# General principles of optical and acoustical instruments

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This article is a summary of sub-section 5.6.4.1 of the Manual Sediment Transport Measurements in Rivers, Estuaries and Coastal Seas [1]. This article describes the principles of three types of optical and acoustic instruments: transmission, scattering and transmission-scattering. Furthermore, the article describes the calibration, measuring range and advantages of remote sensing with optical and acoustical instruments.

## Measuring principles

Figure 1: Basic principles

Optical and acoustical sampling methods enable the continuous and contactless measurement of sediment concentrations, which is an important advantage compared to the mechanical sampling methods. Although based on different physical phenomena, optical and acoustical sampling methods are very similar in a macroscopic sense. For both methods the measuring principles can be classified in (see Figure 1): transmission, scattering, transmission-scattering.

### Transmission

The source and detector are placed in an opposite direction of each other at a distance 1. The sediment particles in the measuring volume reduce the beam intensity resulting in a reduced detector signal. The relationship between the detector signal (It) and the sediment concentration (c) is:

[math]I_t=k_1\,e^{-k_2\,c}[/math]

in which: k1 = calibration constant depending on instrument characteristics, fluid properties and travel distance (l), k2 = calibration constant depending on particle properties (size, shape), wave length and travel distance (l).

### Scattering

The source and detector are placed at an angle relative to each other (see Figure 1B). The detector receives a part of the radiation scattered by the sediment particles in the measuring volume. The relationship between detector signal (Is) and sediment concentration (c) is:

[math]I_s=k_3\,c\,e^{-k_2\,c}[/math]

in which: k3 = calibration constant depending on instrument characteristics, fluid and particle properties (size, shape), wave length and travel distance (l). An important disadvantage of the scattering method is the strong non-linearity of the relation between the detector signal and sediment concentration for large concentrations.

### Transmission-scattering

This method is based on the combination of transmission and scattering, as shown in Figure 1C. If the travel distance for transmission and scattering is equal, a linear relationship for the ratio of both signals is obtained

[math]I=I_s\,/I_t\,=k_4\,c[/math]

in which: k4 = calibration constant depending on instrument characteristics and particle properties.

Important advantages are the absolute linearity between the output signal (I) and the sediment concentration, the independence of water colour and the reduced influence of fouling.

## Calibration

For all measuring principles an in situ calibration for determining the constants is necessary, if possible under representative flow conditions covering the whole range of flow velocities and measuring positions (close to bed and water-surface). Regular calibration is required because the constants may change in time due to variations in temperature, salinity and pollution. In practice, the optical and acoustical sampling methods can only be used in combination with a mechanical sampling method to collect water-sediment samples for calibration. Usually, about 10% of the measurements should be used for calibration. The inaccuracy of field measurements may sometimes be rather large because of calibration problems (Kirby et al, 1981[2]), particularly for optical samplers. The main problem is the lack of synchronity between the optical and mechanical sample collection. To minimize synchronity errors, the optical samplers should be calibrated bij measuring the silt concentration on board of the ship using a pre-collected water-silt sample.

## Measuring range

For an optimal sampling resolution the wave length and particle size must be of the same order of magnitude. Therefore the optical method is most suitable for silt particles (> 50 um). Laboratory experiments using the optical sampler, have shown that the addition of sand particles with a concentration equal to the silt concentration increased the output signal with about 10% (Der Kinderen, 1981[3]). The upper concentration limit for optical samplers is about 25000 mg/1 (Kirby et al., 1981[2]). The acoustic method is most suitable for sand particles (>50 um). The upper concentration limit is about 10000 mg/1.

## Advantages

An important advantage of optical and acoustical samplers is the continuous measurement of the suspended sediment concentration. In combination with a chart recorder for data collection a relatively long period (one month) can be sampled continuously and automatically. When there is very little variation of the silt concentrations in lateral direction of the cross-section, measurements at one point can be considered as representative for the whole cross-section. In that case the sensor can be fixed to a bridge pier or river side installation. The measuring location must be easily accessible for regular cleaning of the sensor and changing of batteries and chart records. Energy consumption and recorder maintenance can be minimized by using a switch system activating the sensor and recorder only for short periods (5 min) at preset intervals (1 hour) as reported by Brabben (1981)[4]. Another advantage of the continuous signal is the possibility of determining continuous concentration profiles by raising the optical or acoustical sensor from the bed to the watersurface (rapid profile method, Kirby et al 1981[2]). Using this latter method a complete concentration profile can be determined in one minute. To check the representativeness of these profiles, occasionally the concentration profile should also be determined by means of a number of point-integrated measurements. The horizontal variability can be determined by towing the sensor at a (monitored) depth below the water surface. Finally, it is remarked that both sampling methods can also be used to measure the instantaneous sediment concentration under wave conditions, provided the respons period is small enough.