Transportion of suspended sediment in the southern mud area of Bohai Bay in summer: Characteristics and mechanism

To understand the transport pattern of suspended sediment in summer in the coastal and adjacent waters of the southern muddy area of Bohai Bay, observation stations were deployed in the bay, the bay mouth, and at the N-E-directed section off the Huanghe (Yellow) River estuary, in which current, thermohaline, and suspended sediment concentration (SSC) were observed in-situ continuously in 25 h. Results show that warm fresh waters travelled from shore to the middle of Bohai Sea in the form of plume flow. High turbidity suspension occurred mostly during periods of maximum flood and maximum ebb, and high SSC value appeared mostly in 5m above sea bottom. The bottom SSC values in the bay and bay mouth stations are the highest, reaching about 130 and 80 mg/L, respectively. The SSC value at 20 m-deep sea area outside the Huanghe River estuary was the lowest at below 40 mg/L. The average single-width sediment transport rate at Station A1 in the bay was 7.8 g·m⁻¹·s⁻¹ toward 280°. However, the sand transport direction of the ebb and flow tide at Station A2 in the bay mouth was opposite, and the sand transport direction was mainly southeastward to the outside of the bay. The average single width sand transport rate of the tide was 7.2 g·m⁻¹·s⁻¹ toward 328°. The sediment transport directions of Station A3 outside the bay during flood tide and flood slack were generally opposite to each other on similar scales, and the sediment transport directions were mainly east-to-northeastward during ebb tide and ebb slack. The tidal averaged sediment transport rate per unit width was 4.7 g·m⁻¹·s⁻¹, and the direction was 77°. At the two stations in SE of the Huanghe River estuary, the sediment transport was in southeastward in flood tide, northeastward in flood slack, northwestward in ebb tide, and north-northeastward in ebb slack. The average single-width sediment transport in the two stations was mainly northward. The average sediment transport rate per unit width of Station A4 was 5.2 g·m⁻¹·s⁻¹, and the direction of transport was 94°. The tidal averaged sediment transport rate per unit width at Station A5 is 7.7 g·m⁻¹·s⁻¹, and the direction of transport was 102°. The Lagrange sediment transport rate contributed most significantly to the tidal averaged sediment transport rate per unit width. In the southern and southeastern parts of the bay-mouth mud area, the vertical net circulation showed a large impact on the Lagrange sediment transport rate, which could offset partially the Lagrange sediment transport rate, thus affecting more obviously the tidal averaged sediment transport rate per unit width than those in the bay and the sea area deeper than 25 m. Other directional components of velocity had smaller orders of magnitude and contributed less to the tidal averaged sediment transport rate per unit width.