Flooding
Murchison is protected from Northerly rain but receives heavy rain from the Western quarter. There is no flood risk to the township from major rivers, unless a major landslip occurred locally to dam the Buller or Matakitaki Rivers. Some smaller risk to property in Murchison exists from local creeks. The main flood risk of any frequency in the region is to roads adjacent to the major rivers in the vicinity, and to farm operations on flood plains and river banks.
There is one flow recorder in the vicinity, on the Buller River at Longford. Historic records exist on the two lakes Rotoiti and Rotoroa, plus the Mangles, Matiri and Matakitaki Rivers.
Risk Profile
The flood risk can be separated into individual catchments:
Buller
The Buller River in its upper reaches is influenced to a great extent by the outflows of Lakes Rotoiti and Rotoroa. These may reduce the magnitude of flooding that might otherwise occur at Murchison, but have the effect of prolonging flood peaks.
The State Highway runs along the Buller Valley, and in at least one position is being eroded by the Buller River. The road has been set back from the river and the riverbank is about to be extensively rock protected to withstand 1% AEP flood events. The Buller is also cutting into the true left of the Four Rivers Plain a little further downstream, causing a loss of land and at least one building to date. Some rock protection exists in this location. Parts of Four Rivers plain is flooded by the Buller River. Surface flooding can occur at Fern Flat Road.
The Johnston geotechnical report identifies a risk of flooding as a result of an earthquake induced slip displacing water from either of the major upstream lakes. No analysis has been done to look at the effect of such displacement. This is unlikely to affect Murchison town unless it was an extreme event. It would have a severe effect on localities closer to the lakes, and river bank areas of the main stem Buller River down below Murchison. The effect would also depend greatly on existing conditions in the river, such as whether it was already in flood.
Mangles
The Mangles enters the Buller some 5km upstream of Murchison. Being a smaller catchment it has less effect on the Buller than do other tributaries local to Murchison.
Matiri
This enters the Buller opposite Murchison. Flows are slightly lowered in peak size by Lake Matiri, and the only likely flooding problems are those which may occur to farmland or roading down the Matiri valley.
Matakitaki
The Matakitaki is a sizable tributary. The original course of the river has been affected by earthquake induced slippage (1929) some 4-5km south of the township, deflecting the riverbed eastwards some 500-600m. The river remains prone to this hazard. The Johnston report however indicates that failure of an earthquake dam is unlikely, so the most likely flood risk is reduced to that affecting farm land close to the river. The township is not directly affected by flooding risk from the Matakitaki River, although land on lower terraces between the township and the river is flood prone in significant flood events.
Maruia
In the Maruia area the river floods at Pointon Road and can also close SH65 at RS4 andRS15.
Slope Instability
Despite the mountainous terrain and high rainfall, the Murchison area has only a moderate risk of large scale rain induced slope instability although small superficial failures are common when the ground becomes water saturated. High rainfall events can trigger debris flows in many mountain streams, more particularly in catchments where basement rocks give rise to numerous boulders. The Tertiary rocks tend erode either as blocks, which are too large to be easily moved by water, or else break down into fine particles. Rock falls can occur in a variety of different rock types and without warning. Even where rock falls can be expected, it may be difficult to eliminate the risk arising from them. For example, the frequency of rock falls on SH6 at Dellows Bluff has necessitated the erection of a barrier fence but this is basically effective in only trapping smaller blocks. Many sources of potential rock falls that could affect roads are even more remote, increasing the difficulty in implementing any mitigation measures.
Slip risks affecting roads are known to be Bluerocks in the Matakitaki Valley (12.2km from Murchison) and Higgins Bluff on SH65 (4.5km from Murchison).
Disruption to communications from slope failure, arising from a severe rainstorm event are likely to be widespread, causing significant damage, by blocking roads and damage to smaller bridges in mountainous terrain. However, unless ill-advisedly located, buildings in the area are generally at low risk. As discussed below, severe earthquake ground shaking will induce widespread slope failures.
Earthquake
The Murchison area has a high earthquake risk, being crossed by the active Alpine Fault and a number of other active faults trending approximately north from that fault. The latter includes the White Creek Fault that ruptured during the 1929 Murchison Earthquake. To the east of the area, the Glasgow Fault ruptured during the 1968 Inangahua Earthquake. The intervening Lyell Fault demonstrates pre-historic rupture of the ground surface. Several other NNE trending faults, such as the Tainui and Tutaki faults, are regarded as potentially active.
