When geomorphology and human existence collide


As a natural hazards postgraduate, and one with a natural interest in geomorophological processes, I have been talking to people about their perceptions of the changes wrought by the Waiau earthquake of 14 November 2016. It has been a topic of conversation that people have generally readily participated in.

From these conversations I have deduced that a lot of people – both elected officials trying to sound positive in the face of adversity, and people not really understanding the hidden geomorphological process – do not understand the scale of the changes wrought. There are perceptions that somehow the road and railways will reopen again shortly. There are perceptions that the landslide dam hazard will last only a few weeks and then everything will be back to normal, that rivers will flow like they had before the earthquakes. Unfortunately that is not going to be the case.

One of the numerous effects that the landslide damming of these rivers will have is to change the ecology of them. Some of the smaller landslides will not affect the rivers in the long term, as the river will carve out a channel around the toe of the landslide, which in turn will be washed away or stabilize as a permanent feature. Larger landslides however may semi-permanently dam the rivers whose catchment they occurred in, so that the ecology downstream and how people use the river as a resource change too. The largest landslides will completely dam rivers, leading to one of two outcomes:

  1. The river stablizes, and water is able to safely drain into the river without undermining the dam
  2. A potentially catastrophic dam burst event where the dam fails and a large volume of water potentially in the 000’s m³s-¹ occurs

Following and during the latter outcome, a large amount of sediment will be deposited in a very short time, which will completely relay the layout of the riverbed channels. With channels that were occupied now suddenly buried, that means the water may start flowing down old channels that might not have been occupied since before European settlement. Rivers in Kaikoura and Hurunui Districts are affected by landslides, a list of which can be seen on the Environment Canterbury website:

ecan-flood-and-landslide-dam-warning

In the short term with summer coming on and these rivers being popular for fishing, canoeing/kayaking, boating, but also supplying irrigation water to farms and drinking water to a number of small towns, the availability of water could be severely impaired because of the reduced flows. In many small towns where rivers provide recreational opportunities that are the livelihoods of local businesses, the combination of severe flash flood hazards from landslide dams and river levels being too low for jet boat tours, and so forth a lean summer might await. There is also a risk that some of the smaller dams may fail in this time causing short sharp flash flood events where anyone and any property on the affected river bed would be in immediate danger.

In the medium term consents for water takes might change as the long term effects of the landslide dams starts to become clear. Rivers that might have had 15-20 cubic metres per second of flow during summer and be able to allow small takes of water for irrigation might not be able to supply that water any longer. Rivers that have landslide dams on them that are stable, may cease to flow or be reduced so much that any remaining water has to remain in the river just to enable it to perform its ecological and geomorphological functions. In the medium term it will also become clear which rivers will now start to rearrange their floodplains as the volumes of landslide debris that fell into the catchments now starts to get flushed out by floods. Heavy rainfall events causing slips and future earthquakes triggering fresh landslides will happen, potentially causing unforeseen aggradation and avulsion events to start.

The long term will see District and Regional Councils reviewing how they manage landslide dam hazards within their boundaries. They will be forced to examine the legal issues that go with land titles suddenly ending up in a river bed when, under the influence of large volumes of sediment being reworked through their systems, a river begins to avulse to a new course, possibly cutting across properties and livelihoods. Ultimately it will be councils and the central Government talking about how to fund remediation works.

The aftershock sequences might be quieter and the slow quakes in progress under New Zealand might be relieving the faults of accumulated stress. However the geomorphological changes that are coming in the next few years and even decades will affect communities and individuals alike.

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