Abstract No : 72

Coastal sediment transport and erosion

Difficulties Of Sediment Transport Modeling In The Coastal Zone

Ruben Kosyan (1), Joachim Grune (2), Boris Divinskiy (1,3) and Igor Podymov (1,4)(1) SBSIO RAS, 353467 Gelendzhik, Russia
Telephone: +78614128281 Email: rkosyan@hotmail.com(2) Coastal Research Centre FZK, University of Hannover, D-30419 Hannover, Germany
Telephone: +49 511 - 762 9227 Email: gruene@fzk.uni-hannover.de(3) Telephone: +78614128281 Email: divin@ecologpro.ru(4) Telephone: +78614128281 Email: podymov@coastdyn.ru

The ultimate purpose of sediment transport studies is the prediction of bottom relief inthe zone of active wave effect accompanied by transport of significant sand volumes. Theenergetic longshore currents induced by the oblique wave approach, transport large
amounts of sand lifted by waves from the sea bottom. This mass sediment transport andits variations finally determine the shore-line configuration and the location ofaccumulative and erosion areas on the underwater slope. Because of great practicalsignificance, the problem of sediment transport has attracted much attention. When
constructing theoretical models of suspended sediment mass transport by water flows,investigators have to face a number of difficulties. Modeling of the sediment transport islimited by the absence of clear physical mechanisms of sediment suspension. The maindifficulties of the modeling are discussed in this report. Detailed measurement ofmechanisms and time scales of sand suspension by irregular waves were carried out
during field experiments at the Black, North and Mediterranean seas and in the largest
wave channel GWK in Europe of the Hannover University. The processes which control
the temporal variations in suspended sediment concentrations near the sand bottom havebeen examined by using field measurements of the suspended sand concentration andthe horizontal current components. Measurements during experiments, revealed, that
there were fluctuations of concentration in a broad band of frequencies near the bottom.
Under the influence of unbroken waves with a typical group structure, the sandsuspension occurred in the moments when the high wave groups passed. Consecutive
sand suspending under the influence of every wave in those groups resulted in theformation of broad-concentration peaks. The peak-duration depended on the quantity ofwaves in the group; it was in the order of several tens of seconds. The fluctuations of thesand-concentrations, with the same period as the waves, occurred within these peaks. In
this case the concentration spectrum of suspended sand is characterized by a local
maximum at the frequency of the approaching waves and by a monotonous increase of
the spectral density at the range of low frequencies, when they decrease. The latter canbe explained by the fact that the spectrum of the envelope of the cross-shore currents isfairly wide and a local maximum is missing. In the breaking zone, where the groupstructure of waves degenerates because of energy dissipation, the concentration

fluctuations mainly take place in the infragravity wave band. Then, in some cases,
significant values of the coherence between concentration and the cross-shore velocity of
infragravity waves can be obtained. In these cases, the concentration spectra of thesuspended sand have no local maxima and they are characterized by a monotonous
increase of the spectral density when the frequency decreases. The observed regularities
can be explained by the fact, that the intensity and time scales of sand suspending in the
breaking zone depends on the turbulence, which is induced from the water surface under
breaking waves. Analyses of the concentration chronograms and the velocity components
have shown that the emergence of sharp concentration-peaks is restricted to themoments, when the turbulent fluctuation of the cross- and longshore velocity starts. The
appearance of such a rise of the turbulent velocity fluctuation and of corresponding
peaks of sand concentration usually is not timed to a definite phase of the approachingwaves and has no periodicity. One of the difficulties of the suspension modeling is theproblem to identify dependence between values of suspended sediment concentrationand the form of wave spectrum. The form of wave spectrum greatly influences on the SSCvalues. With the same wave parameters the spectral steepness can alter theconcentration by 2.5 times. The most effective way in the modeling developing is thedetermination of relations between the suspended sediment concentration and turbulent
kinetic energy, between which there are statistically significant values of coherence. To
obtain such dependencies, the further research in field and laboratory conditions ofspatial-temporal variability of large-scale turbulence and sand suspension under irregularwaves is necessary.