Buildup/Washoff Models
The modelling of the buildup of sediment in the network and the movement of sediment and determinants through the drainage system during a rainfall event is simulated using one of the following models:
 Innovyze model, which incorporates the Surface Pollutant Model and is described in the Surface Washoff and Gully Pot Flushing topic. This model is only applicable to InfoWorks networks.
 SWMM model  which is based on SWMM Buildup and Washoff Land Use described in the document, Storm Water Management Model, Reference Manual, Volume III  Water Quality, EPA/600/R16/093, Rossman L A & Huber W C. The InfoWorks ICM SWMM model includes the buildup, washoff and sweeping functions of the Storm Water Management Model. These can be imported from a SWMM5 text file or defined in the Buildup/Washoff Land Use data fields for InfoWorks networks, or the Land use data fields for SWMM networks.
Note that SWMM model does not include an explicit gully pot model. However, if required for a InfoWorks network, you can define specific buildup and washoff functions for dissolved pollutants to match the behaviour of gully pot buildup and flushing.
In InfoWorks networks, you can choose which model is to be used by the simulation engine in the Washoff section of the water quality and sediment parameters. These parameters can be displayed by selecting Model parameters  Water quality and sediment parameters from the Model menu.
For SWMM networks, the SWMM engine automatically uses the SWMM model.
SWMM Buildup/Wash off Model
For each buildup/washoff land use, defined on the Buildup/Washoff tab on the Subcatchments Grid for InfoWorks networks or the Land use tab on the Subcatchments Grid for SWMM networks, the following processes can be specified:
Pollutant buildup
Pollutant buildup that accumulates within a land use is described by a mass per unit of subcatchment area. The amount of buildup is a function of the number of preceding dry weather days and can be computed using one of the following functions:
Power Function
The pollutant buildup (B) accumulates proportionally to time (t) raised to some power, until a maximum limit is achieved:

where: C_{1} is the maximum buildup possible (mass per unit of area) C_{2} is the buildup rate constant C_{3} is the time exponent t is antecedent dry days 
Exponential Function
The pollutant buildup (B) follows an exponential growth curve that approaches a maximum limit asymptotically:

where: C_{1} is the maximum buildup possible (mass per unit of area) C_{2} is the buildup rate constant t is antecedent dry days 
Saturation Function
The pollutant buildup (B) begins at a linear rate that continuously declines with time until a saturation value is reached:

where: C_{1} is the maximum buildup possible (mass per unit of area) C_{2} is the halfsaturation constant (days to reach half of the maximum buildup) t is antecedent dry days 
Pollutant washoff
Pollutant washoff from a land use occurs during wet weather periods and can be calculated using one of the following functions:
Exponential Washoff
The washoff load (W) in units of mass per hour is proportional to the product of runoff raised to some power and to the amount of buildup remaining:

where: C_{1} is the washoff coefficient C_{2} is the washoff exponent q is the runoff rate per unit area B is the pollutant buildup in mass units 
The buildup here is the total mass (not per area) and both buildup and washoff mass units are the same as used to express the pollutant's concentration.
Rating Curve Washoff
The rate of washoff W, in mass per second, is proportional to the runoff rate raised to some power:

where: C_{1} is the washoff coefficient ^{C}_{2} is the washoff exponent Q is the runoff rate in userdefined flow units B is the pollutant buildup in mass units 
Event Mean Concentration
This is a special case of Rating Curve Washoff where the exponent is 1.0 and the coefficient C1 represents the washoff pollutant concentration in mass per litre.
Sweeping
Street sweeping can be used on each land use to periodically reduce the accumulated buildup of specific pollutants.
Further information about these processes can be found in the Storm Water Management Model, Reference Manual, Volume III  Water Quality, EPA/600/R16/093, Rossman L A & Huber W C.