Korokoro flood warning project

Ethan Coulston - Greater Wellington Regional Council , 20 April, 2020

In December of 2016, Greater Wellington Regional Council (GWRC) was requested by the New Zealand Transport Agency (NZTA) to provide flood warning services in the Korokoro Stream catchment which is located at the north-west end of Petone, Wellington.

Flood warning services were required due to the state highway 2 (SH2) downstream being susceptible to inundation by the Korokoro stream, causing road closures (for example two major floods occurred in 1976 and 2015.

Flooding occurs because the Korokoro stream is confined by a culvert running under a local factory. Provided it is clear of gravel build-up and debris; the culvert has a capacity of 37.5 m3/s. During large floods the carrying capacity of this culvert is exceeded and flooding occurs.

It was intended for flood warning to be provided through the installation of a flood warning network and the subsequent analysis of flood peak timing, to provide a predictive timeframe for when highway downstream should be closed.

The Korokoro flood warning network consists of two sites; a raingauge in the upper Korokoro stream catchment and a water level site in the lower catchment main-stem, on a historic dam.

The network is designed to provide as much warning as practicable for the SH2/Cornish street hotspot. The upper catchment raingauge was intended to provide early warning of heavy rain actually falling in either of the main sub-catchments and provide confirmation of Metservice (New Zealand’s Bureau of meteorology) issued warnings. The river level site is intended to provide confirmation of a physical rise in water level in the stream and the magnitude of the rise. A temporary water level sensor was also installed, for the first year, in the vicinity of Cornish Street to provide data for analysis of travel times of flood peaks within the catchment.

As a newly employed Environmental Monitoring Officer with roughly 6 months of experience, this project was assigned to me so I could learn a number of core aspects of field hydrology (raingauges, bubble water level sensors, pressure transducers, site installs etc.) in a single project. I was thrown in the deep end if you will!

This article will contain all of the aspects of this project (planning, execution, analysis, reflection etc.) and encompasses all of the fundamentals of surface water hydrology.