Karsten, Suggested values from the MIDUSS User Manual are as shown below More details at http://www.miduss.com . Alan A. Smith Suggested Values of Manning 'n' for Pipe Design Description'n' Metal pipe spun concrete lining0.007 Wrought iron0.008 Smooth pre-cast well jointed concrete0.009 Uncoated cast iron; well-aligned glazed vitrified clay0.01 Spun concrete0.011 Monolithic concrete; rough pre-cast concrete; butt-jointed drain tile; slimed sewers0.013 Pre-cast pipes with mortar squeeze at joints; well pointed brickwork0.015 Old brickwork; foul sewers with grease, lime encrusted or sludge0.019 Corrugated pipe D = 0.4 m0.014 + 0.02 (D in metres) Corrugated pipe D > 0.4 m0.020 + 0.002 (D in metres) Suggested Values for Manning 'n for Channel Design Description Manning 'n' Concrete lined, screeded and smoothed0.014 Gunite concrete, not smoothed with sandy deposits0.018 Irrigation canal in hard-packed smooth sand0.020 Canal excavated in silty clay0.024 Channel with cobble stone bottom0.028 Natural channel with fairly regular cross-section0.035 Natural channel, irregular section, grass slopes0.040 Dredged channel, irregular side slopes, grass and weeds0.050 Irregular channel with dense growth, little foliage0.080 Irregular channel with dense growth, with much foliage and vegetation0.110