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,

WashoffLoad);

3rd That mass/sec is converted to mass/ft3 by the runoff

// --- convert from mass/sec to mass/ft3

runoff *= area;

for (p = 0; p < Nobjects[POLLUT]; p++) washoffQual[p] /= runoff;