In some places ditches have been dug across the flood plain.Why?

Question

In some places ditches have been dug across the flood plain.Why?

Answer 1

also ditches will hold more water so that it would take more water to flood the city. the dirt from the ditches could also be used to barricade levees

Answer 2

Mosquito control.

Answer 3

The ditches are used to help slow the progress of a flood across a floodplain and divert the water.  This is only a temporary solution to the problem of building near rivers.  Floods carry large quantities of sediment, as such it is not hard for the ditches to fill with sediment during a flood, in which case, more ditches would have to be dug.

Also, if the flood is large enough, the ditches will have little to no effect as flooding is not only from the river, but from the rise in groundwater.