**Note:** There is a function called ForceMain in SWMM5/InfoSWMM whose purpose is to compute the Darcy-Weisbach friction factor for a force main using the **Swamee and Jain approximation to the Colebrook-White equation**.

f = forcemain_getFricFactor(xsect.rBot, d/4.0, 1.0e12);

return sqrt(f/185.0) * pow(d, (1./6.));

double forcemain_getFricFactor(double e, double hrad, double re)

//// Input: e = roughness height (ft)

// hrad = hydraulic radius (ft)

// re = Reynolds number

// Output: returns a Darcy-Weisbach friction factor

// Purpose: computes the Darcy-Weisbach friction factor for a force main

// using the Swamee and Jain approximation to the Colebrook-White equation.

{ double f;

if ( re < 10.0 ) re = 10.0;

if ( re <= 2000.0 ) f = 64.0 / re;

else if ( re < 4000.0 )

{ f = forcemain_getFricFactor(e, hrad, 4000.0);

f = 0.032 + (f - 0.032) * ( re - 2000.0) / 2000.0; }

else { f = e/3.7/(4.0*hrad);

if ( re < 1.0e10 ) f += 5.74/pow(re, 0.9);

f = log10(f);

f = 0.25 / f / f; }

return f;}

From http://en.wikipedia.org/wiki/Darcy_friction_factor_formulae