Background
The history of flood warning in Southland dates back to the 11th July 1947 when the then Southland Catchment Board issued its first flood warning on radio broadcast. The procedure adopted then has remained largely unchanged since. Flood warning messages broadcast to the general public contain only brief information that a Southland river is in flood, and more information can be found on the website.
Early in the Southland Catchment Board's history it was accepted that the issuing of river level reports during flood events was a service the Board should supply and therefore flood warning, although not a statutory obligation, is a service this Council continues to provide to the general public.
Because of the need for the Council to act quickly and decisively during a flood event, it is essential for the Duty Flood Warning Officer to be provided with a clear, uncluttered picture of the important existing situations and the forecast trends. Only then will they be free to make decisions, to allocate manpower and direct and authorise actions in critical areas with minimum delay.
The Council is ever mindful of the problems that are associated with floods, particularly in major events when residential areas could be in danger of becoming inundated by flood water. The Council policy is therefore to keep organisations such as Civil Defence, City and District Councils advised well in advance of likely problems which may arise.
Flood Warning Operations
The development and provision of flood warning services in Southland is the role of Environment Southland's Flood Warning Operations Centre in Invercargill.
Environment Southland provides flood warnings in the Southland region, predominantly within the Waiau, Aparima, Oreti, Waihopai and Mataura catchments. The aim of the warning system is to minimise loss of life and property damage by warning people of the likelihood and size of a flood so that they may evacuate, shift property or stock to higher ground, or implement other temporary flood loss reduction measures.
Flood Warning Watch service operates on a 24 hour 7 day basis.
Data Collection and Transmission
Rainfall and river level data is collected from over 55 sites throughout Southland via radio and telephone telemetry from automatic stations.
Roles and Responsibilities
The responsibility for flood forecasting and warning services in Southland rests with Environment Southland. Environment Southland coordinates the development and operation of flood warning services in Southland.
In the Southland region, the effectiveness of the flood warning system depends on the cooperative involvement of Environment Southland, Civil Defence and Local Government agencies working with flood-threatened communities.
Flood Classifications
At each flood warning river height station, the severity of flooding is described as minor, moderate or major according to the effects caused in the local area or on the downstream flood plains.
Major Flooding
This causes inundation of large areas, isolating urban areas. Major disruptions occur to road and rail links. Evacuation of many houses and business premises may be required. In rural areas widespread flooding of farmland will most likely occur. Flood protection systems (stopbanks) will run at near 100% capacity.
Moderate Flooding
This causes the inundation of low lying rural areas requiring the removal of stock and/or the evacuation of some houses. Main traffic bridges may be closed by floodwaters. Flood protection systems contain flood water.
Minor Flooding
This causes inconvenience such as closing of minor roads, flooding of low lying rural areas close to main rivers.
River levels
The zero level of a river gauge ("gauge zero") is typically set at the low flow level of the river or stream, i.e. the water level of the stream after a long dry spell. River heights are measured in metres above the gauge zero with normal river level at or close to 1.00 metres. For example, a river level height reading of 5 metres means that the water level has risen 4 metres above its normal level. In tidal areas, the gauge zero may be set at the mean sea level.
Meteorological and Hydrological Data
The collated data is analysed using a range of techniques and software to produce reports and graphs and make the data available through this website. Complex computer simulation models for the Mataura and Oreti Rivers developed by NIWA for Environment Southland are utilized to predict the likely timing and severity of flooding at specific sites. This forecast information is assessed and passed on to emergency organisations.
Flood Warning Services
River Level Bulletins
These contain the latest observed above normal heights at flood warning sites within a river system and are issued regularly, usually 2 hourly during daylight hours.
Flood Information
During a flood event those persons affected, can ring the flood warning office and received up to date river level and rainfall information, and forecast river levels.
Flood Forecasts
Forecast peak flood levels and discharge is available for 8 sites on Southlands main rivers. The Flowroute software produces forecast flood peak data at these sites from upstream information.
Professional Advice
Environment Southland staff provide direct assessments of flood conditions and forecasts to Civil Defence, emergency services and Local Government officers.
Flood Warning and River Height Bulletins
Environment Southland issues flood warnings and river level bulletins regularly during floods for the Mataura River and its main tributaries, the Makarewa River, Oreti River and Winton Detention Dam, Aparima River and Otautau Stream, Waiau River and three Invercargill Flood Schemes. These bulletins are issued normally at 2 hourly intervals during daylight hours, but on a 24 hour basis during the crucial stages of major flood events.
River Catchment Information Brochures
Environment Southland produces catchment brochures for the main river systems in Southland which contain detailed information such as flood warning information for the particular catchment, telephone details for flood warning sites, flood peak travel times, historic peak flood heights and maps showing catchment river level stations and rainfall sites.
Environment Southland is the brand name of Southland Regional Council.