New Review Compares Waimea Community Dam with Alternatives

Monday 24 July 2017

A review of all the reasonable alternatives to a dam in the Lee Valley for solving the Nelson Tasman region’s summer water shortages shows the Waimea Community Dam is the cheapest and most viable solution.

The Waimea River cannot sustain the water supply demands made of it, and in dry summers its flow drops to an unhealthy level. To protect the health of the river, the Tasman District Council either needs to boost the water supply in some way or impose harsh rationing measures most summers.

The review shows three alternative options could close the current urban “water gap”* and cater for future urban demand growth, but only the Waimea Community Dam:

  • increases flows in the Waimea River to improve ecosystem health and meets new national freshwater standards;
  • and holds enough capacity for current and future primary production needs and secures the regional economy;
  • and successfully leverages funding from partners – Waimea Plains water users, central Government (via Crown Irrigation Investments Ltd and the Freshwater Improvement Fund), and Nelson City Council.

The review, carried out by Tasman District Council Engineering Services Manager Richard Kirby, was presented to Tasman councillors in a workshop earlier this month. The findings of the assessment are part of the public agenda at this Thursday’s Full Council meeting.

The assessment examined all the water augmentation options identified since a 1991 Agriculture New Zealand report, including small dams, deep bores, a pipeline from Rotoiti/Buller River to Wakefield and pumping from the Motueka Aquifer. Household rainwater harvesting was also considered.

The four options that would provide enough water to meet current and future urban demand were:

  • the Waimea Community Dam
  • a high dam on the Roding River
  • transfer of water from the Motueka aquifer
  • Storage ponds beside the Waimea River.

The option of a dam at Teapot Valley was also included for comparison of the costs of a smaller dam.

The assessment showed:

 Waimea Community DamRoding High DamPumping from Motueka AquiferWaimea Storage PondsTeapot Valley Dam

Capital cost to Council - urban supply

$9m

$95m - $145m

$35m - $40m

$24.6m

$46.1m

Capital cost to Council – environmental flows

$16m

n/a

n/a

n/a

n/a

Notes on capital cost

 

Includes dam, piping and treatment plant

Serving Mapua only

New ponds needed every 15 years

 

Annual operating cost for Council - urban supply

$300,000

 

$3.4m - $3.8m (includes dam and treatment)

$750,000

$788,000

$1.1m

Water stored

13,000,000m3

1,200,000 – 5,100,000m3

n/a

500,000m3

(More ponds needed for future demand)

500,000m3

Daily supply potential

31,000m3

(could supply up to 60,000m3)

30,000m3

5900m3 (serving Mapua only)

4000m3

4000m3

Capital cost  per cubic metre per day - urban supply

$290

$4000

$6356

$6150

$11,525

 

The $16m Council contribution towards minimum river flows in the Waimea River will allow the Council to meet its legal obligations to protect water quality under the National Policy Statement for Freshwater. The Council’s total operating cost of $714,000 for the Waimea Community Dam includes allowance for the minimum flow provided by the dam.

The assessment confirms small dams such as a Teapot Valley dam would barely satisfy the current water gap and are not cost-effective. The Teapot Valley option could include extra dams in neighbouring valleys to increase the amount of water for future demand, but at significant extra cost.

Weirs have been discounted because by themselves they cannot provide enough water to meet demand. They can have a localised positive effect on recharging aquifers, but not enough to solve the water shortage facing the region. They can cause negative effects downstream.

Rainwater harvesting, while an option that householders should consider to cushion themselves against rationing, could not store enough water to meet the “water gap” and costs about $5000 per property – plus maintenance costs - for a 22.5 cubic metre tank.

For example, if every home installed a rainwater tank there would be a collective cost of $32.4 million for the 6481 urban properties in the Waimea catchment. However, rainwater tanks do not prevent the need for rationing in an extended dry spell, because once tanks are empty there is no way to refill them until it rains again.

Council chief executive Lindsay McKenzie said the information in the assessment was not new, but combined the knowledge collected over the past 26 years into a single review for the first time.

“That’s timely as we prepare for the public consultation on the Waimea Community Dam’s funding and governance. In a process that has been going on for this length of time, it’s easy to lose sight of all the work that has led us to this point but that background information is crucial in understanding the rationale for where we are now.”

*The “water gap” is the difference between urban consumption and the amount of water the Council will be able to provide to urban users under new water take rules that will protect minimum flows in the Waimea River through harsher rationing measures in dry spells. The current summer “water gap” in times of Step 3 rationing (which is likely to occur nine out of 10 years) is 5000 cubic metres a day (equivalent to the amount used by 5000 households).