Land Use Editor

Specify the characteristics of the washing off of pollutants.

Option	Description
Pollutant	The pollutant name. Click the button on the right to display a list of available pollutant objects.
Function	The choice of washoff function to use for the pollutant. The choices are: NONE - No washoff. EXP - Exponential washoff. RC - Rating curve washoff. EMC - Event-mean concentration washoff. The formula for each of these functions is discussed under the Pollutant Washoff topic.
Coefficient	This is the value of C1 in the exponential and rating curve formulas, or the event-mean concentration.
Exponent	The exponent used in the exponential and rating curve washoff formulas.
Cleaning Efficiency	The street cleaning removal efficiency (percent) for the pollutant. It represents the fraction of the amount that is available for removal on the land use as a whole (set on the General page of the editor), which is actually removed.
BMP Efficiency	Removal efficiency (percent) associated with any Best Management Practice that might have been implemented. The washoff load computed at each time step is simply reduced by this amount.

Pollutant Washoff

Pollutant washoff from a given land use category occurs during wet weather periods. This can can be described in one of the following ways.

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, i.e.

W = C1qC2B

Where:

C1 = washoff coefficient
C2 = washoff exponent
q = runoff rate per unit area (mm/hour or inches/hour)
B = pollutant buildup in mass (kg or lbs) per unit area or curb length.

Washoff mass units are the same as used to express the pollutant's concentration (milligrams, micrograms, or counts).

Rating Curve Washoff

The rate of washoff W, in mass per second, is proportional to the runoff rate raised to some power, i.e.

W = C1QC2

Where:

C1 = Washoff coefficient.C2 = Washoff exponent.Q = Runoff rate in user-defined flow 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. The conversion between user-defined flow units used for runoff, and litres is handled internally by SWMM.

In each case, buildup is continuously depleted as washoff proceeds, and washoff ceases when there is no more buildup available.

Washoff loads for a given pollutant and land use category can be reduced by a fixed percentage by specifying a BMP Removal Efficiency that reflects the effectiveness of any BMP controls associated with the land use. It is also possible to use the Event Mean Concentration option by itself, without having to model any pollutant buildup at all.

Typical EMC's for Selected Constituents

Constituent	Event Mean Concentrations
TSS (mg/L)	180 - 548
BOD (mg/L)	12 - 19
COD (mg/L)	82 - 178
Total P (mg/L)	0.42 - 0.88
Soluble P (mg/L)	0.15 - 0.28
TKN (mg/L)	1.90 - 4.18
NO2/NO3-N (mg/L)	0.86 - 2.20
Total Cu (ug/L)	43 - 118
Total Pb (ug/L)	182 - 443
Total Zn (ug/L)	202 - 633

Source: U.S. Environmental Protection Agency. (1983). Results of the Nationwide Urban Runoff Program (NURP), Vol. 1, NTIS PB 84-185552), Water Planning Division, Washington, DC.