For the three coastal segments in detail, the mean values (and ranges) of sand, silt, and clay contents of the samples, respectively, are: 35.6% (1.9–83.9%), 60.5% (14.0–89.7%), and 3.9% (0–9.9%) for the ASYS; and those from the PYS are 10.6% (1.1–30.2%), 82.1% (7.9–94.2%), and 7.4% (11.9–4.6%); those from the PSLB are 68% (29.3–90.9%), 29.6% (7.9–65.5%), and 2.5 (0–5.9%). Generally, the sand, silt, clay contents of the tidal flat samples of the ASYS are significantly different from those of the PYS and PSLB (Figure 2).

The mean values (and ranges) of the mean grain size of the ASYS, PYS, and PSLB are 4.4 (2.8–6.0) Φ, 5.5 (4.9–6.1) Φ, and 3.7 (1.7–4.9) Φ, respectively (Figure 2). To be specific, the number of samples from the ASYS (total of 122) with mean grain size less than 3 Φ (fine sand) is only 1 (less than 1%), 3–4 Φ (very fine sand) is 14 (11%), 4–6 Φ (silt) is 106 (87%), and larger than 6 Φ (clay) is only 1 (less than 1%). The total number of samples from the PYS (44) with a mean grain size between 4 and 6 Φ (silt) is 43 (98%) and only 1 sample (2%) has a mean grain size larger than 6 Φ (clay). The samples from PSLB (72) with a mean grain size less than 3 Φ (medium sand and fine sand) is 2 (3%), 3–4 Φ (very fine sand) is 55 (76%), 4–6 Φ (silt) is 15 (21%), and larger than 6 Φ (clay) is only 1 (approximately 1%).

Two of 10 studied inter-tidal flat transverse profiles are located in PSLB and 8 are in ASYS (Figure 1). As shown by the grain-size composition curves of the 10 transverse profiles, the tidal flat becomes markedly coarser from land to sea. The mean grain size and the sand content increase significantly and the silt, clay, and mud (the sum of silt and clay) contents decrease (Figure 3).

According to the published grain size data, the surficial sediment of the high tidal flat in Zhangzhenhe and Yongfenghe, located in the middle of the ASYS coast, was mainly composed of clayey silt with a clay content of 20–40%, and the low tidal flat was mainly composed of silt with a clay content of 15–20% in 1983 (Liu et al., 1985). Around the same period, the clay content of the high tidal flat surficial sediment from Beihaipu in 1989 was 39.8%, and that of the middle tidal flat was 16.5% (Li et al., 1991). As illustrated in Figure 4, according to the grain size data of the 17 tidal flat surficial sediment samples from the Zimaigou estuary to the Yellow River estuary taken in 2006, the clay content was 5.7–29.5% and the mean content was only 14.0% (Xing, 2007). These results above show that there was no significant variation in the clay content of the surficial sediments of the tidal flat along the ASYS coast in the 1980s. However, we can speculate that the clay content will have undergone significant change after the1980s and dropped considerably, no later than 2006.

The tidal flat surficial sediment dataset of 2006–2007 plotted in a ternary diagram is distributed with a similar shape to that of the 2013−2014 dataset, with a straight band toward the silt–clay axis, encompassing several textural classes from sand to clay (Figure 4). However, the location of the dataset with the highest clay/mud ratios (of more than 10%) deviates clearly from the silt endmember and approaches the clay endmember, which differs from the character of the 2013–2014 dataset. Meanwhile, the distribution range changes little toward the silt–clay axis, without an obvious narrowing tendency. The clay/mud ratios of the 17 samples from the Zimaigou estuary to the Yellow River estuary range from 11.5 to 33.0% with a mean value of 19.0%. Thus, it can be concluded that the surficial tidal flats continued to coarsen from 2007 to 2013.

