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This article is a summary of sub-section 5.6.2.4 of the Manual Sediment Transport Measurements in Rivers, Estuaries and Coastal Seas[1]. This article describes how the USP-61 can be used to determine silt and/or sand concentrations. The USP-61 is an example of a mechanical instrument.

## Introduction

Figure 1: USP-61 sampler

The sampler consists of a streamlined bronze casting (= 50 kg), which encloses a small bottle (= 500 ml), as shown in Figure 1A. The sampler head is hinged to provide access to the bottle. The intake nozzle, which can be opened or closed by means of an electrically operated valve, points directly into the approaching flow. To eliminate a sudden inrush after opening of the intake nozzle, the air pressure in the bottle is balanced with the hydrostatic pressure prior to opening of the valve. This is accomplished by means of an air bell in a body cavity connecting the bottle and the surrounding stream. After opening of the valve, the air in the bottle can escape through a special air-exhaust tube pointing downstream on the side of the sampler head. As a result the hydraulic coefficient is approximately unity during sampling. The filling time varies from 10 to 30 seconds, as shown in Figure 1B. To avoid a circulation flow, the bottle should only be filled for about 75%. The USP-63 is a heavier version (= 100 kg) of the USP-61. The sampler is manufactured by Rickly Hydrological Company.

## Determination of silt and sand concentrations

The silt and sand concentrations are determined as:

$c\,=G\,/V\,$

in which: $G$= dry mass of sediment (mg), $V$= volume of water sample (l).

The silt and sand transport (in kg/s/m2) can be determined as:

$S = \large\frac{G}{FT}$,

in which: $F$= area of nozzle (m2), $T$= sampling period (s).

The sampling efficiency of the USP-61 is strongly dependent on the ratio of the intake velocity and the local flow velocity (hydraulic coefficient). Extensive laboratory measurements, summarized by Dijkman (1978)[2], have shown that the hydraulic coefficient varies from about 0.8 to 1.3 depending on the water temperature, sample height above the bed and the nozzle orientation (maximum deviation with flow direction of 20°). For this range a maximum sampling error in the concentration of about 10% may be expected in case of a steady concentration. In the case of field conditions with fluctuating concentrations the inaccuracy of individual samples may be as large as 50%. To obtain a reliable average value in a statistical sense, a large number of samples (about 10) should be collected at each sampling point.