Havelock North dams and streams – post-Cyclone Gabrielle

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.