Tapei and San Jose, CA have done similar river-widening projects, without the island. Both have parks along a river flood plain, parks that are intended to be flooded during unusually wet periods. Taiwan's riverside park [1] is on both sides of the main channel of the Tamsui River. The edges of the park have high walls to protect the city. Guadalupe Park and Gardens in downtown San Jose[2] is a smaller example of the same concept. In both cases, the parks will take damage when flooded, so they only contain things that can handle flooding or easily replaced.

[1] https://www.google.com/maps/place/Taipei,+Taiwan/@25.030306,... [2] http://www.grpg.org/river-park-gardens