Establishment of the Sediment Monitoring System for the Sava River Basin - page 4

Available Techniques for Suspended Sediment Monitoring

The Core expert group evaluated a number of techniques for suspended sediment measurements, as presented below.

The simplest way of taking a sample of suspended sediment is to dip a bucket or other container into the stream preferably at a point where it will be well mixed. The sediment contained in a measured volume of water is then filtered, dried and weighed. This gives a measure of the concentration of sediment and when combined with the rate of flow gives the rate of sediment discharge. A study of alternative sampling techniques showed that dip sampling in bottles generally gives concentrations about 25% lower than results obtained from more sophisticated techniques. For single samples taken by scooping a sample, a depth of 0.3 m below the surface is recommended as better than sampling at the surface. If the single sample can be taken at any chosen depth, 0.35-0.4 the depth of flow is recommended as giving the best estimate of average sediment concentration.

Using the depth integrating samplers, water and sediment mixture can be sampled continuously while the sampler is moving at a constant transit rate along the vertical. The result is the discharge-weighted average concentration in the vertical. A typical sampler consists of a glass bottle inserted in a fish - shaped frame.

The point - integrating sampler (Figure 3) remains at a fixed point in the stream and samples continuously during the time it takes for the bottle to fill. Samples should be taken at a number of depths at each of several verticals.

Pumping samplers collect samples at regular time intervals or in response to a rise or fall in stream flow at a definite point in the river. The entire variation in sediment concentration during a flash flood may be followed. Sufficient samples can be obtained automatically to define the variations in sediment concentration during a flood.

 

However, all automatic pumping systems are vulnerable to pipe blockages and may also require efficient flushing systems. Portable pumping samplers may be used for taking point-integrated or depth- integrated samples at any point or vertical in a cross-section (Babic Mladenovic M., 2013).

Photoelectric turbidity meter (Figure 4) operates on the principle of attenuation of light transmitted through sediment-laden water. From light scattering theory, the photodensity (the ratio of intensity of the transmitted light and incoming light, I/Io) depends not only on the concentration but also on the particle size existing in the medium. It would be possible to establish a relationship between the sediment concentration and a photo - density reading only if the grain size were relatively constant. In operation, the instrument must be calibrated carefully to establish such a relationship. Determination of sediment concentration on the basis of the photoelectric effect can only be adopted in rivers where variation in grain size is very small and the concentration is fairly low, as in the Sava river.

Acoustic Doppler Current Profiler (ADCP) was originally designed to measure 3D flow structure but they also record the intensity of the return echo. The latter is proportional to the number of backscatters present in the water column and can be used as a proxy for suspended sediment concentration (Figure 5). However, the software for such computation is limited, and considerable post-processing is needed to correct and normalize ADCP data for this use.

Other methods not commonly used for suspended sediment concentration measurements are optical backscatter, optical transmission, focused beam reflectance, laser diffraction, nuclear, spectral reflectance, digital optical, vibrating tube, differential pressure and impact sampler. The overview of advantages and disadvantages of suspended sediment sampling techniques is given in table 2.

 

Fig03
Figure 3: Point-integrating sampler.

 

Fig04
Figure 4: Photoelectric turbidity meter.

 

Fig05
Figure 5: SSC spatial distribution (mg/l), measurement done by DHMZ on Sava river, g.s. Podsused (6.04.2012).

 

Tab02