Coastal Management Tech Notes / Blog


Entries in Storms (1)



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.