Coastal Risk Lines

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.