Grassed Waterways & Water Harvesting Notes

Last reviewed 1 Oct 2026 · 7 min read

Grassed waterways

A grassed waterway is a natural or constructed channel, shaped to a wide, shallow section and lined with sod-forming grass, which carries concentrated runoff safely downhill without gully formation. It serves as an outlet for terraces, diversions and field drainage.

Cross-sections

Shape	Features
Parabolic	Natural-looking; wide and shallow; easy to cross with machines; good for small to medium flows
Trapezoidal	Larger capacity; flat bottom, side slope 3:1 or flatter; common for large flows
Triangular (V)	Small flows only (concentrates flow, higher velocity; erodes)

Width: wide and shallow to keep the depth and velocity low; side slopes about 3:1 to 4:1 (flatter than 4:1 is crossed by implements).

Design considerations

Design flow: peak runoff by the rational formula $Q = C i A /360$ for a return period of about 10 years (for small, grassed waterways; 25 years for large structures).
Velocity: must be below the permissible (non-erosive) velocity, which depends on the grass cover, retardance class and soil erodibility (about 1.0–2.0 m/s for most grass-lined channels; lower on erodible soils).
Roughness (Manning's n): depends on the grass height and density (retardance) — about 0.03–0.06 for medium to dense grass, and it varies with the product $V R$ (velocity × hydraulic radius): a lower n for deeper flow.
Slope: grade is generally below about 5–8 %; drop structures (check) used on steeper reaches.
Freeboard: at least 0.1–0.15 m (or about 20 % of the depth) above the design depth.
Grasses: Bermuda (Cynodon dactylon, doob), Cenchrus ciliaris (anjan), vetiver (Chrysopogon zizanioides), Dichanthium, Napier (bajra × napier hybrid), kikuyu, Paspalum; good, dense sod-forming perennials.
Establishment: shape the channel, apply fertiliser, seed or sod (or use sprigs/slips), mulch and protect with a temporary cover, and divert water until the sod has formed.
Maintenance: mow, remove silt deposits, repair eroded patches, avoid tillage and heavy grazing.

FormulaCapacity check

Use Manning's equation: $V = \frac{1}{n} R^{2/3} S^{1/2}$ and $Q = A V$ , where $R = A / P$ .

For a trapezoidal section with bottom width $b$ , depth $d$ and side slope $z$ (horizontal : 1 vertical): $A = d (b + z d)$ and $P = b + 2 d 1 + z^{2}$ .

Worked ExampleExample — grassed waterway

A trapezoidal waterway has a bottom width $b = 1.5$ m, side slope $z = 3$ , flow depth $d = 0.3$ m, longitudinal slope $S = 0.02$ and Manning's $n = 0.04$ .

$A = 0.3 \times (1.5 + 3 \times 0.3) = 0.3 \times 2.4 = 0.72$ m².
$P = 1.5 + 2 \times 0.3 \times 1 + 9 = 1.5 + 0.6 \times 3.162 = 3.40$ m; $R = \frac{0.72}{3.40} = 0.212$ m.
$V = \frac{1}{0.04} \times 0.21 2^{2/3} \times 0.0 2^{1/2} = 25 \times 0.355 \times 0.1414 = 1.26 m/s$ .
$Q = 0.72 \times 1.26 = 0.90 m^{3} /s$ .

The velocity (1.26 m/s) is below a permissible value of about 1.5–2 m/s for good grass; if the design flow is 0.6 m³/s the section is adequate with margin. A freeboard of 0.15 m makes the total depth 0.45 m.

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