EM 1110-2-1100 (Part III)
30 Apr 02
enough to resist movement under design currents, the porosity and thickness of the riprap must be adequate
to dissipate fluid energy before it reaches the underlying material being protected, and the permeability must
be adequate to satisfactorily relieve pressure buildup at the seafloor-revetment interface.
(c) For long-term protection, riprap must consist of dense, durable material of blocky shape. Porous
carbonate rocks such as coral and some limestones are not satisfactory, and thin slabs of material of any
composition (such as shale) are usually more mobile than blocky shapes having the same weight. However,
in any situation, economics and available materials may make it advantageous to use materials that depart
from the ideal.
(d) It should be recognized that scour protection in coastal engineering differs from scour protection
encountered in typical transportation projects both in the magnitude of the forces and in their reversing
directions. Though scour protection design for highways is a well-developed art with extensive
documentation, the direct transferal of highway riprap experience to coastal problems is usually
unsatisfactory.
(5) Properties important in sediment transport studies.
(a) The underlying physics of how water moves sediment is not well understood. This is, perhaps, one
reason for the large number of formulas (often conflicting) which have been proposed to predict transport
rates. These formulas are usually functions of fluid properties, flow condition properties, and sediment
properties. Sediment properties commonly used include: grain size, grain density, fall velocity, angle of
repose, and volume concentration. Sediment size distribution and grain shape are also important.
(b) One method used to study sediment transport is to follow the movement of marked (tracer) particles
in the nearshore environment. Ideally, tracers should react to coastal processes that move sediment in a
manner identical to the native sand, yet provide some signal to the investigator that will distinguish the tracer
from the native material. In the last two decades, the most common tracer has been dyed native sand grains.
Typically representative samples of sand taken from the site are dyed with a fluorescent dye and then
reintroduced to their environment. Care must be taken to ensure that the dying process does not significantly
alter the sediment size or density. Transport of these tracers is then monitored by sampling. Because of their
dilute distribution, tracers are a very labor-intensive means of studying sediment movement.
(c) Native sand tracers (trace minerals, heavy minerals) have been used to interpret sediment movement,
but usually these tracers have a size and density that differ from the majority of grains on the beach. The
problem becomes more complicated as beach fills become more common, and inadvertently introduce
non-native tracer grains. See Galvin (1987) for a more thorough discussion of this topic.
(d) Recently, a few sensors have become available that can measure the concentration of moving
sediment. These sensors are generally quite sensitive to grain size and require calibration using sediments
obtained onsite. See, for example, a discussion of an optical backscatter sensor (Downing, Sternberg, and
Lister 1981).
III-1-2. Classification of Sediment by Size
a. Particle diameter.
(1) One of the most important characteristics of sediment is the size of the particles. The range of grain
sizes of practical interest to coastal engineers is enormous, covering about seven orders of magnitude, from
clay particles to large breakwater armor stone blocks. A particle's size is usually defined in terms of its
diameter. However, since grains are irregularly shaped, the term diameter can be ambiguous. Diameter is
III-1-4
Coastal Sediment Properties
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