What does the inverted filter protect the weir/barrage from in a downstream protection arrangement?

Groynes are constructed for river bank protection. When a groyne is inclined downstream in the direction of flow, it is designated as a:

What is the available moisture depth for evapotranspiration in a root zone of 1 m depth soil, if the dry unit weight of soil is 1.5 g/cc, field capacity is 30%, and permanent wilting point is 10%?