### Exponential Washoff

Hi,

I wondering to understand the equation of "Exponential  Washoff".

W=C1*q^C2*B

where C 1  = washoff coefficient, C 2  = washoff exponent, q = runoff rate per unit area ( mm/hour), and B = pollutant buildup in mass units (User's Manual Version 5.0)

The washoff (W) is express in term of mass/hour. So there is a problem with the unit. I have a "mm" remaining in this equation to express the W in terms of mass/hour.

Maybe the term C1 is express in mm-1, but I can't find the information.

Thanks for helping me,

Amélie

#### Replies

• Thanks for ansewring me. That helps! Have a great summer!

amélie

• // --- convert runoff to inches/hr (or mm/hr) and
// convert buildup from lbs (or kg) to concen. mass units
runoff = runoff * UCF(RAINFALL);
buildup /= Pollut[p].mcf;

// --- evaluate washoff eqn.
washoff = coeff * pow(runoff, expon) * buildup;

runoff is in units of inches/hour and buildup is in units of pounds

the coeff and buildup are the same as you entered them except coeff is divided by 3600

• 1st remember the internal units of SWMM 5 are old fashioned American Units so please excuse the mess of conversion - the load is utimately converted to kg even though it is computed as pounds

2nd Each landuse on a Subcatchment generates a load in units of mass/sec

// --- get local washoff mass flow from each landuse and add to total
for (i = 0; i < Nobjects[LANDUSE]; i++)
{
if ( Subcatch[j].landFactor[i].fraction == 0.0 ) continue;
landuse_getWashoff(i, area, Subcatch[j].landFactor, runoff, tStep,