St Arnaud area extends southwest from Tophouse to St Arnaud township on the northern shore of Lake Rotoiti. It lies across the Alpine Fault, which separates high greywacke mountains of the St Arnaud Range and Robert Ridge in the southeast from gentler mountains in the north and northwest comprising Beebys Knob and the Buller-Rainy divide. All slopes are subject to movement, but the risk is highest in the higher parts of the St Arnaud Range and the northwestern slope of the Robert Ridge. Most failures are relatively superficial, although (see below) the chances of a large-scale failure occurring increases under seismic shaking. Small failures and the reworking of superficial deposits, such as scree and the products of earlier failures, can give rise to debris flows and the deposition of fans.
Despite receiving high and often intense rainfall, the risk of large-scale, rain-induced, slope failures is only moderate with the northwestern side of the St Arnaud Range and the northern and northwestern flanks of the Robert Ridge being most susceptible. The risk of small-scale failures is high although most are unlikely to have direct serious consequences. The greatest hazard is debris flows arising from small failures, or the remobilisation of older failure, scree or other superficial deposits. The areas at greatest risk are the lower slopes of the ranges, particularly downstream of gullies. On existing fan deposits, creeks can readily change course in response to a severe rainstorm or earthquake event, particularly one that initiates erosion in the upper reaches of the creek, resulting in accelerated erosion and/or deposition. Debris flows or fan deposition could affect culverts or buildings located in their path.
The St Arnaud area has a high earthquake risk, being diagonally crossed by the active Alpine Fault and to the north of the township the Waimea Fault, which trends approximately north across the lower slopes of the Beebys Knob. Both faults, particularly the Alpine Fault, are active with well-preserved scarps arising from surface ruptures during past earthquakes centred on them.
Rupture of the Alpine Fault and, less likely, the Waimea fault, would displace the ground on either side of the faults. On the Alpine Fault, a rupture would likely cause the ground on the southeast side to rise 1-2m relative to the other side. There would also be horizontal displacement with the northwest side moving northeast by several metres. Movement on the Waimea Fault, eastern side uplifted, would likely be less than on the Alpine Fault. Fault rupture would sever all services, including roads such as SH63, that cross them. The Alpine Fault also lies beneath Lake Rotoiti and uplift would raise much of lake, relative to its outlet, causing inundation of the shoreline and flooding in the Buller River.
A large earthquake, including but not confined to rupture of the Alpine or Waimea faults, would cause intense ground shaking of MMVIII or greater on the Modified Mercalli Scale. At such levels of shaking, large landslides can be initiated although failures are likely to be scattered rather than widespread. A large landslide has taken place on the St Arnaud Range and the deposit arising from it is concealed beneath Lake Rotoiti - most likely due to an earthquake. Any large landslide into the lake would result in inundation of the shoreline. Where failures might occur is difficult to predict but the northwestern flanks of the highest ranges are most at risk. Smaller failures will be widespread and will result in an accelerated deposition in water courses, which in turn could lead to flooding and other related problems.
Inundation of the shore of Lake Rotoiti could arise from rupture of the Alpine Fault, severe seismic shaking in the vicinity of the St Arnaud area or major land sliding into the lake.
Severe Weather: Flooding and Snow
The St Arnaud and the surrounding area is vulnerable to severe weather events which have the potential to isolate the village and surrounding valleys. During the event of August 2008, for example, roads, power supplies and telecommunications were all badly affected. Some areas were still without power many days after the event. Tophouse, Rainbow Station, Howard River, Lake Station, Speargrass, and Glenhope were worst affected in terms of power outages. The effect of snow can is particularly damaging for farming operations and other commercial activity. The need for farms and businesses to have backup generators in these areas is extremely important.
St Arnaud appears not to be exposed to intense rainfall from any particular quarter that may give rise to extreme flooding events in the stream network flowing in the vicinity of and through the village. The most severe flooding events in the local stream network tend to arise from convective activity containing intense rainfall cells. Such rainfall activity affected the Upper Motueka and St Arnaud catchment areas in February 1995 and March 2005.
There is no flood risk to the township from rainfall in the upper Buller catchment discharging to Lake Rotoiti and from the Buller River itself. However infrequent flooding of the Kerr Bay lakeshore reserve occurs when lake levels are very high. The main flood risk to some properties in the village and local environs occurs as a result of localised intense rainfall on the north-west facing slope of the St Arnaud range forming the catchments of the Borlase and Black Valley Streams. Unfortunately, there are no rainfall recorders in the village area that have or could record rainfall distribution or intensities that give rise to more extreme flooding in the local stream network.
