Bed Level Changes Research Articles

Sediment management in the Dutch Rhine branches

A brief review of historic normalisation and sediment management in the Dutch Rhine branches is given. Aiming for maintaining navigability and safety against flooding, this river engineering has triggered large‐scale bed level developments that affect for example the use and maintenance of the fairway. The observed trend of bed level changes is explained using simplified theoretical equilibrium approaches. With numerical simulations, future developments are predicted and potential measures are defined to prevent bottle necks in the fairway. Finally a strategy is recommended to develop a sediment management that aims for an economic and sustainable fairway in the Dutch Rhine branches.

Measurement of wave-by-wave bed-levels in the swash zone

A technique is described to observe and quantify wave-by-wave bed-level changes in the swash zone. The ultrasonic instrument system is non-contact with the beach face surface being measured and the sensors remain outside of the fluid flows causing sediment movement. Sensor resolution combined with the electronic noise inherent within a digital network data-logging system results in a (conservative) measurement accuracy of ± 1 mm, equating to a couple of sand grain diameters in height. Illustrative field results demonstrate the practical use of the instrumentation, and a simple data pre-processing method to separate swashes and intervening bed-level ‘events’ is discussed. These example data reveal rather complex fluctuations of the bed observed over time periods of minutes to hours. Rather strikingly, gross bed-level changes per wave are revealed to be up to many orders of magnitude larger than the observed net rate of beach face evolution. It is outlined how observations of successive bed-level changes at multiple locations within a dense grid, combined with a consideration of sediment continuity, will now enable the total net sediment transported per uprush–backwash to be quantified.

The role of Coriolis in sandbank formation due to a headland/island system

Tidal flow past a headland was modelled and the resulting sandbank formation due to bedload transport by eddy systems was examined with particular emphasis on the role of Coriolis. The headland configuration was extended to the more complex case of a headland with a detached island and the role of Coriolis in sandbank formation was again examined. In the headland case, the contribution of Coriolis resulted in the expected change in bed level at the centre of the residual eddies (positive contribution for the cyclonic eddy and negative change for the anticyclonic eddy). In the headland/island case, the role of Coriolis in the resulting bed level change was a function of eddy shedding from the island, defined by the island wake parameter. When eddy shedding occurred, the inclusion of Coriolis had a large magnitude effect on creating flood/ebb sandbank asymmetry. When eddies remained attached to the island, Coriolis had a minor role in the magnitude of this asymmetry.

A model of inter-annual variability in beach levels

Beach profile data, collected twice per year at 19 stations over a 25 km length of coastline in Tremadoc Bay, have been analysed to quantify the inter-annual variability in beach levels over a 7 year period and the results compared against the output of a numerical model. Using hourly wind data as forcing, the morphological development of northern Tremadoc Bay was simulated by wave, tidal, longshore transport, total transport and bed level change models. The modelling methodology was efficient and innovative, allowing realistic simulations of long duration with a time step of 1 h, hence capturing the high frequency nature of wind events. The model was run for each of the 7 autumn/winter periods (generally November–April) and the modelled net change in beach levels compared with the data from all 19 stations. The model results had reasonable agreement with the beach profile surveys. However, the observed magnitude of bed level change in the bay lagged the model output by 1 year, indicating that sediment processes acting over a larger area are important in a relatively localised study of inter-annual variability.

Cross-shore sediment transport and morphological response on a macrotidal beach with intertidal bar morphology, Truc Vert, France

