Grassed Waterway and Vegetated Filter

Grassed waterway and vegetated filter consist of a natural or constructed vegetated channel that is shaped or graded and vegetated to carry surface water at a non-erosive velocity to a stable outlet that, in turn, spreads the flow of water before the water enters the vegetated filter. (The vegetative filter uses guidance from Filter Strip, 393, which is integrated with a grassed waterway.)

A grassed waterway and vegetated filter are used in areas where added water conveyance capacity and vegetative protection are needed to control soil erosion resulting from concentrated runoff. Such areas commonly include draws and other low-lying areas or outlets for other conservation practices (e.g., diversions and terraces). The minimum capacity of a waterway conveys the peak runoff expected from a storm of 10-year frequency, 24-hour duration. In some areas, a combination of high peak runoff and steep slopes may cause water velocities that preclude the use of a grassed waterway.

Vegetation establishment

Establish the waterway vegetation according to Critical Area Planting (342). For the stable, spreading-type outlet, select perennial plant species (native species are encouraged where possible) that are sod-forming plants having stiff, upright stems that act as a dense filter. Use the recommendations for filter strips for the area below the outlet. Establish vegetation before allowing water to flow in the waterway. Use irrigation and mulch to hasten establishment of vegetation as necessary. Use mulch, anchoring, a nurse crop, rock, hay-bale dikes, filter fences, or runoff diversions to protect the vegetation until established. Establish the vegetated filter vegetation according to Filter Strip (393).

Operation and Maintenance

Align tillage and row directions so they are perpendicular to the grassed waterway to allow surface drainage into the waterway and to prevent flow along the edges of the waterway. Provide stabilized machinery crossings, where needed, to prevent rutting. Protect vegetation from direct herbicide sprays, and use plant species tolerant of chemicals used at the site. Minimize damage to vegetation by excluding livestock during periods of soil wetness. The grassed waterway outlet should be kept as wide and shallow as possible to slow the velocity of water, increase infiltration, and spread flows evenly across a wide area before entering a vegetated filter. Inspect grassed waterways regularly, especially following heavy rains. Damaged areas will need to be filled, compacted, and seeded immediately. Remove sediment deposits to maintain the capacity of the waterway. Control noxious weeds.

Site-specific requirements are listed on the specifications sheet. Additional provisions are entered on the job sketch sheet. Specifications are prepared in accordance with the NRCS Field Office Technical Guide. See practice standards Grassed Waterway (412) and Filter Strip (393).