Up to 10 Happy Valley race meetings will be shifted to Sha Tin's all-weather track next year as a major overhaul of stormwater drains underneath the racecourse takes place, and Jockey Club officials are bracing themselves for what could be a half-a-billion-dollar blow to betting turnover.

Three sections of the Happy Valley racing surface will be torn up in early June 2014 and the track put out of play until mid-October next year. The construction period has been timed to include the two-month off-season and therefore minimise its financial impact, but the club is still expecting a significant decrease in betting on the affected dates.

The government project will force between eight and 10 race nights to be transferred from a track where turnover now regularly tops HK$1 billion per meeting, to the less-popular dirt track at Sha Tin, where Wednesday night fixtures this season have yielded around HK$50 to $60 million less.

We did budget lower for the new year, and I'd expect a drop in the magnitude of three to four per cent for those meetings

"We did budget lower for the new year, and I'd expect a drop in the magnitude of three to four per cent for those meetings," Hong Kong Jockey Club director of racing Bill Nader said yesterday.

"I don't think it will be excessive, but I think there will be a slight reduction in turnover. The occasional dirt meeting can hold its own, but a constant diet of dirt races on Wednesday nights, we haven't experienced that in Hong Kong - at least not for some time. On average, I think dirt races perform better from a turnover standpoint than most people think. I think we will be able to absorb it, and under the circumstances, we had no choice."

The HK$1billion stormwater storage project is being undertaken by the Drainage Services Department to prevent flooding in low-lying areas of Happy Valley and Causeway Bay during heavy rain.

A whole race meeting was abandoned because of flooding in 2005, and another race lost in 2002, and the new drainage system will mean the track would be able to withstand similar deluges in the future, with stormwater from the existing culverts under the grandstand to be diverted into massive holding tanks under the infield.

The Jockey Club will attempt to make the most of the lay-off by completing some major infrastructure work of its own. A tunnel will be built between the forecourt of the grandstand and the infield, and a significant upgrade of lighting and work on other facilities is also planned. "It's an inconvenience, but hopefully it helps us long term - and of course we are a massive part of the community and this project will help a lot of people," Nader said.

He said the impact could not simply be measured in "dollars and cents", claiming some of the valuable momentum gained through the successful "Happy Wednesday" promotions could be lost.

"We have a loyal Happy Valley fan base that won't necessarily come to Sha Tin on those eight to 10 Wednesdays. So it's not just about turnover, it's about customer experience," he said. "That's one of the costs of the project, that the Happy Valley racegoer is out of the picture from June to October. We will have to win that customer segment back."

Races can also be run on the Sha Tin turf at night, and Nader raised the possibility of "mixed meetings", with races on both the inner track and course proper. But the grass track's usage would have to be minimal given its already high workload.

The finalised racing fixtures should be known late next month, with the club awaiting approval from the Department of Home Affairs.

An earlier version of the story erroneously stated in the second paragraph that "Three sections of the Happy Valley racing surface will be torn up in early April 2014..." It has since been corrected to "early June 2014."