A 3-week field investigation was conducted on a macrotidal beach with intertidal bar morphology with the aim of quantifying cross-shore sediment transport rates and beach response. Cross-shore transport rates were estimated in four different ways: (1) net transport from bed-level changes; (2) measured suspended sediment fluxes; (3) sediment fluxes associated with migrating wave ripples; and (4) total load transport predicted using the Bailard [Bailard, J.A. 1981. An energetics total load sediment transport model for a plane sloping beach. Journal of Geophysical Research, 86, 10938–10954.] model. Excellent qualitative agreement was found between the different measures under all recorded wave conditions, with onshore (offshore) sediment transport and beach accretion (erosion) observed under low-to- moderate (high) wave conditions. The agreement is due to the strong correspondence between the direction of mean cross-shore flows across the intertidal bar surface and that of the sediment transport. Significant onshore mean flows ( u = 0.1–0.2 m s − 1 ) were persistently observed under low wave conditions, and these were strongest in shallow water depths ( h < 1 m). These flows are attributed to the (weak) three-dimensional water circulation in the surf zone associated with the intertidal bar-rip morphology and caused onshore bar migration. Under high wave conditions, on the other hand, strong mean cross-shore flows ( u = 0.2–0.4 m s − 1 ), due to the bed return flow, were consistently directed offshore and resulted in bar destruction. Under low wave conditions, vertically-integrated suspended fluxes, ripple transport rates and predicted total transport fluxes were remarkably similar, but when the fluxes are integrated over a complete tidal cycle, they are considerably smaller than that derived from bed-level changes based on low tide surveys.

Three-dimensional numerical modeling of reservoir sediment release

For the computation of flow field and bed evolution in a water reservoir during the flushing process a fully three-dimensional hydrodynamic model, using a finite volume method to solve the Reynolds averaged Navier–Stokes equations, has been developed and combined with a three-dimensional sediment transport model. The hydrodynamic model is based the equations of mass and momentum conservation along with a standard k–e turbulence closure model. The sediment transport model is based on the equation of convection/diffusion of sediment concentration and sediment continuity equation for calculating the sediment concentration and bed level change in the reservoir flushing process, respectively. Both the hydrodynamic and sediment transport models are developed in a boundary-fitted curvilinear coordinate system. The grid is adaptive in the vertical direction, and changes according to the calculated bed level. The hydrodynamic section of the model was verified using experimental and direct numerical simulations data, and the sediment concentration calculations compare well with the experimental results. Also a physical model study was carried out to verify the results of bed evolution at the upstream of a sluice gate. Good agreement is found between bed evolution in the numerical and physical models.

The effect of bedform dynamics on computing suspended sediment fluxes using optical backscatter sensors and current meters

The sensitivity of the suspended sediment flux is tested with respect to rapid changes in bed-level across the surf zone of a sandy beach. The suspended flux was computed using a fixed instrument array, but bed-level changes due to ripple migration caused the instrument elevations to be significantly changed during the course of the experiment. The nominal elevations of the instruments were adjusted during data processing (using the MOBS array) to maintain a fixed elevation with respect to bed-level changes. The resultant suspended sediment concentrations and fluxes were significantly different from the unadjusted data, and for the present data set O(35%) less when averaged over the tide. The maximum difference between adjusted and unadjusted fluxes may be O(260%). The results indicate that changes in bed-level, particularly those due to bedform migration, must be accounted for when processing OBS data if reliable estimates of suspended sediment transport are to be obtained in the field.

A numerical scheme for morphological bed level calculations

The typical equation for bed level change in sediment transport in river, estuary and near shore systems is based on conservation of sediment mass. It is generally a nonlinear conservation equation for bed level. The physics here are similar to shallow water wave equations and gas dynamics equation which will develop shock waves in many circumstances. Many state-of-art morphological models use classical lower order Lax–Wendroff or modified Lax–Wendroff schemes for morphology which are not very stable for long time sediment transport processes simulation. Filtering or artificial diffusion are often added to achieve stability. In this paper, several shock capturing schemes are discussed for simulating bed level change with different accuracy and stability behaviors. The conclusion is in favor of a fifth order Euler-WENO scheme which is introduced to sediment transport simulations here over other schemes. The Euler-WENO scheme is shown to have significant advantages over schemes with artificial viscosity and filtering processes, hence is highly recommended especially for phase-resolving sediment transport models.

