Why do debris flows contain a variety of sediment sizes? a. intense rain generates surface flow with the power to move lots of sedimentb. intense rain generates surface flow that can erode small and large sized sediment that was created by rock fallc. intense rain generates surface flow on steep slopes that coalesces to form streams and erodes the soil coverd. intense rain generates surface flow that eventually finds a stream channel and erodes the loose sediment resting theree. all of these