Coasts and Reefs

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Beyond the Foreshore of some beaches is a backshore zone, characterized by dunes. Dunes are constructed by windblown sediment transported from the foreshore and elsewhere.

Sediment can also be transported to the backshore area during storms, when big waves can reach far inland (note that a storm beach face can be seen well away from the normal beach face).

Slide 10

So where does beach sediment come from ?

Not all shorelines are alike. Whereas some shorelines are dominated by deposition (as is the case for sandy beaches), others are dominated by erosion.

Erosional shoreline area

(material removed)

Depositional shoreline area

(material deposited)

Slide 11

Sediment from coastal erosion

Shorelines characterized by exposed bedrock and strong wave activity are important suppliers of beach sediment.

Minerals of beach sediment derived mostly from eroded rocks along the coast match those of the source rocks.

Black sand

Basalt

Black sand beach, Big Island, Hawaii

Slide 12

Sediment from rivers

…But most sediment supplied to beaches along continental coastlines is delivered to the coast by rivers.

When a river enters a large body of water (e.g. ocean), its flow rapidly decreases, resulting in the deposition of sediment at the river mouth.

The resulting sediment deposit is a delta.

Mississippi delta

(a river-dominated delta)

Slide 13

Sediment transport away from river mouth

If wave action is strong, sediment deposited at a river mouth can be transported along the coastline instead of forming a well-defined delta.

Sediment

reworked by

waves and transported

along coastline

Slide 14

How is sediment transported along a coastline ?

Most waves move toward the shore at a slight angle.

Consequently, the uprush of water (swash) from each breaking wave is oblique.

Slide 15

Beach drift

The direction of swash is oblique.

However, the backwash runs back to the water at a right angle.

Sediment particles are therefore transported in a zig-zag pattern along the beach.

This “beach drift” can carry sand and pebbles hundred to thousands of metres per day.

Slide 16

Longshore drift

In a similar manner, water in the shoreface zone flows toward the shore at an angle, and flows back at a right angle to the shore.