Optimising flow–sediment transport parameters for rivers

The unsteady flow–sediment transport equations for a real river–reservoir system contain unknown hydraulic, sediment and geometric parameters. Knowledge of these parameters and input data comprising initial and boundary conditions is required to simulate flow–sediment transport, water surface and river bed profiles. The sensitivity analysis and optimisation procedure, which involves determining these parameters by fitting a model with real river field data, is an important tool for finding the best-fit values of each parameter in a river–reservoir sediment transport model. Sensitivity analysis and optimisation methods have been found to be very useful when applied to sediment routeing problems in river–reservoir systems. This technique was employed for optimisation of some of the important parameters involved in graded sediment routeing and bed armouring processes using a non-linear coupled model (NCMG). The particular parameters studied in this investigation are degradation, bed armouring and grain size distribution of bed material in a river bed downstream of a dam. The effects of bed roughness, sediment parameters and thickness of the active layer on bed level changes are studied. A combined Gauss–Newton and modified Newton method are employed to calculate optimised parameters.

In situ estimation of erosion and deposition thresholds by Acoustic Doppler Velocimeter (ADV)

Field-based estimations of bed shear stress have been made using SonTek/YSI 10 MHz ADVs ( Acoustic Doppler Velocimeter) at the Kongsmark mudflat, Danish Wadden Sea, in order to test if it was possible to estimate erosion and deposition thresholds in situ by use of unidirectional tidal and orbital (wave) currents. The results were promising and erosion thresholds were in the same range as those observed in EROMES erosion experiments carried out at the same site. Similarly, the short-term erosion rates which could be calculated matched closely those obtained with EROMES, and were in the same range as those published for a large annular flume (Sea Carousel) from another fine-grained site. This indicates that the erosion rates obtained with those two erosion-instruments are reasonable estimates of the actual erosion taking place under natural tidal current and waves. One advantage of the use of ADVs to characterize sediment erodibility is that measurements can be carried out for long periods, which makes it possible to examine the temporal variation of erosion threshold, erosion rate and sedimentation. The maximum bed shear stresses, which was observed during a strong storm in January 2005, was only 0.7 N m −2 which is three to four times lower than the highest thresholds measured by EROMES experiments on sediment with biofilms during previous studies. This shows that such strong biofilms at this site will not get eroded unless degraded or physically disrupted by some other mechanism. A prerequisite for the determination of any of the erodibility and sedimentation parameters is that the actual bed level changes at the measuring site are known in detail. This information is obtained automatically by the ADVs used in the present study. Accurate estimates of the bed shear stress under waves in very shallow water are difficult to obtain and, in the present study, a simple method based on instantaneous velocities and assumption of a logarithmic velocity profile was used. Further field-studies are required of the boundary layer under these extreme conditions in order to refine this simple method of calculating bed shear stresses.

Sediment Dynamics in the Meghna Estuary, Bangladesh: A Model Study

A depth integrated two-dimensional numerical modeling was carried out to study the sediment dynamics within the Meghna estuary. The sediment–water dynamics within this estuary are very complex due to its irregular shape, wide seasonal variation, and the changing role of the tide. Both cohesive and noncohesive sediment transport formulations were used to estimate the total transport. An interactive morphological computation was also used to verify the bed level changes over 2years. Sediment transports of both monsoon and dry seasons (the two most hydrologically pronounced periods in this region) were modeled, and a large seasonal variation in sediment transport pattern was observed. Land reclamation dams were tested by the model and found to be effective in enhancing the accretion in its vicinity.

Unified View of Sediment Transport by Currents and Waves. IV: Application of Morphodynamic Model

The TR2004 transport formulations for clay, silt, and sand as proposed in Parts 1 and 2 have been implemented in morphodynamic models to predict bed level changes. These models have been verified using various laboratory and field data cases concerning coastal flow in offshore and near-shore zones. Furthermore, the model has been applied to two complicated sediment environments concerning the flow around a spurdike in a river and the tidal flow of cohesive sediments in the Yangtze Estuary in China. Overall, it is concluded that the morphodynamic models using default settings performs reasonably well. The applied scaling factors of the sediment transport model are in the generally accepted range of 0.5–2.

