Sediment Fall Velocity Research Articles

SEDIMENT TRANSPORT IN THE NIBUTANI DAM RESERVOIR AT 2003 FLOOD LF THE SARU RIVER

Sedimentation in dam reservoirs is a serious matter of great concern with river management works. In general, sediment deposited in the reservoir is hard to move comparing with river bed materials, even if the diamete is equal. Therefore, hydraulic experiments were conducted on the fall velocity of sediment and the entrainment rate of suspended sediment using the material which is deposited in the dam reservoir. It is found that the fall velocity of sediment and the entrainment rate of suspended sediment are smallre then the value currently used conventionally. These results are applied to calculate bed variation of the 2003 flood in the Nibutani Dam resevoir. The calculated results mostly agree with field surveys.

A Simple Universal Equation for Grain Settling Velocity

Abstract A new equation is presented for sediment fall velocity as a function of grain diameter for given values of fluid viscosity and fluid and solid density. Sediment fall velocity is a fundamental parameter in the modeling and interpretation of fluviatile and coastal deposition. The equation applies to the entire range of viscous to turbulent conditions, and its simple explicit form makes it easy to use in computer models and other applications in sedimentology, geomorphology, and engineering. The equation is derived from dimensional analysis and converges on Stokes' law for small grains and a constant drag coefficient for large grains. Its two physically interpretable parameters are easily adjusted for shape effects or for the use of sieve diameter rather than nominal grain diameter. It gives a close fit to published and new experimental data for both quartz sand and low-density materials, with no more error than previous equations of more complicated form.

Three-dimensional hydrodynamic model, study cases for quarter annular and idealized ship channel problems

An extended version of the three-dimensional hydrodynamic model, ADCIRC 3D-DSS, was utilized to simulate both horizontal and vertical flows in a (quarter) annular harbor (QATP and ATP) and rectangular basin with an idealized ship channel (RBSC). Comparison of horizontal and vertical solutions to the analytical solution and results of other researchers are in good agreement. The vertical velocity solution is highly sensitive to the horizontal velocity solutions. The presence of the sidewall boundary may also affect the vertical solutions. Around the sloping bank of RBSC channel with one-third gradient, the vertical velocity becomes important. The maximum vertical velocity approaches ±50% of the sediment fall velocity of fine sand.

Sand suspension, storage, advection, and settling in surf and swash zones

A time‐dependent cross‐shore sediment transport model in the surf and swash zones on beaches is developed to predict both beach accretion and erosion under the assumptions of alongshore uniformity and normally incident waves. The model is based on the depth‐integrated sediment continuity equation, which includes sediment suspension by turbulence generated by wave breaking and bottom friction, sediment storage in the entire water column, sediment advection by waves and wave‐induced return current, and sediment settling on the movable bottom. The hydrodynamic input required for this sediment transport model is predicted using the finite‐amplitude shallow‐water equations including bottom friction. The developed model is compared with three large‐scale laboratory tests with accretional, neutral (little), and erosional beach profile changes under regular waves. The model predicts sediment suspension under the steep front of breaking waves and due to bottom friction in the swash zone. The computed depth‐averaged sediment concentration does not respond to local sediment suspension instantaneously because of the sediment storage and advection. The mean sediment concentration becomes large in comparison to the oscillatory concentration with the decrease of the normalized sediment fall velocity. The net cross‐shore sediment transport rate is shown to be the small difference between the onshore transport rate due to the positively correlated oscillatory components of the suspended sediment volume per unit area and the horizontal sediment velocity and the offshore transport rate due to the product of the mean suspended sediment volume and the mean horizontal sediment velocity. Relatedly, the net accretion or erosion rate of the movable bottom is determined by the small difference between the mean sediment settling rate and the mean suspension rate caused by wave breaking and bottom friction. The present computation is limited to the initial beach profile change, but the numerical model is capable of predicting the accretional, erosional, and neutral profile changes.

