Suspended load is the portion of a sediment load that consists of fine particles carried within the water column of flowing water, remaining in suspension due to turbulence and the upward forces generated by fluid motion. In stormwater management and fluvial systems, suspended load typically includes smaller, lighter particles such as silt, clay, organic matter, and fine debris that do not readily settle under normal flow conditions.
Unlike bed load, which moves along the bottom of a channel, suspended load is distributed throughout the depth of the flow and is transported at approximately the same velocity as the surrounding water. The ability of water to carry suspended particles depends on factors such as flow velocity, turbulence, particle size, shape, and density. Higher energy flow conditions increase the capacity of the system to keep particles in suspension, while reductions in velocity, such as in detention basins or quiescent water bodies, allow these particles to settle out.
Suspended load is a major concern in stormwater management because it is closely associated with water quality impacts. Fine particles in suspension often serve as carriers for pollutants, including nutrients, heavy metals, hydrocarbons, and pathogens, which can adsorb onto particle surfaces and be transported through the system. As a result, controlling suspended load is essential for reducing pollutant loads and protecting receiving waters.
Stormwater best management practices are often designed to target suspended load removal through processes such as sedimentation, filtration, and flocculation. However, because suspended particles are typically very small and slow to settle, they can be more difficult to remove than coarser materials, requiring longer detention times or specialized treatment approaches.