Flap Valve Stormwater

stormwater
Stormwater is a term used to describe water that originates during precipitation events. It may also be used to apply to water that originates with snowmelt or runoff water from overwatering that enters the stormwater system.
Precipitation from rain or snow that accumulates in a natural or man-made watercourse or conveyance system.
Water derived from a storm event or conveyed through a storm sewer system.

valve
device in a brass wind instrument for varying the length of the air column to alter the pitch of a tone
control consisting of a mechanical device for controlling the flow of a fluid
A cylindrical mechanism in a brass instrument that, when depressed or turned, admits air into different sections of tubing and so extends the range of available notes
a structure in a hollow organ (like the heart) with a flap to insure one-way flow of fluid through it
A device for controlling the passage of fluid through a pipe or duct, esp. an automatic device allowing movement in one direction only
A membranous fold in a hollow organ or tubular structure, such as a blood vessel or the digestive tract, that maintains the flow of the contents in one direction by closing in response to any pressure from reverse flow

flap
Move (one's arms or hands) up and down or back and forth
roll: move in a wavy pattern or with a rising and falling motion; "The curtains undulated"; "the waves rolled towards the beach"
dither: an excited state of agitation; "he was in a dither"; "there was a terrible flap about the theft"
(of something attached at one point or loosely fastened) Flutter or wave around
any broad thin and limber covering attached at one edge; hangs loose or projects freely; "he wrote on the flap of the envelope"
(of a bird) Move (its wings) up and down when flying or preparing to fly