HY8:Stilling Basins

The four types of Stilling Basins External Energy Dissipators available in the program:

• USBR Type III Stilling Basin
• USBR Type IV Stilling Basin
• St. Anthony Falls (SAF) Stilling Basin

The maximum width of an efficient 'USBR' type stilling basin is limited by the width that a jet of water would flare naturally on the basin foreslope. The user is given the maximum flare value and is prompted to enter a basin width smaller than this value. If a 'SAF' basin is used, the basin width is set equal to the culvert width and the user is prompted to choose either a rectangular or flared basin depending on site conditions. Stilling Basins resemble the following illustration.

Variables from the figure

• W₀ — width of the channel
• W_B — Width of the basin
• y₀ — Culvert outlet depth
• y₁ — Depth entering the basin
• y₂ — Conjugate depth
• S₀ — Slope of the channel
• S_T — Slope of the transition
• S_S — Slope leaving the basin
• Z₀ — ground elevation at the culvert outlet
• Z₁ — ground elevation at the basin entrance
• Z₂ — ground elevation at the basin exit
• Z₃ — Elevation of basin at basin exit (sill)
• L_T — Length of transition from culvert outlet to basin
• L — Total basin length
• L_B — Length of the bottom of the basin
• L_S — Length of the basin from the bottom of the basin to the basin exit (sill)
• T_w — Tailwater depth leaving the basin

Warning for Stilling Basin Width

Since the maximum basin width is a function of basin depth, the maximum width may decrease as the program increases the basin depth while converging on a solution. Therefore the maximum basin width may fall below the user's first choice for basin width. In this case, the user will be prompted for a new basin width.