Aggradation is the process by which the elevation of a streambed, channel, or floodplain increases over time due to the deposition and accumulation of sediment. It occurs when the supply of sediment to a channel exceeds the channel’s capacity to transport that sediment, resulting in net sediment storage within the system of the watercourse.
In stormwater and fluvial systems, aggradation is typically driven by changes in watershed conditions that increase sediment yield or reduce flow competence. Common causes include upstream land disturbance, such as construction, deforestation, or agriculture, which accelerates erosion and delivers larger volumes of sediment to downstream channels. When these sediments enter a reach with lower flow velocities or reduced energy, they settle out and build up on the channel bed.
Aggradation alters channel geometry by raising the bed elevation, which can reduce flow depth and decrease overall conveyance capacity. As a result, channels become more prone to overbank flooding during storm events, even if discharge levels remain unchanged. Over time, aggradation can lead to channel instability, prompting lateral adjustments such as bank erosion or the formation of new flow paths.
In engineered stormwater systems, aggradation can occur in ditches, swales, culverts, and detention or retention basins when sediment is not adequately managed. Accumulated sediment can clog infrastructure, reduce storage volume, impair hydraulic performance, and increase maintenance requirements. In natural systems, aggradation plays a role in floodplain development and habitat formation, but excessive or rapid aggradation due to human activity is generally considered undesirable.
Effective stormwater management seeks to minimize aggradation by controlling sediment at its source through erosion and sediment control practices, stabilizing disturbed areas, and incorporating sediment forebays or other treatment measures to capture sediment before it enters downstream conveyance systems.