Rill Erosion

Rill erosion is the process by which numerous sub-parallel and randomly occurring small channels are formed on slopes by running water. Usually, if channel cross sections are smaller than 900 cm2 (square centimeters; 30 cm x 30 cm), they are called erosion rills. If they are bigger, they are called erosion gullies, and the erosion process associated with them is called gully erosion. Between the rills, the removal of soil is done by sheet (inter-rill) erosion, dominated by raindrop splash and sheet flow. Rill erosion is the most effective sub-process of soil erosion by water, contributing up to 80% of the total (rill + interrill) soil erosion.

Rills occur when overland flow starts to become concentrated. Overland flow with sufficient kinetic energy generates a shear stress that can detach soil particles. At locations where such detachment occurs, the soil's surface is lowered slightly. Such lowered areas form preferential paths for subsequent flow and, thus, will be eroded further. This process results in small, linear concentrations of flow known as “microrills” or “traces,” with a width and depth of few millimeters. If they grow further, they become erosion rills. Rill networks tend to be dendritic in form on natural soil surfaces but are constrained by the directions of [Page 2465]tillage on agricultural lands. Once rills form, their migration upslope occurs by retreat of the head-cuts on the steep banks at the top of the channel. The rate of retreat is controlled by the cohesiveness of the soil, the height and angle of the head-wall slope, and the discharge and velocity of the flow. Downslope extension of rills is controlled by the flow and the strength of the soil.

Ephemeral gully erosion and severe rill erosion wash away young corn plants from the ground as well as the topsoil and the nutrients from loess soils in this Western Iowa field.

Source: Lynn Betts, USDA Natural Resources Conservation Service

Rills are often associated with the “midslope,” as they may become enlarged on the lower part of the slope to give way to gullies. Therefore, the usual sequence of erosion processes downslope is rill and interrill erosion followed by gully erosion.

The density of rills depends on slope steepness and length, runoff rate, soil texture and erodibility of the soil, and the presence or absence of rainfall. On highly erodible soil, where the limiting factor of erosion is transport capacity, rills have similar size along the slope and exhibit high densities. On less erodible soil, where erosion is detachment limited, the width of rills varies along the slope, and their density is lower.

Typically, rills present no obstacle to normal tillage operation and can be easily removed.