How does flooding occur due to deforestation?

Forest reduces flood risk from the top to bottom. It is a natural absorber of rainwater, as it allows rainwater to seep through them.

Deforestation plays many roles in the flooding equation because trees prevent sediment runoffs and forests hold more water than farms or grasslands. 

When it rains, lots of raindrops that fall on leaves of trees and plants in the forest, evaporate straight into the air- so less water reaches the ground, which also prevents the rain from directly hitting the ground which may result in soil erosion and thus if the soil gets eroded, trees lose its capability to hold or absorb a large amount of water with the help of their roots, and it results in floods.

Hence, if deforestation is done, it causes the flood.