Grassed Waterways

Agdex#:	751
Publication Date:	03/94
Order#:	94-039
Last Reviewed:	03/94
History:	Reprinted July 1997
Written by:	R. Stone - Engineer (Soil Management)/OMAF

Table of Contents

1. Introduction
2. Design Considerations
3. Waterway Shape
4. Waterway Grade
5. Waterway Outlets
6. Draining the Waterway
7. Subsurface Drainage
8. Surface Drainage
9. Waterway Construction
10. Seeding and Fertilizing the Waterway
11. Maintenance of the Grassed Waterway
12. Relevant OMAFRA Factsheets
13. Related Links
    

Introduction

Grassed waterways are broad, shallow, saucer-shaped channels designed to move surface water across farmland without causing soil erosion. The vegetative cover in the waterway slows the water flow and protects the channel surface from rill and gully erosion. Waterways are often constructed in natural depressions where the water collects and flows to an outlet.

Figure 1. A Grassed Waterway Would Control This Erosion.

If properly sized and constructed, grassed waterways will safely transport water down slopes. Waterways may also be used to provide outlet channels for terrace systems, contour cropping operations and diversion channels.

Grassed waterways are a good solution to the potential problem of rill and gully erosion when the contributing watershed acreage is relatively large. As a guideline, consider having grassed waterways when working with watersheds greater than 20h (50 acres) in size; for smaller watersheds, investigate other structural controls such as water and sediment control basins. For further information on these, see OMAFRA Factsheet, "Water and Sediment Control Basins."

The main advantages of a grassed waterway are as follows:

1. The waterway will carry large flows making it suited to large watersheds.
2. Farm machinery can cross it.
3. After establishment of vegetation, maintenance is low.

However, a grassed waterway also has disadvantages. These include:

1. Working around the waterway with farm equipment is difficult.
2. The waterway lacks the depth necessary to serve as a tile drainage outlet.
3. The establishment of vegetation may be difficult.

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Design Considerations

The design of a waterway requires proper determination of the channel dimensions. It must ensure that (1) the velocity of the flowing water will not wash out the waterway and that (2) the capacity of the waterway is sufficient to carry the surface flow from the watershed without overtopping.

The allowable velocity of water in the waterway depends upon the type, condition and density of vegetation, as well as the erosive characteristics of the soil. Uniformity of vegetative cover is important because the stability of the most sparsely covered area determines the stability of the channel. Grasses are a better vegetative cover than legumes because grasses resist water velocity more effectively.

The waterway capacity depends upon the waterway width, depth and grade, as well as soil erodibility and vegetative cover in the channel area. Tall vegetative growth in the waterway resists water flow and must be viewed as a critical factor in capacity design.

In summary, waterway dimensions will generally vary according to the factors below:

1. Waterway width and depth increase as the volume of surface runoff increases.
2. Steeper channel grades result in the creation of a wider, shallower waterway.
3. A highly erodible soil will produce a wider, shallower waterway.

Design charts are available to assist in selecting proper waterway dimensions.

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Waterway Shape

A vegetated waterway is constructed in a parabolic or saucer-shaped form. Such a broad, shallow channel, with the bottom almost flat, spreads the water, slows its velocity and reduces its power to erode. The side slopes are graded to a maximum steepness of 10 horizontal : 1 vertical to permit the crossing of farm machinery.

Errors in constructing waterways include making them too deep, too narrow or too V-shaped, which encourages the water to scour the base of the waterway, leading to further gully formation.

Figure 2. A Parabolic (Saucer-Shaped) Waterway Allows The Water To Spread Out And Not Erode The Channel Bottom.

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Waterway Grade

Waterways are constructed to natural field grade where possible. In practice, the most satisfactory grade range has proven to be one to five percent. With steep grades (greater than five percent), drop structures are installed at critical locations throughout the waterway to reduce the grade to five percent. For excessively steep grades, the waterway centre must be lined with an erosion-resistant material such as rock riprap. However, lack of grade is also a problem. When the natural waterway grade is less than one percent, more grade has to be cut.

Waterway Outlets

The landowner is responsible for ensuring that the waterway exits at an adequate outlet. A landowner who has provided an inadequate outlet onto a neighbour’s property may be liable for any damages caused. The most desirable arrangement is the construction of the waterway across all the landowners’ properties involved to an adequate outlet and the registration of a mutual agreement by all landowners. Grassed waterways often exit into open ditches or streams so the construction of stable, non-erosive outlets at these locations is important. Rock chute spillways or vertical grade control structures will safely convey the water to the outlet.

Figure 3. A Rock Chute Spillway Located At The Outlet Of A Grassed Waterway.

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Draining the Waterway

Waterways should not be continually wet. If they are, the development of good vegetal cover is inhibited and the crossing of the waterway with farm equipment is impossible.

Subsurface Drainage

To prevent the waterways from being wet for long periods of time, install tile drainage lines. Each one should be parallel to the centre of the waterway and offset by at least one-fourth the top width of the waterway from centre line. Depending upon soil type, water table conditions, etc., one or two 100 mm (4") diameter lines per waterway may be required.

