Plunging Flow


Water leaving a perched culvert has a velocity with both horizontal and vertical components. By neglecting all frictional losses, a simplified projectile equation can be applied to the exiting water. Using this equation, FishXing calculates the path of the falling water from exiting the culvert outlet until it impacts the downstream outlet pool. FishXing assumes the location in which the plunging flow impacts the outlet pool is where the fish will attempt to leap from.

The height the water plunges, H, and the horizontal distance the water travels from the outlet, L, is described by the following standard projectile equations:



H = Vertical plunge distance.

L = Horizontal plunge distance

Vout = Exiting water velocity at the culvert outlet [ft/s or m/s]

g = Gravitational acceleration [32.2 ft/s2 or 9.8 m/s2]

t = Time for the exiting water to fall from the culvert outlet to the pool [s],

q = Angle at which the water exits the culvert outlet [degrees or radians]


Since H is known, FishXing solves for the time, t, and the horizontal plunge distance, L.

Exit Angle

The exit angle is the angle the exiting water surface makes with the horizontal.  It is calculated using the last two nodes within the culvert,  which is defined as the Outlet Zone. The water surface slope at the outlet  is determined by the applicable Free Surface Outlet conditions.

See Also:  Defining Normal Depth, Froude Number and Flow States, Open Channel Flow, Perched Outlet