Evaluation of fine sediment settling velocity in an ephemeral stream on Jeju Island, South Korea

Jeju Island is a volcanic island of South Korea. Groundwater is a major water source for Jeju, where most streams are intermittent or originate from springs near the coast. To secure sustainable water resources, artificial recharge technology to capture ephemeral stream water has been developed on the island. However, during Typhoon Nari in 2007, severe turbidity led to stream bed disturbance. Understanding the characteristics of fine sediment suspension and settling velocity is important for the success of artificial recharge technology. This study estimated the physicochemical properties of sediments, including their size distribution, organic content, and mineralogical composition. In addition, settling velocity was quantitatively estimated by an experimental settling test using a 2-m-tall settling column. In comparison with soil samples taken near the stream, streambed sediments had lower organic contents and finer size fractions such as clay and silt. A numerical method based on a one-dimensional mass conservation equation governing the vertical settling of mass was used to calculate settling velocity for different concentrations at each elevation and time. Settling velocity is controlled primarily by the water viscosity, the size and shape of particles, and the concentration of suspended material. Most of the suspended sediment concentration was located in part of the flocculation settling range (0.1 < C < 6.53 g/L) with a settling velocity of 0.001 to 0.588 mm/s. Increasing sediment concentration led to increasing inter-particle collisions and consequently enhanced aggregation within the range. There was no distinct difference in the calculated settling velocity curves among the sampling sites. Our results and analysis provide information necessary for evaluating artificial recharge systems affected by turbid water. More detailed study of the hindered settling range with a high suspension concentration is required.