HY8:Box Inlet Drop Structure

The input variables required for this calculation is the following:

• H_D — Desired drop height. Must be between 2 and 12 ft or between 0.6 and 3.7 m.
• New Slope — The slope that will exist on the channel once the drop structures are in place (The new slope must be subcritical).
• Box Length — Length of box inlet. (USER'S CHOICE)
• W₂ — Width of box inlet. Must fit criteria (.25 < H_D/W₂ < 1)
• W₃ — Width of the Downstream End of Stilling Basin. This must be equal to or larger than the culvert width.
• Flare of Stilling Basin (1 Lateral: Z long) — This value must be greater than or equal to 2, which is to say 1 lateral: 2 Long)
• Length from Toe of Dike to Box Inlet — If a dike is used, the distance from the toe of the dike to the box inlet must be entered. If no dike is used, enter a value of 100 ft or 30.48 m for this distance.

The following figure shows a plan and side view of a box inlet drop structure.

Variables from the figure

• W₁ — Width of the upstream end of the basin
• W₂ — Width of box inlet crest
• W₃ — Width of the downstream end of the basin
• W₄ — Distance from the toe of dike to the box inlet
• L₁ — Length of box inlet
• L₂ — Minimum length for the straight section
• L₃ — Minimum length for final section (potentially flared)
• H₀ — Drop from crest to stilling basin floor
• h₂ — Vertical distance of the tailwater below the crest
• h₃ — Height of the end sill
• y₀ — Required head on the weir crest to pass the design flow
• y₃ — Tailwater depth above the floor of the stilling basin
• h₄ — Sill height