Storm water drainage - How do I define the pipe material?

The input for 'Linear measure of roughness' dictates the pipe material properties. Suggested values are displayed in the information alongside the input and are as follows:

Typical values for roughness are:

Pipe material	Roughness (mm)
Copper, lead, brass, alkathene, glass	smooth
Cast iron, bitumen lined	0.03
Cast iron, concrete lined	0.03
Uncoated steel	0.03
Coated steel	0.06
Galvanised iron	0.16
Coated cast iron	0.16
Uncoated cast iron	0.30
Wet-mix spun precast concrete	0.6
Glazed stoneware	0.6
Precast concrete, mortar not wiped on inside of joint	3.0