Sedimentary processes over an intertidal flat: A field investigation at Hythe flats, Southampton Water (UK)

The sedimentary processes acting over an intertidal flat (Hythe, UK) located in Southampton Water, Hampshire, UK are investigated on the basis of hydrodynamic, morphological and sediment transport field measurements. Bed level changes, patterns of suspended sediment transport and shell movement over the tidal flats were examined in order to identify sediment transport pathways and salt marsh–mudflat morphological changes. Overall, sediment transport patterns at Hythe can be described in terms of suspended sediment and bedload-shell transport. The hydrodynamic complexity of the tidal flow over the flats determines the patterns of suspended sediments transport. During the young flood phase, tidal currents present a SW orientation (shoreward). About 1 h into the flood, a change occurs and the currents start flowing in a SE direction (estuary mouthward). This veering of the tidal current controls the hydrodynamic pattern over the mudflat inducing sediment transport towards the estuary mouth. Conversely, shell tracer experiment showed a net landward movement. Shells are transported over the mudflat, deposited at the base of the cliff, and eventually transported over the salt marsh where they form extensive chernier deposits. Shell transport rates can reach up to 0.7 m/tide over the upper mudflat. Shell movement over the salt marsh (on cheniers) occurred only during storms (up to 3 m in a storm event). Shell-induced erosion can become an important parameter on morphological changes of a coastal area. Shell transport and deposition over the salt marsh surface can contribute to vegetation deterioration, as shell deposits migrate over the marsh surface. As a result, the seaward limit of the salt marsh showed deteriorated vegetation, which leads to scouring and consequent surface lowering. In terms of bed level changes, no seasonal pattern was observed. A relationship between mudflat bed erosion and cliff retreat is suggested. During periods of high cliff erosion there was a tendency for the mudflats to be stable and, conversely, mudflat bed erosion was greatest when cliff retreat rates were lowest. Hence, morphological characteristics of Hythe intertidal area appear to be controlled by a combination of prevailing hydrodynamic conditions and shell transport over the salt marsh.

Simple analytical results for bedload transport due to tides

Abstract Patterns of sediment erosion and deposition arising from tide induced bedload transport are explored using standard bedload formulae and a simple analytic approach. Decomposing the current into a principal M 2 tidal constituent, an M 4 first harmonic and a tidal residual, it is straightforward to derive an expression for the contribution each makes to the integrated tidal flux. The resulting expressions are applied to calculate the net change in bed level induced by bedload transport due to a superposition of a Kelvin tidal wave, a harmonic overtide and a residual tide.

STUDY ON NUMERICAL SIMULATION AND FACTOR ANALYSIS OF FLUSHING RIVER MOUTH BAR

A 2D River morphology model with a curvilinear grid is developed to simulate the flushing river mouth bar occurred at the Ara River in July 2004. In the hydrodynamic model, the 2D free surface flow is calculated by the Saint Venant equations which are solved by an implicit finite difference technique and the ADI method. In the morphological model, the Van Rijin's model is used to evaluate sediment transport including bed load and suspended load. The helical flow and the transverse bed slope modify the bed load sediment transport. The bed level change is computed by a sediment continuity equation of the bank erosion model. The model is validated by comparison with the observations of water level variations and the riverbed topography at the mouth. A factor analysis is conducted to evaluate the effects of discharge, tide level, and grain size. The result shows that differences of sea elevation and peak discharge have significant effects on a process of morphological change at the river mouth.

Wind influence on tidal flat sediment dynamics: Field investigations in the Ho Bugt, Danish Wadden Sea

Sediment dynamics on a mixed microtidal flat in a sheltered backbarrier position are shown to vary strongly on short term, tidal, and monthly timescales. Bed level changes were ± 1 mm per tidal period in the absence of winds. Erosion of 10 to 20 mm within a few hours was possible during high combined current and wave shear stress. Mean grain size in the upper 1 cm bed sediment varied from 34 to 155 μm and was consistently coarser than the mean size of trapped suspended matter (7–10 μm). Threshold for resuspension was low at 0.03 N m − 2 . No clear correlation was found between resuspension activity and suspended matter concentrations as these concentrations also depend on current induced advection. Due to tidal asymmetry, the net suspended sediment flux was flood directed. On average it was 14 kg m −2 per tidal period in the summer and 3 times higher during November when wave activity generally was stronger. Bed micro topography, was followed on digital photos taken every 1.5 h during the two month study period, showed that (1) ripples started to develop when wave shear stress exceeded 0.15 N m − 2 , and (2) their wavelength varied with shear stress.

