Sediment inflow boundaries

A rating relationship for the sediment concentrations, or a sediment inflow record, is required for each inflow boundary.  For finer sediments, a rating relationship can be obtained from sediment concentration measurements using many samples from the flow (in some cases simple bottle sampling techniques are appropriate).  However for sands more expensive and complex measurement techniques, such as pump sampling, are desirable.  Measurements must include periods of higher flow when the majority of the sediment load is carried.  At some sites such measurements can be expensive and even dangerous due to the extreme flow conditions.  For the coarsest sediments bed load sampling is required, which can suffer from very large errors. (Advice on sediment sampling methods is provided in Lawrence, 1996.)

There are many circumstances for which the sediment inflows to the model will not be known.  If it is known that the channel is in equilibrium under existing conditions, and the purpose of the modelling is to assess the impact of proposed changes, then the model itself can be used to set the sediment inflows.  The procedure would be as follows:

• ensure that the bed material in the reaches near the inflow boundaries has been set accurately, using the 'GRADING' option if required
• run the model under existing conditions using the COMPOSITE algorithm and no bed updating (method set to 0)
• use Flood Modeller Interface to study the model output from the model for the 2nd, 3rd and 4th  sections downstream from each inflow boundary, if consistent concentration:discharge relationships are found, then these can be represented using 'QCBDY' boundary data in the sediment input file
• if consistent concentration:discharge relationships are not found, then investigate the reason, it may be that one of the cross sections is not representative of the reach, or that downstream structures are affecting flow conditions; if necessary extend the model upstream, adding more sections, and repeat the steps above
• find the listing in the general output file ('filename.zzd'), the proportions in the cumulative transport for each size fraction at each of the sections used above; copy a typical set of these proportions for each inflow boundary into the appropriate part of the sediment input file, using the 'NEW' option in the inflow datablock
• if, in the step above, the proportions are too heavily weighted to just one or two fractions, consider adding new fractions or adjusting the sizes of existing fractions; if this is done then the default distribution and any local bed material distributions will need re-writing, and the procedure repeated from the first step above
• if, during the modelling exercise, the bed roughnesses, sediment sizes, sediment transport equation or sediment transport calibration coefficient are adjusted, then the procedure must be repeated.

Note that this approach is not appropriate to cohesive sediment transport, for which there is no link between bed material composition and the material in transport.  Also, it will not apply at side inflow locations, where a channel between the inflow boundary and the junction with the main channel is not included in the model. In all cases with graded sediments the 'NEW' option for determining the sediment distribution entering at the boundary should be used.