Flow resistance models for flexible submerged vegetation

Abstract

Summary This paper investigates the variation of hydraulic roughness parameters with flow depth for submerged flexible vegetation. In an experimental study flexible grass canopies are used to examine the impact of stem height, grass type and degree of submergence on the flow resistance properties. Results reveal that Manning’s n roughness coefficient increases with decreasing flow depth reaching an asymptotic constant at lower levels of vegetation submergence. Data suggest that the value of the constant is dependent on the vegetation height and the vegetation properties. Currently the n - UR method underpredicts the Manning’s n and the data from this study extends the USDA method for lower bed slopes and for smaller UR values. The vegetal drag-Reynolds number model is extended for submerged flexible grass linings of differing heights and types. Similar values of the vegetative characteristic number k are evaluated as from previous studies yet for contrasting vegetation types. The vegetal drag coefficient value was found to be strongly correlated to the vegetation height, and follows a power law relationship. The exponent value was found to be independent of the vegetation characteristics.