Trade-wind waves and mud dynamics on the French Guiana coast, South America: Input from ERA-40 wave data and field investigations

The South American coast between Brazil and Venezuela is affected by longshore migrating mud banks derived from the fine-grained Amazon sediment discharge. Onshore mud migration prevails over shallow ‘bank’ areas alternating alongshore with deeper ‘inter-bank’ areas. The transport on the inner shelf, and attachment to the shoreline, of this migrating mud has been attributed mainly to wind waves. However, the lack of in situ data on waves hampers understanding of the relationship between waves and mud dynamics. A 44-yr record (1960–2004) of the ERA-40 wave dataset generated by the European Centre for Medium-Range Weather Forecasts (ECMWF) was used, in conjunction with field investigations in French Guiana, to define both event-scale and longer-term patterns of mud mobilisation induced by waves. The ratio H 0 3 / T 2, combining wave height H and period T, and the angle of wave incidence α, were singled out as the most relevant parameters for describing wave forcing. Typical ‘bank’ and ‘inter-bank’ profiles and corresponding mud densities, and a 3-month record of changes in the thickness of the fluid mud layer in an estuarine navigation channel were monitored by echo-sounding from October 2002 to January 2003. An 80-day record of bed-level changes in the intertidal zone was obtained from August to November 2004 using a pressure transducer. The results on the wave regime of French Guiana confirm a distinctly seasonal pattern, and highlight an increase in H 0 3 / T 2 over the 44-yr period related to an increase in trade-wind velocities determined from corresponding trends in Atlantic wind pseudo-stress off the South American coast. Wave forcing over bank areas leads to the liquefaction of a 1–3 m-thick layer of mud that is transported onshore (and alongshore by the longshore component of wave energy). The episodic nature of high wave energy events generally results in the formation of mud bar features from the shoreward mobilisation of gel-like fluid mud. The effect of waves on mud is particularly marked following long periods of low energy, and especially at the onset of the high wave energy season (October to May), when even moderate wave energy events can lead to significant mobilisation of mud. Significant phases of increased wave energy are attended by higher long-term (annual) rates of longshore mud bank migration but the correlation is rather poor between the wave forcing parameter H 0 3 / T 2 and migration rates because stronger wave forcing is generally associated with low angles of wave incidence. This suggests a complementary role of other hydrodynamic mechanisms, such as geostrophic and tidal currents, in longshore mud bank migration.

Modelling sand–mud morphodynamics in the Friesche Zeegat

The objective of the study presented in this paper is to investigate the predictive capabilities of a process-based sand–mud model in a quantitative way. This recently developed sand–mud model bridges the gap between noncohesive sand models and cohesive mud models. It explicitly takes into account the interaction between these two sediment fractions and temporal and spatial bed composition changes in the sediment bed [Van Ledden (2002) 5:577–594, Van Ledden et al. (2004a) 24:1–11, Van Ledden et al. (2004b) 54:385–391]. The application of this model to idealized situations has demonstrated a good qualitative agreement between observed and computed bed levels and bed composition developments. However, in real-life situations, a realistic quantitative prediction of the magnitude and timescale of this response is important to assess the short-term and long-term impacts of human interventions and/or natural changes. For this purpose, the Friesche Zeegat in the Wadden Sea (the Netherlands) is used as a reference to hindcast the morphological response in the period 1970–1994. Due to the closure of the Lauwerszee in 1969, the tidal prism of this tidal basin was reduced by about 30%. Significant changes in the bed level and bed composition have occurred in the decades following the closure to adjust to the new hydrodynamic conditions. We modeled the long-term bed level and bed composition development in the Friesche Zeegat in the period 1970–1994 starting with the geometry of 1970 by using a research version of Delft3D, which incorporates the sand–mud formulations proposed by [Van Ledden (2002) 5:577–594].The computed total net deposition in the tidal basin in the period 1970–1994 agrees well with the observations, but the observed decrease of the import rate with time is not predicted. The model predicts net deposition in the deeper parts and at the intertidal area in the basin and net erosion in between, which resembles the observations qualitatively. Furthermore, the computed distribution of sand and mud in the basin of the Friesche Zeegat appears to be realistic. Analysis of the results shows that the absence of the decreasing import rate in the basin is caused by a poor quantitative prediction of the changes in the hypsometry of the basin. Because of this, the computed velocity asymmetry in the main channel tends toward flood dominance, whereas the observations indicate that the system is ebb-dominant in 1992. Although the sand–mud model needs to be further improved and verified, the results presented in this paper indicate that the model can be applied as a first step to estimate the effects of human interventions on the large-scale bed level and bed composition changes in tidal systems with sand and mud.

