Swmm - Pollutant Buildup

The accumulation of pollutants that accumulate within a land use category is described (or "normalized") by a mass per unit subcatchment area or per unit curb length. Mass is expressed in pounds for US units and kilograms for metric units. The amount of accumulation is a function of the number of previous dry weather days and can be calculated using one of the following functions:

• Potential function - The accumulation of pollutants (B) accumulates proportional to time (t) raised to some power, until a maximum limit is reached
  B = Min(C1, C2 * t ^ C3)
  Where
  C1 - maximum possible accumulation (mass per unit area or curb length)
  C2 - accumulation rate constant
  C3 - time exponent
• Exponential function - The accumulation follows an exponential growth curve that approaches a maximum limit asymptotically
  B = C1 * (1 - e ^ (-C2*t) )
  Where
  C1 - maximum possible accumulation (mass per unit area or curb length)
  C2 - accumulation rate constant (1 / day)
• Saturation Function - Accumulation starts at a linear rate that continually decreases over time until a saturation value is reached
  B = C1 * t / ( C2 + t )
  Where
  C1 - maximum possible accumulation (mass per unit area or curb length)
  C2 - half saturation constant (days to reach half maximum accumulation)
• External Time Series - This option allows you to use a time series to describe the accrual rate per day as a function of time.
  Values placed in the time series would have units of mass per unit area (or curb length) per day.
  You can also provide a maximum possible accumulation (mass per unit area or curb length) with this option and a scale factor that multiplies the time series values.