Surface Drainage

Surface water inlets, or catch basins, should be installed on the upper end of the waterway and connected to a subsurface tile drain to carry seepage water and/or low flows of surface water underground. This tile line intercepts only a small percentage of the water that the waterway carries. Drainage of surface seepage water will result in a dryer waterway, allowing a better vegetative cover to be established and permitting the crossing of farm equipment during field operations.

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Waterway Construction

Waterways are constructed with equipment capable of moving and smoothing soil. Bulldozers, land levelers and road graders may be used. Farm tillage implements can aid in the final smoothing of soil for seedbed preparation.

The construction procedure and time involved in constructing a grassed waterway depends upon the topographic features of the area and the construction equipment available. If a waterway is to be located in a natural draw or shallow gully with few washout areas, only minimal shaping and smoothing will be required. On the other hand, if the waterway is to reclaim a larger established gully, considerable earthwork will be required.

The timing of construction for the grassed waterway is extremely important. A high percentage of grassed waterways have washed out within the first few weeks or first year of construction because they were installed in the late fall - there was not enough time for the vegetation to become established.

The best time to construct a waterway is late August or early September so that vegetation will have formed a good stand before the late fall rains occur.

The waterway should be seeded at the end of each construction day. Seeding at this time permits the germination of seed because there is still adequate moisture in the soil. The seed should be broadcast with a hand cyclone seeder.

Figure 4. A Grassed Waterway Being Seeded With A Hand Cyclone Seeder.

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If broadcasting is not possible, the seed can be applied with a drill, but be sure to plant across or partially across the direction of the water flow. Farm machinery wheel-tracks made in a new waterway, combined with drilled rows parallel to the water flow, encourage erosion of the waterway.

Taking a temporary measure to protect the newly constructed waterway may be required. The need is especially great when risks of possible washout are high. Severe storms occurring prior to vegetative establishment may cause washouts. The following protective measures may be implemented:

• a temporary diversion may be constructed at the top end of the waterway to prevent large flows from entering the waterway until the vegetation is established. This measure can be used only where a satisfactory alternate route for surface water exists. After the waterway vegetation is established, remove the temporary diversion.
   
• after seeding the waterway, a straw mulch applied at a rate of 1.5 tons per acre (3.4 tonnes per hectare) can protect the growing seeds effectively. The straw must be anchored. Normally, anchoring is accomplished by crumping, discing, rolling or punching the straw into the soil. Now, many new materials are on the market for stabilizing the soil surface during the establishment of grass seedlings. Such materials include chemical soil stabilizers, plastic fibre or mesh covers, asphalt mulches and biodegradable mats. These materials serve as a: 
  • mulch to reduce the rate of drying,
  • barrier to absorb the energy of raindrops or wind, and
  • retarding barrier to reduce the velocity of runoff water.

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Seeding and Fertilizing the Waterway

Waterway seeding mixtures should include quick growing annuals for temporary control as well as a mixture of hardy perennials for permanent protection. Most seed companies have specific seed mixtures for different applications based on such factors as soil drainage, shade tolerance, resistance to flow velocity, maintenance requirements and climatic conditions. A recommended mixture would be creeping red fescue at 20 kg/hectare (18 lb./acre) and birds-foot trefoil (empire) at 12 kg/hectare (11 lb./acre). For further information, see OMAFRA Factsheet, "Seeding of Erosion Control Projects."

Grasses that bunch, such as timothy and orchard grass, are not recommended. Although tap rooted legumes are rarely recommended alone, alfalfa can provide a satisfactory cover when seeded as part of a grass-legume mixture. This recommendation offers a practical benefit to livestock farms where forage harvesting can include that portion of the crop in the waterway.

Under most conditions, grasses or grass-legume mixtures can be used as an erosion-resistant cover for water velocities up to 1.2 m per second (4 ft. per second). Above this speed, consider other linings such as rock riprap or permanent erosion-control mats.

Figure 5. A Grassed Waterway Will Safely Move Surface Water Off Farmland.

Soil in the waterway should be brought to a high fertility level and limed according to soil and plant requirements. Ideally, the fertilizer should be broadcast and worked in fairly shallow during the final stages of seedbed preparation. See OMAFRA Factsheet, "Seeding of Erosion Control Projects," for more details on fertilizer rates.

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Maintenance of the Grassed Waterway

Grassed waterways must receive regular attention, especially during the first year after establishment, to keep them in good repair.

1. Repair and reseed bare or eroded spots quickly.
2. Apply fertilizer to encourage a thick, vegetative growth.
3. Mow the waterway two or three times a year to help thicken the sod.
4. Prevent livestock from having access to the waterway.
5. Be sure not to use the grassed waterway as a travel lane for either cattle or farm machinery.
6. Raise farm implements when crossing the waterway and keep sprayers shut off.
7. Plough the surrounding land at right angles to the waterway to allow surface water to flow into the waterway. Never plough a headland furrow parallel to the waterway, a gully would develop there in future.
8. Follow good tillage and cropping practices that promote conservation to reduce the accumulation of sediment in the waterway and avoid the possible future destruction of the waterway structure.

Relevant OMAFRA Factsheets

Seeding of Erosion Control Projects.
Water and Sediment Control Basins.

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Related Links

• OMAFRA Factsheet, "Water and Sediment Control Basins."
• OMAFRA Factsheet, "Seeding of Erosion Control Projects."

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