Lowering Waihī Beach dam would not have prevented flood, says council deputy CEO

Flooded Marine Avenue in Waihī Beach.

Katikati Advertiser spoke with Western Bay of Plenty Council deputy CEO and general manager of infrastructure Gary Allis on Waihī Beach’s dam and how it is managed.

What happened with the dam on May 29?

The stormwater storage in the dam quickly filled with water from the upper catchment. The dam acted as designed and the secondary spillway was activated, which flows over the grass face of the dam and down the dam access track. The dam has approximately 38 megalitres (ML) of stormwater storage between the standard operating level and the dam crest.

During the event on May 29, it took just 50 minutes for this additional stormwater storage to be filled. At this point, the water flowed over the top of the crest of the dam and the emergency spillway started to operate.

Water flowed over the crest and down the emergency spillway for approximately 40 minutes from 2.15pm.

Over the five-hour period between 1pm - 6pm, approximately 83.5ML flowed into the dam, approximately 220 per cent 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.

How is the dam managed in times of heavy rain?

The dam is self-managing, like any wet stormwater detention pond. This dam has two spillways - one to maintain water at normal level, and the second to allow overflow when the pond/dam is full to protect the wall from water flowing over the crest.

Rainwater flows into the dam, where it is held until the storm has passed. The water is then released slowly over time.

We may lower the level of water in advance if there is an exceptional rainfall event forecast. Normally we do not make any changes as most of the emergency storage is above the normal water level. There was no opportunity to lower the dam before/on May 29 because we received no formal heavy rain warning for the area prior to the event.

Had the dam been lowered on May 29, it would not have prevented the dam from overflowing. Emptying the dam would’ve just slowed the overflowing by an additional 10 minutes. However, the same volume would still have been released through the One Mile Creek catchment.

Why is the dam not lowered before rain anymore?

It hasn’t been our practice for the last number of years as there have been problems in the past where the gate to lower the dam has been caught open under pressure. 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 happening on May 29.

In response to recent events, we lowered the dam in preparation for the rain we were expecting on June 22-23.

It’s also worth noting that emptying the dam has ecological consequences for the down steam freshwater and marine ecosystems, including the eels and whitebait living there. There are also recreational uses that are affected.

More info: westernbay.govt.nz/WB-stormwater-2023.