On many tidal flats, there is a transition from sand flat at the more energetic seaward margin to mudflat further inland. However, the ability of sand flats to attenuate incident wave energy is an important but poorly understood constraint on mudflat development and morphology. This paper presents the results of an instrumented field study of incident wave attenuation across Roberts Bank, the sandy intertidal portion of the Fraser River Delta. The attenuation of wave height was monitored at four stations along a shore-normal transect for a period of 2 months (December 23, 2003, to February 10, 2004). The attenuation varied with the relative wave height ratio (H_s h⁻¹) along the seaward margin, with dissipation increasing as water depths decrease and/or incident wave heights increase. Under the most dissipative conditions observed (H_s h⁻¹ ≈ 0.25), the exponential decay coefficient reached 0.00045. This decay coefficient is an order of magnitude smaller than predicted by a simple wave transformation model due to the relatively large wind fetch over the sand flat. Despite the maintenance of wave energy, the range of wave heights remains constrained in the landward direction, with the frequency of waves capable of entraining sediment on the sand flat decreasing from 11% at the outer flat to 2% at the inner stations. In response, bed elevation change and depth of sediment activation are greatest at the seaward margin and decrease exponentially landward. It is argued that the sand flat provides a natural barrier that defines the extent of mudflat development by limiting the potential for sediment resuspension and morphological change on the mudflat.