The government is committed to minimising the environmental impact of dredging and reclamation works. That's what it pledged eight years ago after excessive toxic mud was removed during construction of Disneyland at Penny's Bay, resulting in environmental damage that led to hundreds of millions of dollars in clean-up costs and compensation. At least 200 times as much dangerous sediment will be removed by major engineering projects starting soon in Victoria Harbour. Assurances have been made that every measure is in place to protect marine life and water quality, but authorities would do well to revisit past mistakes to prevent another catastrophe. Our harbour is, after all, Hong Kong's most important asset. Despite that, we've got an appalling record of taking care of it. Sewage, industrial waste, shipping and oil and chemical spills in a mere three decades turned its waters into an almost lifeless toxic brew. Reclamation shrunk its width, reshaping it from a majestic waterway with intricate bays and inlets to a straight-sided channel. Only now, with sewage treatment schemes well advanced and environmental protection rules in place, are fish of any size returning. The harbour protection ordinance and a court ruling against the government have made plain that no more land will be reclaimed. Water that was once green and hazardous is clearing up. Those who thought the cross-harbour swimming contests held until the early 1960s were a thing of the past may yet be proven wrong. But the scale of upcoming infrastructure projects raises questions about whether such progress can be maintained. There are at least a dozen either already under way or planned to start in coming months or next year. Many of them will be in progress at the same time. They will have a massive impact on the harbour, with an estimated 40 million cubic metres of sediment being dredged out to make way for works ranging from tunnels to bridges. Some have been long planned, others more recently announced. Among them is the bridge to Zhuhai and Macau and the cruise liner terminal at Kai Tak. Most recently, there's been a proposal to deepen the basin at the Kwai Chung container port by two metres to allow for bigger ships. When all are under way, it will be the most dredging work in the harbour at the same time for two decades. Harbour traffic will increase, waters will be muddied and sealife sent scurrying. Heavy metals and other toxins on the seabed and in the sediment will be disturbed, creating a fresh environmental hazard. To be sure, much of this can't be avoided. Development always has an environmental cost. What authorities have to ensure is that it is minimised as much as possible. Impact assessments have to be thorough and every effort made to eliminate risks. With at least seven million cubic metres of toxic-laden sediment being moved, that will be a massive undertaking. Just 30,000 cubic metres was excavated from the Choy Lee Shipyard at Penny's Bay and its improper handling led to the deaths of 189,000 fish in two farms and perhaps six million in open waters. Farmers were paid HK$5.78 million in compensation and an extra HK$428 million more than was anticipated was needed for clean-up work. This was a single project in a confined area; those planned cover all parts of the harbour and many will be in progress at the same time. Attitudes for harbour preservation have never been stronger. Given the many concurrent projects, co-ordinated monitoring is essential. Clear contingency plans to prevent breaches of rules have to be in place. We can't afford another disaster.