Havelock North dams and streams – post-Cyclone Gabrielle

Dams

There are five dams in the hills above Havelock North: Herehere, Mangarau, Te Kahika, School and Karituwhenua.  These five dams all drain via their correspondingly named streams through Havelock North to the Karamū stream.

The dams were built in the late 1970s/early 1980s following devastating floods in 1974 described at the time as ‘the most savage flood in living memory and the worst in the area since 1936’.

NIWA’s historic weather events catalogue describes the three-day storm and its effects on 15/6/1974 here. Most notably, a state of emergency was declared on June 16. Two hundred people were evacuated from Havelock North and Haumoana, with damage caused throughout Napier Havelock North, Clifton, Te Awanga and Haumoana.

As in Cyclone Gabrielle, homes across the district were without telephone and power connection, homes in low-lying areas were flooded, thousands of acres of farmland were inundated, and roads were washed out.

In 1974, Hastings recorded 127 mm of rain in 12 hours (described as a 1/50-year event). During Cyclone Gabrielle, 283.5mm was recorded at the Mangarau Dam over 24 hours on February 13 and 14, 2023, (described as a 1/250-year event).

There were eight significant rain events from 1938 to 1974, with rainfall in a 24-hour period not exceeding 162mm.

All five dams have a culvert allowing ‘normal’ water flows from the headwaters (at the top of the Havelock Hills) through the dam, down to the Karamū stream. Each dam also has a ‘spillway’ which operates to protect the physical dam structure when the volume of water behind the dam nears its maximum capacity. This is designed so that even when flows are greater than what the dam is designed to hold, the flow can be controlled (similar to how the lip on a jug works when you pour a glass of water).

Each dam was designed to cope with a 1 in 100-year storm event.

Watch a video explaining the meaning of a 1 in 100 year event here.

Cyclone Gabrielle

Cyclone Gabrielle was estimated to be a 1 In 250-year event.

The Cyclone Gabrielle flood flow calculation for the Havelock North flood detention dams’ report by Stantec outlines how the five dams performed during Cyclone Gabrielle The report found that:

• Despite the level of rainfall, a post-cyclone report on the dams’ performance found they worked as designed. None of the dams reached full capacity, however the Mangarau Dam did begin to release water via its spillway.
• The Mangarau Dam performed satisfactorily (as designed) and did not breach the dam walls Itt released flood waters, via the spillway designed to control flow, into the Mangarau Stream
• The main contributor to the heightened water flows from the Mangarau Dam was the sheer amount of rain that fell over the 24 hours, with water in the 12.5m deep dam reaching 11.2m. Earlier data from the dam gauge (2007 to 2022) shows the Mangarau Dam did not, over that time period, exceed a water-depth of 5.5m.