Dr Tim Fisher: Dam won't exacerbate erosion

The Ruataniwha Dam. Photo / Duncan Brown

"The issues around sediment management were carefully considered by a board of inquiry."

In response to the Talking Point by Sara Gerard titled "Dam will lead to coastal erosion", I am responding to provide some context and to clarify some points raised by Ms Gerard.

I was the lead author of the Tonkin + Taylor (T+T) Ruataniwha Water Storage Scheme (RWSS) Sedimentation Assessment (May 2013).

To correct Ms Gerard, the primary basis for the RWSS dam height is water storage, not sediment build-up. The estimated time for the reservoir to become half full with sediment is between 175 and 287 years.

The issues around sediment management were carefully considered by a board of inquiry (BoI). To ensure that the sedimentation assessment was as rigorous as it could be, HBRC/T+T included a peer review process.

The peer review process involved three rounds of peer review comments, which led to further analysis included in the sedimentation assessment.

Ms Gerard in her Talking Point has quoted from the first round of peer review, without reflecting that these concerns were subsequently addressed in the sedimentation assessment to the satisfaction of the peer reviewer.

The Tukituki/Waipawa rivers naturally have an excess of sediment. The sediment is generated in the upper catchments but there is insufficient natural river flows to transport this sediment to the coast.

Therefore, the rivers are naturally aggrading (building up), which is the long-term morphological process that has built up the plains.

Development of the land for towns and primary production has led to river management practices that hold the rivers in their alignment.

To ensure the flood conveying capacity of the rivers is maintained it is necessary to remove on average 187,000cu m /year (2005-2012) of this sediment build-up from the Tukituki/Waipawa rivers.

The change in sediment transported to the coast as a result of the project has been estimated at 1700cu m, which is considered to be a conservative estimate (ie it is likely to be less than this).

Twice this amount of sediment, 3400cu m has been offered as beach nourishment to mitigate the effects of the reduction in sediment at the coast.

This sediment is trucked and placed on the beach. The sediment management plan also includes for extensive monitoring of the river and coastline, and morphological modelling.

Where there are effects, these can be mitigated by changes to the gravel management in the rivers (extraction regime) and the beach nourishment programme.

In this way, the effects from the RWSS are monitored and managed into the future.

The coastline in the vicinity of the Tukituki River mouth is retreating, regardless of the RWSS.

The mitigation proposed by the RWSS is to ensure that the project does not worsen the situation.

There are wider issues to consider in terms of managing the coast including the coastline retreat and climate change.

However, the RWSS as a single project can manage only its own activities and mitigate its own effects.

* Dr Tim Fisher is principal water engineer for Tonkin & Taylor.
* Business and civic leaders, organisers, experts in their field and interest groups can contribute opinions.

The views expressed here are the writer's opinion and not the newspaper's. Email us at: editor@hbtoday.co.nz