The main part of the harmful chemical compounds that enter the rivers as a result of human activity is concentrated in suspensions – solid particles in the water. Currents are capable of transporting heavy metals, pesticides and other dangerous substances over long distances. Tracking them is quite difficult, since many different factors have to be taken into account: the speed of the water flow, its depth, the heterogeneity of the distribution of pollutants in the water column and at the bottom. Scientists from the Faculty of Geography of Moscow State University presented an algorithm that will help to cope with a difficult task.
The geographers of Moscow State University proposed to estimate the amount of suspended matter using acoustic methods. This can be done relatively quickly and accurately with an acoustic Doppler current meter. This device, installed on board the vessel, sends ultrasonic waves into the water, which, reflected from the bottom, return back. The device captures the response signal and, by its dispersion, determines the amount of solid suspended particles in the water. The principle of operation of the device can be compared with an echo that a person hears, for example, by shouting into the depths of a well. This approach has already become widely known in relation to measurements of water discharge and current velocities, but so far it has not been used to study river suspensions.
Using an acoustic Doppler current meter, geographers of Moscow State University studied six Russian rivers that differ in depth and length, the nature of the flow and geographical location: Moscow, Yenisei, Lena, Ob, Kolyma and Selenga. The measurements of the current velocity and echo signal intensity were compared with the results of water sampling from different depths in the same rivers. In water samples from the surface, from the bottom and from the thickness of the reservoir, the authors of the study determined the amount of suspended solids using analytical methods. The relationships derived from this comparison later formed the basis of a large turbidity dataset. These are tens of thousands of values for each river.
The approach made it possible to obtain fundamentally new models that describe the distribution of solid particles along the depth of river flows, as well as to evaluate the flows of suspensions from the mainland into rivers at a qualitatively new level. If the estimates that existed until today were based on single measurements of suspended matter concentrations within the cross section of the river, then the developed technology has become an automated way to obtain data from each point of the stream.
In addition, on the basis of the proposed methodology for the rivers under study, geographers of Moscow State University mathematically determined the conditions for the distribution of particles between bottom and suspended forms. Scientists substantiated the hydraulic criteria under which, with a different composition of the transported material, a smooth distribution of sediments along the depth of the stream or their concentration in the bottom layer occurs.
“Our study will allow us to obtain large amounts of data on the amount of suspended matter carried by large rivers at a new qualitative and quantitative level. This will help to determine the features of the formation of suspended sediment runoff in different rivers. The developed technique is a significant step forward in the study of large rivers, and in the future we plan to include other water arteries of Russia in the analysis, — said the project manager, Doctor of Geography, Associate Professor of the Department of Terrestrial Hydrology of the Faculty of Geography of Moscow State University Sergey Chalov.
The results of a study supported by a grant from the Russian Science Foundation are published in the journal Big Earth Data.