Principles, statistics and errors of measuring sediment transport

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3. MEASURING PRINCIPLES, STATISTICS AND ERRORS

3.1 Measuring principles for suspended load transport

Measuring instruments for suspended sediment transport can be classified according to their measuring principle: direct or indirect measuring of the sediment transport.

The direct method is based on the direct measurement of the time-averaged sediment transport (u c ) in a certain point (point-integrating) or over a certain depth range (depth-integrating). This latter procedure implies vertical movement at a uniform speed of the sampler over a certain depth range.

The indirect method is based on the simultaneous but separate measurement of the time-averaged fluid velocity and the time-averaged sediment concentration, which are multiplied to obtain the time-averaged sediment transport. This method implies two assumptions which introduce errors: (1) the turbulent flux terms are zero and (2) the fluid and sediment particle velocity are equal.

Traditional sampling instruments (based on taking samples of water and sediment) as well as electronic sensoring instruments (based on optical and acoustical principles) are discussed.

3.2 Measuring principles for bed load transport

The most widely used method for the measurement of bed load is the direct method by means of mechanical trap-type samplers. Many versions of the trap-type sampler have been used with varying amount of success. The problems of the trap-type sampler are the lowering and raising of the sampler to and from the streambed and the efficiency of the sampler in collecting the particles.

The most widely-known indirect measuring methods for bed-load transport are: bed-form migration studies (tracking), sediment deposition and erosion studies and tracer studies.

3.3 Measuring statistics

3.3.1 General aspects

A critical aspect of any morphological study is the field survey during which the samples to be analysed are collected. It is important to be aware of the fact that the quality of the total study can only be as good as the quality of the information gained through sampling. Thus, any errors incurred during sampling will manifest themselves by limiting the accuracy of the study.

The objective of a field survey is to obtain samples from the project area with the purpose of characterizing the area sampled. The sample size should be small enough to be conveniently handled and transported and yet sufficient to meet the requirements of accuracy. The quality of the sampling process and analysis is dependent upon: selecting representative sampling sites in the project area, collecting sufficient samples at each sampling site, using appropriate sampling methods, protecting the samples during the storage period (sample preservation), and flexibility of the sampling programme. These aspects are discussed in detail.

3.3.2 Sampling site

The selected sites should be well-distributed over the project area and be representative for the (mean annual) prevailing hydraulic and morphologic conditions. Some general requirements are:

- located in a straight reach,

- located in a stable cross-section,

- located normal to the main flow direction, uniform wave characteristics,

- sufficiently deep with respect to the dimensions of the sampling equipment,

- accessible and clear of natural and/or artificial obstacles,

- well-defined geometrical dimensions.

3.3.3 Number of measurements for suspended load transport

The total load consists of bed material load and wash load. The bed-material load can be subdivided in bed load and suspended load transport.

The wash load consists of sediment particles (fines < 63 um) with particle sizes smaller than those found in appreciable quantities in the bed material. The fine particles usually are uniformly distributed over the entire cross-section. The sediment discharge can simply be obtained by multiplication of the flow discharge and the concentration. Since the concentration is approximately constant over the cross-section, the number of samples can be limited to a few samples.

The suspended sediment discharge (particles > 63 um) usually is determined by measuring in some points over the depth and over the width of the river. The cross-section is divided into several subsections. The sediment discharge passing through each subsection is determined by measuring (point or depth-integrated measurements) along one vertical within each subsection. The accuracy of the suspended sediment discharge depends on:

- the number of points over the depth,

- the number of verticals over the bed-form length in each subsection,

- the number of verticals over the width (cross-section),

- the number of verticals over time (flood period, ebb period).

Guidelines are provided to find the optimum number of measurements and the accuracy involved.

3.3.4 Number of measurements for bed-load transport

The number of measurements of mechanical trap type bed-load samplers is presented. Typical sampling problems related to the variability of the physical processes involved are:

- sampling duration of individual measurements,

- number of samples at each location,

- number of sampling locations along the bed form length,

- number of locations over the width of the cross-section.