Hydraulic roughness – links between Manning’s coefficient, Nikuradse’s equivalent sand roughness and bed grain size

This paper presents and reviews the connection between Manning‘s n and Nikuradse‘s
equivalent sand roughness ks, which is well established for pipe flow in the rough turbulent region.
The link with bed grain size is less clear, and a survey is made covering pipelines and channels, river
and coastal engineering. It is concluded that whilst the equivalent n and ks values are useful
alternatives for sewer and culvert design, the link between roughness parameters and bed grain size
for river and coastal purposes should be treated with more caution, particularly because the hydraulic
resistance is likely to include not only a skin friction element which depends on the grain size, but
also a form drag component.

Citation:
Marriott, M.J. and Jayaratne, R. (2010) ‘Hydraulic roughness - links between Manning’s coefficient, Nikuradse’s equivalent sand roughness and bed grain size’ Proceedings of Advances in Computing and Technology, (AC&T) The School of Computing and Technology 5th Annual Conference, University of East London, pp. 27-32.