Build-up/Washoff Models
Water Quality
The modelling of the build-up 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 Build-up and Washoff Land Use described in the document, Storm Water Management Model, Reference Manual, Volume III - Water Quality, EPA/600/R-16/093, Rossman L A & Huber W C. The InfoWorks ICM SWMM model includes the build-up, washoff and sweeping functions of the Storm Water Management Model. These can be imported from a SWMM5 text file or defined in the Build-up/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 build-up and washoff functions for dissolved pollutants to match the behaviour of gully pot build-up 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 Build-up/Wash off Model
For each build-up/washoff land use, defined on the Build-up/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 build-up
Pollutant build-up that accumulates within a land use is described by a mass per unit of subcatchment area. The amount of build-up 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 build-up (B) accumulates proportionally to time (t) raised to some power, until a maximum limit is achieved:
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where: C1 is the maximum build-up possible (mass per unit of area) C2 is the build-up rate constant C3 is the time exponent t is antecedent dry days |
Exponential Function
The pollutant build-up (B) follows an exponential growth curve that approaches a maximum limit asymptotically:
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where: C1 is the maximum build-up possible (mass per unit of area) C2 is the build-up rate constant t is antecedent dry days |
Saturation Function
The pollutant build-up (B) begins at a linear rate that continuously declines with time until a saturation value is reached:
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where: C1 is the maximum build-up possible (mass per unit of area) C2 is the half-saturation constant (days to reach half of the maximum build-up) 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:
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where: C1 is the washoff coefficient C2 is the washoff exponent q is the runoff rate per unit area B is the pollutant build-up in mass units |
The build-up here is the total mass (not per area) and both build-up 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:
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where: C1 is the washoff coefficient C2 is the washoff exponent Q is the runoff rate in user-defined flow units B is the pollutant build-up 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 build-up 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/R-16/093, Rossman L A & Huber W C.
