Contour Trenching and Terraces

Construction of trenches on slope contours to detain water and sediment transported by water or gravity downslope generally constructed with light equipment. These are also known as contour terraces or contour furrowing. lined with geotextiles and filled with rock, stacked or placed to form an erosion resistant structure. 

Purpose: Contour trenches are used to break up the slope surface, to slow runoff and allow infiltration, and to trap sediment. Rills are stopped by the trenches. Trenches or terraces are often used in conjunction with seeding. They can be constructed with machinery (deeper trenches) or by hand (generally shallow). Width and depth vary with design storm, spacing, soil type, and slope.

Relative Effectiveness: Excellent-67% Good-33% Fair-0% Poor-0% (Replies = 3).  Two of the three interviewees who rated trenching considered its effectiveness “excellent;” the other thought it “good”. Trenches trap sediment and interrupt water flow, slowing runoff velocity. They work best on coarse granitic soils. When in-stalled with heavy equipment, trenches may result in considerable soil disturbance that can create problems.

Implementation and Environmental Factors: Trenches must be built along the slope contour to work properly; using baffles or soil mounds to divide the trench reduces the danger of excessive flow if they are not quite level. Digging trenches requires fairly deep soil, and slopes of less than 70 percent are best. Trenches are hard to construct in heavy, clay soils and are not recommended for areas prone to landslides. Hand crews can install trenches much faster than log erosion barriers (a similarly effective hillslope treatment), and crew skill is not quite as important to effective installation. Trenches have high visual impact when used in open areas (and thus may be subject to controversy), but tend to disappear with time as they are filled with sediment and covered by vegetation. On the other hand, more extreme (wide, deep) trenches installed several decades ago are still visible on the landscape in some areas.