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Bio-erosion of a Limestone Breakwater in Arabian Gulf

Authors: Martin Mulcahy. Angus Jackson

April 2012

ICM are involved in the design and construction of a 300-berth fishing harbour in Umm Al Quwain in the United Arab Emirates. The site is on the exposed coast of the Arabian Gulf approximately 50km northeast of Dubai and is subjected to strong shamal [northerly] winds and waves.  Primary armour is in the 3-6t range.

The project involves the extension of an existing breakwater to increase the overall size of the harbour – this required the removal of a large quantity of the original breakwater rock material during construction.  The original rock had been in place for approximately 30 years.

 Much of the original armour rock that had been submerged below low tide level was found to have holes ranging from “pinhole” up to 15mm in diameter and up to 50mm depth on the exposed faces (see Figure 1).  

With water temperatures from ~ 19 degrees to ~ 35 degrees Celsius the area is rich in marine life and the holes were observed to be caused by rock boring bivalves – pholadidae. The evidence of this organism was found by the presence of live and dead shells remaining in burrows in the rock, and observations were consistent with other documented cases of this kind of bio-erosion. Using a simple acid [vinegar] “fizz” test, the original rock was determined to be limestone.  The bivalve uses its exterior shell to grind into the “soft” limestone rock – it then remains in its burrow for its lifespan of up to 8 years.

Figure 1 Bio-erosion of limestone rock face [larger holes are about 15mm dia]

The new rock was specified as the more durable Gabbro.  While the recovered limestone core rock and secondary armour sized rocks [ < 2t] were considered ok for reuse as they would be covered, the long term durability of the larger 3t plus rock that would be exposed to ongoing bio- erosion needed to be assessed. 

The functional purpose of a breakwater is to dissipate the energy of incoming waves and to prevent large waves from propagating inside the harbour where calm water is required. The function of the primary armour is to provide stability and weight to the seaward side of the breakwater structure and prevent movement in high waves in storm events.

Over the 30 years, the original limestone armour had performed satisfactorily and only minor fracturing was observed.  The percentage of weight loss due to boring for a 3 tonne armour rock was estimated at 0.4%,ie its eroded mass is 2.99t after 30 years.  Even if this rate of weight loss due to bio-erosion continues at a linear rate [which is unlikely] with an original fos >> 2 this potential weight loss was not considered significant.  Reuse, mixed with the new gabbro to produce a consistent finish to the new breakwater was approved. 

It was also considered that there would be some environmental benefit in retaining some habitat for the pholadidae and any empty holes would provide a habitat for other small marine creatures such as crabs and molluscs.

This approach saved considerable cost to the project with some environmental benefits.