Bed Material Grading Option

This data block enables the default bed material distribution to be changed. The option can be used to change bed material composition at some sections or all sections in the model. Bed material is assumed constant when the COMPOSITE algorithm is selected, and this option enables it to vary, usually becoming finer in the downstream flow direction. In the SORTED algorithm, bed material is used as the initial composition of the active layer and is used as the parent bed material when erosion below the deposited layer is predicted.

Data is input by specifying a number of representative distributions for the sediment sizes given in the general system parameters datablock at the head of the file. The maximum number of representative distributions (or 'gradings') is 10. Each section label for which the default distribution is to be overwritten is then listed. The new distribution is given as a combination of two of the representative distributions by listing the two representative grading numbers and a factor. In this way the bed material grading can be made to gradually vary down a reach. If the distribution at a certain section is to be set as identical to one of the representative gradings, then the appropriate representative grading number can be given twice and the factor omitted.

Any number of section labels can be given, if one is not listed the original default data is retained, if one is given twice, then the second set listed in the file is used. An example of the structure of this section of the file is shown in Figure 4.

Datafile Format-Bed Material Grading Option

Where

N_grad= number of representative gradings being given

i = representative grading number (must be listed 1, 2, 3 etc)

Pb_i:1= proportion in the bed material of sediment size 1 for irepresentative grading

Pb_i:2= proportion in the bed material of sediment size 2 for irepresentative grading

Pb_i:3= proportion in the bed material of sediment size 3 for irepresentative grading

label = section label for which bed material grading is being set

Ng1 = first representative grading number being used for this section, (the i number above)

Ng2 = second representative grading number being used for this section

factor = the factor specifying the combination: factor=0.0 means all Ng1 grading, factor=1.0 means all Ng2 and factor=0.4 means 0.4* the Ng2 combined with 0.6* the Ng1. If a value for factor is omitted, a value of 0.0 is taken