Tunnel structure

A tunnel structure comprising a plurality of cover support members and a cover member supported by and extending over the cover support members, the cover support members being of generally arcuate profile and each including an upwardly projecting hump, the humps of the cover support members being aligned with one another, and wherein the cover member is anchored in position by a sheet-retaining structure on the upwardly projecting humps, to permit rolling up, such that upon rolling up of the cover member to a rolled up maximum ventilation position adjacent the humps, a portion of the cover member that is not wound up covers the humps, such that the cover member defines a ridge having sides that are more steeply sloped, in use, than the parts of the cover member when in an unrolled condition adjacent the ridge.