Exploring Sediment Compaction in Experimental Deltas: towards a
meso-scale understanding of coastal subsidence patterns
Abstract
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 of 250 μm/h. Subsidence in
the control experiment (on average 54 μm/h) is concentrated in the
lowest elevation (<10mm above sea level) areas near the coast
and is likely due to creep induced by a rising water table near the
shoreface. The treatment experiment exhibits larger (on average 126
μm/h) and more spatially variable subsidence rates controlled mostly by
compaction of recent marsh deposits within one channel depth (_10 mm)
of the sediment surface. These rates compare favorably with _eld 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 in a given decade or century may exceed
thresholds for marsh platform drowning, even if the long term trend does
not.