Channel bed evolution and sediment transport under declining sand inputs

Coupled field and laboratory investigations explore links between bed surface structure development and sediment transport as sand inputs decline. On the Pasig-Potrero River, we investigated channel recovery following emplacement of sand-rich pyroclastic deposits in the 1991 eruption of Mount Pinatubo, Philippines. As sediment inputs declined from 1996 to 2003, surface grain size increased, clast structures formed, and sediment mobility and bed load transport rates declined. Our field results parallel those from flume experiments studying highly energetic channels with similar sand contents. As the feed mixture shifted from 70% to 40% sand, surface grain size and gravel interactions increased, forming clusters and armor patches. Bed reorganization into patches initially accommodated coarsening with minimal effect on bed load transport rates, roughness, or friction angle within sandy transport zones. As sand declined further, gravel jams interrupted the connectivity of sandy transport zones, lowering transport rates. Patch development lowered the threshold for the transition from sand-dominated to gravel-dominated transport, focusing this transition over a narrow range of sand content.