Wave-current bottom shear stresses and sediment re-suspension in the mouth bar of the Modaomen Estuary during the dry season

On the basis of the measurement data pertaining to waves, current, and sediment in February 2012 in the mouth bar of the Modaomen Estuary, the Soulsby formulae with an iterative method are applied to calculating bottom shear stresses （BSS） and their effect on a sediment resuspension. Swell induced BSS have been found to be the most important part of the BSS. In this study, the correlation coefficient between a wavecurrent shear stress and SSC is 0.86, and that between current shear stresses and SSC is only 0.40. The peaks of the SSC are consistent with the height and the BSS of the swell. The swell is the main mechanism for the sediment re-suspension, and the tidal current effect on sediment re-suspension is small. The peaks of the SSC are centered on the high tidal level, and the flood tide enhances the wave shear stresses and the SSC near the bottom. The critical shear stress for sediment re-suspension at the observation station is between 0.20 and 0.30 N/m2. Tidal currents are too weak to stir up the bottom sediment into the flow, but a WCI （wave-current interaction） is strong enough to re-suspend the coarse sediment.

Effects of the Anthropogenic Activities on the Morphological Evolution of the Modaomen Estuary, Pearl River Delta, China

Owing to the intensive human activities, the Modaomen Estuary has been significantly modified since 1950s, which has resulted in considerable changes of hydrodynamics and morphodynamics in the area. In this paper, the effects of the anthropogenic activities on the hydrodynamics and morphological evolution in the estuary at different stages are systematically assessed based on the detailed bathymetric data and field survey. The results show that the human activities have caused the channelization of the enclosed sea area in the Modamen Estuary;fast seaward movement of the mouth bar with high siltation;expansion of the channel volume due to channel deepening. The paper also highlights the main hydrodynamic changes in the estuary, including the rise of the water level;the distinguishing changes of tidal range before and after the 1990s （decrease and increase respectively）; as well as the increase of the divided flow ratio. It is found that reclamation is the main factor promoting the transition of nature of the estuary from runoff dominant to runoff and wave dominant, and sand mining activities are mainly to strengthen the tidal dynamic and to low the water level. The results provide useful guidance for better planning of the future developments in the estuary and further research in the area.

A Long-Term Numerical Model of Morphodynamic Evolution and Its Application to the Modaomen Estuary

Because of the influence of human activities, the evolution of the Modaomen Estuary is no longer a purely natural process. We used a long-term morphodynamic model （PRD-LTMM-10） to study the evolution of the estuary from 1977 to 1988. The model incorporated modules for riprap-siltation promotion and waterway dredging. The model can simulate the morphodynamic evolutionary processes occurring in the Modaomen Estuary during the period of interest. We were able to isolate the long-term influences of various human engineering activities and the roles of natural factors in estuarine evolution. The governance projects had the largest effect on the natural development of the estuary, resulting in larger siltation on the west side. Installation of riprap and reclamation of submerged land resulted in scouring of the main Hengzhou Channel causing deep trough out-shift. Severe siltation narrowed the upper end of the Longshiku Deep Trough.

Impacts of human activities on morphological evolution in the Modaomen Estuary, China

The morphology of the Modaomen Estuary(ME)has undergone drastic changes in recent decades,and quantifying the contribution of human activities and natural processes is crucial for estuary management.Using Landsat images,chart data,and hydrological and meteorological data,this study analyzed the evolution of the shoreline and subaqueous topography of the ME and attempted to quantify the extent of the contributions of human activities.The results show that local human activities dominated morphological evolution in some periods.From 1973 to 2003,the shoreline advanced rapidly seaward,resulting in approximately half of the water area being converted into land.Human activity is critical to this process,with the direct contribution of local land reclamation projects reaching more than 85%.After 2003,the shoreline remained relatively stable,probably due to a decrease in land reclamation projects.Regarding the evolution of subaqueous topography,the shoals in the estuary were heavily silted and gradually disappeared during 1983–2003,and the waterways narrowed and deepened.The average siltation rate decreased from 15.43 mm/a to-1.02 mm/a,indicating that the ME changed from sedimentation to slight erosion.By detecting variations of sediment load,we found that upstream human activities reduced river sediment,while downstream human activities significantly increased sediment input to the ME,leaving little change in the actual sediment input to the ME for a relatively long period.In addition,based on the empirical relationship between the sediment input and siltation rate,local human activities influenced the shift in the siltation state more than upstream and downstream human activities did.These findings suggest that more attention should be paid to local human activities to improve the estuarine management in the ME.