Coastal Management Lines

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 (CMLs), which is a line which indicates a ‘buffer zone’; seaward of which development is controlled.

The determination of CMLs 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 key components in the determination of the coastal risk line

Components

Determinant

Sea-level rise projections

Sea-level rise modelling is based on 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) was used

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

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 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.

Coastal Risk and Vulnerability