Manning’s ‘n’ co-efficient for flexible emergent vegetation in tandem configuration

A comprehensive laboratory study on the hydraulic resistance characteristics due to a group of slender cylindrical members representing flexible plantation has been carried out in a laboratory open channel. The model stem group comprised of individual members arranged in a tandem configuration. The tests with model vegetation were carried out by adopting different sizes for the individual stems and concentrations which were chosen in such a way that they would fall into practical ranges of vegetations in coastal forestry. The results indicate that the flow resistance varies with stem concentration, stem diameter and elastic properties of the individual members and the flow velocity. Based on the flume data, the Manning’s n, for various flow conditions as well as for vegetative parameters was determined. A new empirical equation is proposed for evaluating the Manning’s n friction coefficient for partially submerged flow when, the depth of flow is greater than 0.8 times the un-deflected plant height. It is expected that the Manning’s n thus determined will be useful in modelling shallow flows using shallow water equations.

► The study reveals that the Manning’s n be determined as functions of flow and vegetal parameters including flexibility.
► A new empirical equation is proposed for evaluating the Manning's n friction coefficient for partially submerged flow.
► A modified relative rigidity, VFP (Vegetation-Flow Parameter), is proposed that includes the parameters of structural rigidity of vegetal stems, green belt parameters and flow parameters.
► The n exhibits an excellent correlation with its variation with the proposed VFP.
► A typical sample application of the present empirical model for a real field situation is demonstrated herein.