General overview¶
MIKE 1D models sediment transport as a multi-fraction process. This page describes how fractions are defined, how their type is set, and which parameters each fraction needs.
Sediment fractions¶
MIKE 1D is a multi-fraction sediment transport, morphological and bed layer model. The model operates with multiple sediment fractions with individual characteristics such as grain size, which combined form the local particle size distribution and are transported accounting for effects of the bed surface composition.
MIKE 1D allows both single-fraction and multi-fraction models.
Type¶
All sediments can be specified as cohesive or non-cohesive. Cohesive sediments are transported as suspended load (in some cases wash load) while non-cohesive sediments are transported as bed load and/or suspended load, i.e. optionally only one of the transport modes can be selected, as e.g. gravels will usually be bed load only.
The distinguishing factors between the two types of sediments are the grain size, typically non-cohesive fractions for \(d_{50} \geq 0.1\) mm and cohesive fractions for \(d_{50} < 0.1\) mm.
In MIKE+, the Transport mode is used to select whether a fraction is cohesive or non-cohesive. Set the transport mode to ST Model to get a non-cohesive fraction and AD Model to get a cohesive fraction. Non-cohesive fractions will be modelled using bed load only.
Parameters¶
Parameters can be divided into overall parameters and parameters specific for the sediment fractions.
The grain size factor \(d_{90}/d_{50}\) needs to be specified for single fraction models due to its impact on the skin friction shear stress. If more than one sediment fraction is used, a graded sediment model is used. In this case \(d_{50}\) and \(d_{90}\) are calculated from the sediment content in the top layer.
For each sediment fraction, the following parameters are needed:
Table: Parameters required for each sediment fraction.
| Parameter | Unit | Note |
|---|---|---|
| Grain size (\(d\)) | m | |
| Density (\(s\)) | Relative density, no unit | |
| Porosity (\(n\)) | Common value: 0.3-0.7 | |
| Fall velocity (\(w_{s}\)) | m/s | Settling velocity, default is the use of Rubey formula (1933), while a user defined value is also allowed |
| Critical Shields parameter (\(\theta_{c}\)) | Used in some formulas, van Rijn uses its own Critical Shields parameter from the Shields curve, some formulas do not use it. |