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Chapter 8.3

Combining several constructed erosion controls with the practices discussed in the previous chapter is most effective for reducing slope-related runoff nutrient losses.

Note that much of the information available on constructed erosion control measures is based on experience in warmer areas of the United States with high annual precipitation where the majority of runoff is a result of rainfall. Although most runoff is from spring snowmelt, in Alberta some intense rainfall early in the growing season has caused the most erosive runoff events. Because of these differences in types of runoff, not all measures used in the warmer areas are applicable to Alberta. If control measures are applicable, tips are included to successfully implement these controls under Alberta conditions where possible.

Grassed Waterways

Grassed waterways are broad, shallow, saucer-shaped channels designed to move surface water across farmland without causing soil erosion. There are three features of this control measure:

The key component of this control is the vegetative •

cover in the waterway, which slows the speed of water flow in the watercourse and serves as a physical filter that removes sediment (and sediment bound nutrients) from water flow. As long as sediment deposition is not excessive, the sediment-bound nutrients trapped by the vegetative cover can then be used to supply the nutrient requirements of waterway vegetation. Waterways are typically constructed along the •

natural surface drainage pattern in a field. In many cases these are built to rehabilitate gullies formed as a result of the erosive force of surface runoff events. Ideally, the waterway conducts water to a suitable •

outlet, typically a ditch, water body or other control structure such as a settling basin.

Advantages of Grassed Waterways

Earthwork during construction is minimized •

and potential licensing requirements are reduced when existing field drainage patterns are followed.

If designed properly, they can be safely crossed •

by farm machinery.

Grassed waterways are capable of handling •

large flows, which makes them suitable for handling larger drainage areas.

Once vegetation in the waterway is firmly •

established, maintenance requirements are minimal.

Provided the waterway is properly maintained, •

waterway vegetation can serve as a valuable source of forage for livestock.

As a guideline, use grassed waterways when working with a drainage pathway handling runoff from area greater than 20 ha (50 ac). Grassed waterways are most effective on sites where runoff flow tends to concentrate in identifiable channels, which in some cases may erode to form gullies. Use other controls such as zero till for smaller areas.

If properly sized and constructed, grassed waterways will safely transport water down slopes. This characteristic makes them suitable for use as an outlet for contour cropping operations and as run-on diversion channels. Once the velocity of water running down a grassed channel exceeds about 1.2 metres per second (4 ft/s), the grass channel lining may need to be reinforced or alternative methods such as drop structures to reduce the channel grade should be investigated.

more info

The Association of Professional Engineers, Geologists and Geophysicists of Alberta (APEGGA) regulates the practice of engineering, geology and geophysics in our province under a mandate in the provincial

Engineering, Geological and Geophysical Professions Act.

The association website (www.apegga.org) has some useful advice on how to find a consultant or contact APEGGA with questions:

Phone: Toll free (in North America) 1-800-661-7020

Email: email@apegga.org