when do floods occur?

Answer 1

Floods occur after massive water events (such as snow melts or rain) and the drainage system's capacity are overwhelmed.

This can result from poor drainage or lack of absorbsion in the topsoil.

Answer 2

deforestation
trees are like sponges and sivs at the same time
the stop as well as absorb water
when mountains are deforested and it rains ,all the rain water runs directly with out obstructions int the rivers ,who cannot handle the excess amount and huge quantities of water get too quickly down stream causing the flood ,
canals have the same effect ,that is why nobody is making straight canals any more
too much water too quickly down stream .

so almost all floods are directly caused by deforestation.

Answer 3

during El Nino

Answer 4

When there is an excess of water.