Seasonal changes in beach morphology along the sheltered coastline of Perth, Western Australia

Abstract Seasonal change in beach morphology is traditionally ascribed to a variation in the incident wave energy level with calm conditions in summer resulting in wide beaches with pronounced subaerial berms and energetic conditions in winter causing narrow beaches with nearshore bar morphology. The coastline of Perth, Western Australia, is characterised by a large seasonal variation in the incident wave height and local beaches exhibit a distinct seasonal change in morphology. However, these morphological changes are better explained by a seasonal reversal in the littoral drift direction than by variations in the incident wave energy conditions. In summer, when northward sediment transport prevails due to sea breeze activity, beaches located south of coastal structures, headlands or rocky outcrops become wider due to the accumulation of sediment against the obstacle. These beaches will subsequently erode in winter during storms when the longshore sediment transport is toward the south. In contrast, beaches located north of obstacles will become narrower during summer and wider during winter. The usefulness of the dimensionless fall velocity Ω=H b /w s T (where Hb is the breaker height, ws is the sediment fall velocity and T is the wave period) as a predictor of presence/absence of bar morphology and beach type was investigated. It was found that Ω fluctuates around the threshold of bar formation (Ω≈1.5–2) over a variety of time scales (daily, weekly, and seasonally). These temporal variations in Ω in conjunction with the relatively low wave energy level that characterises the coast negates the development of beach and nearshore morphology that is in equilibrium with the hydrodynamic conditions. As a result, bar occurrence and beach type can not be readily predicted using Ω along the Perth coast.

On the spacing between observed rip currents

An extensive data set of observed rip currents and associated wave parameters collected on Narrabeen Beach, Australia, is re-examined to determine how rip current spacing is related to incident wave conditions. Rip spacing is found to be better predicted by a simulation of surf zone width, based upon observed wave height and a model for beach profile, rather than the visually estimated surf zone width. This leads to a predictive equation for rip spacing which is proportional to H / 3 2 /w 2 , where H is the breaker height and w the sediment fall velocity. Suprisingly, the ratio of rip spacing to surf zone width appears to be relatively insensitive to incident wave period, though there is slight evidence that it increases with increasing period. Possible reasons for the scatter in the rip spacing to surf zone width ratio are discussed.

Prediction of Storm/Normal Beach Profiles

Using the results of large‐scale wave tank tests of monochromatic waves breaking on sandy beaches, Larson and Kraus have shown that storm (barred) and normal (nonbarred) equilibrium beach profiles can be segregated in terms of two‐dimensionless parameters, which involve wave and sediment characteristics. Here, by rearranging their results, a single dimensionless parameter is developed that predicts the occurrence of storm or normal profiles. The profile parameter (P=gH02/(w3T)) uses deep‐water wave characteristics to distinguish the two profile types. Using shallow‐water data the shallow‐water Dean number, Hb/(wT) can serve the same purpose. Further, a Froude number representation of the sediment fall velocity, w2/(gH0), is shown to be an important parameter for equilibrium profiles.

