High-resolution DEMs reveal multi-scale roughness of oyster reefs via smartphone photogrammetry

Oysters, as ecosystem engineers, possess unique bed roughness that considerably influences local hydrodynamics, affecting both surrounding fields of currents and waves as well as the growth of oysters. This study aims to quantify and interpret the surface roughness characteristics of oyster reefs using smartphone-based Structure from Motion (SfM) technology. We constructed millimeter-resolution Digital Elevation Models (DEMs) of two contrasting oyster reefs along the Jiangsu coast, China: a live reef on a submerged breakwater in Sheyang, and a degraded reef at Liyashan, Haimen. Results show similar overall roughness heights for both sites (total roughness height k_t = 19 ± 5 mm; root mean square roughness height k_rms = 4 ± 1 mm). However, multi-scale roughness analysis revealed distinct morphological differences. The Sheyang reef exhibits sharper small-scale features, while the Liyashan reef shows blunter surfaces and more pronounced large-scale undulations. These surface characteristics of the reefs are directly related to the occurrence of oysters, the degree of surface sediment cover, and the bedforms beneath the surface oyster shells. This study provides valuable roughness parameters and introduces a multi-scale analysis method, enhancing our understanding of oyster reef surfaces. These findings contribute to more accurate predictions of hydrodynamic roughness and offer insights into the complex interactions between oyster reef morphology and hydrodynamics.