theory (DET) has been applied to tidal flats to systematically explain intertidal morphological responses to various distributions of bed shear stress (BSS). However, it is difficult to verify this theory with field observations because of the discrepancy between the idealized conceptions of theory and the complex reality of intertidal dynamics. The core relation between intertidal morphodynamics and BSS distribution can be easily masked by noise in complex datasets, leading to conclusions of insufficient field evidence to support DET. In the current study, hydrodynamic and morphodynamic data were monitored daily for one year on two tidal flats with contrasting wave exposures. BSS distribution was obtained by validated numerical models. Tidal flat dynamic equilibrium behaviour and BSS were linked via Empirical Orthogonal Function (EOF) analysis. We show that the principal morphodynamic modes corresponded well with the respective modes of BSS found at both sites. Tide-induced BSS was the dominant force at both sites, regardless of the level of wave exposure. The overall erosional and steepening trend found at the two flats can be attributed to the prevailing action of tidal forcing and reduced sediment supply. Hence, EOF analysis confirmed that tidal flat morphodynamics are consistent with DET, providing both field and model evidence to support this theory.

Digital Modeling of Bar (Char) Incidence and Assessment of Braiding Intensity of the Padma River in Bangladesh

This study has prepared bar (char) incidence map and assessed braided intensity of the Padma River in Bangladesh. GIS techniques have integrated the spatial data (satellite images) and attribute data to identify the char’s location and assess the braiding intensity on the Padma riverbed. According to the bar prepared maps chars have been developed in the reaches in the south of Harirampur, northeast of Faridpur district and upstream of the Padma Bridge crossing. But chars had never existed at the downstream of Paturia and close to the left bank at Mawa. The young chars with ages below eight years are 48%. The longstanding chars between 8-20 years are almost the same (43%) in the char incidence map. But the very long duration chars appeared only 9% on the river bed. In the late 1960s and early 1970s, the braiding intensity of the Padma River was 1.63 and 1.75 which is very close to each other. During 1976 to 1984, the braiding intensity dropped to a minimum, the range of which varied from 1.38 to 1.50. The braiding intensity started to rise very rapidly from the early 1990s and reached its peak value of 2.13 in 1997. The sediment input also cause changes in bed level, width and braiding intensity of the river.

Development of a numerical model for simulating bed-level changes in front of a sea wall

Anticipating the trend of seabed changes and scouring phenomena is one of the most important issues in coastal engineering, and several numerical models have already been presented to simulate coastal seabed evolutions and estimate the depth of scouring in front of coastal structures. The sea wall as a reflective structure plays an important role in protecting the coast, and how the presence or absence of these structures influences the sediment transport rate and consequently seabed changes should be studied more carefully. In this study, bed-profile changes and the potential for scouring were calculated by a numerical model at different locations in the vicinity of a sea wall. Unlike previous models in which the velocity of sediment particles was assumed to be equal to the velocity of water particles, a more detailed process was considered here to calculate the velocity of water and sediment particles. Finally, it became clear that using the sediment transport coefficient parameter did not significantly improve the seabed changes that were predicted by the modified model in the present study. This achievement shows the rationality of considering the velocity of sediment particles as being equal to the velocity of water particles in predicting seabed changes.

An Equilibrium Transport Formula for Modeling Sedimentation of Dredged Channels

The proposed formula for modeling sedimentation is based on a concept of hydrodynamic equilibrium in sediment transport. Hydrodynamic equilibrium occurs when the sediment transport is in a form of grain jumps with the distance traveled dx smaller than the one established in the numerical modeling. A first-order upwind scheme FDM is applied. It provides the solution to equations describing changes in bathymetry in time and space coupled with simultaneous changes of bed sediment distribution. A clear-cut division of sediment transport into those during wave crest and wave trough is proposed. The results of bathymetry modeling coincided satisfactorily with the laboratory data. Moreover, the longer the experiments run, the more the results of measurements are compliant with the calculations. A certain time is needed for sediment transport to reach the hydrodynamic equilibrium, especially when the suspended load is significant. It is reached after going through the initial stages of bed level changes coupled with simultaneous changes of grain size distribution. The application of the model to field conditions shows its ability to predict sedimentation of dredged channels. Furthermore, the calculations show a good agreement with measured bed grain size distributions of sediment samples taken from the sites within the navigational channel of Łeba Port in Poland.