Morphodynamic variability of surf zones and beaches: A synthesis

A synthesis of some results obtained over the period 1979–1982 from a study of beach and surf zone dynamics is presented. The paper deals with the different natural beach states, the process signatures associated with these states, environmental controls on modal beach state, and the temporal variability of beach state and beach profiles. Hydrodynamic processes and the relative contributions of different mechanisms to sediment transport and morphologic change differ dramatically as functions of beach state, that is depending on whether the surf zone and beach are reflective, dissipative or in one of several intermediate states. Depending on beach state, near bottom currents show variations in the relative dominance of motions due to: incident waves, subharmonic oscillations, infragravity oscillations, and mean longshore and rip currents. On reflective beaches, incident waves and subharmonic edge waves are dominant. In highly dissipative surf zones, shoreward decay of incident waves is accompanied by shoreward growth of infragravity energy; in the inner surf zone, currents associated with infragravity standing waves dominate. On intermediate states with pronounced bar-trough (straight or crescentic) topographies, incident wave orbital velocities are generally dominant but significant roles are also played by subharmonic and infragravity standing waves, longshore currents, and rips. The strongest rips and associated feeder currents occur in association with intermediate transverse bar and rip topographies. Long-term consecutive surveys of different beaches with contrasting local environmental conditions provide the data sets for empirical—statistical assessment of beach mobility, direction of change and response to environmental conditions. Conditions of persistently high wave energy combined with abundant and/or fine grained sediment results in maintaining highly dissipative states which exhibit very low mobility. Relatively low mobility is also associated with persistently low-steepness waves acting on coarsegrained beach sediments. In such cases, the modal beach state is reflective. The greatest degree of mobility is associated with intermediate but highly changeable wave conditions, medium grained sediment and a modest or meager sediment supply. Under such conditions, the beach and surf zone tend to alternate among the intermediate states and to exhibit well-developed bar trough and rhythmic topographies. A good association is found between beach state and the environmental parameter Ω = H b ( w ̄ sT ) where H b is breaker height, w ̄ s is mean sediment fall velocity and T is wave period. Temporal variability of beach state reflects, in part, the temporal variability and rate of change of Ω, which, in turn depends on deep-water wave climate and nearshore wave modifications.

A SIMPLIFIED MODEL FOR LONGSHORE SEDIMENT TRANSPORT

An energetics-based longshore sediment transport model is developed which takes the form of a modification to the wave power equation. Instead of being constant, the wave power coefficient is a function of the breaker angle and the ratio of the orbital velocity magnitude and the sediment fall velocity. This modification extends the range of application of the wave power equation to include both field and laboratory conditions.

BEACH AND SURF ZONE EQUILIBRIA AND RESPONSE TIMES

Analyses were performed on a 6% year time series of daily wave data, daily beach state data and monthly beach and surf zone profile data. Beach state changes, which involve the relatively rapid redistribution of sediment already stored locally, are predictable in terms of Dean's (3) simple parameter A - H, /(w T) where H, is breaker height, w is sediment fall velocity and T is wave period. Each of the six beach states has a different equilibrium range of S2 values and the direction of change (erosion or accretion) depends on the departure from the equilibrium association. Empirical eigenvector analyses performed on the profile data permitted separation of different response components. The lower order vectors expressing the grosser aspects of the profile features such as beach volume and surf zone gradient displayed maximum variance at periods in excess of 2 years whereas much shorter response times characterized the higher order components such as bar-trough shapes and asymmetries. We infer that the fast response, and more predictable, components of beach and surf zone change largely involve smaller scale sediment exchanges between the beach and surf zone whereas the slower responses are related to larger scale exchanges between the surf zone and the inner continental shelf.

Fall Velocity of Sediment in High Concentrations

Fall Velocity of Sediment in High Concentrations

Understanding Beach Erosion and Accretion

Improved understanding of beach erosion and accretion has been attained by reanalyzing large-scale, two-dimensional monochromatic laboratory data, using the data to develop an empirical beach volume change model, and systematically studying the influence of each parameter on predicted beach volume changes. The sediment fall velocity is predicted to be especially important in determining the amount and direction of beach volume change.

NUMERICAL MODELLING OF SUSPENDED SEDIMENT

The present paper describes the application of a two-dimensional numerical suspended sediment model to problems having analytical solutions, as well as to laboratory and field situations. The model is based upon an implicit finite-difference solution to a two-* dimensional (longitudinal and vertical) diffusion-advection equation for suspended sediment transport. Horizontal eddy diffusion is neglected in comparison with vertical diffusion and vertical water motion is assumed negligible in comparison with the sediment fall velocity. The various applications indicate that the greatest errors in the model are due to large spatial concentration gradients and that errors can be controlled by a suitable choice of space and time step. In addition, it is considered that the model has great flexibility and seems to have an acceptable level of accuracy, at least in the field situations tested, provided the physical parameters of the model can also be determined accurately.