Guidelines for selection of sediment transport samplers

The selection of the most appropriate sampling technique should be based on a number of criteria, as follows:

1) type of process/parameters to be measured:

  - bed material;

  - bed load; 

  - suspended load; 

  - particle size of bed material and suspended sediments;

  - particle size of flocculated suspended sediments;

  - fall velocity of sediments;

2) type of sampling environment:

  - rivers, estuaries, coastal seas;

  - sediments involved: mud, silt, sand, gravel, mixtures; flocculated materials;

  - depth range and velocity range involved;

3) type of sampling:

  - on-line measurements (short term);
  - stand-alone measurements (long-term) attached to frames, tripods or structures,
  - point measurements using sensors in one point;
  - profiling (or depth-integrated) measurements traversing the flow depth;

4) type of project and required accuracy:

  - reconnaissance study;

  - process study;

  - studies focussing on high spatial and time resolution (dredging and dumping plumes);

  - input for mathematical modelling (boundary conditions) for design purposes;

  - data for verification of models;

5) available instruments and available budget.

To select the most appropriate sampling instrument, quantitative information of the physical parameters to be measured should be available prior to the actual field survey. It is important to have information of the various transport modes at the sampling site such as the (relative) value of the wash and bed material load, the bed load and the suspended load transport rates, the sediment concentrations (low or high) and the particle size ranges of the suspended sediments (clay, silt, sand; flocculated sediments) involved. To get this information, existing data sets should be analyzed or a reconnaissance study should be carried out.

Most measurements in rivers and estuaries are based on on-line data sampling using instruments attached to a survey ship (through cable-winch system). Stand-alone measurements using a package of electronic instruments attached to a frame, tripod or other structure placed on the bed or in the flow are conducted when survey ships cannot be used due to the presence of surface waves (coastal envirionments) or when long-term measurements (weeks to months) are required. The instruments usually are operated in burst mode, which means that measurements of limited duration are taken at regular intervals in time (15 min per hour) to reduce on data storage.

Examples of instrumented stand-alone measuring facilities are:

  - HSM tripod of University of Utrecht;
  - AMF tripod of Rijkswaterstaat (RIZA);    
  - STABLE tripod of Proudmann Oceanographic Laboratory POL. 

The availability of these advanced facilities with electronic equipment greatly improves data quality and data density over the traditional mechanical equipment, and long-term costs will be reduced, as there is no longer need for intensive sample processing and analysis in the laboratory. Another important criterion is the purpose of the study and the accuracy required. For example, the predicted deposition volumes of a planned harbour approach channel should be rather accurate when the maintenance dredging costs of the channel are critical with respect to the economic feasibility of the project. In that case the most accurate instruments and the most sophisticated facilities should be used. Although these type of measurements are expensive, the overall costs of field surveys are only a fraction of the total project costs. When research (process) studies are performed, it is often required to measure the time-series of the fluctuating parameters so that the turbulent fluxes can be determined. For engineering studies it is usually sufficient to measure the time-averaged velocities and sediment concentrations. Simple instruments such as the bottle and trap samplers can then be used, although the analysis costs of the many samples involved are relatively high.