Sheet flow is the movement of water over the land surface as a thin, uniform layer that is not confined to a defined channel. In stormwater management, sheet flow typically occurs during the initial stages of runoff when precipitation exceeds the infiltration capacity of the soil and water begins to move downslope as shallow, diffuse flow across relatively smooth or gently sloping surfaces.
Under true sheet flow conditions, the water depth is very shallow and the flow is distributed evenly across a broad area, resulting in relatively low velocities compared to concentrated flow in channels, swales, or pipes. This type of flow is most common over short distances before it begins to concentrate into small rills or channels as surface irregularities, slope, and flow accumulation increase.
Sheet flow plays an important role in both hydrology and erosion processes. While it generally has lower erosive potential than concentrated flow, it can still detach and transport fine soil particles, especially on disturbed or unprotected surfaces. In stormwater design, maintaining sheet flow is often desirable in certain contexts, such as when directing runoff across vegetated areas or buffer strips, because it maximizes contact with the ground surface, promoting infiltration, filtration, and pollutant removal.
However, sheet flow is inherently unstable over longer distances. As runoff continues, it tends to transition into concentrated flow due to variations in topography and surface roughness. For this reason, stormwater practices that rely on sheet flow, such as vegetated filter strips or level spreaders, must be carefully designed to distribute flow evenly and prevent channelization, which would reduce treatment effectiveness and increase erosion potential.