Flooding has occurred throughout New Plymouth’s history. Because of its proximity to Mt Taranaki and the Pouakai Ranges, our district can experience some of the highest intensity rainfall in New Zealand.
As New Plymouth and surrounding urban areas have grown, development within the stream catchments has meant that both businesses and residential properties are exposed to flooding during extreme rainfall events. New Plymouth experienced major floods in 1935, the 1970s and the 1980s.
The largest flood was in 1971 when 290mm (11.4 inches) of rain fell in 24 hours. Several shop windows in Devon Street had to be smashed by civil defence workers to relieve pressure inside the buildings - goods were then swept into the street by the floodwaters. Homes and businesses were ruined and shops incurred hundreds of thousands of dollars in damage.
In the 1980s three major earth flood protection dams were built on the Huatoki, Waimea and Mangaotuku streams to detain flood flows and to limit the flows into the central business district of New Plymouth. Since that time additional flood diversion tunnels and earth detention bunds (smaller than dams) have been constructed as part of the city’s flood protection scheme.
New Plymouth's flood protection network includes three major detention dams (Huatoki, Mangaotuku and Waimea) and two tributory detention dams (Huatoki and Fernleigh streets) along with culverts and diversion tunnels within the developed area.
The excess water, which previously would have flooded inner city areas, is detained by the dams. While ponding may still occur on some properties after heaving rainfall, our flood protection network means that water should not well up above the habitable (livable) floor level.
For example, in 1990 rainfall equivalent to that experienced in the 1971 flood caused widespread flooding elsewhere in Taranaki. However, flood flows in New Plymouth were controlled by the dam system, with no flood damage to the central business area.
A detention dam reduces flood flows in a stream by temporarily storing water to spread the total flow over a longer period. This is achieved by having a culvert through the dam. The culvert is sized so that the maixmum flow from the dam is kept at a level that will not cause flooding downstream.
Under normal conditions, the whole flow of the stream passes through the culvert. When a storm increases the flow upstream beyond the "safe" level, the excess flow cannot get through the culvert and water builds up behind the dam. This continues until the storm is over and the upstream flow subsides to a safe level again. The stored water then drains away slowly, controlled by the culvert.
Only in the most extreme cases will a storm be long enough for the dam to fill right up. If this does happen, the excess water passes over the a spillway to rejoin the stream below the dam. Even then, the dam will have been effective in reducing the worst of the flooding.
The tributory dams slow the flow of water from tributory streams into main streams and also help to control flooding at culverts within the city centre which, during heavy rainfall, are subject to increased volumes of water runoff from buildings and roads.
There are three detention dams in New Plymouth:
The Huatoki Dam is the largest of the three major detention dams. The dam culvert is constructed with an adjustable inlet plate and the outflow from the dam is restricted to 15m³/second. At this rate, the dam can cope with flood conditions equal to those experienced in the 1971 storm. The storage capacity of the dam is 800,000m³.
Replacement value: $2.5 million
The Waimea Dam, above Tukapa Street, has a storage capacity of 150,000m³ and maximum outflow from the dam is set at 3.8m³/second. The Huatoki and Waimea dam sites are leased for grazing.
Replacement value: $0.6 million
The Mangaotuku Dam has a storage capacity of 336,000m³ and is designed to spill into Barrett's Lagoon, although this is most unlikely. The maximum outflow from the dam is set at 7m³/second. This dam is on Council-owned land which is grazed by the surrounding land owner's stock.
Replacement value: $1.05 million
A diversion tunnel diverts the stream from its normal path during high water flow. There are three diversion tunnels in New Plymouth:
- Bonithon Avenue / St Aubyn Street (Mangaotuku Stream)
- Queen Street (Mangaotuku Stream)
- Gilbert Street / Pukekura Park (Huatoki Stream)
A bund is an earth embankment to prevent flood waters. There are five bunds in New Plymouth:
- Vogeltown Park
- Sutherland Park
- Rugby Park
- Huatoki Street
- Ainslee Street
A telemetry station records the upstream water level and rainfall (every ½mm) every 15 minutes and then transmits the data to Taranaki Regional Council to use for flood warnings. There is a telemetry station at the Huatoki Dam which is one of many stations around Taranaki. The data recorded is also used for water resources studies.
The effects of flood detention dams can be explained by hydrographs. The hydrograph shows the amount of water in cubic meters per second (cumecs) against the duration of the storm (time) and can be used to predict flood flows for storms of varying intensity and duration.
This hydrograph shows how the Waimea Dam would function during a 1971-type storm. The hydrograph shows the capacity of water to be detained by the dam, based on the rainfall and river flows measured in previous severe storms. The blue lines represent river flows during a typical storm. The red line (culvert flow) shows the rate of flow allowed by the culvert.