SWCE-3101: Graded Bunding and Terracing Quiz

Graded Bunding

• Graded bunds are defined as 'bunds laid along a pre-determined longitudinal grade instead of along contours'.
• Suitable for high rainfall areas (> 600 mm) with land slope of 2 to 8% and regions with relatively impervious soil or low infiltration rates.
• Function: Constructing wide and relatively shallow channels across the slope to induce and regulate excess runoff water and drain it with a mild and non-erosive velocity.

Characteristics of Graded Bunds

• Not suitable for farming operations on bunds.
• Recommended in areas with highly impermeable soils like deep, black soils, even in regions with < 500 mm rainfall.
• Narrow-based versions of channel terraces where the design of the channel is important.
• The bund serves only to guide the water spilling from the channel safely.

Grassy Waterways/Vegetated Waterways

• Definition: Natural or constructed water courses covered with erosion-resistant grasses used for safe disposal of runoff from a field, diversion terrace, or other structures.
• One of the basic conservation practices.
• Grass establishment should be before any water is turned into it.
• Outlets/waterways should be ready before bunds, terraces, and diversions are constructed/installed.

Uses of Vegetated Waterways

• To handle natural concentration of runoff.
• To carry the discharge from terrace/bund systems, contour furrows, diversion channels, or emergency spillways of farm ponds/other structures.

Shape of Waterways

• Three shapes: triangular, trapezoidal, and parabolic.
• Parabolic shape is the most common due to its hydrological efficiency and ease of construction.
• Side slopes are kept very flat to permit easy crossing by farm implements.

Terracing

• Definition: An earth embankment or ridge and channel constructed across the slope of the land to intercept surface runoff water.
• May be constructed with a desirable slope to an outlet or with a level channel and ridge.
• Reduces the length of the hillside slope, thereby reducing sheet and rill erosion and preventing gully formation.

Functions of Terraces

• In humid areas: To reduce the length of the hillside slope, decrease rill erosion, prevent gully formation, and allow soil particles in runoff water to settle, improving the quality of surface runoff water leaving the outlet.