MEASUREMENTS IN THE BOTTOM BOUNDARY-LAYER ON THE AMAZON SUBAQUEOUS DELTA

An instrumented bottom tripod (GEOPROBE) recorded flow and suspended sediment data in the bottom boundary layer above the lower foresets of the Amazon subaqueous delta in 65 m mean water depth in February, 1990. After about two weeks of operation the apparent seafloor at the tripod site rapidly elevated over a 14-hour period by about 44 cm. This sudden change, which was detected by an acoustic altimeter and which caused the loss of signals from the lowermost GEOPROBE current and optical sensors, is thought to have been caused by the incursion of a dense bottom layer of fluid mud that migrated downslope from shallower sections of the foresets. The fluid-mud migration across the outer part of the foresets, if a repetitive and occasional process in this region, could be a major mechanism for episodic seaward growth of the delta. Current velocity profiles are used to estimate shear velocities, u*, and roughness lengths, z(0), during the first two weeks of measurements. The extremely energetic semi-diurnal tidal motion required correction of the estimates of u* and z(0) for acceleration. Average values of u*, and z(0) for the two-week period were 1.7 cm/s and 0.3 cm, respectively. The effects of strong stratification due to flocculated, near-bottom suspended sediment on the estimates of u* are significant for the lower values of u* (u* <1 cm/s).