for description purposes I’ll use an exaggerated hourglass shape as the tunnel. larger opening to a reduced sized turbine area for the Ram effect, and then from the reduced sized turbin area to a larger exit area for a suction effect... I think that would cause enough pressure to make a few watts of juice!! Especial in areas like where Lake Huron first drains into the St. Clair River. Current is very fast with thousands of GPM
You are taking about hydrokinetic turbines. Aka run of the river. These also work in ocean currents and tidal currents as well. The Gulf Stream has multiple hundreds of terrawatt hours flowing past it just the Florida portion of the current has 163TWh/y up for grabs. River in the continental USA 120TWh according to the USGS and DOE studies. Marine energy with waves and tides is 2,300 TWh again from DOE. Add in ocean thermal power and that adds.4000+ TWh more. Of course solar dwarfs any of those by a factor of a million times. The earth receives well over a million times the entire human energy use per day every day. 3% of the area of the Sahara desert can power all 8 billion of use at EU levels forever. It’s has never been, never will be a energy source problem it is a energy storage , and transport problem both of which have technical solutions.
that’s multiple hundreds of nuclear plants and these sources are 24/7 currents or 6-8 on switch flow with slack tides of a couple hours with tides being predictable years in advance.
https://www.sciencedirect.com/topics/engineering/hydrokinetic-turbine
https://www.energy.gov/eere/water/articles/doe-funded-study-pinpoints-potential-lies-within-waves