EM 1110-2-1100 (Part III)
30 Apr 02
irregularities in one procedure using fractal geometry types of analysis. See Ehrlich and Weinberg (1970),
and Frisch, Evans, Hudson, and Boon (1987).
(2) Grain shape is important to coastal engineers because it affects several other properties, particularly
when the grains are far from spherical, which is the usual assumption. These include fall velocity, sieve
analysis, initiation of motion, and also certain bulk properties, such as porosity and angle of repose. One
particular area of interest to coastal engineers is in the design of man-made interlocking armor units on
breakwaters that have high stability, even when stacked at a high angle of repose. Grain shape has also been
used to indicate residence time in the littoral environment. See Krinsley and Doornkamp (1973) and Margolis
(1969).
(3) However, most littoral grain shapes are close enough to spheres that a detailed study of their shape
is not warranted. Frequently, a qualitative description of roundness is sufficient. This can be done by
comparing the grains in a sample to photographs of standardized grains (see Krumbein 1941; Powers 1953;
Shepard and Young 1961).
(4) Among the earliest research studies in coastal engineering were investigations of sand grain abrasion,
done because of the worry that abrasion of beach sand contributes to beach erosion. These investigations
found that abrasion of the typical quartz beach sand is rarely significant. In general, recent information lends
further support to the conclusion of Mason (1942) that
On sandy beaches the loss of material ascribable to abrasion occurs at rates so low as to be of no
practical importance in shore protection problems.
(5) To achieve the high stress required for quartz to abrade (to fail locally), very large impact forces are
needed. These forces are developed by sudden changes in momentum, the product of mass and velocity.
Given the small mass of a sand grain, large forces can be achieved only by grains moving at high velocities.
But the drag on a sand grain moving in water increases as the square of its velocity, which limits sand grain
velocity to a low multiple of its fall velocity. Because fall velocities are only a few centimeters per second,
it is extremely difficult to achieve a stress between impacting grains that is anywhere near the strength of
quartz. Thus, the rounding of the corners of angular quartz grains in riverine or littoral environments is a very
lengthy process. Sands, silts, and clays found in the coastal environment can generally be considered as some
of the stable end results of the weathering process of rocks so long as they remain on or near the surface of
the earth.
(6) However, abrasion is common in large particles such as boulders and riprap subject to wave action.
The mass of a particle increases with the cube of its diameter, so a minimum-size boulder (300 mm on
Table III-1-2) compared to a minimum-size coarse sand grain (2 mm) (ASTM classification) will have
(300/2)3 = 3.375 million times more mass. If that boulder was a perfect sphere in shape, and this rock sphere
rested with a point contact on the plane surface of another rock, then merely the weight of the boulder would
crush the point contact of the sphere until the area of contact increased enough to reduce the pressure of the
contact to below the crushing strength of the boulder (say 120 MPa, based on Figure III-1-4). The material
crushed at this contact is abraded from the boulder. This process of stress concentration at points of contact
is considered quantitatively by Galvin and Alexander (1981). If the boulder moves, say, with the rocking
motion imparted by the arrival of a wave crest, the slight velocity of the boulder mass provides a momentum
which can produce impact forces in excess of crushing strength at points of contact between a boulder and
its neighbors, thus abrading the rock.
(7) Because it is probable that a large rock will break along surfaces of weakness, the resulting pieces
after breakage will usually be stronger than the rock from which the pieces are broken. Thus, abraded gravel
III-1-20
Coastal Sediment Properties
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