Page 292 - Nutrient Management Planning Guide

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Constructed Erosion Control Measures to Reduce Nutrient Losses in Runoff

Design and Installation Recommendations

Use the following design and installation

recommendations to maximize effectiveness and ease of maintenance of grassed waterways. The services of a professional engineer may be required depending on complexity and impact of adjacent landowners or infrastructure.

Design and construct waterway to handle the •

required flow of water. The flow of water is influenced by amount and timing of snowmelt or precipitation, site topography, watershed area, soil conditions and crop type. Water flow is also influenced by management practices in the field (e.g., direct seeding and reduced tillage compared to conventional tillage). Ensure that the initial bare-soil condition and the more stable grass-covered condition are considered in the design relative to the potential risk of extreme rainfall events during establishment. The degree of erosion control provided by the •

waterway is related to the density of plant cover in the waterway. Therefore, timing of construction is important. Complete construction and seeding of the waterway in spring to ensure sufficient plant growth before snowmelt runoff the following spring. Construction should begin at the outlet and proceed upstream in the event of a rainfall runoff event during construction. Design waterways to be saucer-shaped (Figure 8.3.1). •

This shape spreads the flow of water over a greater surface area slowing its velocity and reducing its erosive force. This design also makes it easier to cross the channel with machinery and mow or harvest the vegetation. Use a standard width of the bottom of the waterway of 3 m (10 ft). The bottom portion of the channel should not be constructed horizontally but rather tilted slightly to one side so rills do not form in the bottom of the channel during lower flows before grass is established.

Design waterways in conjunction with other limiting •

structures. Sometimes routing waterways through existing or newly placed culverts or other control structures is necessary. Care needs to be taken in the hydraulic design of these structures and also the entrance and exit of the waterways to prevent destruction of the structure. If the slope is very steep other erosion control •

structures and materials can be implemented. Some examples are filter cloth, geotextiles, and various drop structures to dissipate some of the energy of the water.

Figure 8.3.1 Cross Section of a Typical Constructed Grassed Waterway

The side-slopes of the waterway should be 10:1 but •

no steeper than 4:1. A slope of 4:1 means 4 horizontal feet to one vertical foot. This ensures that the waterway functions properly but can still be crossed with equipment safely. Remove brush, rocks and other debris from the •

work area before construction. If these obstructions are not removed or buried, water will erode more quickly around them causing gullies or ponding to form. Any fill that is used in the channel should be packed hard. Remove topsoil from the working area, stockpile •

and replace after construction. Allow extra depth of the waterway to accommodate return of topsoil after construction. After spreading, level and harrow the topsoil to provide a smooth bottom and a good seedbed. Spread excess soil away from the sides, so runoff from adjoining land can flow easily into the waterway.

tip

Appropriate authoritative agencies such as Alberta Environment should be contacted in regards to any licensing requirements for the outlet of Grassed Waterways or diversions. Phone (780) 427-2700.