  We show that the effect of initial non-Gaussian clustering can significantly
enhance the event rate for primordial black hole (PBH) coalescence. The impact
of such clustering is studied in a specific scenario of multi-stream inflation.
Initial clustering enables the possibility of detecting massive PBH coalescence
by space-based gravitational wave interferometers such as LISA and DECIGO/BBO.
The parameter regime for the ground-based detectors to detect PBH coalescence
is also extended.
