HADCP (Horizontal Acoustic Doppler Current Profiler) was deployed in Mahakam River to continuously monitor suspended sediments. To calibrate this instrument, five boat surveys of velocity and SSC measurement were conducted over the river cross-section. It is found that one survey is anomalous in that the maximum SSC values occur at mid-depth. In this study, the plausible cause of this anomaly and effect hydrodynamics on the sediment distribution of the study area is presented.
Mid-depth peaks of suspended sediment concentration in the Mahakam River
Horizontal acoustic Doppler current profilers (H-ADCPs) are being deployed to obtain continuous measurements of water and sediment discharge in rivers and streams. To this end, array data of velocity and suspended sediment concentration (SSC) along a horizontal line need to be translated to cross-section averaged values, based on a calibration procedure. To calibrate H-ADCP data for monitoring the water and sediment discharge of the Mahakam River, five boat surveys have been conducted, in which velocity and SSC were measured over nearly the complete cross-section. It is found that one survey is anomalous in that the maximum SSC values occur at mid-depth. This research sets out to establish the cause of this anomaly. Analysis of the velocity and SSC dynamics shows that intratidally, suspended sediment transport is dominated by ebb tides. SSC values are higher during the ebb, which is due to enhanced resuspension of sediments. Within a spring-neap cycle, velocity and SSC are largest during spring tide. It is found that the campaign showing an anomalous behaviour occurred under conditions of strong flow, and it uniquely appeared during the rising limb of the seasonal hydrograph. Neither the lateral flow circulation patterns, nor the direction of the velocity vectors explain the observed phenomenon. Locally, there is delayed adaptation of SSC to changing flow conditions, which provides a clue that the SSC profiles at a site may be much dependent on upstream conditions. In the H-ADCP range, there is a clockwise hysteresis between flow velocity and SSC. These observations cumulate to suggest the anomaly can be explained by the advection of near-bed water masses from upstream to the mid-depth region, which may relate to the fact that the H-ADCP was located in a relatively narrow section of the river.