(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.

(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:

(III-1-13a)

1

(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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