Why flood plain ditches can have only a limited beneficial effect,even in times when there is no flooding?

Question

Why flood plain ditches can have only a limited beneficial effect,even in times when there is no flooding?

Answer 1

Because as with all ditches, they will fill with sediment and biota.  Floods carry very high levels of sediment, so it doesn't take long for the ditches to fill.  The other problem is that a flood is not necessarily what we see as water coming just from the river, there is ground water the rises too, and the ditches can't protect against groundwater rise as it will fill.