Flooding Information

We’ve budgeted $19.67 million in our Long Term Plan 2021-31 to help manage stormwater at Waihī Beach. The key projects that’ll help provide relief from flooding within the Northern catchments of Waihī Beach are listed below, with more details above.

• Two Mile Creek Bank Protection
• Two Mile Creek Upper Catchment Attenuation
• Darley Drain Upper Catchment Attenuation or Stormwater re-diversion
• One Mile Creek Improved Flow Path
• Increase to the dam stormwater storage capacity

Downer (Councils Maintenance Contractor) complete a round of inspections of drains culverts and other critical infrastructure when we receive a heavy weather warning.

In the period From November 2022 to June 2023 there have been 18 heavy rain warnings. During this period 14 pre-rain inspection rounds were completed, the reason why the checks are less than the warnings is that some of the warnings were over consecutive days and did not warrant a recheck.

A complete list of assets in the network that have been inspected recently and cleared out as required can be found on Councils website.

Councils piped network is typically designed to cater for a 1 in 5 year storm event. In the case of Waihī Beach the low lying nature means it is very difficult to achieve this.

When the stormwater system becomes over whelmed the stormwater will follow an overland flow path to the lowest point. When the overland flow path is not managed appropriately, or stormwater begins to pond in the lowest point, it results in flooding to private property.

Stormwater management at Waihī Beach is very complex. As the area is very low lying (in parts lower than sea level) stormwater is unable to get away.

Council has a number of projects in the Long Term Plan to help better manage stormwater however it is unlikely the risk of flooding to houses can be removed from all properties. In these cases we strongly recommend properties raise their dwelling above the flood level and we will waive processing costs for building and resource consents for anyone in a at risk area who decides to raise their home.

Yes. Grates are often used to prevent large debris from being stuck in drains. But grates can in some cases also result in increasing the risk of flooding. If they are not appropriately designed they can quickly become blocked resulting in stormwater backing up and flooding upstream from the network. We can consider installing these where appropriate.

One of our biggest issues is greenwaste being dumped on the banks of our creeks and around our ponds. It floats down and blocks outlets during flood events.

If you see a cesspit blocked with leaves call us on 0800 923 732 or submit an Antenno to let us know. Alternatively the fastest and most effective is to clear the cesspit grates yourself, especially the ones on the side of the road.

Did you know: Grass doesn’t block drains, when the water gets up and flows higher than normal, the grass just lays down, it actually protects the banks from getting washed away, or caving in, which goes on to cause more issues.

Council will continue to work with the community to better understand the stormwater issues at Waihi Beach. We encourage feedback on our heavy rain warning inspection list if the community identifies an area that often is impacted by a blocked system. View the list online.

Council ensures appropriately qualified experts are engaged by developers to both design the stormwater system and assess the risk. For larger developments Council requires the stormwater design to be peer reviewed.

All developments are required to comply with Councils stormwater consent and to ensure they do not increase the risk of flooding downstream.

Council has a series of projects in the Long Term Plan to help address flooding to private property, including

• Two Mile Creek Bank Protection
• Two Mile Creek Upper Catchment Attenuation
• Darley Drain Upper Catchment Attenuation or Stormwater re-diversion
• One Mile Creek Improved Flow Path
• Increase to the dam stormwater storage capacity

There was no formal rain warning for the Western Bay of Plenty District Council for the 29 May event so we did not do a heavy rain warning inspection immediately before. However our engineers do monitor the rainfall and do ad-hoc inspections if they think it was appropriate. The last inspection was done on the network on the 18th May.

No. There have been problems in the past where the gate to lower the dam has been caught open under pressure. This poses a greater risk than not lowering the dam level. Council has funding in the Long Term Plan to address this issue, but it’s important to remember that lowering the dam would not have prevented the flooding from occurring and the majority of the stormwater storage for stormwater retention is above the normal water level of the reservoir.

The dam has approximately 38 Mega Liters (ML) of stormwater storage between the standard operating level and the dam crest. During the event on 29May 2023, it took just 50 minutes for this additional storage to be filled with the spillway fully operational, at this point the water over topped the crest of the dam and the emergency spill way started to operate. Water over topped the crest and down the emergency spillway for approximately 40minutes from 2:15pm.

Over the 5-hour period between 1pm and 6pm approximately 83.5ML flowed into the dam, approximately 220 percent of the dam’s operational storage. Had the Dam been lowered it would have taken an additional 10 minutes before the Dam started overflowing however, the same volume would still have been released through the One Mile creek catchment.

The Dam was originally designed and built in 1962/63 as a water reservoir to supply water for the Waihī Beach community. The dam has subsequently been re-purposed for flood detention and recreation. Based on the geometry of the spillway, the dam provides for somewhere between a 1 in 50 and 1 in 100 year event. However, the July 2012 storm event and the more recent 29 May event has demonstrated it has insufficient capacity for these larger events. Several improvements have been planned to increase the capacity of the dam and the auxilliary spillway.

Initial investigations have and concept design has been completed for an upgrade of the Waihi Beach Dam. The purpose of the upgrade is to ensure the dam meets new dam safety guidelines. The works include raising the crest providing more storage volume within the Dam and to construct a new emergency spillway which would look to address erosion issues at the dam and 1-mile creek. This upgrade is planned to be completed by 2028.

Tonkin and Taylor investigated damage around the dam and One Mile Creek, through the motor camp and to the surf club. The dam face itself was not damaged, however there was some damage to One Mile Creek, through the motor camp. Council has not investigated the upper catchment of the dam in DOC land and is unaware of slips.

Tonkin and Taylor have undertaken an investigation of the Dam and there was no damage other than damage to the access track and carpark.

The intention of the “Waihī Beach Diversion of Maranui Flood Water” project in Council’s Long Term Plan is to undertake this re-diversion.

Te Mata block drains out through Two Mile Creek which has both capacity issues and erosion issues. However, no stormwater can be re-directed from Marauni towards the Te Mata block until both the erosion issues and capacity issue at Two Mile Creek are addressed. Council also has funding in the Long Term Plan to address these two issues.

Homeowners in this area should be aware that these works are not expected to protect properties in the lower catchment from flooding during the large events as was experienced on 29 May.

Yes. Council assists properties within a floodable area to raise their floor levels above the flood level by waiving the building and resource consent fees required to raise the building. The fees are covered by an internal transfer at Council and funded by the stormwater budget.

Council is currently investigating options to help alleviate flooding at the Pensioner Housing. Investigations include the possibility of raising or relocating the pensioner housing units.

No. Modelling indicates that holding back stormwater in the Te Mata block will not provide any benefit to stormwater management at Waihī Beach.

Typically, when managing stormwater, the water is attenuated in the upper catchment to allow it to drain away in the lower catchment. We aim to get stormwater in the lower catchment drained away as quickly as possible to allow for the stormwater from the upper catchment to flow through. The Te Mata block is too close to the outlet (Two Mile Creek) to provide any benefit.

The Catchment Management Plan is available here.

Council are required to provide a yearly summary report to the Bay of Plenty Regional Council on its compliance with the Comprehensive Stormwater Consent. These can be accessed here.