By Daniel Collins
The Christchurch earthquakes of 2010-2011 had a disastrous effect on Christchurch and its residents. But one effect could not be known until much later – flooding.
The earthquakes changed the height of the ground across the city, raising or dropping the land by several 10s of centimetres in places. This meant that the flood risk maps we had of Christchurch were no longer as true as they used to be, and they needed to be updated.
An opportunity to update the flood maps came when another disaster struck Christchurch. As an intense southerly moved up the country, many parts of the city were flooded, with water levels peaking between late morning and early afternoon on March 5, 2014. Homes were flooded and roads closed. But thanks to the widespread availability of camera phones, as well as our curiosity in freak events, many people were out and about documenting the event by taking photos. In the interests of understanding future flood risk and improving public safety, this presented a new scientific opportunity we could not miss.
On Friday, March 7, NIWA sent out requests via mainstream and social media asking for photos from the public of flooding about the time of maximum inundation. During the days that followed we received over 600 photos from around Christchurch. Along with a terrain map of the city, the photos would help us estimate inundation depths. Not all photos were used or could be used – the flood level may not have been discernible; the time or location were unknown; or flood data may have been available from other photos or another source. In the end we used around 300 photos to get a fix on 289 points.
One of the people who sent in photos did something extraordinary (though it would be great if it became ordinary). He took photos of exactly the same shot at different times during the flood, producing time-lapse imagery of the rise and fall of the waters.
For each of the 300 photos we used across Christchurch, we located the water level shown in the photo on the LiDAR scan of the city to determine the water level. LiDAR stands for Light Detection And Ranging, and is a means of generating high resolution topographic maps. Where we could not locate the water level or where there were uncertainties we sent a field team to measure the position of the photographed water level. We then extrapolated these inundation depths across neighbouring streets where data were still lacking using an inundation model.
The resulting map, pictured below, shows the extent of flooding across a portion of Christchurch looking east. In the foreground, covered in blue, is the Flockton Basin, which received substantial media coverage at the time and is the subject of on-going flood management challenges.
This information is invaluable in understanding the changed flood risk for Christchurch, and it shows that citizen science can be a great asset when the professional scientists just don’t have the time or the resources to mobilise quickly or broadly enough to make the necessary measurements. The resulting hazard map is being used to plan response and evacuation plans and to calculate human and economic costs of flooding using RiskScape (www.riskscape.org.nz). So a big thanks to everyone who sent in photos.
Dr Daniel Collins is a hydrologist and water resources scientist at NIWA.