Manning´s Online Calculator
Calculates liters per second and flow speed for both drenotube®sizes considering 90 and 140 mm for inner diameters.
Results are for a totally filled pipe section. It is possible to choose between three different degrees of internal pipe roughness. "Nearly smooth pipe" can be chosen for HDPE with smooth interior pipe (Manning´s coefficient= 0,012). To be realistic keep a security margin of 20 % and choose "Slight roughness" (0,018) or "Some roughness" (0,025) for a slooted HDPE drainage pipe such as drenotube®.