Coastal Management Tech Notes / Blog
FOR PUBLISHED CONFERENCE PAPERS GOT TO http://www.coastalmanagement.com.au/icm-papers/
Stability during overtopping or avoidance of overtopping is a key design issue for coastal structures. Raising the crest height to try to avoid overtopping completely can increase the cost considerably and impact on recreational and visual amenity of the beachfront. Low crested structures are designed to function with overtopping.
It is difficult to design for overtopping and often overtopping, especially with strong onshore winds and storm surge, is underestimated and failure of the crest and then quickly the whole structure can occur. It is always interesting to observe overtopping events as the power of the sea in storm mode is always humbling.
A strong shamal [northerly winds] with storm surge event occurred in mid January The land elevation of the UAE along the Arabian Gulf is not much above HAT and various esplanades and structures were overtopped (Figure 1). The impacts on a sand filled geotextile bag wall and a rock breakwater were observed and photographed.
Wave and water level data is not presently available but the following data has been obtained from various sources:
Hs = 2.8m, Hm = 4.5m, T = 7-8 sec. Water level = ~300mm above HAT
Photo 1 Ajman cornice flooded. The photo was taken several hours after peak water level. It appears that much of the flooding was due to backflow from stormwater drains and not overtopping of the seawalls which were sufficiently high to cope.
Photo 2 Inundation of Jumeirah beachs [JBR]
Photo 3 and 4 Overtopping of 2.5m3 sand filled geotextile seawall. Crest ~ 1.4m above HAT
Photo 5 After storm. No damage to structure. Erosion of perched backfill only.
Photo 6 Overtopping of seaward breakwater near entrance of fishing Harbour. Crest ~2m above HAT and designed to overtop. Rock 3-6t armour. The entrance remained navigatable - but not recommended.
Photo 7 Overtopping of seaward wall – intermittent and in sections only only during “sets”. Solitary waves generated by overtopping but not large enough to cause risk to vessels.
Photo 8 Localised overtopping
Photo 9 After storm. Crest undamaged.
Angus Jackson, International Coastal Management, email@example.com
Innovation requires confidence to "give it a go" and that often comes from trust and mentoring of others who have travelled the road ahead of us. I have been able to implement a number of innovations in my career and I owe much of my successes to a number of extraordinary Engineers who mentored me and gave freely of their time, support and trust. They lifted me up to stand on their shoulders, the shoulders of innovative giants.
These technical notes [or blogs as the younger generation label them] are dedicated to all of my mentors . I can not name them all – but they included Jack Cronin, Sam Smith, Frank Goetsch, John King, Phil Hill, Roy Starkey and Barry McGinnity – THANK YOU. Of these great Engineers, 2 who have now passed on deserve special mention for their trust and patience with me:
Jack Cronin, RIP, who was the chief Engineer of the Gold Coast Council from 1947 until sometime in about the mid-1980’s. Jack was not trained as a coastal engineer but he was a natural one. He was a leader and mentor to many like myself. He was a real gentleman and lived his chosen profession. Once while sick in hospital he took the time to ask a young nurse about herself and found out her husband was at uni studying engineering and was interested in becoming a Coastal Engineer. He suggested he [me] work with the Council in his next vacation. I did and I was sold on Coastal Engineering.
Sam Smith, RIP, who was the Gold Coast City Council coastal engineer from 1967 to about the late 1970’s. Sam “got his feet wet” to solve many problems in implementing a practical and economically achievable coastal management plan for the then small and young City of Gold Coast. He had an inquisitive mind and whenever he came across a fascinating problem or observation in his professional career he wrote one of his legendary “coastal engineering notes”. Often the content of these hand scribbled notes showed a brilliant insight into coastal processes and coast dwelling humans who try to conquer the forces of the sea [or “fart against the wind” as he often put it.]. Copies of his notes are now kept with his collection of reference books at Griffith University.
The older generation of engineers need to make sure we too pass on our successes and failures to the next generation. As part of my efforts, I will attempt to keep Sam's “blogging” practice alive and publish some selected notes of his and new, random, ones from myself and my staff.
Authors: Martin Mulcahy. Angus Jackson
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.