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As recreational spaces go, the Tai Hang Tung playground is a welcome green lung in the crowded urban landscape that is Mong Kok.

What lies beneath  its football and rugby fields  is a huge cavern that is the size of about 2? football pitches.

At 136 metres long, 130 metres wide and 7.5 metres deep, this underground storage tank is engineered to intercept and temporarily hold rainwater from black storm warning downpours - rainfall exceeding 70mm an hour - for up to 10 hours. The rainwater stored in the tank, which has a capacity of about 100,000 cubic metres, is pumped back into the drainage system for discharge when the water level in the downstream drains recedes.

This reservoir, built two metres below the playground, has significantly alleviated the flooding problem in Mong Kok.

'The storage tank is useful in preventing floods,' said Derek Arnold, regional practice leader and project director of Black & Veatch Hong Kong.

The watershed storage tank, the first of its kind to be used in Hong Kong, was completed in October 2004 as part of stage two of the West Kowloon drainage improvement project.

Black & Veatch, a global engineering, consulting and construction company, is the designer and engineering consultant of the ongoing project, one of the many flood prevention measures undertaken by the Drainage Services Department.

'Normally, rainwater flows through the drainage system under gravity,' said Mr Arnold. But in an urban, built-up city such as Hong Kong, 'sometimes it is necessary to increase the [drainage system] capacity with pumping stations and even temporary storage tanks. One such example is the Tai Hang Tung storage tank.'

Nothing that is worthwhile is easy and there are many difficult challenges and constraints to take into consideration when constructing a sustainable drainage system.

To start with, there has to be 'an accurate identification of problem areas and the prioritisation of the resolution of problem areas', Mr Arnold said.

Then come 'the design and construction of cost-effective solutions'.

Tasked with reviewing the drainage master plan carried out by other consultants, Black & Veatch proposed alternative schemes to reduce the scope of drainage upgrading works in congested areas and reduce disruption to traffic. The schemes included the storage tank and a tunnel between Waterloo Road near Kowloon Tong and the Kai Tak nullah that transfers storm water away from the flood-prone areas near Mong Kok.

Drainage system construction must also aim at 'preserving or enhancing the environment, including the landscape environment', Mr Arnold said.

Work has to be done with a view to minimising the impact and inconvenience to the public and to traffic. The Tai Hang Tung storage scheme and the Kai Tak transfer scheme avoided extensive excavations. Steps were also taken to avoid disturbance to park users and football players during the construction of the storage tank.

The company has a workforce of 350 civil engineering professionals and technicians, and hires several fresh graduates who will train and work towards their qualification as chartered engineers. The firm is now on a recruitment drive.

Black & Veatch is involved in a number of drainage improvement projects. One is in East Kowloon, where the firm, as the lead consultant, will carry out the detailed design and construction supervision of 11km of drains and 8km of box culverts.

The firm is undertaking a project to improve two main drainage channels in Yuen Long, and in Kam Tin and Ngau Tam Mei, there is a project to alleviate the flooding problems by improving drainage.

The firm will also do a feasibility study on the rehabilitation of Yuen Long nullahs, and will examine various options and produce specific proposals to make the best use of the Staunton Creek and Fuk Man Road nullahs and the space they occupy.

The West Kowloon project has been rewarding. The Tai Hang Tung underground storage tank was recognised as an innovative flood alleviation scheme, winning the firm the Hong Kong Institution of Engineers Innovation Award for the Construction Industry 2001-02.



Pondering what lies beneath