Edgecumbe and the Bay of Plenty have shaky, flood-prone histories
Edgecumbe lies on a flood plain and an active seismic zone.
The township and the surrounding area has an unenviable history of natural disasters: quakes, floods, and debris flows.
Many parts of the plains are below sea level.
A little over 30 years ago, a 6.5-magnitude quake struck the town and the Bay of Plenty. At the time, it was the largest quake in New Zealand in 45 years.
It dropped parts of the town and surrounding rural land by up to two metres, which means when the Rangitaiki river floods, it runs higher than the town's ground level.
According to a 2007 report, this substantially increased an already elevated flood risk, particularly in the southwest of the town.
Areas previously drained by gravity must be pumped, the report said.
Historically, Edgecumbe is prone to floods, earthquakes, and debris flows. Before the region was settled the plains were basically a big swamp.
"Whakatane District communities have been severely impacted by a number of natural disasters over the last 20 years including the Edgecumbe earthquake in 1987, the floods of 1998 and 2004, and the Matata debris flow in 2005."
A flood inundated rural land in 1925, an old newspaper report says.
In the 1987 quake, according to GeoNet, the ground ruptured along a 15km long surface fault on the Rangitaiki plains. "The fault displaced the ground vertically and up to 3m of height change was measured in places."
The plains lie within the Taupo Volcanic Zone, a rift zone running from Ruapehu in the south to White Island and it opened by 1.2 metres during the quake, which was widely felt on a Monday afternoon on March 2, 1987. Its epicentre was at a depth of 10km, 20km southwest of Whakatane.
A GeoNet summary says the hardest hit towns were Edgecumbe, Te Teko, Kawerau, Matata and Thornton, where chimneys toppled and poorly constructed houses suffered serious damage.
"In Edgecumbe the intensity of the shaking cracked asphalt in roads and footpaths, damaged river embankments, toppled a locomotive and tore large power transformers, weighing up to 20 tonnes each, from their mountings.
"In the Rangitaiki Plains the quake ruptured the ground to create a spectacular 7 km surface rift that was over one metre wide."
Edgecumbe's plains cover about 30,000 hectares, flat land with the Rangitaiki river - a large river with three dams - running to the sea at Thornton.
The plains are gradually subsiding and, at the same time, the coast has been uplifted. Flood control has been ongoing since the 1960s.
In 1998, there was a one-in-15 year flood, which was not powerful enough to spill into the floodway, but caused concern about the integrity of some stopbanks.
Then, in July 2004, there were three days of heavy rain dumped on an already saturated catchment, leading to failed stopbanks, spillage, and widespread flooding.
A Niwa summary of the 2004 flood said rain was prolonged, 17,000 people faced drinking water shortages, 3200 homes were evacuated, 450 farms and orchards were damaged and the Rangitaiki burst its banks in a 1-in-100 flood.
There were landslips "aggravated by an earthquake swarm of over 200 shallow earthquakes around Lake Rotoehu, coinciding with the flood impact." At Opotiki, there was 280mm of rainfall in 48 hours.
A year later near Edgecumbe, in a rain-related disaster, there was the Matata debris flow, in which landslips in the catchments behind the town caused debris and tree avalanches, and around 700,000 cubic metres of debris surged towards the town and destroyed more than 100 homes. There were no fatalities.
A Whakatane District Council report, in 2012, said the town was build on old debris flows: "It has been estimated that the May 2005 debris flow event at Matata was caused by a rainfall event with a return period of 200 to 500 years and rainfall intensities of greater than 2mm/minute.
"GNS estimate that it would take a rainfall event of similar magnitude or greater to cause significant future debris flows or debris avalanches in the vicinity of Matata. It is possible that climate change may progressively affect weather patterns to the extent that rainfall events with high rainfall intensities similar to the 2005 Matata rainfall event may occur more frequently than the presently estimated 200 to 500 year return period."
GeoNet, in an analysis, said debris flows were more dangerous than floods. When landslips and rain flood a river, the water gets clogged with silt, trees, and other debris.
"Debris floods contain water so highly charged with sand and silt that it no longer behaves like normal water; it flows faster and is more dense, and is capable of moving larger boulders than could be moved by normal flood flow."