Sediment Compaction in Experimental Deltas: Toward a Meso-Scale Understanding of Coastal Subsidence Patterns

We present the first investigation of subsidence due to sediment compaction and consolidation in two laboratory-scale river delta experiments. Spatial and temporal trends in subsidence rates in the experimental setting may elucidate behavior which cannot be directly observed at sufficiently long timescales, except for in reduced scale models such as the ones studied. We compare subsidence between a control experiment using steady boundary conditions, and an otherwise identical experiment which has been treated with a proxy for highly compressible marsh deposits. Both experiments have non-negligible compactional subsidence rates across the delta-top, comparable in magnitude to our boundary condition relative sea level rise rate of 250 mu m/hr. Subsidence in the control experiment (on average 54 mu m/hr) is concentrated in the lowest elevation (<10 mm above sea level) areas near the coast and is likely related to creep induced by a rising water table near the shoreface. The treatment experiment exhibits larger (on average 126 mu m/hr) and more spatially variable subsidence rates controlled mostly by compaction of recent marsh deposits within one channel depth (similar to 10 mm) of the sediment surface. These rates compare favorably with field and modeling based subsidence measurements both in relative magnitude and location. We find that subsidence hot spots may be relatively ephemeral on longer timescales, but average subsidence across the entire delta can be variable even at our shortest measurement window. This suggests that subsidence rates over a short time frame may exceed thresholds for marsh platform drowning, even if the long term trend does not.

Automated summary

This study presents the first investigation of subsidence in laboratory-scale river delta experiments, revealing significant differences in subsidence rates between the control experiment and the treatment experiment. The treatment experiment exhibits higher and more variable subsidence rates, consistent with field and modeling measurements. This suggests that short-term subsidence rates may exceed the threshold for marsh platform drowning.