Blog Post #5
Did you feel that Earthquake…
…I was sitting on the sofa on Saturday night and immediately knew it was an earthquake. Possibly the largest I have felt, at least definitely in the top 2. I arrived after the Kaikoura quake of November 2016 or the devastating Christchurch quakes at the beginning of that decade. So I went straight online to Geonet to log how the quake felt for me and actually see how close it was. Where was it? Over 350km away in the very place I had come from?! I still find it hard to comprehend, but logically it makes sense as you have numerous fault lines across the country, and in Gisborne’s case either side of the district. In particular the Hikurangi trough which almost mirrors the coastline as it runs parallel to the east of Gisborne.
As I have been progressing with this project in Gisborne it has become very clear that seismic activity can, and has, influenced the floodplain that the river flows across. The 2007 Gisborne earthquake plays evidence to this. The lower Waipaoa River meanders across the Poverty Bay Flats which is a broad basin infilled with a thick sequence of river alluvium (deposits of clays, silts and sands) with alternating strips of beach alluvium and estuarine alluvium.
Along with the evidence in the photo below, geotechnical investigations have indicated how an earthquake may impact on the land around the lower Waipaoa River. Moreover, there is plenty of literature that informs us of the potential failure modes, with respect to stopbanks, resulting from seismic activity (Bainbridge, 2013).
The question that I have is, is there a way to integrate seismic risk into an asset performance assessment, for a specific design flood event? The two are of different magnitudes. Can you compare, for example, a 100 year flood event with a 500+ year earthquake event? Can you compare apples with oranges? CAN YOU? Would it be prudent to at least acknowledge the risk an earthquake poses? Yes. Can you integrate it within the same assessment for a design flood event? Je ne sais pas!
What I have explored with is creating a function whereby you can flick between the two different events to gauge a high-level understanding of what impacts an earthquake of a significant magnitude (~>6.0RM) may have on the river scheme, based on existing geotechnical investigations. Not only does this come with limitations and uncertainty, but the aforementioned must be acknowledged and reported on.
Earlier this week, as part of the asset performance assessment, I undertook an Engineering Integrity Assessment. A non-intrusive investigation that involved detailed inspections of known critical locations along the river scheme. Walking the length of each site, inspecting the assets present across a range of criteria that will indicate any deterioration since previous inspections or any signs that may pose a risk to the integrity of the assets.
It was a whole day affair (in 34 degrees heat!) that was a pleasant change of working environment. I have always enjoyed getting out on site and seeing what beauty the river and floodplain have to offer. As I approached the end of my final site inspections I came across a nice clearing which provided a great view down the river accompanied by a herd of sheep (although relentlessly my engineering brain queried why there was no vegetative edge protection). On my way back along the river’s edge, on the hunt for some elusive culverts, I saw some rippling in the water and thought “Alas! I have found the culvert”. Nope. A sheep was half in the water casually grazing. Upon noticing me it bolted and ended up fully in the river, as I backed away to avoid scaring it further it returned to the river’s edge but did not climb out of the water. Now my albeit limited farming knowledge concluded that sheep do not swim. From a distance I perched to watch whether it could get out the water, after all it was a steep edge with large boulders to contend with. In the 10 minutes I observed , it continued to casually graze the river’s edge, fully submerged but for it’s head. Showing no signs of struggling I continued on my way, but I’ll always wonder what happened to that sheep. Hopefully it doesn’t drive me too baaarmy…
