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Coastal Risk Lines

Project Duration: Long Term

About this category:

Risk is defined as the probability of harmful consequences or expected loss (deaths, injuries, property, livelihoods, disrupted economic activity or environmental damage) resulting from interactions between natural or human-induced hazards and vulnerable conditions. Risk is expressed as the combined effect of the hazard (uncontrollable factor e.g. natural hazard) and vulnerability (controllable factor): Risk = Hazard * Vulnerability

Coastal risk is a reality globally, with KZN being no exception. As such we need to plan for and better manage in light of coastal risk. In South Africa, one of the approaches to this is the development of coastal management lines (CML’s), which is a line which indicates a ‘buffer zone’; seaward of which development is controlled. 

The determination of CML’s requires a number of considerations and inputs, one of the key inputs is that of coastal risk – understanding the physical risk along the coast in light of climate change (sea-level rise), long and short term coastal changes (erosion; extreme events) and natural coastal dynamics (waves climate). 

The risk lines determined considered three time horizons: 

 Scenario Time Frame  Motivation 
 Short Term  5-10 Years  Period correlates with current risk to motivate action
 Medium Term  50 Years  Period correlates with the time span of a generation. Typical design lifetime of non-critical infrastructure
 Long Term  100 Years  Period for long term forecasts. Design lifetime of more critical infrastructure

 

The key components in the determination of the coastal risk line 

 Sea-level rise projections  Sea-level rise modelling is based IPCC projections as well as local level projections (Mather et al.)
 Extreme events (Water levels)  Assessed long term water levels and extreme water levels. Important consideration of the March 2007 event
 Short-term coastal changes  Applied a coastal buffer for short term erosion based on the physical characteristics of the coastline
 Long-term coastal changes  KZN coastline is dynamically stable in the long term at present - No additional buffer was applied for this component
 Wave Climate  The wave run-up was estimated with the approach of Mather et al (2011) for sandy beaches. For rocky shorelines, the method depicted in the EurOtop manual (2007)

 

Drawing the risk line

  • Step 1:  Profiles were drawn every 200m along the coastline in the dominant wave direction (SE). 
  • Step 2: The -15m CD contour line based on the SANHO admiralty charts was assumed to be equal to the depth of closure 
  • Step 3: The position of the estimated wave run-up level for each SLR Scenario was mapped at each profile location. 
  • Step 4: The position was further shifted landwards with based on the short-term erosion buffer 
  • Step 5: For each of the Scenarios, the estimated positions of the Coastal Processes along the profiles were plotted and connected to form the Preliminary Coastal Processes Line.