EM 1110-2-1100 (Part III)
30 Apr 02
coarser, sand-sized material. The dredging of sand usually encounters less severe environmental objection,
provided that there are few fines mixed with it and that the site has no prior toxic chemical history.
(c) Environmental regulation is changing, and many regulatory questions are outside the usual experience
of coastal engineers. However, a basic coastal engineering contribution to facilitating the progress of a
project through regulatory review is the early collection of relevant sediment samples from the site and
obtaining accurate data on their size, composition, and toxicity.
(3) Properties important in beach fills.
(a) Beach fills have two primary functions: to provide temporary protection to upland property, and to
increase temporarily the recreational space along the shore. Neither function can be well satisfied with
sediment finer than sand. Because the recreational function is inhibited by material coarser than sand, and
because fill coarser than sand is frequently less available, the primary beach fill material is usually sand.
(b) The source sediment for a beach fill is known as the borrow material, and the sediment on the beach
prior to the fill is known as the native material. The sediment property most important for design is the size
distribution of the borrow and native sands. The art of beach fill design consists of calculating the volume
of borrow with a given size distribution that will produce a required volume of beach fill.
(c) Ideally, the median size of the borrow sand should not be less than the median size of the native sand,
and the spread of the sizes in the borrow size distribution should not exceed the spread of sizes in the native
sand. Often it is impossible to meet these ideal conditions because suitable borrow material does not exist
in adequate volume at a reasonable cost. Further, on severely eroded beaches, the native sand may be skewed
to coarser size ranges because the fines have eroded out, producing unrealistic requirements for borrow sand
size distribution.
(d) Beach fill design aims to compensate for the differences between borrow sand and native sand,
usually by overfilling with borrow sand and assuming preferential loss of the fine fractions. A favorable
feature of beach fill technology is the accidental, partial loss of the fine fraction during the dredging and
handling between borrow and beach. There have been cases (mostly anecdotal) where such handling losses
have produced sand fill on the beach that is coarser than the borrow sand from which the fill was derived.
(e) The shore protection and the recreational qualities of a beach fill conflict when coarser sediment sizes
are used. Usually, a beach provides more protection against erosion when its particles are coarser (also when
they are more angular and more easily compacted). However, fill material larger than sand size (about
2.0 mm) will lessen the recreational value of the beach. Also odor and color of beach fill may be objectional
to recreational users; but usually, if the grain size of the material is adequate, the objectional odor and color
are temporary.
(4) Properties important in scour protection.
(a) Scour, the localized removal of bed material below its natural elevation, usually occurs near (and is
usually caused by) marine structures such as jetties, seawalls, bridge pilings, etc. These types of structures
can accelerate tidal currents, focus wave energy, and increase turbulence in the water column.
(b) To prevent scour, it is usual to place a layer of less erodible material on the surface of the sediment
that is subject to erosion. Such a layer is called a revetment or scour blanket. A typical revetment consists
of broken rock, known as riprap, which is essentially a sediment consisting of large rock particles. The
sedimentary properties of riprap which are important as scour protection include its size distribution, density
of the rock material, and the porosity and permeability of the material as placed. The riprap must be heavy
Coastal Sediment Properties
III-1-3
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