Long-term and high-resolution measurements of bed level changes in a temperate, microtidal coastal lagoon

This study presents the results of a long-term monitoring program of bed level changes measured during 8 yr at an intertidal mudflat in a microtidal, temperate coastal lagoon. Additionally, bed level measurements obtained at a 10-min temporal resolution at the same tidal flat and at the bed of a nearby tidal channel are presented. Short-term changes in bed level are one or two orders of magnitude larger than the annual net-deposition rate, which shows that the environment is highly dynamic with respect to erosion, transport and deposition of fine-grained sediment. Some seasonality in the bed level changes was observed and there is a tendency for mudflat deposition in spring, summer and early autumn and erosion during the rest of the year, but interannual variations are large and different parts of the mudflat show different seasonal signals. A close coupling between sub- and intertidal deposition and erosion was observed. The time-series showed that some of the material eroded from the mudflat was not exported to the open sea, but instead temporarily deposited in a nearby shallow tidal channel and later returned to the mudflat during calmer weather conditions. These findings support previously published hypothesis and results of modelling studies. Based on the observed abundance of fine-grained sediment at the study sites and the high accretion rates generally found on fine-grained tidal flats in the Danish Wadden Sea area, it is argued that these fine-grained tidal flats are not seriously threatened by the expected sea level rise in the 21st century.

Hydrodynamic and morphodynamic response of isolated and multiple low crested structures: Experiments and simulations

Low crested structures for beach defence purposes are frequently configured in the form of cells. This paper investigates the hydrodynamic and morphological response of such defence schemes by means of laboratory experiments and numerical simulations. Experiments were carried out at the large mobile-bed basin of the “Laboratorio di Idraulica Costiera” of the “Politecnico di Bari”, IT. Tests peculiarity is the simultaneous simulation of three different types of lateral confinement: a channel, representative of an indefinitely long structure, and two areas with narrow and wide gaps, representative of a common scheme and of an almost isolated structure respectively. Submerged, zero freeboard and emerged conditions were tested. A typical storm of the Northern Adriatic sea was reproduced by 6 wave attacks. Water level and currents were measured in front and behind the structures, at gaps and roundheads. Results, focusing on rip current intensities and piling-up, are presented and discussed. An extensive plan view of wave intensities and flow patterns inside the basin is provided through numerical simulations with MIKE 21 numerical suite. The sand bed was surveyed along 30 profiles covering the narrow gap area and the isolated structure roundheads. For the submerged conditions tests, 15 of these profiles and the shoreline position were monitored after each step of the simulated storm. Average and maximum values of local erosion at the structure toe are presented. Time evolution of erosion at gaps is related to the main hydrodynamic variables through Bijker sediment transport formula. Evaluation of volume changes in the protected area shows that sand is in average trapped behind the barriers, being transported offshore from gaps and returning inshore over the crest. The observed and reconstructed transport mechanism is a combination of bed load and suspended transport, the latter being slightly more important. Bed level changes are well predicted by morphodynamic simulations performed with MIKE 21 CAMS.