VOLUME 19 NUMBER 1 (January to June 2026)

Effects of fish rafts on hydro-morphological changes: A case study in the Tien River, Vietnamese Mekong Delta

The Vietnamese Mekong Delta, a critical global delta system, is experiencing severe morphological degradation marked by increasing riverbed incision. This instability is largely driven by large-scale anthropogenic factors, specifically upstream dam construction and sand mining. While these basin-wide impacts are well-documented, the localized hydro-morphological modification due to fish cages/rafts remains quantitatively underexplored. This study utilized a high-resolution two-dimensional (2D) hydro-morphodynamic model (e.g., MIKE 21 Flow Model FM) to quantify the effects of dense fish cage clusters on hydro-morphological changes along a critical segment of the Tien River in Dong Thap Province. The model, which demonstrated good performance (the Nash-Sutcliffe efficiency is equal to 0.78, 0.68, and 0.76 for discharge, suspended sediment concentration, and total sediment load, respectively, in model validation), compared the actual condition (Scenario A: with cages) against a baseline where fish cages were excluded (Scenario B: without cages). Simulation results showed that fish cages, clustered along the right bank, created a localized "bottleneck effect" forcing flow velocity to increase significantly in the open channel (mid-channel and left bank). This hydraulic alteration generated a strongly asymmetric sedimentation pattern. The accelerated flow intensified scour and erosion along the opposite (left) bank, with an mean incision depth reaching -0.8 m. Conversely, the right bank, located in low-velocity wake regions behind the structures, experienced substantial sediment accumulation, reaching more than +2.8 m in areas of the densest clustering. These findings confirm that concentrated fish cage systems act as direct catalysts for localized morphological instability. This emphasizes the critical need to integrate hydro-morphological modelling into floating aquaculture spatial planning to optimize cage placement and mitigate the resulting cross-sectional imbalance.

The Vietnamese Mekong Delta, a critical global delta system, is experiencing severe morphological degradation marked by increasing riverbed incision. This instability is largely driven by large-scale anthropogenic factors, specifically upstream dam construction and sand mining. While these basin-wide impacts are well-documented, the localized hydro-morphological modification due to fish cages/rafts remains quantitatively underexplored. This study utilized a high-resolution two-dimensional (2D) hydro-morphodynamic model (e.g., MIKE 21 Flow Model FM) to quantify the effects of dense fish cage clusters on hydro-morphological changes along a critical segment of the Tien River in Dong Thap Province. The model, which demonstrated good performance (the Nash-Sutcliffe efficiency is equal to 0.78, 0.68, and 0.76 for discharge, suspended sediment concentration, and total sediment load, respectively, in model validation), compared the actual condition (Scenario A: with cages) against a baseline where fish cages were excluded (Scenario B: without cages). Simulation results showed that fish cages, clustered along the right bank, created a localized "bottleneck effect" forcing flow velocity to increase significantly in the open channel (mid-channel and left bank). This hydraulic alteration generated a strongly asymmetric sedimentation pattern. The accelerated flow intensified scour and erosion along the opposite (left) bank, with an mean incision depth reaching -0.8 m. Conversely, the right bank, located in low-velocity wake regions behind the structures, experienced substantial sediment accumulation, reaching more than +2.8 m in areas of the densest clustering. These findings confirm that concentrated fish cage systems act as direct catalysts for localized morphological instability. This emphasizes the critical need to integrate hydro-morphological modelling into floating aquaculture spatial planning to optimize cage placement and mitigate the resulting cross-sectional imbalance.