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The AAPG/Datapages Combined Publications Database
Journal of Sedimentary Research (SEPM)
Abstract
Research Articles
On the Stabilizing Influence of Silt on Sand Beds
Abstract
In marine environments, sediments from different sources are stirred and dispersed, generating beds that are composed of mixed and layered sediments of differing grain sizes. Traditional engineering formulations used to predict erosion thresholds are however, generally for unimodal sediment distributions, and so may be inadequate for commonly occurring coastal sediments. We tested the transport behavior of deposited and mixed sediment beds consisting of a simplified two-grain fraction (silt (D50 = 55 µm) and sand (D50 = 300 µm)) in a laboratory-based annular flume with the objective of investigating the parameters controlling the stability of a sediment bed
. To mimic recent deposition of particles following large storm events and the longer-term result of the incorporation of fines in coarse sediment, we designed two suites of experiments: (1) “the layering experiment”: in which a sandy
bed
was covered by a thin layer of silt of varying thickness (0.2–3 mm; 0.5–3.7 wt %, dry weight in a layer 10 cm deep); and (2) “the mixing experiment” where the
bed
was composed of sand homogeneously mixed with small amounts of silt (0.07–0.7 wt %, dry weight). To initiate erosion and to detect a possible stabilizing effect in both settings, we increased the flow speeds in increments up to 0.30 m/s. Results showed that the sediment
bed
(or the underlying sand
bed
in the case of the layering experiment) stabilized with increasing silt composition. The increasing sediment stability was defined by a shift of the initial threshold conditions towards higher flow speeds, combined with, in the case of the mixed
bed
, decreasing erosion rates. Our results show that even extremely low concentrations of silt play a stabilizing role (1.4% silt (wt %) on a layered sediment
bed
of 10 cm thickness). In the case of a mixed sediment
bed
, 0.18% silt (wt %, in a sample of 10 cm depth) stabilized the
bed
. Both cases show that the depositional history of the sediment fractions can change the erosion characteristics of the seabed. These observations are summarized in a conceptual model that suggests that, in addition to the effect on surface roughness, silt stabilizes the sand
bed
by pore-space plugging and reducing the inflow in the
bed
, and hence increases the
bed
stability. Measurements of hydraulic conductivity on similar
bed
assemblages qualitatively supported this conclusion by showing that silt could decrease the permeability by up to 22% in the case of a layered
bed
and by up to 70% in the case of a mixed
bed
.
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