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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 rate of 250 μm/hr. Subsidence in the control experiment (on average 54 μm/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 μm/hr) 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 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.
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Altered surface hydrology as a potential mechanism for subsidence in coastal Louisiana
Abstract. The natural wetlands of coastal Louisiana areexperiencing rapid subsidence rates averaging 9±1 mm yr−1. Recentmeasurements based on GPS data and CRMS surface elevation tables (SETs) haveshown that most of the subsidence is shallow and occurs in the uppermost 5meters. Sources of subsidence and the origin of their spatial variabilityare strongly debated. Here we use CRMS SETs together with historic maps ofcoastal Louisiana to explore two hypotheses: (i) shallow subsidence is aresult of accommodation created by (long-term) deep subsidence processes andself-weight consolidation, and (ii) changes in marsh hydrology (groundwaterand surface water flows) have led to a recent increase in shallowsubsidence. First, we find that, although self-weight consolidation would result ingenerally high observed shallow subsidence rates, it does not explain therates nor the spatial variability of the CRMS SET data. Second, based onhistoric maps, we find that shallow subsidence rates are significantlyhigher for CRMS sites where shipping canals have reduced their distance tothe marsh edge. This is potentially a result from increased sedimentdeposition, but CRMS data also show altered groundwater levels near themarsh edge. We find some indication that prolonged periods of low watercould have led to increases in effective stresses that explain some of therapid rates of shallow subsidence observed along Louisiana's coastline.
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- Award ID(s):
- 1810855
- PAR ID:
- 10167448
- Date Published:
- Journal Name:
- Proceedings of the International Association of Hydrological Sciences
- Volume:
- 382
- ISSN:
- 2199-899X
- Page Range / eLocation ID:
- 333 to 337
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/piahs-382-333-2020
