Washoff is the process by which pollutants that have accumulated on impervious and pervious surfaces are mobilized and transported by rainfall and runoff into a stormwater system.
In stormwater management, washoff represents a critical component of pollutant loading because it describes how contaminants that build up during dry periods, often referred to as antecedent dry weather accumulation, are removed from surfaces once precipitation begins. As rainfall intensity and duration increase, runoff forms and exerts shear forces on surfaces such as roadways, rooftops, parking lots, and landscaped areas, dislodging and entraining materials into overland flow.
The types of pollutants subject to washoff include suspended solids, nutrients such as nitrogen and phosphorus, heavy metals, hydrocarbons, pathogens, organic debris, and other urban contaminants. The rate and magnitude of washoff depend on several factors, including rainfall intensity, rainfall duration, surface characteristics, slope, land use, and the nature and quantity of accumulated pollutants.
Washoff is often characterized by a “first flush” effect, in which a disproportionately high concentration of pollutants is transported during the initial phase of a storm event. This occurs because readily mobilized materials are quickly washed from surfaces at the onset of runoff.
From a modeling and design perspective, washoff is frequently represented using empirical relationships that relate runoff rate or rainfall intensity to pollutant removal rates. Understanding washoff processes is essential for designing effective stormwater Best Management Practices, such as street sweeping, infiltration systems, and treatment devices, that aim to reduce pollutant loads entering receiving waters.