Abstract: Although the sediment flux from small and medium-sized rivers is low, these sediments are an important source of sediment along China's long coastline. Large amounts of fresh water and dissolved matter runoff into the sea have a huge impact on the ecological and chemical environments in estuarine and coastal areas. Observations were undertaken during the 2013 flood and dry seasons in Luanhe Estuary, Bohai Sea. The seasonal variation characteristics of hydrological environmental factors and suspended sediment transport mechanisms were analyzed based on hydrodynamics and sediment surveys. A comparison of the measurements in different seasons shows significant variations in temperature, salinity, turbidity, Chl-a, and DO distributions in shallow microtidal estuaries.
During the flood season, the temperature, turbidity, and Chl-a in the water column and the bottom DO decreased from shallow water to deep water, and the salinity and surface DO increased from inshore to the sea. The water column was steady, and the thermocline and halocline occurred at approximately 5 m to 10 m. The surface temperature was greater than the temperature at the bottom. However, the surface salinity was lower than the salinity at the bottom because of summer runoff and rainfall. Surface Chl-a and DO were greater than those at the bottom. Salinity, turbidity, Chl-a and DO were greater in the dry season than in the flood season, but the temperature was significantly less in the dry season than in the flood season. The temperature and salinity increased from shallow water to deep water, and the turbidity, Chl-a and DO decreased from inshore to the sea. Under the stronger winter monsoon conditions, the water column was well mixed and the stratification disappeared. The temperature, salinity and DO indicated a relatively consistent pattern in the water column. The maximum turbidity occurred in the estuary, and the turbidity on the bottom was far greater than that on the surface. The suspended sediment concentration and sediment transport scope were greater in the dry season than in the flood season, which indicates that the small and medium-sized rivers had the same distribution of “storage in summer and transport in winter” as the large rivers. The changes in hydrological environment factors were closely related to the tidal current. The hydrological environment factors clearly varied periodically with tidal changes. Almost all of the factors reached their peak values during ebb or flood tides and were bimodal with different peak values in a single day. A significant linear relationship between velocity and turbidity was found, which indicates a strong re-suspension event when the velocity was high. The flow was characterized by an irregular semidiurnal tide and reciprocating flow in the study area. The suspended sediment was transported with strong asymmetric characteristics during tidal cycles, which resulted in net sediment transport on the shore. The calculation of flux decomposition showed tidal pumping, especially in the southern part of the estuary. In addition, the suspended sediment transport patterns in the subtidal area may have been related to fine-grained sediment trapping over the intertidal area.
Key word: Luanhe River, hydrological environment factors, seasonal variation, tide current, wind
