Flooding is arguably the most significant natural hazard in
Southland. It is certainly one of the
earliest hazards that Southland’s pioneers had to contend with.
Over time, flood alleviation efforts have
reduced the frequency of damaging floods. They still occur, albeit on a
less frequent basis. Modern floods tend
to result from large floods that exceed the protective capacity of the flood
protection works or, in other words, overtop the stopbanks.
Local flooding / stormwater flooding
All properties are at risk of flooding during particularly intense rainfall, whether they are located on a floodplain or not. When the rain is particularly heavy, sheet flooding can occur and this soon gets concentrated in relatively low areas to form watercourses in the most unexpected places. Normal stormwater flooding occurs in a similar manner, but the quantity is less. In all cases, barriers or restrictions across flow paths, for example roads, races, hedges and culverts, can exacerbate the flooding that occurs.
Intense rainfall is usually reasonably localised, but it can give rise to the largest floods that ever occur in small streams. Such streams are usually located outside the normal river monitoring network and, as such, they are not well recorded. Nevertheless, they do occur, some areas seem to be particularly prone to them and with climate change, they are predicted to become more frequent and more intense. As such, they are a form of flood hazard to be watched in the future. What's more, they normally occur in watercourses where it is neither practical nor economic to protect against them.
The Mataura and Waimea Valleys seem to be particularly susceptible to high intensity localised rainfall. Mataura, Gore and Riversdale have experienced this in the past, but the Wendon, Nokomai and Lumsden areas seem to have suffered the most from very heavy rain and sometimes hail as well.
Marine inundation / coastal flooding
When spring tides, strong winds and low barometric pressures coincide/combine, the sea can rise to much higher levels than expected, as much as 700 millimetres higher based on past observations. On top of this lift are the waves that inevitably occur in windy conditions. The elevated sea level and waves threaten to overtop coastal stopbanks, flooding low lying coastal land.
The margins of the New River Estuary are particularly susceptible to such flooding, as the pictures below show.
The highest known flooding (2.6 metres above mean sea level) occurred in June 1958, just a month after the previously highest known flooding. This hazard is exacerbated by sea level rise – there has been approximately 100 millimetres of sea level rise since then and a lot more is predicted in the future. Any land lower than 3.5 metres above mean sea level is considered to be at risk in the near term and, looking 100 years out, there is some risk for land up to five metres above mean sea level.
A king tide caused flooding on Stead Street on 11 March 2016.
Lacustrine flooding / lake flooding
Flooding in and around the margins of lakes is often overlooked, but there have been issues around the margins of Lakes Te Anau, Manapouri, McKerrow and Waituna in the past.
Lake levels on Lakes Te Anau and Manapouri have been recorded since 1926.
One of the key differences between lacustrine and riverine flooding is the persistence of the former. High lake levels can persist for days, even weeks, whereas most riverine flooding seldom lasts more than a couple of days. Lake McKerrow has been high enough in the past to flood some adjoining buildings, but such flooding is not well documented.
Lake Te Anau on 29 April 2010.