City Prepares For Predicted Rise In Sea Level

A predicted half metre rise in sea levels in the next 100 years has prompted a call for development restrictions in coastal and low-lying areas.

Maintenance of existing natural protection systems such as sand dunes, beaches and vegetation should also be implemented or stepped up before costly or irreversible losses occur.

These recommendations are contained in the Study of the Effects of Sea Level Rise for Christchurch by Derek Todd of Tonkin and Taylor Ltd.

In 1995 the Inter-governmental Panel on Climate Change prepared predictions for sea level rise. Its "best estimate" was a rise of 0.2m ± 0.15m in 50 years and 0.5m ± 0.3m in 100 years.

To better understand what this might mean for the City's coastline, the Council commissioned the study to obtain a "best guess" of the effects on the coast and adjoining areas including Brooklands Lagoon, the Estuary, rivers and groundwater.

Derek Todd's findings suggest that while the Waimakariri River stopbanks should provide sufficient protection for sea level rise over the next 100 years, water levels will rise in Brooklands Lagoon, putting an additional 420 ha at risk of inundation by 2100. The tidal gates on the Styx River will need to be upgraded.

The dune system will be subjected to an increased frequency and intensity of coastal storms, requiring greater dune maintenance. However, with the exception of some low areas, the dune system should continue to provide adequate protection from inundation. Sand encroachment may increasingly become a problem in areas where the dunes are strictly confined by Marine Parade.

Water levels in the Estuary will rise putting an additional 80 ha and $208M of assets at risk from inundation from a 1:100 year tidal event by 2100. Ecosystems along the Estuary margins will suffer from a loss of biodiversity and natural character unless provision is made for their landward migration.

Assets at risk from flooding in the lower reaches of the Avon and Heathcote Rivers will increase by an estimated factor of 600 per cent for a 1:100 year tidal event by 2100. The saline interface with freshwater will move upstream causing die-off of plants which are not salt-tolerant.

Due to increased trapping of sand in the tidal delta of the Estuary, less sand will be available to beaches south of the Estuary and coastal erosion may become an increasing problem at Clifton, Sumner and Taylors Mistake. With rising sea levels there is also an increased risk of saltwater intrusion into groundwater.

The challenge now is how to plan for the potential effects of sea level rise, particularly when considering proposals for the development of coastal areas. The report recommends restrictions on new land-use development in areas that may be affected within 100 years and restrictions on infill in areas that have already been developed.

To boost natural protection systems ongoing maintenance programmes such as dune restoration, vegetation control and beach renourishment should be implemented or enhanced now, says the study.

Existing protection structures requiring upgrading after 2050 to maintain the present level of protection include the Heathcote and Styx tidal gates, the Lower Avon stopbanks and protection walls around the Estuary. Before considering longer-term protection options the Council should undertake a cost- benefit analysis of implementing these options against the alternative option of doing nothing and planned retreat, says the study.

(jenny.ridgen@ccc.govt.nz) Jenny Ridgen
Environmental Scientist