Sediment Size and Gradation

MIKE 21 ST assumes that the bed material is formed by different fractions whose sizes are log-normal distributed. In this way, a grading curve can be built based on two parameters: the median grain size d (sometimes repre­sented as d₅₀) and the geometric standard deviation s_g, defined as:

(5.32)   

where d₁₆ and d₈₄ are the particle sizes for which 16% and 84% of the sedi­ment by weight is finer.

The grain size and the geometric standard deviation can be defined in two alternative ways:

·         As constant values of d and s_g for the whole simulation area

·         As a type 2 file containing the values of d and s_g for every node of the grid

If you choose to specify the sediment size and gradation as a type 2 file, MIKE 21 ST expects a file containing 2 items as input. The first item must be the median grain size and the second one the geometric standard deviation.

The specified grain size d should be within the usual limits for sand.

Uniform bed material is indicated by setting s_g equal to one.

You can identify areas where no sediment transport takes place (e.g. areas of the bottom covered by rocks or revetments) by setting the grain size d to -99 in your sediment map.