EM 1110-2-1100 (Part V)
31 Jul 2003
(h) Sediment suitability. The grain-size distribution of the borrow material will affect the cross-shore
shape of the nourished beach profile, the rate at which fill material is eroded from the project, and how the
beach will respond to storms. Typically, borrow material will not exactly match the native beach (except
perhaps in some bypassing projects). An analysis is required to assess the compatibility of the borrow
material with the native beach, from a functional perspective. A comparative analysis of sand suitability is
also required to economically evaluate alternative borrow areas for a given project.
Early research into compatibility of borrow area material by Krumbein (1957), Krumbein and James
(1965), James (1974, 1975), and Dean (1974) addressed this issue by various comparative analysis
techniques that utilize the sand-size distributions of the natural beach in the fill area and the borrow
material in the candidate borrow sites. These approaches develop a factor, or parameter, indicating
how much fill material is required in light of the different sediment characteristics between the
borrow and native beach materials. They assume that borrow material placed on the beach will
undergo sorting as a result of the coastal processes; and given enough time, will approach the native
grain-size distribution. The portion of borrow material that does not match the native sediment grain-
size distribution is assumed to be lost to the offshore. James (1975) developed this concept into a
method to calculate an overfill factor, RA, and a renourishment factor, RJ. Conceptually, the overfill
factor is the volume of borrow material required to produce a stable unit of usable fill material with
the same grain size characteristics as the native beach sand. The renourishment factor addresses the
higher alongshore transportability of the finer grain sizes in the borrow sands and provides an
estimate of renourishment needs. Use of the renourishment factor is no longer recommended in
beach-fill design calculations; however, details concerning the renourishment factor and it's
calculation may be obtained from the Shore Protection Manual (1984).
Recent research and beach nourishment experiences have questioned the continued use of these
grain-size-based factors, RA and RJ, to estimate beach-fill performance (Dean 2000). Present guidance
recommends that design be based on equilibrium beach profile concepts, an assessment of storm-
induced erosion, and an assessment of wave-driven longshore transport losses; and that these
methods be used to replace or complement the overfill and renourishment factor approaches
(National Research Council (NRC) 1995). In practice, these recommended methods treat sediment
characteristics using a single grain size parameter, the median grain diameter. They do not consider
natural variations in grain size that occur on natural and nourished beaches. However, they have the
advantage of incorporating more of the physics of coastal processes into the design, much more so
than use of the overfill and renourishment factors. The overfill factor attempts to consider the
distribution of grain sizes. Therefore, it does provide an additional piece of information on the
amount of borrow material that might be needed to construct a beach nourishment project in more
difficult design cases where the grain size characteristics of the borrow material differ significantly
from those of the native beach material, especially the case where the borrow sediments are finer than
the native sediments. The overfill factor is discussed in more detail in the next section.
As a general recommendation, a nourishment project should use fill material with a composite
median grain diameter equal to that of the native beach material, and with an overfill factor within
the range of 1.00 to 1.05. This is the optimal level of sediment compatibility. However, obtaining
this level of compatibility is not always possible due to limitations in available borrow sites. Both
the overfill factor and equilibrium beach profile concepts indicate that sediment compatibility is
sensitive to the native composite median grain diameter. As such, the compatibility range varies
depending on the characteristics of the native beach material, with coarse material being less sensitive
to small variations between the native and borrow sediments than fine material. As a rule of thumb,
for native beach material with a composite median grain diameter exceeding 0.2 mm, borrow material
with a composite median diameter within plus or minus 0.02 mm of the native median grain diameter
is considered compatible. For native beach material with composite median diameter between 0.15
V-4-24
Beach Fill Design
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