This is where the drain comes in. Uachdar and Gramsdale lie along the north coast of Benbecula. Immediately to the south of them the land rises steeply. When it rains, as it does, the water needs to drain north across the low lying machair to the sound between Benbecula and North Uist. Rather than just waterlog all the farmed lands, if you have or can get hold of a map, you will see that it collects in a lochan that lies beside where the road from North Uist forks, one branch heading due west to Dunganichy and Balvanich and the other turning southwards up the hill towards Griminish, Lionacleit and, ultimately, South Uist. That lochan has an outlet north into the sea, the outlet forming the boundary between Gramsdale and Uachdar. As the land is so low lying, there is virtually no fall on the outlet so the fear has always been that a high tide or storm might see the flow of water reversed and the sea rush in to flood the crofts with salt water. Accordingly, these drains were always gated and had to be managed by someone who opened or closed the gate as necessary. That was the job that Norman had and he will also have been responsible for chasing up the surrounding crofters to clear the drain on occasion. His location, the Uachdar of Gramsdale puts his lands alongside the drain. Even today, the 'Old Gramsdale Inn' is immediately to the west of the drain and in Uachdar rather than Gramsdale.
Norman and family had indeed left by 1851. There is no sign in either township of Ann having remained behind. She must either have married though I think I would know of her later if she had married in Benbecula, or have died between 1841 and 1851. It does mean that she was not left by the family when 15 as she would have been approaching 25 by 1851. In fact, the family almost certainly left by about 1844 as that was when the migrant ships that, till then, had headed to Pictou/CB changed their routine and made their first landfall at Quebec. At the end of the 1840s, all the migrants found themselves and settled, in Ontario.
Two questions. Do you know that Ann stayed behind or did she simply not arrive in CB with the rest of the family? And where did they settle? Was it among the other Catholic settlers broadly in Northwest CB in places like Sydney and Mira? Hope this helps. Angus