Patterns and drivers of daily bed-level dynamics on two tidal flats with contrasting wave exposure

Short-term bed-level dynamics has been identified as one of the main factors affecting biota establishment or retreat on tidal flats. However, due to a lack of proper instruments and intensive labour involved, the pattern and drivers of daily bed-level dynamics are largely unexplored in a spatiotemporal context. In this study, 12 newly-developed automatic bed-level sensors were deployed for nearly 15 months on two tidal flats with contrasting wave exposure, proving an unique dataset of daily bed-level changes and hydrodynamic forcing. By analysing the data, we show that (1) a general steepening trend exists on both tidal flats, even with contrasting wave exposure and different bed sediment grain size; (2) daily morphodynamics level increases towards the sea; (3) tidal forcing sets the general morphological evolution pattern at both sites; (4) wave forcing induces short-term bed-level fluctuations at the wave-exposed site, but similar effect is not seen at the sheltered site with smaller waves; (5) storms provoke aggravated erosion, but the impact is conditioned by tidal levels. This study provides insights in the pattern and drivers of daily intertidal bed-level dynamics, thereby setting a template for future high-resolution field monitoring programmes and inviting in-depth morphodynamic modelling for improved understanding and predictive capability.

Effects of seabed morphology on oscillating water column wave energy converters

This paper presents a numerical model to analyse the effects of changes in the bedforms morphology on Oscillating Water Column (OWC) wave energy devices. The model was developed in FLUENT® and based on the Actuator Disk Model theory to simulate the turbine performance. The seabed forms were reproduced with the morphodynamic model XBeach-G for a series of characteristic sea states in Playa Granada (southern Spain). These bedforms were used as input bed geometries in FLUENT® and compared with a hypothetical flat seabed to analyse the effects of changes in bed level on the OWC performance. Results of the simulated sea states reveal the influence of the seabed morphology in the power take–off performance, affecting the relationship between pressure drop and air flow rate through the turbine. Energy dissipation was found to be directly dependent on the bedforms unit volume. This lead to lower mean efficiencies for the cases with evolved morphologies (up to 15%) compared to those obtained for the hypothetical flat cases (19%). The effects of seabed formations on the power take–off performance presented in this paper can be of interest in planning control strategies for OWC devices.

EFFECT OF FUTURE STORM CLUSTERING ON BEACH/DUNE EVOLUTION AND COASTAL FLOODING

Future storm impacts on dune evolution due to changing storm frequency were simulated in XBeach at an exemplary dune system, Formby Point, UK. Probabilistic approaches were used to establish the future storm clusters from 2015 to 2065 in three scenarios, using measured data in Liverpool Bay. Cross-shore profile simulations were carried out in two series: Recovery and Cluster. Recovery used the same initial profile assuming that the profile is fully recovered when the subsequent storm event occurs. Cluster used the modified profile from the previous storm event. Within a single event, the maximum erosion and accretion of the profile occurred under the Recovery conditions due to the presence of a pronounced nearshore ridge-runnel pattern that evolved during severe storm events. Only a few storm events impacted on the upper dune area resulting in a bed level change, which under the Cluster approach was more noticeable when compared with the Recovery approach. The inter-tidal area experienced erosion while the sub-tidal area showed accretion in both the Recovery and the Cluster approaches, and the agreement of bed level change was considerably higher than that in the upper dune area. Vulnerability of the upper dune area increases in the Cluster approach as the initial storm events flatten the nearshore ridge-runnel pattern, and then the severe storm events directly impact on the dune front. High dune elevation at Formby Point prevents lowering of the dune crest due to the storm cluster erosion and therefore it can still withstand against flooding. Spatial modelling of the dune system is required to gain more insights of erosion and flood prone areas along this coast.

The effect of sea level rise on beach morphology of caspian sea coast

Study of beach morphology has been one of the most important issues in coastal engineering research projects. Because of the existence of two important coastal areas located in the north and south parts of the Iran, in the present study an analysis of the coastal zone behaviour is made. Bed level elevations are measured and compared with the theoretical equilibrium profile. It is shown that the behaviour of the coastal zone in the region is consistent with the Dean (1991) equilibrium profile. In the next stage, following extensive investigations, the bed level changes due to arise in sea level at different locations in the surf zone are estimated. The mechanism of beach re-treatment due to a rise in sea level is considered based on the simplified model of Dean (1991) in which the mass balance of the sediments is taken into account. Comparison of the equilibrium profiles for different cases of sea level rise, clearly shows that because of the sediment transport induced by the fluctuation of the water level, the beach profile in the surf zone changes accordingly resulting in an erosion in the inner region of the surf zone and an accumulation of sediments towards the offshore.

