Extensive Flooding and Damage to Coastal Infrastructure along the KwaZulu-Natal CoastMar 20, 2007
Author: Ian Hunter
SA Weather Service
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The Bluff and port entrance to Durban at the height of the storm surge, 19 March 2007. Picture courtesy Clinton Wyness. CLICK IMAGE TO ENLARGE
By Ian Hunter, SA Weather Service
Late in the evening on Sunday 18 March 2007 a cut-off low reached its peak intensity off the KwaZulu-Natal coast. Significant wave height was at its maximum at around 00h00 UTC on Monday - in excess of 7m close to the coast. It was at this time that most of the serious damage to coastal infrastructure was inflicted. To cite just a few examples :
restaurants on the lower marine parade suffered millions of rands damage. In one case there were still staff inside the building who were trapped by the incoming waves and had to be rescued. A restaurant in Margate was also severely damaged
houses located close to the beach along the North and South Coast were flooded and also sustained wave damage. People had to be evacuated - several people had to be rescued from a hotel in Isipingo
in Umkomaas the beach road was destroyed
although the cost of the damage to coastal infrastructure has already been estimated at half a billion rand, the full extent has yet to be determined. For example it is possible that outfall pipes dumping waste material into the sea have also suffered serious damage
amazingly enough only one fatality has been recorded, a man who drowned at Shelley Beach. However there were many injuries - some serious - when people ventured too close to the water and were washed against obstructions by the waves
Fortunately there were no major shipping casualties, even though vessels off Richards Bay dragged their anchors in winds that averaged over 90 km/ hr on Sunday. Both Durban and Richards Bay ports had to be closed to all traffic.
Cut-off Lows (COL's)
The cut-off low, captured by Meteosat-8 at 06h00 UTC on Monday 19 March 2007. CLICK IMAGE TO ENLARGE
These are one of the most important synoptic-scale weather systems to affect South Africa. They are responsible for most large-scale extreme precipitation events. For example in some areas of the KwaZulu-Natal interior the cut-off low of September 1987 caused more flood damage than Tropical Cyclone Demoina in January 1984.
From a marine aspect COL's are no less important. They are typically paired with a high pressure system that effectively blocks the progress of the low - and increases pressure gradients on its peripheral. Some of the highest coastal wind speeds are associated with cut-off lows. From a wave generation point of view, a semi-stationary fetch zone makes for an increased duration factor and enhanced sea state. Furthermore, the swell generated by these systems is often more perpendicular to the coastline and the greater the angle of the swell to the coast, the less the energy-sapping effect of coastal refraction. The anomalous swell directions associated with COL's can also pose a problem for the ports, which have their breakwaters designed for a heavy SW'ly swell.
The development of the cut-off low that intensified rapidly south-east of East London on Saturday, and reached maximum intensity some 700 km south-east of Durban early on Monday morning, was already being captured by the global numerical weather prediction models on Thursday 15 March - i.e. 3-4 days ahead. Not only was the location correctly predicted at this stage - the forecast intensity was also very close to what actually transpired.
Similarly, the global wave models were also foreseeing the 10m waves analysed off the east coast, with a lead time of 3-4 days.
The contribution of the vernal (21 March) equinox and the new moon should not be overestimated - the damage was mainly the result of the storm surge generated by the low. Consulting the sea level tables compiled by the Hydrographic Office, it will be seen that the predicted spring high tide during the period 19-21 March is roughly 2.3 m above the Lowest Astronomical Tide (LAT). However the average high tide for Durban is close to 2m above LAT. These predictions assume average atmospheric conditions - i.e. take away the storm surge and the water level at high tide on Monday morning would have been less than 0.5m above an average high tide.
Some mention has also been made of the nodal tidal signal (period 18.6 yrs), implying that this was a major factor during the above event. However this component, the result of changes in the lunar orbit and the orientation of the moon's axis, is merely related to a long-term cycle in the tidal patterns.
North Beach, Durban - 19 March 2007. This photo was taken several hours after most of the damage occurred (~ 3 am). CLICK IMAGE TO ENLARGE
The fact that the conditions improved steadily after the high tide of Monday morning, despite little change in the astronomical effects on sea level, confirms the major role played by the storm surge. It remains to break down storm surge into its components:
Wind set-up. This increase in sea level is a function of wind speed (Ws), fetch length (the distance which the wind blows over water at speed Ws), the coastal bathymetry and the wind direction relative to the coastline. The wind direction need not necessarily be directly onshore as the coriolis effect would have caused the alongshore SW'lies to generate a shoreward drift in the water column.
However, the wind off Durban had decreased significantly following the gale force winds on Sunday afternoon.
Wave set-up. The coastal bathymetry also affects the way in which the surf increases the mean water level. In the present case, where the significant wave height of swell approaching the beaches was probably in excess of 5m during its peak, wave set-up would have played a major role in increasing the mean sea level.
Wave run-up. This is the highest water level reached due to the extreme, individual waves in a group (the inland extent is the 'inundation'). It increases with significant wave height and wave period - and is inversely proportional to beach slope. Much of the damage on the KZN coast would have been the result of wave run-up.
Inverse barometer effect. When atmospheric pressure drops, sea level rises : 1 cm for every 1 hPa. This can play a significant role in the case of intense tropical cyclones but in this case, where the central pressure did not go below 990 hPa and the lowest pressure on the coast was significantly higher, it would not have played a significant role.
When it comes to tropical cyclones, storm surge is the by far the biggest killer. For example in 1970 a tropical cyclone resulted in 300,000 people being drowned along the Bangladeshi coast. The storm surge associated with Hurricane Katrina was over 6m. Fortunately major storm surge events and extensive coastal inundation do not in general occur along the Southern African coast.
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