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Natural Resources Conservation Service                                                                          
September 2001
Des Moines , Iowa

How to Design, Construct, Seed, and Maintain Small Grassed Waterways
Iowa Job Sheet

[Image: Photograph of grassed waterway between crops.]

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.
Small grassed waterways, those with drainage areas 30 acres and less, can be designed, built, and seeded using this job sheet.

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.

1. 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.

2. 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.

3. Determine the zone in which the waterway is located. Use the map below.

4. Look up the required waterway top width and depth for your particular drainage area, waterway slope, and zone. Use the Small Waterway Design Chart on the right. 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-ft. top width and a center depth of 1.2 ft.

5. Choose fertilizer and seeding mixture.

[Image: Map showing zones for 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.

[Image: Drawing of a parabolic waterway: Shape a parabolic waterway so that the depth halfway to the center is ¾ of the depth at the center. Example—if center depth is 1 ft., depth halfway to center is .75 feet.]

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 frozen earth, brush, roots, 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 track of excess excavated material so it will not interfere with the functioning of the waterway.
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 fertilize according to instructions on the next page of this job sheet.

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

1. 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.

2. 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; the next best is a figure-8. If you broadcast, cover the seed lightly by harrowing. 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 April 1 and June 1.

3. 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.

4. 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.

5. 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.

Recommended statewide seeding species and rates*

Dry or tiled waterways

• Smooth bromegrass   25 lbs/ac
• Smooth bromegrass   15 lbs/ac
  Switchgrass                3 lbs/ac
• Smooth bromegrass   15 lbs/ac
  Tall fescue                  6 lbs/ac

Alternatives for wet/dry waterways

• Smooth bromegrass  15 lbs/ac
  Reed canarygrass      5 lbs/ac

Alternatives for wet waterways

• Reed canarygrass       10 lbs/ac
• Switchgrass                 7 lbs/ac

* A local recommendation may be preferable. If so, it is:
Species                         lbs/ac


Recommended fertilizers
For bromegrass, fescue, and reed canarygrass

• Nitrogen                   80 lbs/ac
• Phosphate (P205)    150 lbs/ac
• Potash (K 2 O)        100 lbs/ac

The fertilizer rates are for average waterway conditions in Iowa after construction. It is preferable to determine your own rates by using soil test results. When soil testing is not possible, the general rates shown above may be used as a guideline. If considerable subsoil is exposed, increase rates by 50 percent. If sufficient topsoil is remaining, reduce rates by 50 percent. If land is in the Loess Hills, recommendations will vary considerably from these listed. Please consult a conservation professional for assistance.

Design and Construction Record Sheet                                     Tract #_________
                                                                                           Landowner ______________

Small Grassed Waterways (Drainage area 30 acres or less)
Instructions— Design

1. Record landowner and tract number on upper right corner. Record field numbers for the proposed waterways from the conservation plan in the first column on the left below.

2. Record the waterway number assigned on the conservation plan map.

3. Record drainage area in acres for the waterway. See top width determination method in NRCS job sheet.

4. Record slope of waterway.

5. Record needed top width in feet (obtain from design chart in Small Grassed Waterway Job Sheet) for drainage area and zone applicable.

6. Record needed waterway depth in feet from small waterway design chart.

Instructions— Construction

7. Record the constructed length of the waterway in feet

8. Record the actual top width of the waterway, in feet.

9. Record the waterway center depth, in feet.

10. Record waterway depth half way to center (see cross section drawing).

[Image: Drawing of parabolic waterway: Shape a parabolic waterway so that the depth halfway to the center is 3/4 of the depth at the center Example— if center depth is 1 ft. depth halfway to center is .75 ft.]

Seed and Fertilizer Records

Waterways number (s)________________________________

                                  Species / lbs. pure live seed/acre

Species 1 _____________________ / _____________________
Species 2 ____________________ / ____________________
Species 3 _____________________ / _____________________

Nitrogen _________lbs/acre
Phosphate (P205) ________lbs/acre
Potash (K20) ________lbs/acre
Lime _________lbs/acre

I certify this/these waterways have been designed and constructed according to the records provided on this sheet. Seeding was completed (check one),:

_____ 3/1 - 5/15 ______ 4/1 - 6/1

_____ 8/1 - 9/15 _____ Other date

Signature                                               Date

Check one ______ landowner ______ contractor

______ Other _____ Tenant

Seed and Fertilizer Records

Waterways number (s)________________________________

                                    Species / lbs. pure live seed/acre

Species 1 ______________________ / ______________________
Species 2 ____________________ / ____________________
Species 3 _____________________ / _____________________
Nitrogen _________lbs/acre
Phosphate (P205)________ lbs/acre
Potash (K20) _________lbs/acre
Lime _________lbs/acre

Note: If this record form is used for cost-share purposes, make a copy and return with receipts to the NRCS office. NRCS must certify upon completion.

NRCS employee signature                                               Date

Notes

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, sex, religion, age, disability, political beliefs, sexual orientation, or marital or family status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA's TARGET Center at 202-720-2600 (voice and TDD). To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326W, Whitten Building , 14th and Independence Avenue, SW , Washington , DC 20250-9410 or call (202) 720-5964 (voice and TDD). USDA is an equal opportunity provider and employer

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