The screw is of course an Archimedes Screw as used to raise water (vital in the former Zuiderzee area).
I surmise that the azure half represents the sea, and the vert the recovered land. The flag of Flevoland, which approximates to the former Zuiderzee, has a similar blue + f-de-l and green half:
though it arms treat the watery link somewhat differently:
The reason for the f-de-l is at first sight obscure, until one learns that they are canting for Cornelis Lely, who was responsible for the draining of Zuiderzee.
https://en.wikipedia.org/wiki/Cornelis_LelyIt is hard to think of another engineer or government minister (Lely was both) who has been responsible for such a sea-change in the physical geography of their native land.