Coral Reef Hydrodynamics: A Critical Review of Roughness Indices and Their Characteristic Complexity

This study provides a systematic review of hydrodynamic roughness in coral reef systems, focusing on drag coefficient (Cd), Manning’s roughness coefficient (n), friction factor, and rugosity metrics. A structured bibliometric analysis was conducted using Scopus and Web of Science databases, including field measurements, laboratory flume experiments, and numerical studies validated with physical data. Results show substantial variability in roughness parameters across scales and environments. Reported drag coefficients range from 0.002 for smooth sandy beds to values exceeding 7.0 in complex mangrove–reef systems, with typical reef crest values between 0.05 and 0.30. Manning’s n varies from 0.01 in smooth channels to greater than 0.30 in vegetated reef flats. Laboratory studies consistently report higher roughness values due to shallow flow depths (0.1–0.3 m) compared to field conditions (0.5–2.0 m), indicating strong scale effects. Measurement techniques include acoustic Doppler velocimetry, particle image velocimetry, and photogrammetry-based methods. The findings demonstrate that reef roughness is highly site-specific and cannot be represented by uniform parameters. Improved standardization and site-based calibration are essential for enhancing hydrodynamic modeling and supporting coastal protection strategies.