Turning water into snow – responsibly and sustainably

Snowmaking machines can 'kick-start' a ski season – laying a solid base of snow on the main trails of a ski field to pre-empt and supplement natural snowfall. NZSki Ltd operates The Remarkables ski field and has consent to take up to 60cm of water from nearby Lake Alta for snowmaking.

To comply with this consent, NZSki must monitor the water level and may only take water while the lake surface is above the consent threshold. The company must also provide lake level data to Otago Regional Council, to show that it's meeting the consent requirements.

NIWA has designed and installed a unique lake monitoring system to help NZSki manage its water use, stay within its allocation and prove that it has.

The challenges

Maintaining Lake Alta in pristine condition requires accurate measurement, monitoring and management of the lake level.

For NIWA, designing and installing equipment that accurately and reliably measures water level is usually straightforward. However, a number of unusual requirements made this monitoring task anything but straightforward:
Department of Conservation (DOC) approval is conditional; any equipment installed at Lake Alta must have minimal impact on the environment and be as unobtrusive as possible. Everything needs to be portable. The equipment may be installed in early winter but must be removed at the end of the ski season.

In winter, Lake Alta may be covered with up to a metre of ice. But before the lake freezes over, the submersible water abstraction pump is installed in the lake using a helicopter. At the end of winter, ski field operators have to wait for ice to melt to remove equipment. If repairs are required at any time it can be quite a challenge. Therefore all installed equipment needs to be very robust!

Usually, measuring lake level is easy. But measuring the water level of a lake with a metre of ice floating on a surface that rises and falls, presents several challenges:

• The point on the lake bed where the depth is measured must be fixed, absolutely, and referenced to its height above sea level.
• The bottom-fixed depth-sensor connection must exit the lake in such a way that it can't be torn out by ice, as it rises and falls.
• The accuracy of the lake level measurement must be independent of the presence or absence of ice.

Our solution

We decided to keep the consent-monitoring equipment functionally separate from, but co-located with, the pumping equipment.

To measure the lake level reliably, we considered that the most robust solution would be to use a continuous gas bubbler pressure sensor. That way, we could avoid deploying vulnerable instrumentation in the lake and could locate the pressure sensor in the protective enclosure on the lake shore. The only thing we'd need to put out in the lake would be a gas (compressed air) line to couple the lake level pressure to the sensor in the enclosure.

However, the gas line would not survive if it had to pass through the destructive floating ice layer, as this could tear it out.

The best way to avoid the ice, and at the same time satisfy DOC's 'invisibility' requirement, was to bury the bubbler tube along its entire length – from its position on the lake bed, to where it emerged and entered the portable equipment enclosure near the lake edge.

But because we needed to maintain the lower end of the bubbler tube at a fixed point on the lake bed, and because removing and replacing the gas line every year wasn't practical, we sought DOC's approval to leave this in place permanently. DOC agreed, provided we covered the bubbler line with stones – a job for divers.

Measuring water level when it's iced over

The bubbler depth sensor senses the weight of the water pressing down on the lower end of the submerged bubbler tube. A data logger records the pressure every 15 minutes, converts it to depth and calculates the height of the lake surface above sea level (ASL).

Unlike most other substances, when water freezes it becomes less dense. Therefore, despite being a solid, ice floats on the water surface. When water freezes it increases in volume by about 9 per cent, but its weight stays the same. So even though 9 per cent of the ice sits above the water surface, if it were to melt, it wouldn't cause the water level to change.

So this system will accurately record the position of the water surface... with or without ice.

The benefits

Data from the Lake Alta consent monitoring station is web-enabled, so NZSki and other authorised parties can easily view real-time and historic water levels via the Internet.

The 'dashboard' of the Lake Alta consent monitoring station, showing the current height of the lake surface (above sea level; stage) and 'station health' parameters. To view historical data, you simply click the relevant button. From there you select a data date range and choose either a graphical or tabular display. The small green icon at the left of the screen changes to red to indicate an alarm condition.

This graphical display shows two abstraction events where water was taken from the lake to make snow. This can only be done when the temperature is low enough for freezing. If the lake level ever drops to the 'stage decrease' threshold at 1799.6m ASL, an alarm is generated and text messages are sent to selected NZSki employees to warn that the lake level is down to within 20cm of the consent limit at 1799.4m ASL.

The station connects via the Telecom Cellular Network and sends data to a Neon server. This archives the data and makes it available for consent compliance audits.

The NIWA monitoring system enables NZSki to manage and stay within its consented water allocation, while allowing it to make enough snow to 'kick-start' and prolong a ski season.

"The system has worked flawlessly for us and is a great bit of technology," says Ed Bezett, Assistant Ski Area Manager.