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Estimation of suspended sediment concentrations in natural water bodies from Secchi disk measurements. Lewis, Charles Peter

Abstract

The Secchi disk is one of the simplest and least expensive instruments for measuring the rate of attenuation of light in natural water bodies. Because this rate of attenuation is often closely related to the concentration of suspended sediment in the water, disk measurements may, in some situations, provide a useful and inexpensive alternative to the standard gravimetric methods for determining sediment concentration. Published empirical studies indicate that satisfactory relationships between Secchi disk transparency and concentration can be established but that these relationships differ from area to area and become more complex as turbidity increases. Results presented in this paper based on data from the Mackenzie delta, N.W.T. support these conclusions. These results cover a range of turbidities which extends well above those previously examined. Physical interpretation of these empirical results is made difficult by the large number of uncontrolled variables associated with Secchi disk readings. This interpretation is facilitated by the introduction of objective attenuation coefficients (more variables controlled) as variables intermediate between transparency and concentration. The two coefficients most commonly used are the volume attenuation coefficient α and the diffuse attenuation coefficient for downwelling irradiance Kd. Empirical relationships between either of these objective attenuation coefficients and disk transparency are reciprocal in form but depart from the simple reciprocal model at high sediment concentrations. Tyler (1968) has shown theoretically that transparency is dependent upon both α and Kd. Since the ratio α/Kd increases with turbidity, the observed departures from the simple reciprocal model can be explained. Burt (1954, 1955) has developed a theoretical expression relating α and sediment concentration in which he shows that α and thus Secchi disk transparency are dependent upon total particle surface - ie., on particle size as well as concentration. This dependence on particle size provides an explanation for the increasing complexity of empirical transparency-concentration relationships as the turbidity range considered widens and for the change in relationships with location. Tyler's and Burt's expressions have been combined in this paper to enable estimations of particle size to be made given only an empirical transparency-concentration relationship and knowledge of the ratio α/Kd. The resulting equation has been applied to the Mackenzie delta data and, considering the large number of untested assumptions made, the particle size estimations are remarkable close to those obtained using Postma's (1961) purely empirical relationship. Secchi disk measurements, then, can provide a surprising amount of information about sediment concentration and particle size. There are, of course, many limitations to the method and the theory associated with it is largely untested. Within the context of these limitations, however, the disk can be very useful. It is simple to use in the field, is remarkable insensitive to changes in the underwater light field and is not greatly influenced by observer bias

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