I was one of the few who first arrived on the  scene ( change into simple sentence)

That person would be called the "First Responder"

Explanation:

Huge fragments vaulted like rebounding hail,or chaffy grain beneath the thresher's flail : the water flowing out of the chasm is strong and powerful. the purpose of these similes is to show how the water sprays and bounces around similar to the way hail falls and bounces once it hits a surface. it is also compared to grain being turned and tossed in a thresher, rolling and flying everywhere.