Laboratory Monitoring of Bedload Transport Rates by Means of Hydro-acoustic Techniques

Accurate estimation of bedload transport has been an engineering challenge since the previous century. The traditional measurements are highly uncertain and often statistically invalid. Several studies have investigated the use of the bottom tracking (BT) mode of acoustic current Doppler profilers (ADCPs) for evaluating bedload transport. Some previous laboratory experiments have proved that the filtered apparent bedload velocity efficiently matched the spatially averaged bedload velocity. One of the objectives of these experiments is to confirm the validity of the already established methods in the previous studies by using different bedload sediment size and concurrent deployment of two ADCPs working with four frequencies (0.5MHz, 1MHz, 3MHz M9, by Sontek, and 2MHz StreamPro, by RDI). Simultaneously, the bedload transport was monitored by high-speed cameras, and continuous bedload transport rate measurements were conducted at the end of the measurement section. Besides the apparent bedload velocity, the ADCPs also register the backscattered (BS) signal from the sediment bed, which appears to be sensitive to the bedload transport conditions and the type of the sediment particles. The results confirmed the capability of these acoustic instruments to measure the bedload velocity by demonstrating a strong correlation with the physical transport measurements. The apparent velocities measured by the 3MHz and 1MHz demonstrated similar results; the 2MHz gave lower values with 2-4.5 times magnitude difference. The corrected BS signal documented a clear correlation with the apparent bedload velocity, more precisely with the change of the bedload transport condition. The variation between the results from the two instruments can be assigned to the different acoustic geometry, internal processing and availability of the acoustic parameters needed for correction of the backscattered signal.

The final goal of this study is to explore more possibilities for measuring different bedload transport characteristics and eventually developing a technique to estimate the bedload transport rate using only ADCPs.