On the international stage Turkey’s President Recep Tayyip Erdogan can be a divisive figure, but his latest proposal – a new shipping canal to ease the volume of traffic transiting the Bosphorus Strait – is raising hackles locally.

The Istanbul Canal – some 45km in length – would run north of Istanbul, linking the Sea of Marmara to the Black Sea. Construction is meant to begin mid-year. Engineers estimate it will take seven years to build and cost between US$9.3bn and $14.6bn.

Erdogan’s argues that the Bosphorus Strait is a bit like Auckland’s Harbour bridge at rush hour – far too many ships trying to negotiate a narrow strip of water simultaneously. About 41,000 vessels use the Strait annually. On a typical day dozens of vessels are anchored at either end, waiting for a transit window. It’s also a tricky waterway to navigate.

A new canal – allowing up to 160 vessels to transit a day – would ease the Strait’s congestion and generate plenty of dosh (in transit fees) for the cashstrapped country and its ailing economy.

But the project has plenty of critics, many of them nervous that the canal is yet another of Erdogan’s ‘Think Big’ ideas in an infrastructure spending spree he launched in 2013. With a cost of $200bn, this 10-year plan has so far seen the construction of the massive new Istanbul airport, rail/road tunnels under the Bosphorus and one of the world’s biggest suspension bridge (connecting Europe and Asia).

Nay-sayers also point to the environmental cost: the canal project would eliminate Lake Durusu (it supplies 20% of Istanbul’s drinking water), destroy agricultural and forest land and potentially contaminate groundwater with salt.

There are also sensitive geo-political implications. Critics believe the Canal flies in the face of an international treaty – the Montreux Convention – governing the Bosphorus and Dardanelles. The treaty guarantees access for civilian vessels during peacetime while limiting the passage of warships.

Istanbul’s mayor (Ekrem Imamoglu) says the canal represents one of the biggest risks the city has faced in its entire history. He suggests the money would be better spent on public transport, water supply, social projects, education and earthquake protection measures.

In a recent Istanbul survey, 80.4% of the respondents were against the canal project. Only 7.9% supported it.