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Small Grassed Waterways: Iowa Job Sheet

How to design, construct, seed and maintain Small grassed waterways

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What is a grassed waterway?

Grassed waterways are areas planted to grass or other permanent vegetative cover where water usually concentrates as it runs off a field. Grass in the waterway slows the water and guides it off the field.

How it helps the land

Grass or permanent vegetation established in waterways protects the soil from concentrated flow. Grassed waterways significantly reduce gully erosion.

Where the practice applies

Grassed waterways should be used where gully erosion is a problem. The most common areas are in draws between hills, and other low lying areas on a slope where water concentrates as it runs off a field.

Where to get help

Conservationists can provide guidance for controlling small gullies with grass. Design charts for waterways in larger drainage areas are available from the Natural Resources Conservation Service (NRCS) to allow design and lay out of larger waterways.

Your local NRCS can assist in design of waterways for larger drainage areas. For more job sheets and conservation information visit the NRCS website at www.ia.nrcs.usda.gov.

Design

Grassed waterways are designed to safely carry runoff water from the area that drains into them to a stable outlet.

Small waterways are designed in a parabolic, or dish shape, wide enough and deep enough to carry the peak runoff from a 24-hour storm that would be expected to occur once in 10 years. By keeping the flow within the grassed waterway area, maintenance of the waterway is significantly reduced.

Even under the best conditions, grassed waterways tend to either silt in or cut into gullies.

The goal of waterway design is to minimize both siltation and gullying of the waterways. This includes directing water down the center of the waterway, preventing it from spilling out and cutting a path along the outside edges. Considerations in waterway design include:

Use a stable, natural outlet where possible. A new gully will form from the outlet end of a waterway if the outlet is unstable.
Provide tile drainage where necessary to prevent prolonged surface flow or seepage problems, which could start a new gully. Excessive wetness can prevent establishment of grass, which could lead to formation of new gullies.
Preserve as much of the natural drainage system as possible. Natural waterways tend to have the flattest grade in the area and the most stable outlets.
The better the erosion control on land that drains to the waterway, the less siltation of the waterway. Well protected cropland in the waterway drainage area will make it easier to establish and maintain the waterway.
Proper design will produce a waterway that can be crossed with farm equipment.

Steps to waterway design

Note: As you design and construct a grassed waterway, you are encouraged to keep records of design and construction. The records are helpful in determining whether a waterway should be functioning properly. Use the “Design and Construction Record Sheet for Small Grassed Waterways” provided by NRCS.

Determine the drainage area of the waterway, in acres. On smaller drainage areas, field observations may produce accurate estimates. A simple estimation method on larger areas is to determine the distance from the highest point in the drainage area to the outlet of the waterway (hydraulic distance). Estimates can then be made either on the land or by using maps for drainage areas. Use the chart below. Example: If the actual hydraulic distance is 800 feet, the drainage area is in the 7-10 acre drainage area design class. Using a conservation plan map at a 660 ft. per inch scale, a hydraulic distance of 1.1" would also be in the 7-10 acre class.
Determine slope of the waterway channel, in percent. Percent slope is the amount of vertical drop in the waterway in 100 feet of length. Example: A hand level shot, survey, or USGS contour map showing 2 feet of drop in 100 feet of waterway length is a 2 percent slope.
Determine the zone in which the waterway is located. Use the map below.
Look up the required waterway top width and depth for your particular drainage area, waterway slope, and zone. Example: A drainage area of 8 acres with a 2 percent slope in Boone County (zone 2) would call for a waterway design with a 36-foot top width and a center depth of 1.2 feet.
Choose fertilizer and seeding mixture.

Outlet considerations

If water flow at the lower end of the proposed waterway tends to spread evenly over the land, it will likely be best to blend the outlet of the waterway into the landscape.

However, if the waterway will outlet into a larger waterway or gully, several hundred feet of the lower end of the new waterway should be graded to blend into the flow line of the larger gully, or a grade control structure should be installed.

Consult with NRCS if there are drastic changes in the grade of the waterway. Significant breaks in grade could lead to overfalls and a new gully within the waterway.

Construction specifications

Remove all trees, brush, stumps, and other objectionable material from the site. In areas of the waterway where a deep gully will call for earth fill, trees and stumps may be sawed off within 12 inches of the ground surface if the final surface is three or more feet above the top of the stumps.

Construct the waterway to the dimensions selected from the small waterway design chart. The fill material should be free of frozen earth, brush, root, and other materials which will not allow for the desired compaction.

Place all earth fill in layers of 9 inches or less, with each layer compacted by the wheels and/or tracks of the construction equipment. Spread or dispose of excess excavated material so it will not interfere with the functioning of the waterway.

If infertile subsoil will be exposed by construction operations, strip and stockpile topsoil. When excavation is complete, spread the topsoil over the exposed infertile soil.

Grade and shape the soil so that the area to be seeded is reasonably smooth, free of rills and gullies, and has the designed cross section, After the seedbed is prepared, seed and fertilizer according to instructions on the next page of this job sheet.

Outlet considerations

If water flow at the lower end of the proposed waterway tends to spread evenly over the land, it will likely be best to blend the outlet of the waterway into the landscape.

Consult with NRCS if there are drastic changes in the grade of the waterway. Significant breaks in grade could lead to overfalls and a new gully within the waterway.

Seeding and fertilizing

After shaping and smoothing a waterway, it’s important to get a protective seeding established before heavy rains form new rills or gullies.

In area where grass is difficult to establish, divert water from the waterway during the seeding establishment period. Follow these guidelines:

A firm seedbed will greatly increase the chance of a good grass stand. So before you plant, be sure the seedbed is firm. A good way to check is to step on the seedbed. If your footprint is less than 1/2" deep, you’ve got a firm seedbed. You may need to use a roller or cultipacker to get a firm seedbed.
Apply fertilizer and seeding at rates recommended in the chart. Plant grass seed 1/4" to 1/2" deep. The best row direction is across the area. If you broadcast, cover the seed lightly with a harrow. Seeding may be completed during the spring seeding period, March 1 to May 15; during the late summer seeding period, August 1 to September 15; or during the dormant seeding period, November 15 - freeze up. Warm season grasses should be seeded between March 1 and June 1.
If construction is completed at any time other than the above seeding periods, seed 1-2 bushels of wheat or rye, or 2-3 bushels of oats per acre as a temporary cover. Then seed permanent species during the next seeding period.
Oats may be seeded as a nurse crop for introduced grasses at the rate of one bushel per acre during the spring. Mow oats before they head out.
Mulch the area with straw, if possible, to protect the area until a seeding is established. Anchor the mulch using a straight disk. Make only one trip over the mulch.

Maintaining the practice

Proper maintenance will protect your investment in a grassed waterway. The following tips will help ensure longevity of the waterway:

Lift implements out of the ground before crossing the waterway.
Bring row crop patterns into the waterway nearly level, or use it as the turn area. Don’t plant end rows along the side of the waterway, as they contribute to failure.
Fertilize periodically.
Inspect the area frequently for places needing reseeding and eroding areas. Repair minor rills or gullies by reshaping and reseeding.
Maintain the width of the grass area when tilling and planting surrounding fields.
Don’t let herbicide spray continue into the waterway.
Don’t use the waterway as a road. Vehicle tire tracks can lead to the formation of a gully.
Don’t mow the grass until a good sod is established. Once it is established, the waterway should be mowed. To benefit wildlife nesting, delay mowing until August 1.
Maintain outlets to prevent new gullies from forming at the outlet. This may include reshaping and reseeding the outlet, or repairing or replacing components of structural outlets.