Author(s): SLAVEN CONEVSKI, COLIN RENNIE, MASSIMO GUERRERO

Linked Author(s):

Keywords: ACDP

Abstract:

Originally designed to profile water velocity and discharge in rivers and estuaries, ADCPs now play a central role in sediment studies. Backscatter intensity and signal attenuation are used to estimate suspended-sediment concentration and flux (Chapter 3). This chapter describes how the instrument’s bottom-tracking BT mode, intended for vessel navigation, has been adopted to infer bedload transport rate and characteristic grain size. This leverages the same acoustic principles presented in Chapter 2: Doppler shift and backscatter amplitude applied to the BT return. The BT mode estimates the velocity of the ADCP relative to the riverbed using the Doppler shift of acoustic echoes reflected

from the bed. If the river bed is mobile, BT is biased by the moving bedload particles. Figure 4.1 shows divergence of the BT speed from the boat speed measured by Global Positioning System GPS-GGA and VTG data, illustrating the “moving-bed bias” noted in the 1990s [109] and later formalized and applied for bedload estimation [146]. The GGA string is a standard NMEA GPS message that provides time-stamped geographic position, typically latitude and longitude, together with information on fix quality, number of satellites, dilution of precision, and antenna altitude, depending on the receiver type and correction mode. When consecutive GGA positions are available, the boat velocity over

ground can be derived from the change in position with time. In contrast, the VTG sentence provides the GPS-based course over ground and speed over ground directly, without requiring differentiation of successive positions. Therefore, both GGA and VTG can be used to estimate the earth-referenced velocity of the ADCP platform, although their accuracy depends on the GPS receiver, update rate, satellite geometry, correction type, and signal quality. For simplicity, herein “GPS” is used to refer to all Global Navigation Satellite Systems (GNSS).

DOI: IAHR, 2026. 10.64697/iahr.watermonograph.010.ch04

Year: 2026