Acoustic techniques for the indirect measurement of sediment transport

Measurement of bed-sand transported in river streamflow is desirable for the evaluation of many issues related to river hydro-morphodynamics, such as climate change impacts, maintenance of navigation way and water intakes capacity. Suspended- and bed-load have traditionally been investigated by deploying traps and pressure-difference samplers. These measurements procedures are difficult, especially in large rivers, where permanent installations like pits or slots are unfeasible and water sampling may take from hours to days for a single cross-section. More recently, the ADCP was used to assess (i) the backscatter from suspended particles to be related to suspended-load, and (ii) the Doppler velocity induced from moving particles at the river-bed thus inferring the bed-load rate. Time and space resolutions of the ADCP’s methods are particularly attractive for the investigation of flow field-sediment patterns. Unfortunately, these techniques strongly depend on acoustic properties (e.g., frequency, pulse length) coupling to in field features (e.g., sediment size and concentration, bed sedimentology and roughness). Aiming to solve the inherent ambiguities in acoustic methods, some laboratory and field tests were conducted. Regarding the suspended-load, the calibration of backscatter at different frequencies versus water-sediment samples demonstrated the ability of multi-frequency in reducing the ambiguity in sand concentration and size assessments at the large Parana River (Argentina). For the bed-load rate, tests were performed aiming to parameterize the Doppler velocity detected near a flume sand-rippled bed on the basis of acoustic properties of used device. This relationships may be applied for the interpretation the induced bias of bed-load on ADCP’s bottom tracking (i.e., the instrument ability of measuring its velocity relative to a fixed bottom) to finally infer the actual rate of transported sediment near bed.