In situ measurements of shear stress, erosion and deposition in man-made tidal channels within a tidal saltmarsh

A field study was conducted in man-made ditches in a tidal saltmarsh in Lewes, Delaware, USA. Ditches are prevalent throughout tidal marshes along the Atlantic US coast, and influence hydrodynamics and sediment transport. The field study focused on measuring near-bed velocity, shear stress, sediment concentration, and bed level variability at 5 stations over a 3-week period. Velocities in the ditch (2–5 m wide, 1 m deep) peaked between 0.4 and 0.6 m/s and were slightly ebb dominated. Velocity and shear stress were maximum during a storm event, with peak shear stresses of 2 N/m2. Bed levels were estimated from acoustic amplitude return of a downward-looking velocity profiler. The bed level in the ditch at the landward locations increased ∼ 0.03 m over 3 weeks, while there was ∼ 0.01 m bed level decrease at the most seaward site suggesting a net import of sediment into the channel. At all sites, erosion (∼ 0.005–0.015 m) occurred during the accelerating phase of the flood tide, and accretion of a similar magnitude occurred during the decelerating phase of the ebb tide. This erosion-deposition sequence resulted in small net changes in bed level at the end of each tidal cycle. The intratidal behavior of the bed level was simulated using erosion and deposition flux equations based on shear stress, critical shear stress, and suspended sediment concentration. Erosion was predicted well with RMS errors on the order of 2⋅10−3 m. The bed level during the deposition phase could not be reproduced using the simple approach. Model inaccuracies for deposition were attributed to advection and variations in fall velocity due to flocculation that were not modeled due to lack of ground-truth observations.

Towards better design of riprap bed sills – an experimental study

Flexible bed sills are extensively used in rivers in several applications: to protect pipeline crossings, for slope control and to enhance environmental aspects. Use of riprap provides the flexibility required to adjust to small changes in river bed level and makes this material one of the most commonly used. Shear failure of riprap has been extensively studied, but little knowledge is available on edge failure of flexible bed sills and on the design of the required extent of sills to protect assets. Experimental work in a large test facility was carried out to gain improved understanding of flat sills and sills protruding above the bed. Significantly lower depth and length of the scour hole were observed at the downstream edge of flat riprap sills than at rigid sills. Existing equations for rigid bed sills were found not to be appropriate for estimating scour at the flexible bed sills tested. As expected, protrusion enhances scour potential significantly: it was found that scour depths were of the order of 100% or greater than those associated with flat sills. The research has provided useful evidence for avoiding placement of pipelines straight on to river beds and instead to adopt trenches whenever feasible.

Bed-level changes on intertidal wetland in response to waves and tides: A case study from the Yangtze River Delta

Short-term bed-level variability in tidal wetlands has important implication both for ecology and engineering. In this study, we combined in situ measurements with model simulations to quantify short-term bed-level changes on a meso-macrotidal wetland in the Yangtze River Delta. On the middle flat, we observed erosion during neap-to-mean tides under onshore moderate-to-strong winds, and bed recovery during subsequent spring tides, when winds were both offshore and weaker, suggesting that winds can overturn the neap–spring cyclicity of bed-level changes even on meso–macrotidal mudflats. The magnitude of bed-level changes was smaller on both sides of the middle flat, while the smallest changes occurred on the salt marsh. Observed bed-level changes were reconstructed using a single-point bed-level change model, which incorporates in situ measured parameters of hydrodynamics (waves and currents), suspended sediment concentrations, and bed sediment properties. We conclude that the relative importance of waves and tides in intertidal wetland erosion and accretion can vary temporally (due to changes in balance between wave and tidal energies) and spatially (because of changes in elevation and vegetation in the cross-shore profile). Our study also reflects the advantage of combination of in situ measurement with simulation in detecting short-term variability of tidal flats.

One Dimensional Numerical Simulation of Aggradation-Degradation in a Channel Using Finite Difference Method Case Study Chashma Right Bank Canal (CRBC)

A one-dimensional MacCormack explicit finite difference model is developed for simulating hydraulics and bed changes in irrigation channels. The Saint-Venant equations describing unsteady flow in open channels and the continuity equation for the conservation of sediment mass are numerically solved. These equations are highly nonlinear and therefore do not have analytical solutions. For this purpose the MacCormack scheme is used. The scheme is second order accurate; it is a coupled solution as it is a two-step predictor corrector method. Model gives results in terms of bed level changes, flow depth and discharge provided physical boundaries of the system are valid for simulation time. Model execution and accuracy is very sensitive to time step and stability. The simulated results show a good agreement with previous studies in the downstream section, and can predict an average value of measured profile in the upstream section. The application of this model to Chashma Right Bank Canal in Pakistan and results of the model are compared with the published results gave very convincing result.

