Meeting the flood challenge in post-quake Christchurch
In June 2011, the Christchurch landscape was physically deformed by a Magnitude 6.2 earthquake and its sequence of violent aftershocks. The Port Hills were raised by about half a metre and slippage along buried faults caused uplift and subsidence, redistributing ground levels throughout the city. More than 80 percent of central and eastern Christchurch suffered subsidence—in some areas the ground dropped by more than a metre.
The gradients of pipes and open-channels used to drain surface-runoff water had suddenly changed. Systems that worked during storms and high tides were now ineffective putting some parts of Christchurch at serious risk of flooding. City authorities needed to establish ‘the new downhill’ to ensure satisfactory water flow.
The evolution of a solution
Christchurch City Council (CCC) quickly realised the potential problems, particularly in “at risk” areas such as Dudley Creek where nearly 600 properties were exposed to heightened flood risk.
New models of post-earthquake waterway behaviour and flood mitigation strategies were urgently required. To achieve this, the council needed new accurate, city-wide water level and flow data along with aerial laser surveys of the city's changed topography.
NIWA proposed a solution for monitoring open-channel water-ways across the entire city. This involved the supply and installation of 42 new solar-powered hydrometric monitoring stations to provide the necessary rainfall, water-level and flow data to assist with long-term drainage recovery planning.
Gathering the data
NIWA technicians assembled the equipment and field team members then installed the network of Neon-based monitoring stations at selected waterways, city-wide. These record water-level and flow data and this may be viewed remotely on the Neon server, via Internet connection, in near-real time. Two rain sites were also installed. Field gaugings were completed at all flow sites during a range of storm events to assist rating development for modelled flow data.
NIWA sub-contracted the subsurface pipe-flow component to an external contractor who monitored flow at 16 additional locations across the city.
Waterflow data from two monitoring stations on Dudley Creek showing water peaking at 1.2 metres—twenty times normal flow levels. [Photo: NIWA]
Several of the Neon-based surface water monitoring sites are measuring water level and velocity, with the new generation Unidata Starflow QSD ultrasonic doppler instrument. These are used in combination with precision water level sensors. Starflows are inexpensive, completely sealed and can tolerate any bidirectional backwater effects resulting from the city’s generally-flat terrain.
Left: A Starflow, mounted in the creek bed, measures water velocity, depth and temperature. Right: The solar-powered monitoring station on Dudley Creek. [Photos: NIWA]
Monitoring stations are installed in a wide range of locations—from suburban streets to an estuary drain affected by tidal flow. [Photo: NIWA]
Overview of present monitoring across Christchurch
Christchurch City Council now has a $48M flood-remediation scheme well underway. The scheme will offer increased protection from flooding to more than 500 properties.NIWA’s water flow monitoring network ensures a continuous supply of accurate data to steer the council’s models toward practical remediation strategies.
The NIWA monitoring stations will remain in place for at least two years and provide flood level warnings while the council uses the archived data to continue developing effective drainage strategies to lower risks for residents.
If you have a flow-monitoring need that you think might be resolved by this type of application, please contact the NIWA Instrument Systems team.
Jeremy Bulleid – NIWA Instrument Systems
Mike Bargh – Environmental Information