Many benthic organisms form very rough surfaces on the seafloor that can bedescribed as submerged canopies. Recent evidence has shown that, compared with aunidirectional current, an oscillatory flow driven by surface waves can significantlyenhance biological processes such as nutrient uptake. However, to date, the physicalmechanisms responsible for this enhancement have not been established. This paperpresents a theoretical model to estimate flow inside a submerged canopy driven byoscillatory flow. To reduce the complexity of natural canopies, an idealized canopyconsisting of an array of vertical cylinders is used. The attenuation of the in-canopyoscillatory flow is shown to be governed by three dimensionless parameters defined on thebasis of canopy geometry and flow parameters. The model predicts that an oscillatoryflow will always generate a higher in-canopy flow when compared to aunidirectional current of the same magnitude, and specifically that the attenuation willmonotonically increase as the wave orbital excursion length is increased. A series oflaboratory experiments are conducted for a range of different unidirectional and oscillatoryflow conditions, and the results confirm that oscillatory flow increases water motioninside a canopy. It is hypothesized that this higher in-canopy flow will enhance rates ofmass transfer from the canopy elements, a problem formally investigated in a companionpaper (Lowe et al., 2005b).