Surface Pollutant Build-Up

This topic describes the calculation of pollutant buildup on a subcatchment surface before and during a simulation.

Prior to the start of the simulation a Build-Up Time is set on the Globals Page of the Rainfall Event Editor. InfoWorks ICM calculates sediment buildup over this period prior to the simulation.

Sediment buildup continues during the simulation, and is calculated every timestep.

The water quality calculations may not take place on every hydraulic timestep. The calculation frequency depends on the value set for the QM Multiplier field on the QM Parameters Dialog.

The Buildup Equation

The buildup equation is based on the hypothesis that on a clean surface the rate of pollutant accumulation is linear but as surface mass increases the accumulation rate decays exponentially.

The buildup equation is used to determine the mass of sediment buildup only. The mass of attached pollutant is determined by multiplying sediment mass by a potency factor. See Surface Washoff and Gully Pot Flushing for more details.

The buildup equation is written:

(1) where M is the mass of deposit per surface unit (kg/ha) Ps is the Buildup Factor (kg/ha.day). You enter a value for the Buildup Factor for each Pollution Index in the Surface Pollutant Editor. See below for some typical surface buildup factors. K1 is the Decay Factor (./day). Enter a value for this in the Surface Pollutant Editor. The default value for Decay Factor, deduced from experimental calibration, is 0.08. This is the value used in the default surface pollutant parameters file (default.wpf) distributed with InfoWorks.

The equation gives a maximum surface mass of:

Ps/K1(kg/ha) (2)

This maximum limit is never exceeded. If you define an initial mass of sediment, and then set a Build-Up Time, sediment buildup will stop at this limit. If you define an initial sediment mass greater than this limit, the initial mass will be reduced to this limit.

During a simulation, InfoWorks ICM calculates the buildup of sediment and the erosion of sediment in parallel for each timestep. Both calculations begin with the initial sediment mass at the start of the timestep. See Surface Washoff and Gully Pot Flushing for details of the erosion calculation.

The sediment mass at the end of a timestep is given by:

(projected mass without erosion) - (amount of eroded sediment)

The equation used within InfoWorks to determine the mass of sediment deposit at the end of the buildup period, or the projected mass at the end of a timestep, is obtained by integrating the buildup equation above. The mass of deposit is written:

(3) where: M0 is the mass of sediment at the end of the buildup period or the projected mass at the end of the timestep (kg/ha) Md is the initial mass of sediment deposit (kg/ha). When calculating the initial buildup, this value is taken from the catchment sediment data defined on the Rainfall Event Editor. InfoWorks uses zero if you do not specify catchment sediment data. When calculating for each timestep, this value is the sediment mass at the end of the previous timestep. K1 is the Decay Factor (./day). See above. NJ is the duration of the dry weather period, or the timestep length (days) Ps is the Buildup Factor (kg/ha.day). See above.

During a rainfall event surface mass also depends on the erosion rate and the buildup equation is solved using an Euler approximation. See Surface Washoff and Gully Pot Flushing for more details.

Typical Surface Buildup Factors

Here are typical surface buildup factors for various land-uses:

Land Use	Surface Buildup Factor (kg/ha/day)	Origin
Residential (dense)	25	French Calibration
Residential	6	French Calibration
Town Centre (commercial)	25	US Calibration (EPA)
Industrial	35	US Calibration (EPA)
Mixed Suburban	6	French Calibration