EM 1110-2-1100 (Part III)
30 Apr 02
e. Other factors.
(1) Several other factors affect the fall velocity. A tight clump of grains in an otherwise clear fluid will
fall faster than a single grain because the adjacent fluid is partially entrained and thus the drag on each particle
decreases. However, if the grains are uniformly distributed in the fluid, each will fall slower because, as each
grain falls, replacement fluid must flow upward and this flow impedes the other grains. Likewise, an adjacent
wall will decrease the fall velocity. These effects must be considered in the design and calibration of a
settling tube used to measure the relationship between grain diameter and fall velocity. Theoretical
calculations have shown that an increase in turbulence levels in the fluid should reduce the fall velocity for
large particles. However, this has not been shown experimentally. These and other effects are hard to
measure and parameterize. However, taken together, they are probably of minor importance and only serve
to somewhat decrease the accuracy of fall velocity predictions.
(2) There are no ASTM standards for estimating sand size from fall velocity (see ASTM volume 4.08),
and equipment to measure such fall velocity is not usually offered by the construction materials testing
industry (Soiltest 1983). However, the fall velocity of grains is an essential parameter in coastal engineering
research on sediment transport. Where large grain sizes are of importance and where shell material makes
up a large percentage of the sample, fall velocity may be a preferable way to characterize the material as
opposed to sieve size.
III-1-5. Bulk Properties
a. Porosity.
(1) Porosity, bulk density, and permeability are related bulk properties that arise from the fact that
aggregations of sediments have void spaces around each grain. The porosity P is defined as the ratio of pore
space, or voids, to the whole volume. It is related to the volume concentration N, which is the ratio of the
solid volume to the whole volume; and to the voids ratio e, which is the ratio of pore space to solid space;
by the equation:
P
e'
(III-1-13a)
N
1
N'1&P'
(III-1-13b)
1%e
(2) Porosity is a function of how tightly the grains are packed together, and thus, is not a constant for a
given sediment. As a grain settles to the bed, the greater the effects of gravity relative to the effects of the
lateral fluid stresses over the bed, the lower the volume concentration. That is, the grains have less
opportunity to roll around and find a position of maximum stability (= most tightly packed position). Thus,
grains in the surf zone are typically compacted to near their maximum volume concentration while this is not
the case in many quiet estuaries.
(3) In natural sands, volume concentration is essentially independent of grain size within the sand size
range. However, the volume concentration is complicated by the irregular shape and nonuniform size of the
grains. In general, an increase in nonuniformity of grain sizes increases the volume concentration (decreases
the porosity) because small grains can fit into the pore spaces of the large grains. In engineering terms,
III-1-26
Coastal Sediment Properties
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