Shambles, I had to look up weare, which spelling of weir is unknown over here.
I found scanty information, but a system of weirs and obstacles (ridges), with overflows, etc., were built at several ports as a form of fortification. Sounds like Portland was one of these sites. In 1610, someone complained that the movement of salmon was impeded on the Wie (haven't looked this up) by weares that were constructed there.
Any more information?