School of Geography, Earth and Atmospheric Sciences - Theses

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    Transfer processes over natural surfaces : wind and crop interactions
    Bennett, J. M. (John Michael) (University of Melbourne, 1975)
    This work is an investigation into some aspects of the transfer of momentum from the atmosphere to the ground. It is concerned with the reactions between the wind and long slender crops such as wheat and barley. It shows that the behaviour of the latter, such as flexing and oscillating, is dependent on properties of both the wind structure and the plant. It shows that wave-like motion of the canopy occurs when there is close coupling between the plants, near a damped fundamental resonance frequency, and certain structures in the wind. The wind gust structure is known to be stability dependent, and patches of the canopy deflect to indicate certain features of this structure. Canopy waves are observed to occur in particular in lapse conditions, although plant motion can exist at all conditions provided the plant's physical and mechanical properties are such that large deflections can occur. The role of the plant's properties in its response to the wind is examined and shown to be particularly dependent on the maturity of the plant. Changes induced in the structure of long slender crops, by the plants flexing in them, are shown to be a source of variations in the atmospheric wind velocity profile parameters, the roughness length and zero plane displacement. It is suggested that in high winds, an increased plant volume density due to flexure is as important as streamlining, in interpreting the dependence of these profile parameters on windspeeds. It is demonstrated that the loss of momentum from the mean flow is greatest in the vicinity of the heads of the cereal crops considered here, due principally to the large area of the heads in comparison to the stems. The role of close coupling between the turbulence structure and plants is discussed in the last chapter in relation to damage to crops.