Coastal hazards
Coastal hazards can cause damage and destruction in estuaries, river mouths and to our coastline as well as to assets and communities in Canterbury/Waitaha.
Here are the current issues faced around coastal hazards in Waitaha.
Issue: Coastal erosion and related impacts on communities and assets in Waitaha
Coastal erosion is the loss of coastal land due to the removal of sediments from the shoreline. It is generally a natural process which occurs whenever the transport of sediment away from the shoreline is not balanced by new material being deposited onto the shoreline.
Coastal erosion can occur over very short periods (hours, days, weeks) or over much longer timeframes (years, decades, centuries). In some coastal environments, coastal landforms naturally undergo cycles of erosion and accretion (the opposite of erosion) on timescales ranging from days to years.
Waitaha has a long and varied coastline, and each different coastal setting has its own unique way of functioning in response to natural drivers of change such as waves (including storms), tides, sediment movement, and changing sea levels.
Natural coastal processes such as erosion only constitute a hazard when they interact with human activities, the built environment and sites of cultural infrastructure.
Coastal erosion at Patiti Point Timaru
Current state of coastline stability
Excluding Banks Peninsula/Horomaka, approximately 60 percent of the coastline of Waitaha is in a long-term erosional state (see image - Current state of Waitaha coastline stability). Most of the coastline between the Waitaki River and Banks Peninsula/Horomaka is eroding.
Shoreline retreat rates are highest for the mixed sand and gravel beaches north of Timaru. Between Waitarakao/Washdyke Lagoon and the Opihi River, the coastline can retreat up to 2.5 metres each year due to erosion.
The remainder of the eroding beaches south of Banks Peninsula/Horomaka have annual erosion rates of around 0.5 metres per year.
Current state of Waitaha coastline stability
North Waitaha and the Kaikōura District
Apart from the extensive, eroding coastal cliffs of the Hurunui coastline, areas of coastal erosion north of Banks Peninsula/Horomaka are more localised. This is the case along the northern few kilometres of northern Pegasus Bay around Amberley Beach and some small stretches of coastline in the Kaikōura District.
However, even beaches that are not eroding (because they are either stable or they are accreting/building out) can still undergo periodic, short-term cycles of erosion and recovery in response to coastal storm events. It is important acknowledge these natural storm-recovery cycles as they can still be very damaging.
The shorelines of sandy spits such as at South Brighton, Brooklands Lagoon and Ashley Estuary/Te Aka Aka can fluctuate widely in response to changes in river and estuary mouth dynamics.
Coastal erosion north of the Rangitata River
Assets at risk from coastal erosion
In areas subject to erosion, there are a wide range of interests at risk. Road and rail corridors are at risk at several locations along the coast and farmland is regularly lost. Some sites of cultural significance and natural resources such as coastal wetlands throughout the region are at risk from long-term erosion.
Sea level rise impacting Waitaha coasts
Sea level rise will have an impact on the way our coastline moves.
The open coast sandy beaches of Waitaha, such as those north of Banks Peninsula/Horomaka, generally have a surplus of sediment and are growing. For our sandy beaches, it is likely that any increased erosion due to sea level rise will be balanced by the rate at which sediment is supplied.
Therefore, most of the sandy open coast beaches in Waitaha are unlikely to experience long-term permanent erosion within this century, but the rate of accretion will begin to slow down.
Many of the region’s mixed sand/gravel beaches are in a state of long-term erosion. Sea level rise will generally exacerbate erosion rates on these beaches but only modestly in most places.
Climate change is also likely to increase the frequency of short-term storm events. Short-term storm events will impact on our coastlines and generally increasing the rate of erosion however different parts of the coastline are likely to be affected differently.
The avoidance of areas prone to coastal erosion in the future and other natural hazards cannot always be achieved. The regional planning framework has an important role helping us decide how and when we may respond in those situations.
Current actions or approach
The Canterbury Regional Policy Statement (CRPS) provides direction that the responsibility for managing coastal hazards in the region is shared between us and the Territorial Authorities.
District Plans and the Regional Coastal Environment Plan (RCEP) for Waitaha place restrictions on subdivision, use and development in areas that are subject to coastal erosion or are likely to be subject to coastal erosion in the future, taking into account the effects of climate change.
We also undertake monitoring and investigations, coastal profile surveying, sea level monitoring, and future shoreline modelling to help implement and inform the regional planning framework.