MIKE 21 Non-Cohesive Sediment Transport

MIKE 21 ST is the module of the MIKE 21 modelling system that calculates the rates of non-cohesive sediment (sand) transport for both pure current and combined waves and current situations.

Apart from the sediment transport components, the initial rates of bed level change associated with the time-averaged - over the user-defined simulation period - transport field are also output from a MIKE 21 ST simulation.

Requirements

An output file obtained from a MIKE 21 HD simulation is required as input in all cases. In addition, a data file containing wave parameters as calculated by MIKE 21 NSW or PMS is needed if the sediment transport rates in combined waves and current are to be calculated.

It is important to keep in mind that the simulation is performed on the basis of the hydrodynamic conditions that correspond to a given bathymetry. No feed­back of the rates of bed level change on the waves and the hydrodynamics, as in the case for a full morphological model, exists. Hence, the results pro­vided by MIKE 21 ST can be used to identify potential areas of erosion or deposition and to get an indication of the initial rate at which bed level changes will take place, but not to determine an updated bathymetry at the end of the simulation period.

Application Areas

MIKE 21 ST can be applied to a wide range of sediment-transport related phenomena, including modelling of sediment transport fields in the littoral zone, in the vicinity of coastal structures, in tidal inlets, and under the sole or combined effects of tidal-, wind- and wave-driven currents in estuaries or coastal areas.

Computational Features

The main features of the MIKE 21 non-cohesive sediment transport module are as follows:

·         Constant or spatially varying characteristics of the bed material (i.e. median grain size and gradation)

·         Five different sand transport theories available for the calculation of transport rates in pure current conditions:

–         The Engelund and Hansen total-load transport theory

–         The Engelund and Fredsøe total-load (determined as bed load + suspended load) transport theory

–         The Zyserman and Fredsøe total-load (bed load + suspended load) transport formulation

–         The Meyer-Peter and Müller bed-load transport theory

–         The Ackers and White total-load transport formulation

·         Two methods available for the calculation of the sediment transport rates in combined current and waves:

–         Application of DHI's deterministic intra-wave sediment transport model STP

–         Bijker's total-load transport method

·         User-selected sand transport method (two-dimensional (2DH) or quasi three-dimensional (Q3D)) in combined waves and current when the STP model is used. Computation of the transport rates are speeded up through the use of sediment transport tables created beforehand

·         Use of STP allows to account for the influence of the following phenom­ena on the computed sediment transport rates:

–         Arbitrary direction of wave propagation respect to the current

–         Breaking/unbroken waves

–         Geometric properties of bed material described through a single grain size or a grain size distribution curve

–         Plane/ripple-covered bed

·         Finite differences technique on space-staggered rectangular grid

·         Courant-Friedrichs-Lewy stability criterion