Environmental Engineering Reference
In-Depth Information
Fig. 2.14
A channel cut through the beach and dune on Collaroy-Narrabeen beach, Sydney,
Australia caused by storm water discharging from a drainage outfall. © Nick Lewis
2.14 Increased Scour by Wave Reflection from an Artificial
Structure
Waves breaking against a solid shore structure, such as a sea wall built of concrete,
stone blocks, boulder ramparts, steel sheeting or timber, are reflected, and generate
seaward currents that carry sediment away from the foot of the wall (Fig.
2.15
).
This reflection scour is prevented as long as a beach is high and wide enough to
stop waves reaching and reflecting from the solid structure, but beach erosion
occurs rapidly once waves reach the backing wall (Fig.
2.16
).
2.15 Extraction of Sand and Shingle from the Beach
Sand or gravel has been quarried from many beaches for use in road and building
construction (Fig.
2.17
), and the result of lowering the shore profile is to allow
larger waves to attack the beach more strongly during storms. Beaches in Jersey
and Guernsey were much reduced by the extraction of sand from beaches by the
German occupying forces during the Second World War for use to build bunkers
