*2.1. Study Area 2.1. Study Area*

The study area was located in the offshore area of the subaqueous Yellow River Delta in China, as shown in Figure 1. The surface sediment of the subaqueous Yellow River Delta consists mainly fine particulate silty clay [18], which is prone to liquefaction. The waves in this sea area are mainly stormy waves, with the annual strongest wind direction from the northeast, an average wind velocity of 6.8 m/s, and a maximum wind velocity of 20.9 m/s. Storm surges occur easily under these conditions. The study area was located in the offshore area of the subaqueous Yellow River Delta in China, as shown in Figure 1. The surface sediment of the subaqueous Yellow River Delta consists mainly fine particulate silty clay [18], which is prone to liquefaction. The waves in this sea area are mainly stormy waves, with the annual strongest wind direction from the northeast, an average wind velocity of 6.8 m/s, and a maximum wind velocity of 20.9 m/s. Storm surges occur easily under these conditions.

**Figure 1.** Location of the study area. **Figure 1.** Location of the study area.

#### *2.2. Experimental Facilities 2.2. Experimental Facilities*

Experiments were conducted in a "T" shape wave flume (3.5 m × 0.4 m × 1.0 m), which consists of a water flume and a sediment tank (Figure 2). The flume was equipped with a wave generator at the right end and a dissipating gravel beach at the other. Experiments were conducted in a "T" shape wave flume (3.5 m × 0.4 m × 1.0 m), which consists of a water flume and a sediment tank (Figure 2). The flume was equipped with a wave generator at the right end and a dissipating gravel beach at the other.

As shown in Figure 2, a capacitive wave gauge and a turbidimeter was fixed along the central of the water flume to collect the wave height, wave period, and the turbidity in the overlying water during the experiments, respectively. The turbidimeter (RBR, Canada) was fixed with a self-design bracket on the top of the flume. Two turbidity probes of the turbidimeter were fixed at 5 cm and 20 cm above the sediment surface (Figure 3), which were the same as the water sampling points, to record the variation of suspended sediment concentrations. Before the experiment, the turbidimeter was calibrated with suspended sediment solutions configured at setting concentrations of 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 g/L. Turbidity data was collected once per second, and the suspended sediment concentration in the overlying water was calculated as the average value of the 60 values collected every minute. The sediment tank (0.6 m × 0.4 m × 0.3 m) is located in the middle of the flume bottom and the right side of the tank is 1.6 m from the wave generator plate (Figure 2).

*J. Mar. Sci. Eng.* **2019**, *7*, x 4 of 14

**Figure 2.** Sketch of the wave flume used in the present study. **Figure 2.** Sketch of the wave flume used in the present study.

#### As shown in Figure 2, a capacitive wave gauge and a turbidimeter was fixed along the central *2.3. Experimental Preparation and Procedures*

2.3.2. Wave Flume Experiment Processes

**Stages Wave** 

Static diffusion

7 cm wave height (Stage II)

height (Stage III)

min in each stage.

parameters and sampling intervals in the experiments.

**Height***/***cm** 

#### of the water flume to collect the wave height, wave period, and the turbidity in the overlying water during the experiments, respectively. The turbidimeter (RBR, Canada) was fixed with a self-design 2.3.1. Sediment Preparation

bracket on the top of the flume. Two turbidity probes of the turbidimeter were fixed at 5 cm and 20 cm above the sediment surface (Figure 3), which were the same as the water sampling points, to record the variation of suspended sediment concentrations. Before the experiment, the turbidimeter was calibrated with suspended sediment solutions configured at setting concentrations of 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 g/L. Turbidity data was collected once per second, and the suspended sediment concentration in the overlying water was calculated as the average value of the 60 values collected every minute. The sediment tank (0.6 m × 0.4 m × 0.3 m) is located in the middle of the flume bottom Sediments used in the experiments in the present study were collected from a lobate tidal flat area near the Diaokou flow route of the Yellow River Delta. The soil was classified as sandy silt with amedian particle size of 44 <sup>µ</sup>m. The soil has a fine sand content of 29.57%, a silt content of 62.97%, and a clay content of 7.8%. To ensure the homogeneity of the experimental sediment, the soil was air driedand sieved to remove gravel. Artificial seawater with salinity of 35‰ (which was called standard seawater hereinafter) was used as overlying water in the wave flume and to mix the heavy meatalsolution with soil in the experiments.

and the right side of the tank is 1.6 m from the wave generator plate (Figure 2). *2.3. Experimental Preparation and Procedures*  2.3.1. Sediment Preparation Sediments used in the experiments in the present study were collected from a lobate tidal flat area near the Diaokou flow route of the Yellow River Delta. The soil was classified as sandy silt with a median particle size of 44 μm. The soil has a fine sand content of 29.57%, a silt content of 62.97%, and a clay content of 7.8%. To ensure the homogeneity of the experimental sediment, the soil was air dried and sieved to remove gravel. Artificial seawater with salinity of 35‰ (which was called A total of 567 g Cu (NO3)2·3H2O was dissolved into 8.5 kg of standard sea water to produce a heavy metal solution, which was subsequently well mixed with 30.0 kg of sieved dry soil to form a uniform polluted slurry with a water content of 30%. The slurry was sealed in darkness for seven days to the ensure Cu reached a stable state. Clean slurry was prepared with sieved dry soil and standard seawater. In order to eliminate the effect of wave orbital shear stress on the resuspension of polluted sediment particles, the polluted slurry was designed to be covered by a layer of clean slurry. The clean slurry was first backfilled into the soil tank until a depth of 20 cm. Then, a uniform 5 cm thick layer of polluted slurry was deposited above the clean slurry. Another 5 cm thick layer of clean slurry was finally laid on the top. Standard seawater was then gradually added into the wave flume up to a depth of 50 cm above the soil surface (Figure 3). *J. Mar. Sci. Eng.* **2019**, *7*, x 5 of 14

**Figure 3.** Layout of the sediment tank and sampling points. sediments. **Figure 3.** Layout of the sediment tank and sampling points.

**Table 1.** Wave parameters and sampling intervals in the wave flume experiments.

**Period/s Duration Sampling Intervals** 

every 10 min for the first 60 min and every 20

**Wave** 

7.0 2.3 180 min

13.5 1.2 180 min

120 min. The sampling points and method were the same as that in Stage I.

(Stage I) - - 40 h 8 h

13.5 cm wave min for the last 120 min

In Stage I, the overlying water was left still for 40 h and the sediments remained in a consolidation state. Stages II and III were the wave action stages, with the wave actions lasted for 180

In Stage I, water samples were collected every 8 h using self-design water sampling devices at 5 cm and 20 cm above the center line of sediments, namely at water depths of 30 cm and 45 cm (Figure 3). Each water sample was 50 mL in volume. Since the volume of water samples were relatively small, standard seawater was not replenished into the wave flume. Three parallel samples were collected at each sampling point. Nitric acid was added to the samples, which were stored at low temperatures (4 °C) for analysis of dissolved Cu concentrations. During Stages II and III, after the wave height was steady, water samples were collected every 10 min for the first 60 min and every 20 min for the last

At the end of each stage, a polyvinyl chloride (PVC) tube was inserted in the sediment at the central position of the sediment tank to collect columnar samples of sediments (Figure 3). These PVC tubes were remained in the sediment tank until the end of Stage III. All sediment columnar samples were incised into 2 cm segments for the analysis of Cu concentrations at different depths of the
