By Daniel Collins
As a follow-up to Waiology’s article in June, here is a full map for the flooding of Christchurch on 5 March 2014.
The map is derived from photos sent in by the public which were generally taken within a few hours of peak inundation. The blue colours represent the depth of water above the ground surface – the darker the deeper. Flood levels are extrapolated around the photo locations using ground levels taken from LiDAR and field survey measurements. Because the analysis hinges on the public’s photos taken at different times of the day, the map can only approximate the depth of flooding across the city, because either we did not receive photos in a particular area or the photos were taken when water levels were not at their highest.
It is natural for some people to want to make decisions about their property based on just this map. But it’s important to remember that this flood map is not an exact blueprint of what flooding will look like during future floods. Some areas flooded in March because of high tide, some because of clogged drains, and of course some because of changes in the ground surface due to the earthquakes.
Alternatively, there may have been isolated islands amongst flooded areas that we did not pick up. The distribution and timing of rainfall would be different during future storms, and some flood protection measures have already been implemented, for example in the Flockton Basin. Therefore basing decisions of future flood risk solely on this map is not recommended.
For a thorough assessment of flood potential around the city you should consult the Christchurch City Council.
Water depths across Christchurch during the 5 March 2014 storm as derived from photos received from the public. Where houses are shown there is no modelled surface flooding on the property; where there are no houses shown surface flooding is indicated by the shade of blue.
Dr Daniel Collins is a hydrologist and water resources scientist at NIWA.
