Sedimentary Budget - Longshore Drift

Longshore Drift

Longshore drift of sediment is important for distributing sediment along the coast, and is considered one of the most important mechanisms. The longshore drift of sediment can be considered both a source and a sink due to the fact that in some case it can add sediment to a coastline but in others transport sediment away from a coastline. An example of both extremes of longshore drift can be found on the Canterbury coastline in New Zealand, either side of Banks Peninsular. Both the Waimakariri River in the north and the Canterbury Bight in the south of the Banks Peninsula provide large amounts of sediment respectively. The difference is the sediment supplied by the Waimakariri River is a source for the coast's New Brighton spit due to the reversal of southern currents transporting sediment to the south. In contrast the Canterbury Bight has a combination of high energy environments and strong southern longshore currents that transport large amounts of sediment north, which can be classified as a sink, putting a deficit to the coasts sedimentary budget. As a result there is erosion of the Canterbury Bight and a predominantly balanced New Brighton Spit.

Models have been developed for measuring longshore drift which can assist in determining a sedimentary budget, if they are integrated over the correct timescale.

The sediment budget takes into consideration sediment sources and sinks within a system. This sediment can come from any source with examples of sources and sinks consisting of:

• Rivers
• Lagoons
• Eroding land sources
• Artificial sources e.g. nourishment
• Artificial sinks e.g. mining/extraction
• Offshore transport
• Deposition of sediment on shore

This sediment then enters the coastal system and is transported by longshore drift. A good example of the sediment budget and longshore drift working together in the coastal system is inlet ebb-tidal shoals, which store sand that has been transported by long shore transport. As well as storing sand these systems may also transfer or by pass sand into other beach systems, therefore inlet ebb-tidal shoal systems provide a good sources and sinks for the sediment budget.