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WQ4.2.4 Comparison of Models - Coggle Diagram
WQ4.2.4 Comparison of Models
Table 4-1 lists the units of the washoff coefficient Kw for 3 washoff models
note
the units of washoff rate w(Kw)
mass/hr for exponential
mass/sec for the other 2
the runoff rate here
based on the runoff for the entire subcatchment
before any internal routing between the impervious and pervious subareas takes place
when the internal routing option is used
The runoff rate actually leaving the subcatchment will always be a lower number
assuming pollutant mass units of mg
Figure 4-2, shapes of the runoff pollutgraphs for the 3 different washoff functions
base condtions
an initial buildup of 20 lbs of pollutant
a one acre catchment
a 2- inch, 6-hour storm with a triangular-shaped runoff hydrograph
their coefficients were selected so that the storm would remove about 45 percent of the initial buildup
To make the functions comparable
how to get values for Kw of the 3 models
estimate a Kw for rating curve washoff
to produce results roughly similar to those for exponential washoff
by exponential Kw × an average buildup
seen over a storm event and
converting from mass/hr to mass/sec
So for this example,
assuming an average buildup of 15 lb over the event
to achieve the target of removing 45 percent of the initial buildup
The exponential KW value of 0.45 was selected by trial and error