By Bev Mortimer:
There is a high probability that the canal dune Spit will be breached this year. “Should a large wave event occur around the equinox spring tides in September this will be of particular danger to the Spit,” says Dylan Anderson (MSC.) Coastal and Environmental Scientist Afri-Coast Engineers SA. Port Elizabeth.
Aerial Photos: By Mike Miller
This is the alarming conclusion that Anderson comes to after considering all factors and due to “the narrow and vulnerable nature of the sand Spit”. He points out the Spit is 20m wide in places and single erosion events have resulted in a retreat of 20mn in the past. Anderson makes this conclusion “as we enter the particular stormy winter period followed by the spring SON (September. October, November) season with increased probability of cut-off low activity in the region.”
Anderson has nine years experience in the field of coastal and environmental systems. He has extensive experience and knowledge relating to the coastal processes in St Francis Bay having been involved in several coastal studies and projects in the area in recent years. Current coastal projects include: various beachfront and coastal rehabilitation work in Nelson Mandela Bay; a study to establish coastal set-back lines for NMBM; and beach profiling and evaluation of the sand bypass system for the Port Elizabeth and Ngqura Harbours.
Anderson* provides the following interpretation of coastal processes responsible for beach erosion and erosion of the Spit:
St Francis Bay beach lies on the southern shores of the greater St Francis Bay, one of several crenulated bays on the east coast of South Africa. Stabilisation of large headland bypass dune fields, through the development of the Santareme Township, during the 1970s and 1980s, lead to a dramatic reduction in sediment supply to St Francis Bay beach and resulted in rapid shoreline retreat along the St Francis Bay beach which stretches for 3 km between Cape St Francis headland in the South and the Kromme River mouth in the North.
Analysis of aerial photography and beach profiling data indicates that the beach has retreated at 0.5-3 myr -1. Since the 1990s erosion has threatened to undermine beachfront properties and infrastructure leading to the placement of rock revetments along much of the beach.
Where properly maintained these revetment structures have provided effective land protection. In addition significant erosion has been experienced along unprotected areas where pronounced “end effects” have also been experienced.
The northern most 800 meters of beach which is backed by a narrow artificial sand Spit has not been protected by rock revetments and has experienced significant erosion over recent years (Anderson, 2009). This barrier dune is only +/- 20m wide on average and is in imminent danger of breaching, with potential severe implications for the Marina Glades canal system. Erosion of St Francis Bay beach is characterised by acute erosion events caused by large storm waves, occurring over short duration (hours or days), where retreat as much as 20m has been measured during a single event.
These erosion events are separated by intervening longer periods (months or years) of accretion or relatively stable conditions during calm wave conditions. However due to the disruption in sediment supply the long term erosion trend has persisted since stabilisation of the Santareme headland bypass dune field. Recent significant erosion events were experienced in 2006, 2007 and 2008, subsequently the situation has been fairly stable for the past few years. This calm and stable period over the past few years is a natural occurrence and has also been recorded on the beaches in Algoa Bay 80 km east of St Francis Bay.
Due to the orientation of St Francis Bay and the protection offered by the large headland of Cape St Francis, swell conditions within the bay differ from the open coastline. Although offshore waves from the south westerly quarter occur for more that 60% of the year offshore of St Francis Bay, within the bay waves from this direction are much reduced in height due to the large degree of refraction and diffraction around the Cape St Francis headland.
While short period waves generated by local Easterly winds may be of secondary significance on the open coast, waves from this direction are allowed direct approach and therefore account for major wave events within the bay. In addition although fairly infrequent, longer period waves from a southerly to south easterly direction have led to severe cross shore erosion at St Francis Bay Beach.
Beach erosion and erosion of fore dunes is most likely when large waves coincide with raised water level due to: high spring tides, low atmospheric pressure and storm surge conditions. (Storm surge is the phenomena of raised water levels at the coast as a result of sustained strong onshore or side-shore winds). Of particular relevance is the cut-off low pressure system which occurs periodically off the east coast of South Africa causing sudden strong onshore or side shore winds, which can generate large waves and raised water level (through storm surge and low atmospheric pressure) resulting in acute erosion.
The probability of the Spit being breached this year:
Taking into consideration the above factors responsible for erosion on St Francis Bay Beach and the Spit, the following can be noted in terms of the probability of the Spit being breached this year:
* Waves
As part of previous studies, the long term offshore wave conditions (NOW WWIII) for the period 1997 to 2006 were transformed to the inshore using numerical modelling to create near shore wave statistics for St Francis Bay. Analysis of the near shore wave data indicates that large waves greater than 3m in height are most likely to occur in autumn and winter with 39% and 46% of waves over 3m height occurring in these seasons respectively.
* Tides
Two spring tides occur each month with extreme high spring tides experienced at the autumn and spring equinox, in March and September respectively.
* Low atmospheric pressure
The predominant waves from the south westerly quarter are generated by low pressure systems travelling in an easterly direction across the mid-latitudes; therefore the probability of low atmospheric pressure occurring in conjunction with large south westerly or southerly swells is high.
* Storm Surge
Strong onshore winds are more likely to occur during spring and summer due to prevailing high pressure systems off the east coast of South Africa at this time of year.
* Cut-off lows
Cut-off low pressure systems have the highest frequency of occurrence in the SON (September. October, November) Season, although these features do occur during other seasons.
Anderson who has an impressive CV, holds an MSc. (cum laude) Thesis Title: A hybrid approach to beach NMlWU erosion mitigation and amenity enhancement, St Francis Bay, South Africa; BSc Hons Physical Oceanography UCT BSc. Majors: Ocean and Atmospheric Science, UCT
Meanwhile the Riparian Home Owners Association is continuing with its revetments plan to save the spit from being breached in severe sea storms.
Asked whether enough revetments will be in place by September to protect the spit, Tony Butler of the RHOA says: “Our intention is to construct as much of the revetments before winter as our funds will allow, which is why we are asking for donations pending the municipality raising the levy on all canal homeowners.”
Meanwhile on its website, the St Francis Bay Residents’ Association notes that the repair and construction of the revetments to the spit will soon start and “the SFBRA is firmly in favour of these revetments”.
Donations needed
In view of the fact that winter storms are approaching the RHOA is asking home owners to donate funds to it or to pay the R10 000 levy directly to it until the levy issue is resolved. The appeal follows the halting of the raising of a R10 000 levy by Kouga Municipality from each home owner because a few canal home owners objected to the levy and the rock revetments.
In the light of this dispute Kouga Municipality halted the process until the issue is resolved.
Bank details are as follows: Name: Riparian Homeowners Association; Bank: FNB; Humansdorp branch; Code: 210115; Account Number: 6212 924 3466. Reference: your ERF No. (NB: Please ensure this is included to identify your payment).
“We sincerely hope that canal homeowners will give our appeal their full support in order to ensure that we can protect the St Francis canals and thereby protect the value of our properties. In the meantime we will continue in our efforts to resolve the Special Levy issue with Kouga Municipality in order to ensure that the financial burden is shared by all canal homeowners,” a letter to all canal home owners says.
Aerial Photos: By Mike Miller
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