Question 1

What is a straight-drop spillway?

Accepted Answer

A straight-drop or 'box-inlet' spillway is a vertical-walled drop with a horizontal apron at the toe. The free-falling jet impacts the apron and forms a hydraulic jump that dissipates the falling-head energy. NRCS uses straight drops as principal spillways for small embankment dams (NEH-11), and for grade-control structures in stable channel design.

Question 2

What is the drop number?

Accepted Answer

The drop number DN = q²/(g·d³) is a dimensionless group that scales the geometry of the impinging jet downstream of the drop, where q is unit discharge, g is gravity, and d is the drop height. Rand (1955) showed that brink-pool depth, jet impact distance, and required apron length all collapse onto power-law functions of DN. Typical drop spillways have DN between 0.001 and 0.5.

Question 3

How long should the stilling basin be?

Accepted Answer

NRCS NEH-11 (and Rand 1955) gives x_a/d = 4.30·DN^0.27 as the distance from the drop to where the supercritical jet completes its plunge, plus an additional 6·y₂ for the hydraulic jump itself. Together, the apron should be at least L_a = x_a + 6·y₂. For typical small dams with DN ≈ 0.05, L_a is about 8–12 times the drop height d.

Question 4

What tailwater is required?

Accepted Answer

For the hydraulic jump to remain on the apron and not sweep downstream, tailwater depth must equal or slightly exceed the conjugate depth y₂. Rand gives y₂/d = 1.00·DN^0.22. If actual tailwater is below this, add an end sill or block apron, lower the apron elevation to depress the jump, or use a Type III stilling basin instead.

Question 5

When should I use a drop instead of a chute spillway?

Accepted Answer

Drop spillways are simplest, cheapest, and most common at small heads (d ≤ 8 ft for NRCS designs). For larger drops, the impact forces become large, and a chute or stepped spillway with a Type II/III stilling basin is more efficient. Above d ≈ 20 ft, a chute spillway is almost always preferred.

Question 6

What's the difference between a drop weir and a drop spillway?

Accepted Answer

In NRCS usage, a 'drop spillway' is a complete principal-spillway structure with crest, vertical wall, apron, and end sill — designed to safely pass the design storm from a small dam. A 'drop weir' or 'grade-control structure' is the same vertical-wall drop installed in a channel to control bed slope, without an embankment behind it. The hydraulic equations are identical.

D_N	x_a/d	y₂/d	L_basin/d
0.001	1.04	0.30	2.8
0.005	1.51	0.42	4.0
0.01	1.82	0.49	4.7
0.05	2.66	0.68	6.7
0.10	3.21	0.79	7.9
0.50	4.71	1.10	11.3
1.00	5.68	1.28	13.4

Drop Spillway Calculator

How to use this calculator

Drop-number scaling

Worked example

Example — small NRCS sediment-dam principal spillway

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