Heavy weekend rain offered Northland Regional Council staff the first real chance to measure the impact of upgrades made since the NRC took the scheme over from the Far North District Council 13 months ago.
The Regional Council is part-way through a several-year, $900,000 upgrade of the century old scheme – one of the region’s biggest - which is designed to help protect Kaitaia and surrounding farmland from flooding.
Bob Cathcart, the Regional Council’s Land Operations Manager, says initial works have concentrated on a several-kilometre long stretch of the river running from Rongopai Place to the Whangatane spillway. That section runs past Kaitaia’s central business district, with Rongopai Pl in particular prone to regular flooding.
Work to date – funded via a special rate on approximately 5000 properties that make up the Awanui Scheme catchment - has included the removal of trees and other debris as well as tonnes of accumulated silt from riverbanks.
Mr Cathcart says a team of four Council staff took more than 100 river flow measurements in Kaitaia during the weekend’s heavy rain and is very pleased with the results.
Typically water about one metre deep flows down the river during summer, rising to about 2.5 metres during the wetter winter months. However, heavy rain can boost that figure considerably and once the river level reaches about 7.5 metres at Church Rd there is a very real risk of serious flooding in urban Kaitaia.
Mr Cathcart says at the weekend the river had peaked at about 5.3 metres, with a flow of about 135,000 litres per second. Prior to the partially-completed upgrade, the river would have been roughly another 0.5 metres higher (5.8 metres) to carry that sort of flow.
“While a drop of 0.5 metres might not sound like much to a layperson, it means the river can already carry almost 12 percent extra before there is a serious risk of flooding to Kaitaia.”
The Council’s hydrologists estimate that prior to the partially-completed upgrade of the scheme, Kaitaia was likely to face flooding once the flow in the river reached about 228,000 litres per second. That figure had now increased to about 255,000 litres a second – an increase of 11.84%.
Mr Cathcart says while restoring the scheme to its as-built condition is expected to take several more years, Council staff expect it will eventually offer Kaitaia’s urban area protection from a one-in a hundred year flood.