Risk Profile
The earthquake risk in the Murchison area is high with damaging earthquakes originating within and adjacent to the area resulting in extensive slope failures with the possibility of surface rupture if one of the active faults is involved. Small to moderate landslides will be widespread with the probability under severe ground shaking several very large landslides. Small scale failures will be even more numerous should a large earthquake event occur when the ground is water saturated. Large landslides are unlikely unless ground shaking exceeds MMVII on the modified Mercalli Scale. MMVII or greater and MMVIII or greater levels of ground shaking are expected to affect the Murchison, on average, every 34 and 230 years respectively. While large landslides can occur in any rock type, the Tertiary rocks are particularly susceptible where bedding planes are unfavourably orientated. Such failures may originate on only moderate, or even gently dipping, hillsides as was demonstrated during the 1929 Earthquake. Major landslides may ride on a cushion of air, as well as incorporating water from rivers on the valley floor, resulting in extensive run out over flat land. They also have the potential to dam and divert rivers as, for example, occurred in 1929 in the Matakitaki Valley and Lake Matiri was formed by a prehistoric landslide. The Maruia Falls resulted from the diversion of the Maruia River by a landslide in 1929 and there is evidence to show that similar diversions had taken place prior to that. Subsequent erosion of the landslide deposits, particularly those in the mountains southeast of the Alpine Fault, will result in increased bed load in the rivers causing aggradation that can affect farm land and bridges.
Although landslide dams have the potential to fail catastrophically, the large size of the blocks they are expected to be composed of makes sudden collapse leading to flooding unlikely. Landslides into permanent or temporary lakes can result in tsunami-type events and prehistoric landslides are known on the eastern side of Lake Rotoiti and at the head of Lake Rotoroa. Similar scale landslides into the lakes would result in devastating water displacement with overflows down the valleys draining them.
Failures from the slopes above roads and collapses of road batters can severely disrupt communications by blocking lengthy sections of roads. However, it is likely that most of the damage to roads will be from debris falling onto them rather than the collapse of the carriageway, except where side cast fill may be present. On flat land collapses of terrace edges can locally result in minor blockages to roads and creeks. Liquefaction is possible on the valley floors were waterlogged sand is present. The collapse of poorly compacted fill, such as forming the approaches to some bridges, is likely.
As well as severe ground shaking, movement on one of the active faults in the area would result in the displacement of the ground surface. In 1929 Murchison Earthquake the road, now SH6, where it crosses the White Creek Fault was uplifted about 2.5m, east side up thrown. In addition, it was offset horizontally by approximately 5m due to the western side of the fault moving northwards relative to the eastern side. Future displacement on the Alpine Fault could be 1m or more, southeast side up thrown. Horizontal movement, with the northwest side moving to the northeast, is likely to be several times the vertical offset. The amount of uplift will gradually diminish away from the fault involved. For example, the uplift associated with the White Creek Fault in 1929 was measured at a distance of 15km east of the fault. Rupture of the Alpine Fault would also displace the waters of lakes Rotoiti and Rotoroa. If the southeast side of the fault is uplifted this would temporarily raise the head of Lake Rotoroa and much of Lake Rotoiti causing serious flooding in the Gowan and upper Buller rivers severe and disturbances of the lakes themselves.
Volcanic
It is possible that an eruption originating from Mt Taranaki or the Taupo Volcanic Zone could result in a minor fall of ash in the Murchison area. This would depend on the size of the event and the wind direction. The risk of volcanic ash reaching the Murchison area is low and should it do so it would possibly result in temporary inconvenience by affecting sensitive machinery and equipment using unfiltered air or water.
Bushfire
A risk exists for significant fire events in the Murchison area but there are sufficient natural barriers to make escalation to a civil defence emergency unlikely.
In the event of a major earthquake it is possible that multiple fire events could occur in combination with infrastructure damage. In this situation allocation of fire fighting resources would need to be prioritised to meet the needs of all the emergency response functions.
Chemical Event
The potential for a chemical related event that is beyond the control and management of the emergency services is considered low. The petrol companies have plans in place for such events, and one outcome could be the need for immediate evacuation of defined areas. The potential for a LPG incident or major petrol spill, for example, could lead to the activation of the community response plan and possibly evacuation measures being implemented.
Pandemic
The threat of an influenza pandemic continues to be rated as a moderate to high risk. This is in spite of the reduced attention in the media to pandemic influenza. Murchison’s relative isolation will not necessarily provide protection against the spread of the disease.
Planning is in place at the national level and regional levels for a possible influenza pandemic. The lead agency in planning and responding to this emergency is the Ministry of Health (at the national level) and the Nelson Marlborough District Health Board (at the regional level). The Nelson Tasman CDEM Group is closely involved in work to support the NMDHB in our region. The Nelson Tasman CDEM Group Pandemic Plan 2006 sets out regional CDEM arrangements.
In general terms, the risk for Murchison from pandemic influenza is the likely restriction on travel and public assembly. This would impact on the community’s ability to conduct business as usual with organisations such as schools, shops, farms, commercial businesses, and voluntary organisations likely to halt. There would be a need for the community to provide support to individuals and families impacted by the disease.