Terraces
Terraces
Definition
Earth embankments or combination ridges and channels, constructed across the field slope. Terraces intercept, detain and safely convey surface runoff to stable passages that include grass waterways or underground outlets.
Purpose
Terraces are used to reduce runoff and erosion potential and the delivery of sediment and attached nutrients (phosphorus) to downslope streams, rivers, and lakes. These structures also promote infiltration and moisture conservation, prevent gulley development and alter the land surface to improve farmability.
How Does This Practice Work?
Terraces reduce soil erosion by decreasing excessive slope lengths, minimizing runoff velocity, and allowing sediment particles to deposit. The resulting runoff which contains less sediment is then transported from the field in a nonerosive manner.
Terraces reduce phosphorus transport by reducing soil erosion and excess runoff. Water erosion moves soil particles that may have phosphorus attached. As sediment reaches downstream water bodies, phosphorus particles may be released into the water, which can significantly impair water quality.
Where This Practice Applies and Its Limitations
Terraces can be used on moderate to steep slope fields that are susceptible to water erosion. They can also be employed where runoff or sediment could impair water quality or cause damage downstream.
Terraces may be used where:
- Soil erosion from water and excessive slope lengths are problems.
- Excess runoff is a concern.
- There is a need to conserve water.
- A suitable outlet can be provided.
- Soils and topography are such that terraces can be constructed and the area farmed successfully.
Effectiveness
Terraces have been shown to provide a 70% reduction in surface runoff and a 40% decrease in nutrient loads (nitrogen, phosphorus). Terraces are most effective when used as part of an erosion control system, which may include contour farming, no-till or reduced till residue and tillage management, and field borders.
Types of Terraces
There are two basic types of terraces:
- Storage terraces: capture and store runoff until it can infiltrate into the soil or be removed through an underground pipe (tile drain).
- Gradient terraces: designed as a channel to direct runoff towards a stable outlet, such as a grassed waterway.
Terraces require careful design, layout, and construction to provide erosion control while maintaining farmability. Some commonly used cross-section types of terraces include:
- Broad-based terraces are flatter looking and farmed on both slopes. They require a uniform gentle slope less than 8%.
- Narrow base terraces have steep 2:1 slopes (2 feet horizontal for every foot of vertical drop) with both the front slope and backslope and are seeded with perennial grasses.
- Grass backslope terraces have a farmable gentle front slope with a steep 2:1 backslope that is seeded with permanent cover.
- Grass front terraces have steep front slopes seeded with perennial grasses, and a mild downslope that is farmed.
Terrace Outlets
Stable and adequate outlets are required to convey the surface runoff that is intercepted by terraces. The following types of outlets are used for terrace systems:
- Surface outlets: designed to handle peak discharge for a 10-year frequency, 24-hr duration storm event. Examples include grassed waterways (most common), existing road ditches and borders.
- Underground outlets (UGO): consists of an aboveground inlet, underground conduit, and a temporary water storage pool. UGOs discharge the terrace design storm to stable surface outlets like open-channels, road culverts or grade-control structures.
Erosion can be a problem at the outfall of an underground outlet unless it is stable. Outlets from terraces may provide a direct conduit to receiving waters unless filter areas are established. Intakes from underground outlets can be easily damaged during farming operations and, therefore, they should be brightly colored.
Cost of Implementing the Practice
The cost of constructing terrace systems includes earth work for the embankment structures and the construction of adequate outlets (grassed waterway or UGO) and establishing vegetation on the respective terrace slopes on grassed backslope or narrow base terraces. Terraces require high initial capital investments, with costs ranging from $100 to $250 per acre depending on the type of terrace system. Potential losses in production because of construction and some reduction in crop acres may also result from terrace and waterway placement.
Operation and Maintenance
Terraces have a minimum expected life of 10 years. The operation and maintenance plan should include inspection and repair of the terraces and outlets, maintenance of terrace height and vegetation, and the removal of accumulated sediment. Where vegetation is specified, complete seasonal mowing, control of trees and brush, and reseeding and fertilization may be necessary. It may be necessary to repair damages from burrowing animals. Inspections of the terrace system should be performed periodically and after major storm events.
Tillage should be avoided in close proximity to intakes. Farming operations can create ridges that block drainage of the terrace channel. Sediment within the terrace channel should be removed to maintain required water holding capacity. Sections of the terrace that have eroded or have excessive settlement should be repaired. It may be necessary to extend intake pipes to prevent burial.
References
Al-Kaisi, M., & Hanna, H. M. (2001). Install and maintain terraces for conservation.
McLoud, P. R. (2011). NRCS Terrace Design Tool. ASABE.
Schottman, R. W., & White, J. (2023), Choosing Terrace Systems. University of Missouri Extension.
USDA-NCRS (2001). Terraces: Iowa Job Sheet.
USDA-NRCS (Feb 2021). 210-National Engineering Handbook, Part 650, Chapter 8, Terraces, Washington, D.C.
USDA-NRCS (2023). NRCS Conservation Practice Standard – Terrace Code 600. Washington, D.C.
For Further Information
Contact your local soil and water conservation district, USDA-NRCS or Cooperative Extension Service office. To find your local USDA Service Center, visit https://www.nrcs.usda.gov/contact/find-a-service-center. Funds for cost share may be available for terrace construction from NRCS or local conservation districts.
Current Authors
| Ken Wacha USDA-ARS ken.wacha@usda.gov |
John Gilley USDA-ARS john.gilley@usda.gov |
Previous Author
| Dennis Carmen USDA-NRCS |
Editing and Design
| Deanna Osmond NC State University |
Forbes Walker University of Tennessee |
Citation:
Wacha, K. and J. Gilley. 2023. Terraces. SERA17 Phosphorus Conservation Practices Fact Sheets. https://sera17.wordpress.ncsu.edu/terraces/