A tentative sediment budget for the Red River subaqueous delta in the Gulf of Tonkin: A synthesis of existing data

A mass balance for sediment in the subaqueous delta of the modern Red River has been constructed to identify major sources, sinks, and the magnitude of sediment available for export to the adjacent coastal zone. The Red River is the largest sediment source in the subaqueous delta, and its input amounts to 63.957 � 6.935 Mt/yr (Mt = 106 tonnes) with about 91.39% contribution of the total sediment inputs. Contributions from the atmosphere (0.004 � 0.001 Mt/yr), the coastal erosion (1.124 � 0.112 Mt/yr), the anthropogenic input (4.550 � 0.339 Mt/yr), the primary production (0.114 � 0.039 Mt/yr) and the seabed erosion (0.232 � 0.044 Mt/yr) account for only 8.61% of the total input. About 47.72% of total sediment inputs has been deposited in the subaqueous delta. The remaining 52.28%, primarily from the dissolved load (54%) of the Red River, is believed to have been transported southward by the dominant southward flow along the coastline. These results confirm earlier evidence indicating that most of particulate matter from the Red River is deposited close to the river mouth. Comparing accumulation rates over different timescales suggests that the inverse relationship between long- and short-term accumulation rates is the result of the changing balance of sedimentation and erosion processes. Under present conditions we believe that the surficial sediment coarsening in the northern part of the delta front is the result of a combination of the decreasing sediment load of the Red River basin and the dominant southward flow along the coastline This makes contribution to the subaqueous delta change from sedimentation to erosion status. These findings contribute to our understanding of sediment movement within the subaqueous delta, and may also provide a baseline for the evaluation and comparison of future anthropogenic or natural change in the subaqueous delta of the Red River. � 2019 Elsevier B.V.