This page provides information on what we know about the performance of the dams and streams following Cyclone Gabrielle. It is focused on the Mangarau dam and stream, as that stream was the one that flooded homes during the cyclone. Using the links to the right, you can find newsletters that have been sent to residents, historic and current reports, related information, and contact Council if you have questions.
The dams in the hills above Havelock North 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.
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 Karamu stream.
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 Karamu stream. Each dam also has a ‘spillway’ which operates to protect the physical dam structure when the volume of water behind the dam nears it maximum. 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 was estimated to be a 1 In 250-year event. Despite the level of rainfall, a post-cyclone report on the dams’ performance found that they worked as designed. None of the dams reached full capacity. However, the Mangarau dam did begin to release water via its spillway. The ‘Cyclone Gabrielle flood flow calculation for the Havelock North flood detention dams’ report, by Stantec, is available here. It outlines how the five dams performed during Cyclone Gabrielle.
The report finds that the Mangarau Dam performed satisfactorily (as designed) and did not breach the dam walls. It released flood waters, via the spillway designed to control flow, into the Mangarau Stream.
The report notes that 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 dam did not, over that time period, exceed 5.5m in water depth in the Mangarau Dam.
The Government has been working on new regulations for dams since 2019. They will come into force in 2024. The regulations are designed to ensure that dams are well operated, maintained, property and the environment. More information is available here
There are five naturally occurring streams which take water from the surrounding hills, and flow through the correspondingly named dams. They are the Herehere, Mangarau, Te Kahika, School and Karituwhenua streams.
Responsibility for the management of the streams sat with Hawke’s Bay Regional Council until the early 2000s. A governance agreement moved responsibility to Hastings District Council in 2003.
A report commissioned by HBRC (Tonkin + Taylor) investigates measures that may provide flood mitigation that would enable recategorisation of affected Havelock North properties (noting that the organisation responsible for the initial categorisation and any recategorisation is Hawke’s Bay Regional Council). This report is still in progress. HDC will provide a link to the report when HBRC makes it publicly available.
The draft version of the report does note the difficulties related to accessing the streams. Records show that 92 per cent of the land abutting the streams (and in many cases reaching into the streams) is privately owned.
It references a number of matters that contributed to the flooding including the amount of rain, debris and structures in the streams prior to the cyclone, and debris that fell into the stream as a result of the cyclone.
Once finalised, that report will be used by Hastings District Council to inform an action plan for the Havelock North streams.
Hastings District Council has commissioned a work programme to improve the performance of the streams during heavy rain events.
Over a 24-hour period on February 13 and 14, 2023, 283.5mm was recorded at the Mangarau Dam. The previous highest recording was 162mm.
The dam filled to above the spillway; an estimated 300,000m3 (300 million litres) of water at its peak. The stormwater was released into the Mangarau Stream (as designed) via the throttle culvert at up to 20,000 litres per second. A further up to 15,000 litres per second was released via the spillway at its peak, releasing 35,000 litres. The volume of water behind the dam did not breach the dam wall nor overtop the dam crest.
At this time no, the Managrau dam does not need to be made bigger. The dam was designed for a 1 in 100 year event yet operated satisfactorily during the larger (1 in 250 year event) Cyclone Gabrielle. The dam did not over-top the crest, which tells us it was adequate for Cyclone Gabrielle even though the cyclone was greater than what the dam was designed for. However, the event operation report for the dam does recommend investigating enlarging the dam given the potential for even larger storms in the era of climate change, new upcoming dam legislation and changing risk profiles.
The spillway works like the overflow hole near the top of some bathroom and kitchen sinks. Once the water gets to that height, the water is released into the receiving environment – in this case the Mangarau Stream. The 1.8m diameter outlet culvert acts as a ‘throttle’ to stop vast quantities of water being released at once. If flows behind the dam are greater than flows through the culvert the water is stored behind the dam wall. If the flows are still larger than what the dam can store and discharge via the culvert, water is released via the spillway. The Mangarau spillway operated as designed, beginning to operate once the water level rose above the 10.2m depth mark.
Where the streams are within private property and not visible to Council we have to rely on reports from private landowners adjoining the stream or other members of the public to advise of debris that needs clearing. Whether debris can be cleared then depends on accessibility – related to both land ownership and the ability to get machinery and vehicles to the site. A recommendation in the HBRC-commissioned draft report on the streams is that ownership/accessibility issues are addressed. HDC regularly clears debris and vegetation from the stream in areas abutted by Council reserve land. Council is in the process of contracting a maintenance contractor dedicated to the streams and dams, both planned and reactive.
Risk will never be able to be completely designed out, particularly given the changing climate with the expectation of more severe weather events. The priority is to put mitigations in place that reduce the risk to life. Owners of properties across the region moved from category 2 or 3 into Category 1 have been advised that being in Category 1 does not remove all risk of flooding.
The planned stream work is focused on providing a level of service to a 1 in 100-year event as a goal, noting previous standards have worked to a 1 in 50-year event design standard.
It is clear there are parts of the Mangarau stream (and likely other Havelock North streams) that cannot cope with 1 in 100-year stormwater flows, and these constrained areas will need modifications, alongside formalised access locations, improved and scheduled maintenance using a dedicated contractor. Enhanced monitoring , including stream gauges and an alarm system, are planned.
Yes. Council will be discussing work with mana whenua, given there will be significant decisions in relation to land and waterways. A Cultural Aspirations report prepared for a Reserve Management Plan for the Havelock North hills reserves includes a number of aspirations relating to the Havelock Hills waterways, and these must be taken into consideration as the work programme is prepared.
The project team set up to drive the improvements will regularly engage directly with affected landowners, and the wider community more broadly. A communications and engagement plan will be prepared with input from representatives of the community.
The work specific to the Category 2C area, anticipated to enable HBRC to move the properties from 2C to 1, is expected to be funded through the government flood protection scheme, subject to approved business cases. Future improvements and ongoing maintenance uplift will likely see a review of Council funding for the streams and the possible introduction of a targeted rate or some other mechanism to fund upgrades and agreed upon levels of service. Landowners, community and other key stakeholders will involved in discussion on this.
Disclaimers and Copyright
While every endeavour has been taken by the Hastings District Council to ensure that the information on this website is accurate and up to date, Hastings District Council shall not be liable for any loss suffered through the use, directly or indirectly, of information on this website. Information contained has been assembled in good faith. Some of the information available in this site is from the New Zealand Public domain and supplied by relevant government agencies. Hastings District Council cannot accept any liability for its accuracy or content. Portions of the information and material on this site, including data, pages, documents, online graphics and images are protected by copyright, unless specifically notified to the contrary. Externally sourced information or material is copyright to the respective provider.
© Hastings District Council - / +64 6 871 5000 / firstname.lastname@example.org