Sediment Mixture Controls on Morphodynamics of Experimental Deltas
Abstract
The creation of small scale deltas in the laboratory has proven challenging but significant progress has been made in several labs in recent years. Various types of river and fan deltas have been created, ranging from fine- to coarse-grained, non-cohesive to cohesive sediment, and from sheet flow dominated to strongly channelised. These experiments have elucidated a number of important factors and autogenic behaviours under constant boundary conditions, such as channelisation and mouth bar formation and backsedimentation leading to avulsion. However, there has been very little systematic investigation of scale effects (Kleinhans et al., this conference) and of experimental parameter space, particularly the link between the sediment mixture and autogenic delta behaviour. Here we describe systematic pilot experiments designed to investigate the link between sediment mixture properties, such as grain size, sorting, addition of fines or polymers, and delta morphodynamics, described by channel dimensions, network pattern, planform delta shape, and bifurcation mechanisms. These experiments were designed to be small, fast and repeatable, so that large quantities of data can be collected for different mixtures. They are run in conjunction with other small-scale tests on the same sediment mixtures to quantify the link between the mechanical properties of delta substrate, such as bank erodibility, and delta organisation. Discharge and sediment feed rate were kept constant and the same in all experiments. Delta evolution was recorded by high-resolution time-lapse photography, emphasising the channels by coloured flow and showing sorting patterns in cases with sediment mixtures. Topography was measured at sub-mm resolution at a few time steps for each delta. For sediment we used various combinations of well-sorted fine sand or poorly sorted sand with some fine gravel, and silica flour and/or polymer with benthonite. The uniform fine sand invariably exhibited sheet flow or formed unwanted scour holes because of hydraulic smooth conditions. In contrast, the poorly sorted sand formed well-channelised deltas without scour holes, because the coarser particles cause hydraulically rough conditions.and bifurcations were temporarily stabilised by gravel deposition. Both silica flour and polymer increased channelisation and channel depth in the poorly sorted sand. At the largest polymer concentrations knick points and backward steps appeared similar to those in deltas on cohesive lake beds or bedrock. The polymer percolated to underlying layers which became stronger over time. We conclude that poorly sorted sediment removes the unwanted scale effect of scour holes in fine sand and facilitates sorting-related bifurcation and bar formation. Silica flour increases channelisation by strengthening the banks, whilst polymer increases bank strength and channelisation much more but causes unwanted backward step erosion and channel bed hardening. Strong banks and hardened beds prevent dynamic meandering; hence a major challenge in landscape experimentation remains to scale the properties of the fluvial system and the deltaic system at the same time.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMEP43F..02C
- Keywords:
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- 1825 HYDROLOGY / Geomorphology: fluvial;
- 1861 HYDROLOGY / Sedimentation;
- 4203 OCEANOGRAPHY: GENERAL / Analytical modeling and laboratory experiments;
- 4217 OCEANOGRAPHY: GENERAL / Coastal processes