Think Veale Garden on Steroids with a Riverwalk canal built along South Terrace 20 feet below road level..
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I think the plan would be that the canal is Concrete based ( for easy clean) and that the Water is predominately Wetland / Recycled and Aquifer supplied. Building the canal at a gradient to induce flow and then went it reaches the end would be pumped back to the source / wetlands. The canal and riverwalk would still be 20 feet below the street level however because building shadows encroach - might need to be further away from South Terrace. It should be viable as you can build an economy around it to fund its maintenance.