Coastal Management Tech Notes / Blog

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Friday
Jun102016

June 2016 Erosion stress testing of beaches along Australia's east coast

Figures to be added

June 11;  The last week of wild weather has stress tested beaches and coastal management strategies along the east Australia coast line. As per wave buoy plot from wave rider off Gold Coast, maximum wave heights reached almost 10m in the peak of the storm.

 It is not surprising that developed areas with long term pro-active coastal management strategies to protect beaches and foreshore assets have weathered the test with very little impacts and developed coastlines with “do nothing” or ad hoc strategies have fared badly. 

Examples of pro-active coastal management plans are Gold Coast and Noosa.  Both use a combination of terminal seawalls combined with beach nourishment and coastal control structures such as groynes to widen the beaches seaward of the walls.  In extreme events, the walls are an important element of the plan as they limit the extent of erosion to protect beachfront public and private property.  

Seawalls are a very common type of structure used to protect beachfront assets from erosion damage and are often the first works implemented.  Once beachfront assets are protected, there is a more stable political and economic environment to plan and fund broader coastal management works. 

There is a very simplistic and sweeping statement often heard that building seawalls [rock or other] = No Beach.   Like most things in nature, particularly on a high energy coastline like the east coast of Australia it is not that simple. In reality, seawalls don’t cause erosion problems – they are constructed along beaches with erosion problems.  The impact of seawalls on the beaches depends on how far seaward they are located.  Certainly walls that are badly designed and located too far seaward into the surfzone can exacerbate the erosion in front of the wall and downdrift.

Walls as part of an effective and integrated coastal management strategy should be located sufficiently landward to have no significant adverse effects on the beaches.  Such “terminal” or asymptotic walls located landward of general wave action are dormant most of the time but reduce the extent of erosion during severe storms and eliminate the zone of reduced foundation capacity (Figure 1).  

 

Figure 1 Cross sections of typical terminal seawall for fine weather and storm erosion event conditions

It is my observation over some 40 years of working along the coast that:

  •  Terminal walls that are landward of the surf zone, except for limited periods in severe storms, have little impact on the beach processes and beach recovery after storms. 
  • Seawalls that are located in the surf zone that act as long headlands and do have a significant impact adversely.

 Two examples, which I am very familiar with, are the Gold Coast, and Noosa where terminal seawalls are part of the Coastal Management Plans.  These engineered walls are located on an alignment as far landward as practical and are generally buried under dunes to protect public and private beachfront infrastructure in severe erosion events. Both these councils benefit financially from the high rateable values of the protected beachfront properties.

The Gold Coast has over 20km of terminal rock walls that are mostly buried in the dunes (Figure 2).  The only exception is Palm Beach where there are walls that effectively protected beachfront properties over the last week but beach widening is still to be fully implemented. Walls such as shown in Figure 2 may only be exposed in an event greater than a 1 in 25 year event and the recent event did very little damage.   The City of Gold Coast is progressively protecting public area and the private walls are a requirement for any beachfront development and must be funded by owners to Council’s standard. The beach and dunes in front of these walls are managed in a variety of ways by the Council.  They have been doing this for over forty years and the walls are effective in protecting properties against severe erosion events.  The walls are part of the Town Planning Scheme and private properties are required to be protected by a seawall to the approved design at the owner’s expense.  These walls effectively “draw a line in the sand” as to the location of the wall to prevent ad hoc works too far seaward and emergency works in dangerous conditions.

 

Figure 2 Top of buried terminal seawall at Tugun on the Gold Coast.  This wall was a critical line of defence exposed to wave attack in storms before coastal management works were implemented in the 1980s and 1990s by City of Gold Coast. (Photo ICM 6/5/16)

During the event, the nourished beaches absorbed the wave energy and the artificial submerged reef breakwater off Narrowneck not only protected the beach but produced great conditions for tow in surfing (https://www.youtube.com/watch?v=V6UxtSzN70U). 

The Noosa embayment is downdrift of a large natural headland and suffers from erosion shadows at times (similar to Byron Bay). Walls that were constructed in the 1970’s to protect the beachfront properties along Hastings Street now provide a line of last defence since coastal management works were implemented to protect the beaches.  These works include groynes and beach nourishment (Figure 3).

 

Figure 3 Left; Seawalls in 1970s exposed to waves before coastal management works. (From ICM photo archives) Right: Present beach with walls buried (GoogleEarth).

 

The wave heights were similar but the erosion impacts were more severe in the Sydney area.  The media concentrated on Collaroy where a section without seawalls suffered severe property damage while adjacent areas with seawalls, including the Council street ends, were protected satisfactorily (Figure 4).

 

Figure 4 Collary - WRL photo (published www.news.com.au)

At Byron Bay the recently constructed terminal seawall at Manfred Street protected the public street end, and private property to the north (Figure 5).  Without this wall it is likely that the dunes would have breached causing large scale damage to the roads, public utilities, properties and wetlands behind the dune.

 

Figure 5  Manfred St wall Byron Bay

 

SUMMARY: The recent event showed weaknesses along many sections of the east coast of Australia and many areas failed nature’s stress test. Not surprisingly, developed areas with protection works in place performed the best. 

 

 

Monday
Jun022014

Going beyond the brief

At ICM we pride ourselves on going "beyond the brief".  One of our Engineers, Aaron Salyer, supervising a coastal protection project at Ullal in India recently put this culture into action on a personal level.  During 2 days of rough seas, while about 100 non-swimmers of bystanders and the work crew watched, Aaron rescued 3 persons from drowning over the two days.  Getting them ashore was just the start, he then successfully applied CPR.  3 lives were saved.  He is a good surf swimmer but it still took huge effort and risk.  We are very proud of his efforts - well done, Aaron!

 http://www.coastaldigest.com/index.php/news/64848-australian-engineer-who-rescued-three-at-ulall-beach-is-now-a-local-hero

http://timesofindia.indiatimes.com/city/mangalore/Australian-engineer-saves-2-women-from-drowning-at-Ullal-beach/articleshow/35487985.cms

 

Angus Jackson

2/6/14

Monday
Jun102013

NARROWNECK POST-STORM INSPECTION 5-6-13

Ongoing monitoring of Narrowneck reef is being undertaken by ICM. This is the report on post storm montoring on 5-6-13.

Click to read more ...

Saturday
Feb022013

Gold Coast Beach Inspections after Australia Day Long Weekend Storms January 2013

Storms in January 2013 on Gold Coast caused more upper beach erosion.

Click to read more ...

Saturday
Jan192013

OVERTOPPING OF COASTAL STRUCTURES - Jan 2013 storm, UAE

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.

 

 

 

 

 

 

 

Saturday
Jun302012

PROFESSIONAL MENTORING TO FOSTER INNOVATION

Angus Jackson, International Coastal Management, a.jackson@coastalmanagement.com.au

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.

Friday
Apr272012

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. 

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