An improved method for roughening floodplains on physical river models

Physical (hydraulic) modelling of river systems subject to overbank flow require the simulation of floodplain roughness. Significant full and reduced scale experiments have now been carried out to establish the flow resistance of a range of real and simulated floodplain vegetation. One use for this improved knowledge of floodplain hydraulics is to provide a basis for an improved method for overbank roughening in hydraulic models. It can also be of value in multi-dimensional numerical models The first part of this paper reviews briefly this work on floodplain vegetation resistance, leading to both empirical and theoretical relationships to describe floodplain roughness, and assesses the appropriate use of these methods. The second and greater part of the paper examines in more detail two methods used recently to roughen floodplains in large scale hydraulic models: non-submerging vertical rods, and a method based on expanded aluminium mesh units. A comparison is made between these two methods by reference to recent tests in the UK Flood Channel Facility at HR Wallingford. Practical recommendations are made for designing a roughness system appropriate for use in future river model tests.