What is the correct Douma’s equation for rectangular tunnels or conduits?

Outlet works through dams and rivers are openings that allow water to be withdrawn from reservoirs. These openings can be pipes or tunnels. Outlet works are used to regulate the release of water at a controlled rate, and to empty dams for safety reasons.  Here are some types of outlet works: Pressure conduits: A common type of outlet, these are made up of a pressure conduit or tunnel that leads from an intake structure to an emergency gate chamber. The flow is then carried through a free-standing steel pipe to a control structure. Sluices: These are outlet works through concrete gravity... Show more

Outlet works through dams and rivers are openings that allow water to be withdrawn from reservoirs. These openings can be pipes or tunnels. Outlet works are used to regulate the release of water at a controlled rate, and to empty dams for safety reasons. 

Here are some types of outlet works:
Pressure conduits: A common type of outlet, these are made up of a pressure conduit or tunnel that leads from an intake structure to an emergency gate chamber. The flow is then carried through a free-standing steel pipe to a control structure.
Sluices: These are outlet works through concrete gravity dams.
Conduits: These are outlet works through embankment dams that are laid below the dam or constructed by cut-and-cover.
Tunnels: These are outlet works through embankment dams that are excavated through the abutment or foundation. 

Outlet works can also be used to release surplus inflows in conjunction with a spillway. 

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