EurOCEAN 2000 - Vlaams Instituut voor de Zee

transport rate has been measured for normal and enhanced turbulence levels over flat and
rippled sand beds. Two further practical aspects have been considered; (1) given the magnitude
of the extra turbulence, can the sediment transport rate be calculated?, and (2) can the
magnitude of the extra turbulence be predicted without recourse to in-depth investigations? To
address these issues a baseline study of the detailed processes of suspended sediment transport
in a turbulent tidal flow has been completed. To extend these data, the results from numerical
simulations have been applied to quantify the effect of enhanced turbulence levels on
suspended sediment transport (1DV model), and bedload (CFD code). The effects of breaking
waves, turbulence and vortex shedding on bed stability and scour have also been assessed
(Simons, 1999), and an analytical approach for predicting sediment transport proposes. The
data from the above studies will be input to the project database, and the key results will
provide input to the design guidelines. The new findings will be collated with existing
available information when formulating the guidelines (e.g. Whitehouse, 1998).
SEDIMENT BEHAVIOUR CLOSE TO THE STRUCTURE AND
SCOUR (TASK 2)
The work in this part of the project (Task 2, led by SINTEF ) concentrates on the processes
associated with the sediment, which is closely related to the wave-generated pore pressure in
the sediment bed. This task is sub-divided into six topics:
TOPIC 2.1. WAVE-GENERATED PORE PRESSURE AND SCOUR AROUND
BREAKWATER ELEMENTS. UOX has carried out field measurements on wave generated
pore pressures in a sand seabed. A typical element of a breakwater (a rubble mound stone) is
simulated with a truncated cylinder placed on the seabed with the axis vertical. Wave-induced
pressures are measured on the cylinder as well as in the seabed. There is also an altimeter and
an inclinometer to monitor the scour at the cylinder as well as the tilt of the cylinder. UOX has
actually achieved four field deployments of an instrumented cylinder. Due to experimental
problems, the two first were unsuccessful, but the two following ones successfully produced
data. Considerable erosion and consequent tilting of the model breakwater element have been
observed in all the deployments. The data is being analysed.
TOPIC 2.2. NON-LINEAR MODELLING OF WAVE INDUCED PORE PRESSURES.
SINTEF has modelled pore pressure and effective stresses using non-linear material models.
The non-linear material is a swelling soil model.
Based on numerical results, the following conclusions have drawn: (1) Elasto-plastic soil
behaviour reduces gradients out of the seabed when critical gradients are reached. (2) Critical
gradients out of bed can be reached with a small content of gas in the pore fluid. (3) Shallow
water non-linear breaking waves gives lower gradients out of bed than "linear non-breaking"
waves of similar height and period in the same water depth. See also Ilstad et al. (2000).
TOPIC 2.3. WAVE BOTTOM PRESSURES AND WAVE KINEMATICS IN THE SURF
ZONE. NTNU and SINTEF have obtained spatial and temporal variations of the dynamic
bottom pressures and the wave kinematics in the vicinity of the wave breaking point in shallow
water. Different numerical schemes have been tested to calculate the surface elevation, wave
kinematics and bottom pressures FIRAM (Finite Element Reynolds Average Model), VOF
(Volume Of Fluid) and COBRAS (Cornell Breaking - Wave And Structures). The latter model
seems so far to give reasonable agreement with measurements. See also: Arntsen et al. (2000).
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