Dredge Option

The dredge option allows simulated dredging of any part of the model either at a specified rate or to a given level. The start and completion times of the dredging operations are also variable. This option can be specified in any location after the General System Parameters.

In order to use the option when method 2 or 3 is used for bed updating, the character 'D' must be placed in the 'filename.dat' cross section data to define the lateral limits of dredging. The first 'D' is placed in column 41 at the data line which defines the left bank extreme of dredging and a second 'D' is placed in column 41 of the line which defines the right bank extreme. Cross section points at and between the 'D's will be lowered (Figure 8 ).

The 'D' markers should not be placed at the extreme edges of a section, especially when dredging to a level is specified. Otherwise dredging will result in the channel shape being lost altogether and a horizontal bed at the dredged level will be produced.

If method 1 is used for bed update, then the 'D' markers are not required because dredging is assumed to apply uniformly across the whole section. When dredging to a level, the level specified is the minimum elevation in the section.

Dredging can be specified for cross sections at upstream boundaries or immediately downstream from junctions or structures. Dredging at these locations will affect the cross-section shape and the flow predictions, but it will not affect the sediment balance due to the algorithm used to relate sediment loads with bed level changes (see under 'Sediment continuity equation' earlier in the manual). An example of the structure of this section of the file is shown in the figure below.

Datafile Format - Dredge Option

Line 1                                                      keyword 'DREDGE'

Line 2                                                      n1, itype, [t_m]

Line 2                                                      label_i, start_time, end_time, itype_value

Where

n₁ = number of ensuing node labels

itype = integer defining the method of dredging. Enter one of the following:

1 dredge to constant level

2 dredge at constant rate

tm = optional keyword or value for units of time in following data set. Can be any numerical multiplier or one of the following: seconds (default), minutes, hours, days, weeks, months (30 days) or years.

label_i= node labels of dredged sections

start_time= start time of dredging in units of t_m

end_time= end time of dredging in units of t_m

itype_value= if itype = 1, required bed level in m

if itype = 2, dredge rate in m/time (in units of t_m)