PORT WORKS DESIGN MANUAL PART 5 Guide to Design of ...

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exposure to waves. With the same sand size, the beach face slope is usually gentler in
exposed wave condition.
2.3 Types of Beach Sediment Transport
Beach sediment transport is generally characterised by a combination of sediment moving
along the seabed, the so-called bed load transport, and of sediment in suspension, the socalled
suspended load. It can be classified as longshore transport and cross-shore transport,
and is usually expressed as m 3 /year.
2.3.1 Longshore Transport
Longshore transport refers to the movement of sediment in the direction parallel to the
shoreline. It is mainly due to wave breaking at oblique angles to the shoreline, generating
wave-induced currents that transport the sediment in the longshore direction. The direction
and magnitude of the longshore currents and hence the transport are governed by parameters
including wave height, period and direction, seabed friction and beach slope. Since larger
waves break at deeper water and smaller waves break at shallower water, the magnitude of
the longshore current and transport vary across the shore as shown in Figure 2.
It should be noted that waves can approach the shore from a wide range of directions,
depending on the site conditions. Therefore, longshore transport can take place in either
direction of the shore as the directions of incident waves change. The full wave climate in
addition to the extreme wave climate should be investigated when calculating the longshore
transport. The total annual amount of material transported along the shore, irrespective of
directions, is termed the gross longshore transport. The difference between the annual
amount of material transported in each direction, or the net results of the ‘to’ and ‘fro’
movements of the sediment along the shore, is called the net longshore transport. The net
longshore transport is important in assessing the development of the beach plan shape.
2.3.2 Cross-shore Transport
Wave breaking also induces cross-shore transport or onshore-offshore transport in which the
movement of sediment is in the direction perpendicular to the shoreline. While longshore
transport is the primary mechanism for changes in the plan shape of the beach, cross-shore
transport is the means by which the beach profile changes. The response time of beaches to
variation in cross-shore transport can be as short as one tidal cycle during storms or as large