Lost in the flood

By Gareth Renowden 09/03/2014


CantyfloodsNASAEO

This morning’s NASA Earth Observatory image of the day shows the impact of last week’s heavy rain in Christchurch and Banks Peninsula on the sea around. The light blue colours show sediment washed off the land. If you visit the EO page, they provide a helpful reference image: the region snapped from space in late February, when there’s no sign of any sediment at all.

The heavy rain brought flooding to many parts of Christchurch, as NASA notes:

Christchurch’s flood control infrastructure has been under increasing pressure in recent years because a series of earthquakes struck the area in 2010 and 2011. According to University of Canterbury researchers, the quakes caused land in some areas to drop, while narrowing and uplifting certain river channels. The result is an increased risk of flooding.

This rainfall map from NIWA shows rainfall over the last 15 days (right) compared with the average for the same period (left) and the anomaly (centre). The rain event is immediately obvious as the blue thumb sticking out of the South Island east coast:

CantyfloodsNIWA15day

The Weather Underground’s Christopher Burt provides numbers for the storm:

The powerful storm pounded the Christchurch area between March 3-5 with wind gusts up to 119 km/h (74 mph) and rainfall of 151.6 mm (5.97”) as officially measured at Christchurch’s weather station. Of this amount 100 mm (3.94”) fell in just a single 24-hour period on March 4-5. The suburb of Lyttelton received 160 mm (6.30”) in 24 hours and other suburbs reported storm totals of 170 mm (6.70”). The normal monthly rainfall for Christchurch in March is just 45 mm (1.77”).

For a selection of pictures, see these galleries at Stuff.co.nz: Christchurch, Lyttleton and Banks Peninsula.

The severe flooding in parts of Christchurch – notably the “Flockton Basin” – was caused or made worse by a number of factors. The earthquake sequence caused ground levels to fall by up to half a metre in parts of the eastern suburbs and along the Avon River (see map here), raised and narrowed river and stream beds and damaged or destroyed storm water infrastructure. Add to that a heavy rainfall event that would have taxed the drainage system in pre-quake times, not to mention the tail end of a sequence of high spring tides causing water to back up in the estuary, and you have all the makings of a historic flood event.

Local and national politicians have rushed to promise action to address the flooding, but Christchurch’s problems will not be solved by a crash programme to defend homes that now flood every time there’s a rainstorm. Continuing sea level rise and increasing rainfall intensities — both already observed and projected to get much worse — suggest that serious consideration should be given to managed retreat in some areas, rather than rebuild and defend. How high should you make a stop bank when you expect sea level in a hundred years time to be a metre higher than now?

Christchurch is facing the sort of problems that all coastal cities are going to have to confront over coming decades, brought forward by the earthquake sequence that caused so much death and destruction. Unfortunately for the citizens of the city, the earthquake recovery programme is being overseen by Gerry Brownlee, a cabinet minister who is on the record as a climate sceptic. If he fails to consider the big picture, and neglects to plan for a future when the waters have risen far above today’s levels, then Christchurch will be even deeper trouble every time it rains old women and sticks1.

[Brooce, at his best.]

  1. Mae hi’n bwrw hen wragedd a ffyn – It’s raining old ladies and sticks: Welsh idiom.