Laboratory results and field examples of the interplay between sieving, sheetflow and debris flow processes in alluvial settings and their possible change

MILANA, J. P. AND TIETZE K.-W.

Leipzig

Congreso; Sediment 2011; 2011

Universität Leipzig

An experimental plan was designed to study the transition between sheetflood and sieving. That transition was observed in a natural moderate flood as a result of discharge increase. The mechanism was simply that the rate of percolation was overwhelmed by the flood rising hydrograph and as a result the sieve lobe was recycled into a sheeflood alluvial plane. At a laboratory scale, the study done at Marburg University was focused in changing the sediment grain size from 100% fine to medium natural sand (fluvial) till 100% very coarse sand (beach) in 10% steps, and back. These sediments represented the impermeable to quite permeable end members at laboratory scale. The first series was 11 runs, having a subsurface impermeable ground and variable recent sediment permeability and the second series was representing a subsurface permeable ground. Underground permeability was expected to favor taller sieve lobes. Processes were sheetflood dominated with ample lobes, until a 40% of coarse sand, where channel jamming start to make smaller lobes. At 80% of coarse sand added the first sieve lobes occur. Distributary channels capture the fine sand selectively in order to occlude permeability and foster lobe progradation, while lobes are made up of coarse sand. At the run 100% coarse sand, the sediment makes impressive backfill of inlet channel flanked by artificial levees. On the other hand, during the following series, sieving was kept until 70% of coarse sand showing how important is the permeable ground. The backfill is re-incised and fan progradation occurs through distributary channels and sieve lobes. At Run=60% coarse sand, sieve lobes occur together with debris flows, that is, sometimes the transport is clast-by-clast and strands as water percolates, and other times, the mix of coarse and fine sand becomes a slurry mix and advances en-masse. Detention of this debris flow is basically due to water loss and not due to lack of slope. Water loss creates a frictional freezing. At 50% coarse sand debris flows replace completely sieve lobes, while some sheetflood starts to occur due to the surface flow start to reach lower fan positions due to less permeability of the ground due to the added fine sand. Field revision of some alpine and Andean floods suggest that there is a field of coexistence of transport modes, or where the own flood variations (discharge, sediment yield, channelization) are enough to oscillate at the boundary conditions of these transport mode. This show how important is the sediment yield, discharge, and also bed permeability in regulating type of flows. As discharge is a major control on transport and deposition modes in alluvial settings, the expected changes in modal hydrographs in many mountain basins due to the expected climate change could create a conflict between previously expected flood types and the ones that will be fostered the changing conditions. Therefore, a reappraisal of flood risk at populated drainage basins should be a major concern of the societies.