WQ4.2.5 Wet Deposition and Runon

subcatchment runoff

contains

  1. washoff of constituents deposited during dry periods
  1. pollutant loads contributed by direct rainfall and by runon from upstream subcatchments.

The instantaneous loading rates from these two streams

a wrong way

simply be added onto the loads computed from the washoff functions described earlier

because they must first be routed through the volume of water (shallow as it may be) that ponds atop the surface of the subcatchment

condtion

See Volume I for a description of how SWMM uses a nonlinear reservoir model to describe surface runoff.

Consistent with the way that the flow from direct rainfall and runon is treated

This mass flux is added to the mass flux computed from the washoff functions to arrive at a total washoff amount.

these pollutant streams are completely mixed with the current contents of the ponded water

and a mass balance is performed to find the pollutant mass from these sources leaving the ponded surface water over the computational time step.

two-stream approach

to handling washoff from both pollutant buildup and from rainfall/runon

Figure 4-3 depicts

written of mass balance for the pollutant and volume of the washoff stream

originating from the ponded surface water that receives upstream run-on and direct deposition can be

note1

applied to the subcatchment as a whole
not to its separate impervious and pervious sub-areas

Precipitation, infiltration, and evaporation rates have been converted from their more conventional units of inches/hr to cfs

by multiplying by the subcatchment’s area

Infiltration removes a proportional amount of mass

regardless of constituent

Evaporation removes volume but not mass causing Cponded to increase

Qout

It can be lower than the Qrunoff used in the buildup washoff functions

is the total runoff flow leaving the subcatchment.

if internal routing between sub-areas is employed

The only unknown to solve for is Cponded

all flow rates and volumes are known from the runoff calculations done prior to washoff analysis

W_washoff is total washoff rate

by adding together the washoff rates w

computed for the buildup on each land use

Wponded

is the runoff load from ponded surface storage

is Qout ×Cponded.

Wout

is the total mass flow rate of pollutant leaving the subcatchment

is Wwashoff + Wponded

Wout / Qout.

is the concentration of pollutant in the subcatchment’s runoff

note2

this scheme requires

namely the ponded mass (mp=Vponded×Cponded) for each pollutant in each subcatchment

an additional set of state variables be kept track of over a simulation