Land Use Washoff Tab

Washoff refers to the amount of pollutants washed off from a catchment during a wet weather period. The user can select any of the three washoff functions described below:

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.

C 1	=	washoff coefficient
C 2	=	washoff exponent
q	=	runoff rate per unit area
B	=	pollutant build-up in a mass per unit area or curb length

The effect of coefficients on washoff is shown in the following figure:

Effect of Exponential Function Coefficients on Washoff

Rating Curve Washoff: The rate of washoff W in mass per second is proportional to the runoff rate raised to some power.

C 1	=	washoff coefficient
C 2	=	washoff exponent
Q	=	runoff rate, cfs (m 3 /s)

Event Mean Concentration (EMC): This is a special case of Rating Curve Washoff where the exponent (C2) is 1.0 and the coefficient C1 represents the concentration of any and all runoff in mass per liter.

The effect of coefficients on washoff is shown in the following figure:

Effect of Rating Curve Coefficients on Washoff

This tab contains a table with the following attribute columns:

Column	Description
Pollutant	This menu contains all of the pollutants that have been defined in the Pollutants dialog box for the current model. Select the one that should be used for the land use currently highlighted in the list pane.
Washoff Function	This menu allows you to specify which of the buildup functions described above will be used for the land use currently highlighted in the list pane.
Washoff Coefficient	This field allows you to define C 1 in the equations described above.
Washoff Exponent	This field allows you to define C 2 in the Exponential and Rating Curve equations described above.
Cleaning Efficiency	The fraction of available buildup for each pollutant removed by cleaning.
Removal Efficiency	Washoff loads for a given pollutant and land use category can be reduced by a fixed percentage by specifying a Removal Efficiency in this field which reflects the effectiveness of any BMP controls associated with the land use.