Flooding and erosion

An approximately 7 km-long coastal stopbank runs parallel to the coast from the northern margins of the lagoon to Beach Road. It was built in the mid-1980s, after an earlier version was compromised by coastal erosion.

The stopbank is managed by Environment Canterbury on behalf of a small rating district made up of farm owners to the north of the lagoon, and mostly eastward of Seadown Road. Those in the rating district also contribute to the maintenance of the drainage scheme and management of the piped outlet to sea, at the southern end of the lagoon.

The stopbank protects the land behind from seawater flooding and also protects the Seadown Drainage Scheme – the main drainage channel which runs parallel to the stopbank.

The coastline in this catchment is eroding at an average rate between around 1.7m to 2m per year. There can be periods of slow erosion, followed by shorter bursts of significant erosion during and after a storm.

The coast has been eroding throughout recent history, but the rate accelerated significantly after the first Timaru Port breakwater was built in the 1870s. The breakwaters restricted the movement of sediment from the south – starving the Waitarakao/Washdyke beach of gravel.

Erosion has caused the Waitarakao Washdyke lagoon area to reduce by about 90 percent. As the lagoon continues to shrink, its ability to act as a buffer against natural hazards - and as a healthy environment and habitat for wildlife - will be further compromised.

The effect of erosion and the shrinking of the lagoon is exacerbated by land use and development, which prevents the lagoon from expanding inland or to the north. Old North Road was named as such because it was historically used as the main north road for Timaru, in part because the current State Highway One would be inundated regularly by the lagoon.

To the north of the lagoon, erosion primarily impacts on farmland, with the historic response being the periodic retreat of the coastal stopbank and drainage infrastructure (and associated loss of private land).

The lagoon has some capacity to absorb seawater that overtops the beach barrier into this area. The coastal stopbank north of the lagoon also provides some protection against the risk of seawater flooding.

However, coastal defences (both natural and built) can only handle so much, and in large storms there is a risk of significant seawater flooding into the Washdyke industrial area and onto farmland to the north of the lagoon.

Ongoing coastal erosion continues to put pressure on, and will eventually compromise, the coastal stopbank, while also shrinking the capacity of Waitarakao Lagoon.

Sea level rise will increase the potential frequency, depth, and extent of seawater flooding in the area.

Decisions need to be made around the use of, functionality, and location of potential coastal defences.

In extreme flood events in the Ōpihi River, it is expected that deep breakout flows will travel into the coastal stopbank then run parallel to the coastline, through coastal farmland, past the Washdyke industrial area and into Waitarakao Lagoon.

There is currently enough space between the existing coastal stopbank and most built development for this water to reach the lagoon without flooding the Washdyke industrial area, though farmland will be significantly flooded.

However, as the coastline and coastal stopbank retreat, and development remains fixed in place, the space for the water to reach the lagoon will reduce. The ongoing shrinking of the lagoon also reduces its capacity to absorb such flooding without affecting surrounding development. For these reasons, it is likely the Washdyke industrial area will become at direct risk of flooding from the Ōpihi River in future.