Self-organization mechanisms for the formation on nearshore crescentic and transverse sand bars

The formation and development of transverse and crescentic sand bars in the coastal marine environment has been investigated by means of a nonlinear numerical model based on the shallow-water equations and on a simpli ed sediment transport parameterization. By assuming normally approaching waves and a saturated surf zone, rhythmic patterns develop from a planar slope where random perturbations of small amplitude have been superimposed. Two types of bedforms appear: one is a crescentic bar pattern centred around the breakpoint and the other, herein modelled for the rst time, is a transverse bar pattern. The feedback mechanism related to the formation and development of the patterns can be explained by coupling the water and sediment conservation equations. Basically, the waves stir up the sediment and keep it in suspension with a certain cross-shore distribution of depth-averaged concentration. Then, a current flowing with (against) the gradient of sediment concentration produces erosion (deposition). It is shown that inside the surf zone, these currents may occur due to the wave refraction and to the redistribution of wave breaking produced by the growing bedforms. Numerical simulations have been performed in order to understand the sensitivity of the pattern formation to the parameterization and to relate the hydro-morphodynamic input conditions to which of the patterns develops. It is suggested that crescentic bar growth would be favoured by high-energy conditions and ne sediment while transverse bars would grow for milder waves and coarser sediment. In intermediate conditions mixed patterns may occur

Land-ocean interaction Measuring and modelling fluxes from river basins to coastal seas

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Long wave forcing on a barred beach

We present new laboratory data on long wave forcing over a barred beach profile under random wave breaking conditions. The data include incident and radiated wave amplitudes, wave set-up, and detailed measurements of the cross-shore variation in long wave amplitude, including shoreline (swash) amplitudes. The total surf zone width was varied via changes in both wave height and the water level over the bar crest. The data obtained from the barred beach are also compared with previous data obtained from a plane beach under essentially identical short wave forcing conditions. The presence of the bar induces a frequency downshift in the spectral peak of the radiated long waves, a consequence of the increased surf zone width on the barred beach and a clear signature of long wave forcing by a time-varying breakpoint. Further comparisons of the two data sets suggest that the bar leads to resonant trapping and amplification (or suppression) of the shoreline motion at discrete long wave frequencies. Well-defined standing long wave motion occurs at discrete frequencies inside the bar and the resonant response is consistent with a simple seiche between the bar crest and shoreline, in agreement with previous numerical model studies. The long wave structure offshore of the breakpoint depends on the relative positions of the bar, shoreline and breakpoint, and is inconsistent with a numerical solution for a free standing long wave over the barred beach profile

A morphodynamic mechanism for transverse bars in the nearshore

Sand bars which are normal to the coastline have been described on sandy beaches Irom low to moderate wave energy (Niedoroda and Tanner, 1979; Konicki and Holman, 2000). A generating mechanism lor such bars is here proposed that is based upon the coupling between water motions and the evolving topography in case 01 normal wave incidence. 6asically, the momentum carried by the incident waves is locused by the bars because 01 topographic relraction. This causes a strong onshore current on the bars and return Ilow at the troughs in between. Since wave stirring is typically decreasing shoreward, the onshore current over the bars produces accretion and the offshore current at the troughs produces erosiono A positive leedback is thus created between circulation and morphology and this explains the growth 01 the bars. A nonlinear numerical model 01 sur! zone morphodynamics has been set up to show the growth 01 sueh bars Irom a number 01 dillerent small amplitude initial perturbations 01 the alongshore unilorm equilibrium

Measurements of the semi-diurnal drag coefficient over sand waves

Simultaneous measurements of tidal flow and pressure gradient over a 10 km distance have been used to estimate the frictional drag coefficient over sand waves in the southern North Sea. The measurements were made over a 27-day period in October–November 1988 at 52°10?N, 3°46?E, within a field of essentially two-dimensional sand waves approximately 3 m high and of wavelength 250 m.The M2 drag coefficient for depth-averaged flow normal to sand wave crests is found to be 2.95 × 10?3, in good agreement with values used in numerical models of the region. The uncertainty in this value is estimated to be of the order of 10%, primarily due to possible errors in the phase of the flow relative to the pressure gradient.The time series of daily-averaged semi-diurnal (Z2) drag coefficients over the 27-day period shows surprisingly little correlation with nearbed wave orbital velocities. The time of highest waves is associated with a decrease in drag coefficient rather than the increase predicted by most wave-current interaction theories and some previous observations. It is suggested that this behaviour is caused by stratification near the bed due to sediment resuspension under high waves. Predictions using theGlenn andGrant (1987, Journal of Geophysical Research,92, 8244–8264) theory provide qualitative support for this hypothesis, though the magnitude of the predicted effect is smaller than observed.The relative insensitivity of the drag coefficient to wave conditions suggests that incorporating simple wave-current algorithms into numerical models may be misleading

Measuring and modelling sediment transport on a macrotidal ridge and runnel beach: an intercomparison

Observations of hydrodynamics, fluorescent tracer dispersal and beach morphology were acquired in the intertidal zone of a macrotidal ridge and runnel beach. High frequency hydrodynamic data from pressure transducers and electromagnetic current meters were used to describe flow patterns in the intertidal zone while sediment transport rates were estimated using energetics and empirical models. Results from fluorescent tracer experiments provided information on net sediment movement over periods ranging from one to five tidal cycles whereas morphometric analysis was carried out to determine net beach movement during a period of 24 tidal cycles. Comparison of the results showed that sediment transport based on the hydrodynamic measurements did not agree with sediment movement derived using the tracer and morphometric methods. This disagreement is because the latter methods integrate processes occurring throughout the whole tidal cycle including those at very low water depths (swash zone processes). Hydrodynamic data were limited to periods of the tidal cycle where the mean water depth was greater than 0.5 m. Such limitation, imposed by the physical dimensions, principle of operation and installation procedures of the instruments is common in nearshore studies. Sediment transport results obtained by using hydrodynamic data obtained in macrotidal areas would be incomplete if swash-zone processes are not covered by the sampling scheme. However, comparison of results obtained for shorter periods (i.e. excluding shallow water) with those from other methods that integrate over the whole tidal cycle can be used to extract information on sedimentary processes for periods where no direct data are available

Video observations of beach cusp morphodynamics

Beach cusps are a common feature of steep reflective and intermediate beaches. Although many observations of beach cusp spacing exist, there are few observations quantifying the incipient formation, evolution and eventual destruction of these features. Beach cusp morphodynamics were analyzed using a 3-year dataset of video images collected at Tairua Beach (New Zealand). Twenty-four beach cusp episodes were selected to monitor the cycle of beach face changes, from planar to the appearance of the cusp patterns and back to planar again. Observations show that beach cusp disappearance can be ascribed not only to the erosive influence of storms but also to the persistence of accretionary conditions leading to the infilling of beach cusp bays and the development of an alongshore continuous berm. We also report observations of changes in beach cusp spacing over time which can be attributed to the merging of adjacent cusps within the cusp field, with the overall cusp spacing re-adjusting to accommodate the disappearance of a horn. Although the self-organization theory provides a better fit to the data and theory, we were unable to conclusively refute any of the mechanisms causing beach cusp formation since both existing theories, subharmonic standing edge wave and self-organization, can predict the trend in the observed beach cusp spacing. These observations show that initial beach cusp formation primarily occurs (around 70% of the episodes analyzed) under mildly accretionary conditions and, when accretion persists, the pattern disappearance is likely to occur as a result of bay infilling to form a featureless berm