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Urban Mannings Values

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Hi All,

What is the max number of decimal places that is sensible to set manning's n values to? We normally use 0.03 for car parks and 0.02 for roads but have been asked to deliniate further by an approving authority. Having reviewed the data in Chow (1959) we're looking to delinate abandoned car parks with heavy vegetation to 0.037 (allowing for vegetation and irregaularity of surface) and an area of delomished buildings used as a car park (so some surface dressing) to 0.033.

For information we are looking at a number of breach scenarios and this area is located at the breach point. Any help or comments would be greatly appreciated.



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Hi SjB.

3 decimal places is the norm as far as I'm concerned. i.e. 0.016, 0.025, 0.033. Obviously the impact of 0.025 compared to 0.026 would be almost negligible so if the model is uncalibrated then perhaps 0.015, 0.020, 0.025 might be appropriate.



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Agree. Never more than three decimal places, and for n values greater than 0.04, probably to the nearest 0.005 unless there is very good calibration data to support something more accurate. For values greater than 0.1, two decimal places would be appropriate.



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