Terracing is a land modification practice that transforms steep or sloped terrain into a series of level or nearly level steps, resembling giant staircases cut into a hillside. This earthwork practice is a long-established method of land conservation and agricultural management, particularly in mountainous regions or on erodible lands. Terracing fundamentally alters how rainfall interacts with the soil surface. Its primary function is to enable cultivation on slopes that would otherwise be unusable and to protect the soil from being washed away by rainwater runoff.
How Terracing Controls Water Flow and Erosion
The mechanism by which terracing controls erosion centers on reducing the speed and volume of surface water flow, the main driver of soil loss. On an uninterrupted slope, rainfall runoff accelerates, building erosive power that leads to sheet and rill erosion. Terrace construction effectively breaks one long slope into a sequence of much shorter, disconnected segments.
This reduction in slope length prevents water from accumulating enough speed and volume to develop highly erosive force. As water flows across the flat bench of a terrace, its velocity drops significantly, causing suspended sediment particles to settle out behind the terrace ridge. This process conserves topsoil by capturing it before it can be washed into lower areas or waterways.
Terraces also significantly improve the soil’s capacity to absorb water, a process called infiltration. The level platforms hold the runoff temporarily, giving the water more time to soak into the ground rather than running off immediately. This increased infiltration conserves soil moisture, which is beneficial in arid or semi-arid environments, and reduces the total volume of runoff leaving the field.
Research has shown that well-maintained terracing systems can substantially reduce surface runoff, sometimes by as much as 70%, compared to original sloped land. The temporary storage of water behind the terrace ridges also reduces peak flow rates during heavy storms. This hydrologic control minimizes the risk of gully formation and reduces the delivery of sediment and attached pollutants, such as phosphorus, to downstream water bodies.
Essential Structural Elements of a Terrace System
A functioning terrace system is composed of three interconnected components that manage water and soil. The bench or platform is the leveled or gently sloping surface created for cultivation, where water is intended to be detained. This surface is where crops are grown and where the majority of water infiltration occurs.
The riser is the steep bank of earth that connects the bench of one terrace to the bench immediately below it. This vertical drop defines the “step” appearance of the system. Risers are often stabilized with vegetation, stone, or concrete retaining walls to prevent erosion of the bank.
A channel or drainage way is integrated into the system to manage excess water safely. This is typically a shallow ditch or swale constructed along the upper edge of the bench. The channel intercepts runoff from the slope above and conveys it slowly and non-erosively to a stable outlet, such as a grassed waterway or an underground pipe system.
In systems utilizing underground outlets, a riser pipe is installed in the channel to control the discharge rate of the stored water. This mechanism ensures that water is held long enough for sediment to settle out before the clearer water is slowly released through the subsurface drain. The careful design of these elements ensures that the system can withstand intense rainfall events without catastrophic failure.
Categorizing Terrace Designs by Landscape Use
Terracing systems are adapted based on the steepness of the slope, soil type, and intended land use. Bench Terraces are the most recognizable form, characterized by their distinct, stair-like appearance and are used on very steep slopes, often exceeding 15% to 20%. These terraces feature a level or slightly outward-sloping bench and a steep riser, designed to create maximum cultivable area on difficult terrain.
For more gentle slopes, typically less than 8%, Broad-Base Terraces are employed. These designs feature a wide, low earthen embankment and a broad channel. The cultivated area extends over both the front and back slopes of the embankment. The gentler slopes allow for easy crossing by farm machinery, making them suitable for large-scale mechanized agriculture.
A third category includes Conservation or Level Terraces, designed for maximum water retention where moisture conservation is a greater concern than runoff disposal. These terraces are constructed nearly level along the contour, acting as small reservoirs to capture and hold all rainfall on the slope. They are effective in arid regions where the goal is to increase soil moisture content and groundwater recharge.