Geological hazards

Geological hazards

Geological factors can have a profound influence on any major land-use or development project. The suitability of foundation conditions for an industrial site, the likelihood of landslides disrupting road or rail linkages, the stability of slopes in a hillside subdivision, and the effects of earthquake shaking, faulting, or volcanic eruption, are all geological considerations that should be evaluated when decisions are made to develop the land.

Land instability

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New Zealand is a geologically young country with an actively developing topography. The continuing uplift and accompanying severe erosion has led to the formation of steep slopes, many of them potentially unstable. Causes of instability are many and varied, but mainly depend on the rock composition, the attitude of the rocks relative to the slope, fractures, and the amount of water present. Major landslides can also be caused by both earthquake and volcanic activity.

In some areas rocks have been exposed to the weather for many thousands of years, and the weathering process has produced a mantle of weak material many metres thick on top of the solid unweathered rock. The weathered mantle is very prone to slumping, as is debris (such as slope-wash) which collects over many years on hill slopes. Water is usually a very important factor in land stability. It is no coincidence that many slips occur after heavy rain or prolonged wet periods. The water reduces friction in the sliding process, while also making the material heavier and more likely to succumb to gravitational forces.

Human activities often interfere with the natural processes. Slopes that were stable at one angle, may not be at a steeper one; removing vegetation and the natural debris cover allows greater infiltration of water and exposes rock to accelerated weathering which may cause failure several years. Changing the surface water run-off pattern can cause scouring.

Natural erosion of river banks and coastal cliffs is a common cause of land slipping, and undercutting is particularly severe during storms when the sea or river is carrying more debris and is therefore more abrasive. Development along cliffs has to be investigated very carefully as coastal erosion may only be alleviated by the expensive construction of a sea wall which often has only a temporary effect.

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Developments involving steep slopes need careful engineering to minimise the risk of failure. Planning the routes of roads, railways, and pipelines should take the local geology into account; it may be better to take a longer route and avoid unstable country. Where there is no choice, the cut slopes must be designed with the least likelihood of failure and, if necessary, extensive support systems installed.

Siting of hydro-electric dams has to take into account the strength of the rock because the abutments have to bear the weight of the structure. The conditions of the slopes around a hydro-lake must also be considered, because extensive wetting of the lower part of a slope may cause failure. Foundation conditions for industrial complexes and large structures are important as some materials decrease in volume whe loaded, causing buildings to settle and crack.

Land stability also plays an important role in agriculture. Wholesale clearing of forests has caused increased run-off and infiltration of rainwater and in many areas landsliding has resulted. Grazing of large numbers of stock on steep slopes may cause superficial failures; the effect is to remove pasture, and as grass does not readily grow back onto bare rock, the amount of production is decreased. Landsliding in the heads of river valleys can lead to choking of river systems further down the valley and more loss of productive land later.

Earthquake hazard

Mercalli

There are two types of earthquake hazard: (a) ground shaking which can have a wide-ranging effect and cause failure of structures, distortion of roads and railways, and landsliding, up to a considerable distance from the earthquake epicentre; (b) ground rupture which is a local effect causing damage to structures and ground close to the rupture.

Earthquakes are generated by movement within the Earth's crust and upper mantle ranging from a few tens of metres to hundreds of kilometres below the surface. Shock waves travel through the rock strata and release energy at the surface, causing different materials to respond in different ways. In general, soft, loose or weak material shakes more than hard consolidated rock. Thus movement is felt more intensely on alluvial plains than on surrounding hill country formed of solid rock. The hazardous effects of shaking vary in distance and intensity from the epicentre depending on the magnitude and depth of the earthquake and the geological structure of the area. The Modified Mercalli Scale is one way to quantify the felt effects of an earthquake. In New Zealand, building codes are enforced so that structures can withstand shaking from all but the most severe earthquakes.

Some large shallow earthquakes may cause the ground surface to rupture and crack, sometimes along well defined lines (faults) where movement has occurred before. Structures built across these faults will be destroyed in the case of rupture, so it is important to be able to identify them. Geological mapping can identify faults by recognising reference features that were once joined, such as stream courses or terraces; knowing the age of the reference feature helps assess when the last movement took place and how often movement occurs. Any movement within the last 125 000 years, or repeated movement in the last 500 000 years, classifies a fault as "active". It is recommended that no structures should be built within 20 metres of an active fault. The areas most subject to faulting and earthquake activity are Fiordland and the Southern Alps, the Cook Strait area and the North Island axial ranges, and the Taupo Volcanic Zone; but no part of New Zealand can be considered free of the risk of earthquake activity.

Volcanic hazard

At present the area most obviously susceptible to volcanic hazard is the Taupo Volcanic Zone, where there have been continuing eruptions in historic time. Most eruptions over the last 100 years have been minor events erupting mainly ash and little lava. From the geological record it is obvious that past eruptions have been very violent indeed and such events can be expected again. The explosive eruption of Tarawera in 1886, which caused widespread devastation and the loss of 150 lives, shows the type of eruption that could happen. It is certain that much larger eruptions have occurred, such as the eruption that formed the caldera now occupied by Lake Taupo. The emptying of the crater lake at Mount Ruapehu is certain to occur reasonably frequently, though there are now warning devices which should prevent another disaster like Tangiwai in 1953. Although the immediate hazard is close to the volcano, volcanic ash can have an effect a considerable distance from the source by killing off vegetation and animals, and polluting water supplies. Other areas considered potentially hazardous are Taranaki and Auckland. Although these areas have not experienced an eruption for some time, they are certainly not considered extinct. The last eruption at Mount Egmont was 200 years ago as dated by an ash shower burying Maori ovens, while Rangitoto certainly erupted about 600 years ago and may have erupted since. These may seem long intervals to humans, but geologically they are very short indeed.

Tsunami

This is the name given to unusually high sea waves which are generated by movement of the sea floor, as distinct from those caused by storms. Movement may be caused by earthquakes, landslides, volcanic activity or meteorite impact. They can create havoc as they may devastate low-lying areas, which happened in Indonesia after the Krakatoa eruption in 1883. Several tsunami have reached New Zealand in historic times, but no major damage has ensued. An early warning system is established.

Flooding

This is a hazard that, although not strictly due to geological activity, can be assessed with the help of geological evidence. River flood plains give clues as to the extent of past floods, before rivers were affected and channelled by human activities. Although many rivers prone to seasonal flooding are now controlled by stopbanks, the banks are usually only high enough to contain normally expected floods. There will always be the extraordinary flood which will breach the stop-bank and take the river back to its old flood plain. Indeed, where a river is confined it may actually build up its bed within the stopbanks so that unless dredged, less water is required to top the bank. Where there is geological evidence of flooding before European settlement, it is prudent to site buildings and structures on higher ground.