Ero­sion a shore thing - sur­vey

Western Suburbs Weekly - - News -

THE sec­ond year of Cottes­loe coun­cil’s sur­vey of po­ten­tial ero­sion and global warm­ing threats to its beaches in­di­cates sand is be­ing lost at the north end of the town’s 4km coast.

“It does go, but it seems to re­gen­er­ate in the sum­mer, but we have a loss in win­ter that often re­gen­er­ates af­ter a storm,” Mayor Jo Dawkins said.

Con­sul­tants Cardno’s Coastal Mon­i­tor­ing Sur­vey 2015-16 fol­lows ground­break­ing work by the coun­cil in 2008 to iden­tify cliff paths, cafes and util­i­ties un­der threat, and the first sur­vey in 2014-15.

Sur­vey re­sults will aug­ment a beach ero­sion study by Mos­man Park and Fre­man­tle coun­cils started jointly this month.

In Cottes­loe, the 2015-16 sur­vey stated that sec­tions of the beach at North Cottes­loe, iden­ti­fied in 2008 as be­ing at risk from ero­sion, needed con­tin­ued mon­i­tor­ing be­cause there were now signs of a net loss of sand.

How­ever, un­til March, av­er­age shore­lines moved lit­tle at North Cottes­loe and Cottes­loe beaches.

Cam­eras at both sites recorded a sig­nif­i­cant storm the pre­vi­ous May, while another re­cently sent wave wash up to beach shelters and beach walls.

En­gi­neer­ing ser­vices man­ager Doug Elkins said the sur­vey would not iden­tify if storms threat­ened the sea­wall at the In­di­ana Restau­rant.

“It will show us what hap­pens if sea lev­els rise, and what hap­pens if sea lev­els rise and there is also a storm,” Mr Elkins said.

Data was gath­ered us­ing pictures taken hourly by two cam­eras at both beaches, and mea­sure­ments of the height of sand across the beaches at 39 lo­ca­tions in Septem­ber 2015 and Fe­bru­ary-March this year.

It was found the beach from North to Napier streets was mostly sta­ble, ero­sion at three lo­ca­tions near North Street, sand gath­ered near Napier Street, while the each to Mudu­rup Rocks was sta­ble.

Ero­sion up to 1.5m was found twice be­tween the Cottes­loe and Beach Street groynes, and the beach was sta­ble in south­ern Cottes­loe.

“The re­sults gen­er­ally in­di­cated a trend of ac­cre­tion to the north of fea­tures, like Cottes­loe and Beach Street groynes, and ero­sion to their south dur­ing win­ter, and vice-versa dur­ing sum­mer,” the sur­vey’s re­port said.

Rec­om­mended was that the sur­vey con­tinue for at least 5-10 years, more cam­eras in the south­ern half of Cottes­loe, mea­sur­ing sand off­shore to gauge nat­u­ral re­plen­ish­ment, and mea­sur­ing sand grains.

Jo Dawkins and Doug Elkins over­look North Cottes­loe beach.

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