Creation of small weirs to store water during dry season a better alternative to big dams?

The installation of a weir of 20,000litres holding capacity in Kenya

seems to be working well to serve dual purpose of acting as a storage for excess rain water thereby preventing floods and serving as a water source during droughts.Should the new focus on IWRM be on installation of weirs with small capacity over building larger dams with greater evaporation and ecological concerns?