Numerical modelling of the erosion and deposition of sand inside a filter layer

This paper treats the numerical modelling of the behaviour of a sand core covered by rocks and exposed to waves. The associated displacement of the rock is also studied. A design that allows for erosion and deposition of the sand core beneath a rock layer in a coastal structure requires an accurate prediction method to assure that the amount of erosion remains within acceptable limits. This work presents a numerical model that is capable of describing the erosion and deposition patterns inside of an open filter of rock on top of sand. The hydraulic loading is that of incident irregular waves and the open filters are surface piercing. Due to the few experimental data sets on sediment transport inside of rock layers, a sediment transport formulation has been proposed based on a matching between the numerical model and experimental data on the profile deformation inside an open filter. The rock layer on top of a sand core introduces a correction term in the Exner equation (the continuity equation for sediment and change in bed level). The correction term originates from the fact that the sand can only be deposited in the pores of the filter material.The numerical model is validated against additional data sets on the erosion and deposition patterns inside of an open filter. A few cases are defined to study the effect of the sinking of the filter into the erosion hole. The numerical model is also applied to several application cases. The response of the core material (sand) to changes in the wave period and wave height is considered. The effect of different layouts of the filter is studied in order to investigate the effect of different filter profiles on the resulting erosion. Finally, it is studied how much the design of a hydraulically closed filter can be relaxed to obtain a reduction in the design requirements of the filter thickness, while the deformation to the sand core remains acceptably small.

3D Layer-Integrated Modelling of Morphodynamic Processes Near River Regulated Structures

Sedimentation and erosion can significantly affect the performance of river regulated reservoirs. In the vicinity of flow control structures, the interaction between the hydrodynamics and sediment transport often induces complex morphological processes. It is generally very challenging to accurately predict these morphological processes in real applications. Details are given of the refinement and application of a three-dimensional (3-D) layer integrated model to predict the morphological processes in a river regulated reservoir. The model employs an Alternating Direction Implicit finite difference algorithm to solve the mass, momentum and suspended sediment transport conservation equations, and an explicit finite difference scheme for the bed sediment mass conservation equation for calculating bed level changes. The model is verified against experimental data reported in the literature. It is then applied to a scaled physical model of a regulated reservoir, including the associated intakes and sluice gates, to predict the velocity distributions, sediment transport rates and bed level changes in the vicinity of the hydraulic structures. It is found that the velocity distribution near an intake is non-uniform, resulting in a reduction in the suspended sediment flux through the intake and the formation of a sedimentation zone inside the reservoir.

The role of grain-size ratio in the mobility of mixed granular beds

The main goal of the study was to understand the effects of grain-size distribution on the stability of beds in the sand-silt range, which is a critical subject for the understanding of geomorphological processes in aquatic environments. Although theoretical models can explain the mobilization of a mixed bed, there is a clear lack in knowledge regarding the stabilizing effect of non-cohesive fine material. To connect existing findings, we analysed bed stability in relation to grain-size distribution in laboratory experiments. Erosion experiments in an annular flume were conducted using beds of different size compositions of spherical glass beads, i.e. a) the grain-size ratio RD=D50,coarse/D50,fine (the relative size of coarse and fine grains; D50=39–367μm) and b) the amount of fines. Several glass-bead combinations with unimodal and bimodal grain-size distributions (RD=3.9, 5.8, and 9.4) and varying fine fractions (10–40% dry weight) were subjected to increasing flow speeds (0.01–0.19ms−1). Using acoustic Doppler velocimetry (ADV) and optical backscatter, the flow profile in the vicinity of the bed surface, the changes in bed morphology, and the suspended sediment concentration (SSC) were measured. A new method was developed to evaluate the bed-level changes detected by the ADV as a proxy for the bed mobility. We found different modes of bed mobility depending on the grain-size ratio. For low grain-size ratios, an increase in the fine fraction (to 40%) led to increased bed-level changes during the experiment and the mobilization of the mixed bed at the highest flow speed. For high ratios an increase in fine fraction (to 40%) led to a decrease of bed-level changes and the beds remained stable, i.e. no bed forms developed even at the highest flow speed. Therefore, increasing the amount of fine particles can lead to different modes of behaviour depending on the grain-size ratio. For a bimodal sediment bed with spherical grains under unidirectional flow conditions, the grain-size ratio can be used to estimate the bed mobility, i.e. to evaluate if a bed is likely to mobilize at flow speeds of up to 0.19ms−1. Hence, for the stability of a non-cohesive, mixed bed in the sand-silt range, the grain-size ratio plays a fundamental role, while the amount of fine grains is a secondary factor.