The present paper reports of a field experiment over a shallow, roughness-varying barrier reef at Maupiti island, French Polynesia. The depth-averaged momentum balance is used to estimate the bottom drag coefficient Cd, which varies from 0.01 to 0.3, with dependence on both depth and reef structure. The depth effect on Cd is well predicted by a log dependency, as used in previous laboratory and fields observations. The present results extend the approach to a system with more wave exposure and higher roughness-to-depth ratio. Additionally, the statistical relationship between high-resolution reef topography and hydrodynamical parameters is discussed.