HY8:Box Inlet Drop Structure

The input variables required for this calculation is the following:

• HD -- 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 < HD/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