EM 1110-2-1100 (Part III)
30 Apr 02
seaward) during part of the year and to the left during the remainder of the year. If the left and right
transports are denoted respectively QR L and QR R, with QR R being assigned a positive quantity and QR L assigned
a negative value for transport direction clarification purposes, then the net annual transport is defined as QR NET
= QR R + QR L. The net longshore sediment transport rate is therefore directed right and positive if QR R > QR L,
and to the left and negative if QR R < *QR L*. The net annual transport can range from essentially zero to a large
magnitude, estimated at a million cubic meters of sand per year for some coastal sites. The gross annual
longshore transport is defined as QR GROSS = QR R + * QR L *, the sum of the temporal magnitudes of littoral
transport irrespective of direction. It is possible to have a very large gross longshore transport at a beach site
while the net transport is effectively zero. These two contrasting assessments of longshore sediment
movements have different engineering applications. For example, the gross longshore transport may be
utilized in predicting shoaling rates in navigation channels and uncontrolled inlets, whereas the net longshore
transport more often relates to the deposition versus erosion rates of beaches on opposite sides of jetties or
breakwaters. (It is noted, however, that the latter may capture the gross transport rate in some cases.)
b. Modes of sediment transport.
(1) A distinction is made between two modes of sediment transport: suspended sediment transport, in
which sediment is carried above the bottom by the turbulent eddies of the water, and bed-load sediment
transport, in which the grains remain close to the bed and move by rolling and saltating. Although this
distinction may be made conceptually, it is difficult to separately measure these two modes of transport on
prototype beaches. Considerable uncertainty remains and differences of opinion exist on their relative
contributions to the total transport rate.
(2) Because it is more readily measured than the bed-load transport, suspended load transport has been
the subject of considerable study. It has been demonstrated that suspension concentrations decrease with
height above the bottom (Kraus, Gingerich, and Rosati 1988, 1989). The highest concentrations typically
are found in the breaker and swash zones, with lower concentrations at midsurf positions. On reflective
beaches, at which a portion of the wave energy is reflected back to sea, individual suspension events are
correlated with the incident breaking wave period. In contrast, on dissipative beaches, at which effectively
all of the arriving wave energy is dissipated in the nearshore, long-period water motions have been found to
account for significant sediment suspension. For dissipative beaches, the suspension concentrations due to
long-period (low-frequency) waves have been measured as 3 to 4 times larger than those associated with the
short-period high-frequency incident waves (Beach and Sternberg 1987).
c. Field identification of longshore sediment transport
is placed herein upon field,
type, measurement and identification of littoral transport. Laboratory measurement of longshore sediment
transport is generally thought to underestimate prototype transport rates, primarily because of scale effects.
Laboratory measurements are also complicated by the need to establish a continuous updrift sediment supply
in the model.
(1) Experimental measurement.
(a) Longshore sediment transport relationships are typically based on data measured by surveying
impoundments of littoral drift at a jetty, breakwater, spit, or deposition basin; bypassing impounded material
(e.g., at an inlet); or measuring short-term sand tracer transport rates. Other techniques focus upon
measurement of only the suspended load transport. Longshore sediment transport estimates using
impoundment are believed to come closest to yielding total quantities (i.e., the bed load plus suspended load
transport), and typically represent longer-term measures (i.e., weeks to years). It is these longer-term, total
transport quantities that are of central importance to practical coastal engineering design. Impoundment
techniques are discussed below in Part III-2-2.c.(2).
Longshore Sediment Transport