Gully Control Structures — Drop, Chute & Pipe Inlet Spillways — Soil & Water Conservation Engineering Notes

Last reviewed 1 Oct 2026 · 6 min read

Principles of gully control

Treat the catchment above the gully — cover, contour bunds, terraces — to cut the runoff.
Divert or safely handle the flow away from the headcut (diversion channel or grassed waterway).
Stabilise the headcut and the channel bed by structures, and the banks and gully floor by vegetation.
Exclude grazing and fire; seed and plant grass, shrubs and trees.

Structures are chosen by the drainage area (design discharge), the drop height, soil, and cost.

Drainage area/drop	Typical structure
Small catchment (below about 8–10 ha) and small drops	Temporary check dams, gully plugs, brushwood dams
Medium catchment, drop up to about 3 m	Drop spillway (masonry or concrete)
Large drop on long slopes, large flows	Chute spillway
Moderate catchment, drop up to about 3–5 m, earthen dam as embankment	Pipe-inlet (drop-inlet) spillway through the embankment

Temporary (gully control) structures

Used in the early stages, on small gullies, to trap silt and establish vegetation:

Structure	Material and use
Brushwood check dam	Brush/branches held by stakes across the gully
Loose-rock (boulder) check dam	Local stones, flanks keyed into banks, a spillway notch in the centre
Woven-wire (gabion) check dam	Stones in wire-mesh cages; flexible and permeable
Log and sod structures, sandbag	Light, temporary
Gully plug	Earthen or stone plug at the top or mouth of a small gully
Sod or vegetative barriers (vetiver)	Cheap, living check dams

Spacing of check dams. A series of dams is placed so that the sediment deposited behind each dam reaches the toe of the next one upstream. If the deposit forms at about a gully-bed slope $S$ (decimal) behind an effective dam height $H$ (above the bed up to the spillway crest), the spacing is

L = \frac{H}{S}

Worked ExampleExample — spacing

Effective height of each dam = 0.6 m; gully-bed slope = 5 % (0.05). Spacing $L = \frac{0.6}{0.05} = 12 m$ (when silt deposits level; if the deposit forms at a slope half as steep, the spacing is twice as great).

Design features: the spillway notch must pass the design flow, the foundation should be keyed (cut-off), apron or stone pitching below the crest prevents scour, and wings are keyed into the banks.

Permanent structures

Drop spillway (straight drop)

A vertical or nearly vertical masonry/concrete wall over which water falls freely into a stilling basin (pool) below, where energy is dissipated by a hydraulic jump; used for drops up to about 3 m (commonly less than 3–4.5 m).

FormulaMain components of a straight drop spillway

Inlet (control section) — a weir (rectangular, notch or box inlet) with the crest at the upstream bed; wing walls guide flow.
Vertical drop wall (headwall) with side walls.
Stilling basin (apron) — horizontal floor below the drop, with end sill (and baffles/blocks, if required) to produce the jump.
Cutoff walls upstream and downstream to prevent undermining (piping) and flank and end protection (riprap or pitching) against scour.
Aeration (vents) / nappe supply — ensures air under the falling nappe so it does not cling to the wall.

Discharge over the weir (rectangular):

Q = C L H^{3/2}

( $L$ = crest length, $H$ = head over the crest; $C \approx 1.7$ – $1.84$ for a free-flowing sharp or broad weir with $Q$ in m³/s, $L, H$ in m).

Critical depth at the brink, per metre width $q = Q / L$ :

d_{c} = (\frac{q ^{2}}{g})^{1/3}

Drop number $D = \frac{q ^{2}}{g h ^{3}}$ ( $h$ = drop height) is used in empirical formulas (for example Rand's) for the length of the free-falling jet and the pool, which govern the length of the stilling basin.

Worked ExampleExample — drop spillway hydraulics

A drop spillway of crest length $L = 2$ m carries $Q = 3.0$ m³/s over a drop $h = 1.5$ m.

Unit discharge $q = \frac{3.0}{2} = 1.5$ m²/s.
Critical depth $d_{c} = (\frac{1. 5 ^{2}}{9.81})^{1/3} = (0.2294)^{1/3} = 0.612 m$ .
Head over the crest (taking $C = 1.7$ ): $H = (\frac{Q}{C L})^{2/3} = (\frac{3.0}{1.7 \times 2})^{2/3} = (0.882)^{2/3} = 0.92$ m.
Drop number $D = \frac{1. 5 ^{2}}{9.81 \times 1. 5 ^{3}} = \frac{2.25}{33.1} = 0.068$ . Rand's relation for the jet-landing distance $L_{d} = 4.30 h D^{0.27} = 4.30 \times 1.5 \times 0.484 \approx 3.1 m$ downstream of the wall — the stilling basin is made longer than this to contain the jump.

Chute spillway

A channel (open chute, rectangular, steep, with side walls) carrying the flow down a slope at high velocity to a stilling basin; suits larger drops and flows (the drop is large and the slope not vertical), and needs a good, stable foundation.

Components: inlet (entrance) section (a control weir/crest, often flared) → chute (sloping channel, 1:2 to 1:3 slope or steeper) → stilling basin (with baffle piers and end sill) → outlet. Flow in the chute is supercritical; the basin forces the hydraulic jump to dissipate energy; freeboard and side-wall height must account for air entrainment (bulking) and flow waves.

Pipe-inlet (drop-inlet) spillway

A vertical riser (drop inlet — box or hood) at the upstream end connected to a conduit (barrel pipe) laid through the earth embankment, discharging to a stilling basin or plunge pool on the downstream side.

Box-inlet — rectangular box at the pipe entrance; hood inlet — sloping hood over the pipe mouth (the hood gives full-pipe flow); drop inlet (riser) with anti-vortex baffle.
Advantages: suits earth embankments and heights up to about 5–8 m or more, no vibration issue at moderate heads, cheaper than the masonry chute for medium flows; disadvantages: clogging by debris (trash racks) and conduit leakage/piping along the barrel (use anti-seep collars).
Capacity: as an orifice (when the pipe flows full): $Q = C_{d} A 2 g H$ ( $C_{d} \approx 0.6$ for sharp inlets and up to 0.8–0.9 for well-shaped ones).

Worked ExampleExample — pipe-inlet capacity

A 0.6 m diameter pipe flows full under an effective head of 1.2 m with $C_{d} = 0.65$ :

$A = \frac{π \times 0. 6 ^{2}}{4} = 0.283$ m²; $Q = 0.65 \times 0.283 \times 2 \times 9.81 \times 1.2 = 0.65 \times 0.283 \times 4.852 = 0.89 m^{3} /s$ .

Comparison

Feature	Drop spillway	Chute spillway	Pipe-inlet spillway
Typical drop	up to ~3 m	larger	up to ~5 m or more through embankments
Flow handled	moderate	large	small to moderate
Material	masonry/concrete	concrete	pipe (RCC/steel) + riser
Stilling arrangement	Basin + sill	Basin with baffles	Plunge pool/basin
Weak point	undermining	foundation, waves	trash blocking, barrel leakage

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Principles of gully control

Temporary (gully control) structures

Permanent structures

Drop spillway (straight drop)

Chute spillway

Pipe-inlet (drop-inlet) spillway

Comparison

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