The Trojans came on in a mass, led by Hector, 160 always charging forward, like a rolling boulder, which some river in a winter flood dislodges from a cliff beside its banks, its great flood eroding what supports that lethal stone.
Using a simile to explain how massive the charge Hector is leading with the Trojan army. Like a boulder they will smash into the water where the Greeks are and destroy everything as if creating a flood.
