Te Horo coastal hazards

Te Horo coastal environment

The open coast at Te Horo Beach is a composite (sand and gravel) beach with a low gravel ridge behind the beach. Sand is supplied to this area by the persistent southward longshore transport of sediments from the four large rivers to the north (Whanganui, Whangaehu, Rangitikei and Manawatu Rivers), and the gravel at this beach comes from the Ōtaki River. The supply rate is greater than the transport losses to the south, resulting in long-term shoreline growth. Mangaone Stream at the northern end of the beach settlement is the main pathway that water can travel up to flood the settlement

Present-day erosion and flood hazards

The present-day erosion hazard is what could occur in an extremely large storm (which has a 1% chance of occurring each year in the immediate/ near future). At Te Horo Beach, this is ‘most likely’ to be 9 to 11 m of erosion, and ‘unlikely’ to be more than 13 m.

The area most susceptible to flooding in a very large storm is the Mangaone Stream mouth and the low-lying area around it. A small number of properties at the northern end of the settlement are potentially susceptible to flooding through the road drain at the northern end of Rodney Avenue.

Key findings

The coastline at Te Horo Beach has a good sediment supply that has historically resulted in shoreline growth.
If this trend continues, only a small amount of erosion is projected to occur under low relative sea level rise (RSLR) scenarios associated with extremely large storms. However, the shoreline is projected to erode under higher RSLR scenarios over the next 30–100 years.
For coastal flooding, Te Horo Beach settlement is protected by a low gravel ridge but is susceptible to flooding through a pathway up the Mangaone Stream and from wave overtopping.
Under higher RSLR scenarios, properties within the settlement become susceptible to coastal flooding, as well as low-lying farmland behind the settlement and the main evacuation route out of Te Horo Beach.

A. Composite beach at Te Horo, looking north.

B. Composite beach at Te Horo, looking south.

C. Mangaone Stream

Future coastal erosion hazard

The shoreline at Te Horo Beach has good sediment supply which is projected to continue. As a result, it is projected that under the lower RSLR scenarios for each timeframe shoreline growth will continue and that any erosion is likely be limited to small distances (less than the present-day hazard) if an extremely large storm occurred near the end of the timeframe. However, under higher RSLR scenarios at 30, 50 and 100 years, the shoreline is projected to erode.

Te Horo Beach

The shoreline is projected to:

By 2050, erode by 5 to 21 m
By 2070, grow by 2 m under the lower RSLR scenario, but erode by 30 m under the higher RSLR scenario
By 2120
- grow by an average of 15 m under the lower RSLR scenario in 2120
- erode by 5 m with 0.85 m of RSLR
- erode by 38 m with 1.25 m RSLR
- erode by 70 m with 1.65 m RSL

Future coastal flood hazard

0.4 m and 0.65 m RSLR

There is an increase in the area flooded around the mouth of Mangaone Stream, with Rodney Avenue and some properties around this road becoming potentially vulnerable from direct inundation and wave overtopping. Farmland behind the settlement between Sims Road and Harakeke Road becomes vulnerable to flooding.

1.65 m RSLR

There is an increase in the area flooded which is directly connected to the sea. Flooding that occurs along Rodney Ave has the potential to worsen with additional flooding caused from waves overtopping the beach. There are extensive areas of susceptible land behind the settlement up to Pukenamu Road. Many properties around Dixie Street and Gawler Street are not mapped to become flooded, however the main evacuation route out of the settlement is vulnerable to flooding.

Vulnerability

For coastal erosion, no council infrastructure was mapped as being vulnerable to coastal erosion in the future. Private properties only become potentially vulnerable to coastal erosion under the highest RSLR scenario in 100 years.

The number of properties potentially affected by flooding increases with RSLR. Te Horo Beach Road starts to become susceptible to flooding in extremely large storms with 0.4 m RSLR (30 to 50 years’ time).

How these hazards have been assessed for Te Horo

Coastal science experts from Jacobs assessed the susceptibility and vulnerability of coastal erosion and flooding hazards across the entire Kāpiti District.

The assessment follows Ministry for the Environment’s ‘Coastal Hazards and Climate Change Guidance for Local Government (2017)’. It advises councils to use the high [RCP8.5H+] scenario to stress-test dynamic adaptive pathways, policies and new greenfield and major infrastructure developments.

Coastal erosion

The components used to calculate a potential coastal erosion distance along the Te Horo Beach coastline include:

the natural long-term trend of the shoreline movement (i.e. eroding, growing, or stable)
the amount of erosion which could occur as a direct result of the rise in sea levels compared to land level (termed the Relative Sea Level Rise (RSLR)) over time frames of 30, 50, and 100 years
short-term storm erosion from an extremely large storm which has approximately a 1% chance of occurring in each year, based on observations from the September 1976 storm, and
erosion from dunes restabilising to their natural stable slope following a large storm.

Jacobs used a ‘probabilistic approach’ to tie likelihoods to the erosion distances calculated for each scenario of RSLR. The ‘most likely’ range of shoreline positions has a 33–66% chance of occurring. The ‘unlikely’ shoreline position is where there is a 10% chance that the erosion would reach or be greater than this position.

Coastal flooding

To assess the coastal flood hazard for Te Horo Beach Jacobs mapped the area which is susceptible to flooding by a large storm tide which has a 1% chance of occurring in any year. The maps show the flooding that could occur at the present time and in the future, for RSLR scenarios of +0.4 m; +0.65 m; +0.85m; +1.25m; and +1.65 m. Jacobs used a simple ‘bathtub’ approach, where all land below the storm tide water level is mapped as susceptible to flooding, regardless of connection to the sea. The maps also show areas which could be affected by additional flooding due to wave run-up overtopping the dunes.