Experimental results are presented concerning boundary shear stress and force distributions in a compound rectangular channel comprising one main river channel and one or two symmetrically disposed smooth floodplains. A comparison is made between the boundary shear stresses on the main channel bed for channels having either rigid or mobile beds of the same geometry. The influence of the main channel mobile bed on the momentum transfer between the main channel and the floodplains, and on the boundary shear stresses and forces acting on different elements of the compound channels are investigated. The mobile main channel bed causes the boundary shear stress values on the main channel bed to increase once overbank flow takes place for a given flow depth, and all values are always greater than those in an equivalent rectangular channel i.e. the isolated floodplain case. The rigid main channel bed case shows the opposite trend to the mobile bed case, in that the boundary shear stresses decrease once overbank flow occurs.
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