SWMM - Pollutant Washoff
The washing of pollutants from a given land use category occurs during periods of rain and can be described in one of the following ways:
- Exponential Wash - The wash load (W) in units of mass per hour is proportional to the product of the flow raised to some power and the amount of accumulation remaining, i.e.
W = C1 * q ^ C2 * B
Where
C1 - washing coefficient
C2 - wash exponent
q - flow rate per unit area (inches/hour or mm/hour)
B - Pollutant Buildup in mass units.
Buildup here is total mass (not per area or curb length) and the buildup and washout mass units are the same as used to express pollutant concentration (milligrams, micrograms, or counts).
- Wash Curve - The wash rate W in mass per second is proportional to the flow rate raised to some power, that is:
W = C1 * Q ^ C2
Where
C1 - washing coefficient
C2 - wash exponent
Q - flow rate in user-defined flow units
- Event Average Concentration - This is a special case of rating curve Washdown, where the exponent is 1.0 and the C1 coefficient represents the mass washout pollutant concentration per liter.
Conversion between user defined flow units used for runoff and liters is handled internally by SWMM.
Polluter |
Average Event Concentration |
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 - 2.2 |
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.
Note that in each case, the backlog is continually depleted as the wash proceeds, and the backwash ceases when no more backlog is available.
It is also possible to use the Event Mean Concentration option by itself, without the need to model any Pollutant Buildup.
- BMP Removal Efficiency - Wash loads for a given pollutant and land use category can be reduced by a fixed percentage, specifying a BMP Removal Efficiency that reflects the effectiveness of any BMP controls associated with the land use.
Pollutant removal in surface washing can also occur when runoff is captured by Low Impact Development controls (LID). The concentration of a pollutant released from the underground flow of an LID unit can be reduced by a user-specified percentage. These removal percentages are assigned through the LID Control Editor for each generic LID project.