sheath_ the cover for a blade

A friend and I had gotten in near where the outlet of the tunnel emptied into Jamaica Bay (which is part of the Atlantic Ocean, and so it is affected by tides). We hiked inland —upstream — for a few hours, and I was excited because I realized that this huge drain tunnel approximately followed the route of an old stream that had existed on the surface long before the city developed over it. But then the water suddenly started rising, and we realized that it had started raining on the surface above us, and all that water was pouring off the streets and sidewalks and rooftops of Queens and we would be drowned if we didn’t get out as quickly as possible.