Risk Profile for Flooding
The flood risk can be separated into individual sources:
This stream rises from two gullies, one either side of Parachute Rock ridge, on the northwest facing slope of the St Arnaud Range. The two tributaries combine approx. 300-400m southeast of the village then flows through the new Brookvale Drive subdivision, passing under Borlase Avenue and Bridge Street to join the Black Valley Stream just downstream of the State Highway Bridge. The stream is steep and could transport significant volumes of slope material down the gullies during an intense rainfall event. This sediment transport could potentially cause the tributaries to divert onto a different flow path once the lower slopes of the range are reached. Extreme flows may also break out of the existing bed at a variety of points on the lower slopes and flow overland, potentially affecting the roading network and properties in the upper Borlase Avenue and Brookvale subdivision and/or properties adjacent to the stream between Borlase Avenue and the Black Valley Stream. Floodwater could overflow the road at the Borlase Avenue and Bridge Street culvert crossings if culvert capacity is exceeded or the culverts become blocked by debris.
Black Valley Stream
This stream has a rather larger catchment than the Borlase Stream and has a number of tributaries arising on the St Arnaud Range and hillside opposite. Flooding risk in an extreme rainfall event is generally limited to shallow overland flow and localised flooding of pasture and lower lying land adjacent to the stream. Once the stream passes beneath the highway bridge, it remains essentially within its bed down to the lake, although floodwater flow will occur over lower-lying land adjacent to the stream, and in particular of the lower land adjacent to the stream within the Alpine Lodge complex. Floodwater flows in the 2005 event destroyed the two timber footbridge crossings of the stream below the highway bridge, caused bank scour of the stream between the highway and the lake, and scoured out the bridge approaches of the stream crossing in Kerr Bay adjacent to the lake. A limited amount of floodwater also escaped the channel to flow overland through the Kerr Bay camp. In a similarly extreme event, these events could recur.
Other local drainage networks
There are other local drainage networks within the community whose capacity will occasionally be exceeded. Localised flooding of adjacent low lying land and overland flow may result at such times.
Ash from Mt Taranaki or the Taupo Volcanic Zone could deposit a thin layer of ash over the St Arnaud area, depending on the size of the eruption and the wind direction.
The likelihood of significant amounts of volcanic ash reaching the St Arnaud area is very low but if it did it could possibly affect sensitive machinery and equipment.
A risk exists for vegetation fires to commence in or around the St Arnaud village, including areas to the west around to the West Bay Camping Area. There is a strong likelihood in such an event that the fire would threaten all or part of the Village Peninsula area and the safety of the occupants. Evacuation of residents from the Village may be necessary. (The St Arnaud Fire Village Evacuation Plan covers this contingency.) In areas other than the St Arnaud Village natural barriers are likely to mitigate any need for escalation to a Civil Defence emergency.
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 firefighting resources would need to be prioritized to meet the needs of all the emergency response functions.
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 an LPG incident or major petrol spill, for example, could lead to the activation of the community response plan and possibly evacuation measures being implemented.
The threat of an influenza pandemic continues to be rated as a moderate to high risk. St Arnaud’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 St Arnaud from pandemic influenza is the possible restriction on travel and public assembly. This would impact on the community’s ability to conduct business as usual. Organisations such as schools, tourism and accommodation operators, farms, businesses, and voluntary organisations may have their operations interrupted. There may be a need for the community to provide welfare support to individuals and families impacted by the disease.
Avalanche is a risk in both the park and/or Rainbow Ski field areas. The Department of Conservations Nelson Lakes Area Office Avalanche Rescue Plan (2010) exists for this eventuality. The plan is reviewed in May each year (and/or as required) and is to be implemented in conjunction with Rainbow Ski field and the NZ Police (Nelson and Blenheim) avalanche plans.
Note the overall control and organisation of an avalanche rescue rests with the Police. If this event was to transpire a rapid response is critical to survival. A person’s chance of survival is 91% in the first 18 minutes, 34% at 35 minutes, and 7% after 120 minutes.
This section describes the characteristics of the major infrastructure in the St Arnaud area - namely power, telecommunications, and roads. The intention is to highlight key dependencies and vulnerabilities.