Grassed water ways and their design

Grassed waterways are natural or constructed channels designed to transport concentrated water flow safely and efficiently while preventing soil erosion. These waterways are lined with vegetation, which helps stabilize the channel, reduce the speed of water flow, and minimize erosion. Grassed waterways are used to manage excess runoff from agricultural fields and prevent damage to soil, water quality, and downstream environments.

Purpose of Grassed Waterways

Grassed waterways serve multiple purposes:

1. Prevent Rill and Gully Formation: They prevent the formation of rills and gullies, which are erosive features in the land.
2. Soil Erosion Control: The vegetation in the waterways slows down water flow, reducing the risk of soil erosion.
3. Safe Transport of Runoff: These waterways provide a controlled channel for excess water, safely directing it to outlets like rivers, streams, or reservoirs.
4. Sediment and Pollutant Reduction: The vegetation in the waterways helps reduce the amount of sediment and pollutants (like nitrogen, phosphorus, herbicides, and pesticides) carried by the runoff.
5. Wildlife Habitat: Grassed waterways can provide a habitat for various wildlife species.

Design of Grassed Waterways

The design of grassed waterways is similar to the design of irrigation channels and is based on their functional requirements. Proper design is essential for efficient operation and the long-term stability of the waterway.

1. Size of Waterway

The size of a grassed waterway depends on the expected runoff. For a 10-year recurrence interval, the expected maximum runoff is calculated using the rational formula. As the catchment area increases towards the outlet, the waterway’s cross-sectional area is designed to accommodate the increasing runoff.

• Formula for cross-sectional area: a = Q / V
• ​ Where:
  • a = cross-sectional area of the channel (m²)
  • Q = expected maximum runoff (m³/s)
  • V = velocity of flow (m/s)

2. Shape of Waterway

The shape of a grassed waterway is determined by field conditions and the equipment used for construction. Common shapes include:

• Trapezoidal Shape: This shape is preferred when farm machinery needs to cross the waterway.
• Parabolic Shape: This is common over time due to sediment deposition and flow patterns.

The design dimensions for each shape vary and need to be carefully selected based on the slope, expected runoff, and other conditions.

3. Velocity of Flow

The velocity of flow in a grassed waterway is critical for erosion control. Vegetation cover in the waterway helps resist erosion and slow down the flow. The permissible velocity depends on the type of vegetation and the soil. Typical velocities range from 1.0 m/s to 2.5 m/s.

• Recommended velocity based on vegetation type:
  • Low velocity (1-1.15 m/s): Sparse green cover
  • Medium velocity (1.5-1.8 m/s): Good quality cover
  • High velocity (1.8-2.5 m/s): Excellent quality cover

4. Cross-Section Design

The cross-sectional design is based on the required flow capacity and the channel’s geometry. Manning’s formula can be used to calculate the velocity of flow through the channel:

V = 1/n (R²/3 S ^½ / 1)

Where:

• V = velocity (m/s)
• n = Manning’s roughness coefficient
• R = hydraulic radius (m)
• S = slope of the channel

A trial and error method is used to determine the appropriate dimensions for the waterway, ensuring that the design velocity is met.

5. Construction Procedure

The construction of grassed waterways involves the following steps:

1. Shaping the Channel: The waterway should be shaped as smoothly as possible, avoiding sharp turns or sudden drops in grade. This step ensures efficient flow and reduces erosion.
2. Planting Grass: Suitable grasses (preferably local species) are planted to establish a vegetative cover. Seeding is often used, and mulching can help protect young plants.
3. Ballasting: In areas with steep slopes, stones can be used to stabilize the channel. This is particularly useful in small farms and areas prone to erosion.

6. Maintenance

Maintaining grassed waterways is essential for their continued effectiveness. Common maintenance tasks include:

• Weed Removal: Preventing unwanted vegetation that could reduce the efficiency of the grass cover.
• Filling Patches: Ensuring bare spots are reseeded or sodded to maintain complete coverage.
• Grass Cutting: Regularly mowing the grass to prevent overgrowth and maintain channel stability.

Selection of Suitable Grasses

When selecting grasses for a grassed waterway, several factors must be considered:

• Soil and Climate Conditions: Choose species that are well-suited to the local environment.
• Runoff Volume and Velocity: Grasses that can withstand higher velocities and more runoff are preferred.
• Establishment Time: Quick-growing grasses are ideal for rapid establishment.
• Erosion Resistance: Rhizomatous grasses, which spread quickly, are often chosen for their superior erosion control.