Changes in Bathymetry and Their Implications to Sediment Dispersal and Rates of Sedimentation in Manila Bay

Abstract

The predominant sediment dispersal pathways and lateral variations in sedimentation rates in Manila Bay, specifically off Cavite, Manila, and Pampanga were determined primarily by using water depth changes over the period 1901 to 1950 and secondarily from sediment-distribution patterns. High-resolution reflection seismic profiles and piston cores have provided information on longer- term sedimentation. Off Cavite and Manila, sediments are transported predominantly to the north of the source. The zones north of the Cavite Spit and west northwest of the Pasig River mouth are major sinks for sediment accumulating at rates as high as 9 cm/yr. Shoaling rates vary across the three study areas: Pampanga Bay showing the least amount of shallowing and the Pasig River area, the greatest. Apparently low rates of sedimentation in the Pampanga Bay could be due to rapid subsidence. A general increase of sedimentation rate in the offshore direction is also indicated by the bathymetric changes. This trend implies low retention of sediments near the coast, which might be due primarily to a relative sea level rise in the bay. The high-resolution reflection seismic data indicate that the relatively high sedimentation rates along the deeper central portions are not a recent trend. However, this long-term trend is probably controlled by the bay's morphology rather than by sea level fluctuations.