Terrace
Definition
A ridge and channel constructed across a slope and used to convey runoff water. Reduces erosion damage by intercepting surface runoff and conducting it to a stable outlet at a non-erosive velocity.
Where Practice Applies
Terraces are utilized on slopes having a water erosion problem. They should not be constructed on deep sands or on soils that are too stony, steep, or shallow to permit practical and economical installation and maintenance. Terraces should be used only where suitable outlets are or will be made available.
Advantages
Terraces lower the velocity of runoff, increase the distance of overland flow, and reduce slope length. They also hold moisture and minimize sediment.
Disadvantages/Problems
May significantly increase cut and fill costs and cause sloughing if excessive water infiltrates soils.
Design Recommendations
Spacing
The maximum recommended spacing is a vertical distance of 20 feet between terraces or other practices such as a diversion, dike, or sediment fence at the top or bottom of a slope.
Channel Grade
Channel grades may be either uniform or variable with a maximum grade of 0.6 feet per 100 feet length. For short distances, terrace grades may be increased to improve alignment. The channel velocity should not exceed that which is non-erosive for the soil type with the planned treatment.
Outlet
All terraces should have adequate outlets. Such an outlet may be a grassed waterway, vegetated area, or subsurface drain outlet. In all cases the outlet must convey runoff from the terrace or terrace system to a point where the outflow will not cause damage. Vegetative cover should be used in the outlet channel. The design elevation of the water surface of the terrace should not be lower than the design elevation of the water surface in the outlet at their junction, when both are operating at design flow. Vertical spacing may be increased as much as 0.5 feet or 10 percent, whichever is greater, to provide better alignment or location, to avoid obstacles, to adjust for equipment size, or to reach a satisfactory outlet.
Capacity
The terrace should have enough capacity to handle the peak runoff expected from a 2-year, 24-hour design storm without overtopping.
Cross-Section
The terrace cross-section should be proportioned to fit the land slope. The ridge height should include a reasonable settlement factor and "freeboard" (vertical distance between top of ridge and water elevation in the channel at design flow). The minimum cross- sectional area of the terrace channel should be 8 square feet for land slopes of 5 percent or less, 7 square feet for slopes from 5 to 8 percent, and 6 square feet for slopes steeper than 8 percent. The terrace should be constructed wide enough to be maintained using a small bulldozer.
Maintenance
Maintenance should be performed as needed. Terraces should be inspected regularly; at least once a year, and after large storm events.
References
Minnick, E. L., and H. T. Marshall, Stormwater Management and Erosion Control for Urban and Developing Areas in New Hampshire, Rockingham County Conservation District, August 1992.
Washington State Department of Ecology, Stormwater Management Manual for the Puget Sound Basin, Olympia, WA, February, 1992.