According to Dyer (1986), Van Ledden et al. (2004), and Law et al. (2008), the transition from non-cohesive to cohesive sediment occurs when clay content exceeds a threshold of 5–10%. This is because the erodibility of the sediment decreases when the clay content increases to exceed this threshold, as does sorting during erosion (Dyer, 1986; Van Ledden et al., 2004). Thus, small changes of clay content in surficial deposits can have a significant influence on the erosion/sedimentation and the sorting in tidal flats. The mean clay content of the 17 samples was 14.1% along the tidal flats from the Zimaigou estuary to the Yellow River estuary in 2006. The clay contents of only two samples were less than 10% (Xing, 2007). However, along the same coast, the clay contents of only three samples were more than 10% among the 152 samples in 2014 with a maximum of 11.9%. The results revealed that the significant coarsening from 2007 to 2013 achieved the transition from cohesive to non-cohesive surficial sediment in the tidal flat.

The water depth on the seafloor east of the ASYS changed from 0 to 5 m and its surficial sediment also had coarsened significantly from 1980 to 1997 (Liu and Yan, 1998). The water depth of the seafloor east of the northern ASYS ranged from 2 to 10 m prior to the 1980s. There existed a “mire,” which was a region covered by rapidly deposited silty clay with its surficial sediment because the fluid mud was apt to be re-suspended under the action of waves (Pang and Si, 1980; Liu and Gao, 1986; Wu, 1992). However, the silty clay region corresponding to the so-called “mire” in 1999 had shrunk sharply compared to that in 1985 (Lv and Li, 2004). Thus, both the surficial tidal flat and seafloor sediments near the ASYS have been coarsening significantly in the last 30 years.

The main reasons for surficial sediment coarsening of tidal flat include hydrodynamic enhancement and the change of sediment source. Based on hydrodynamic model proposed by Pejrup (1988), the location of a dataset in a ternary diagram reflects the specific hydrodynamic conditions of the sedimentary environment, with coarser sediment representing more energetic conditions. The hydrodynamics of Laizhou Bay had weakened before 1996 because of the increasingly enclosed bay mouth that resulted from the eastward extension of the PYS sandspit; it has tended toward stability since1996 and the formation of the Qing 8 channel (Figure 1). Thus, hydrodynamic enhancement cannot be the main reason for the tidal flat sediment coarsening and the sediment coarsening is more likely to be the result of the sharp decrease in sediment load from the Yellow River.

According to previous studies (Collins, 1987; Mazzullo et al., 1988; Ergin and Bodur, 1999), the mud contents and the clay/mud ratios of tidal flat sediments can jointly reflect the change of hydrodynamic intensity, and the relationship between them should be inversely proportiona. However, there does not appear to be a significant relevant relation between the mud contents and the clay/mud ratios for the ASYS and its adjacent zones (Figure 5), implying that the erosion-accretion factors controlling silt and clay contents of the tidal flat surficial sediments are different.

The sorting coefficients of samples along the ASYS and PSLB increase as mean grain size increases (Figure 6A), indicating that the sorting of finer sediment is poorer and the sorting of coarser sediment is better. However, this relation between sorting coefficient and mean grain size does not exist in the PYS (Figure 6A), which implies that the sediment is derived mainly from in situ sedimentation of suspended clastic material in the Yellow River without long-distance hydrodynamic sorting. The skewness of the PYS is negatively correlated with the mean grain size (Figure 6B), which implies that coarser sediment of inter-tidal flats corresponds to a higher accumulation degree of coarse particles. However, this correlation between skewness and mean grain size does not exist for the ASYS and PSLB coasts (Figure 6B), which signifies that the accumulation of coarse particles in the tidal flats is independent of sediment grain size.

Tidal flats are not isolated morphodynamic units in the coastal zone. There must have been some influences to support the phenomenon of tidal flat sediment coarsening, such as the local coarsening of seafloor sediments (Eisma, 1998). After the construction of a large number of reservoirs in the upper and middle reaches of the Yellow River, the sediment runoff into the Yellow River estuary decreased greatly. The grain size of tidal flat of the ASYS changes little along coast and significantly different from that of PYS and PSLB, so the ASYS has almost no longitudinal sediment exchange with its adjacent coastal sediments (Figures 2, 5, 6). Because of the abrupt reduction in sediment, the flood currents from the submarine coastal slope carry few fine particles into the tidal zone, whereas the ebb currents with reverse direction remove some fine particles from the tidal flat. This is a major cause of sediment coarsening in the tidal flat and the decrease in clay content and clay/mud ratio.

In contrast to the tidal flat surficial sediment coarsening from land toward the sea (Figure 3), the surficial sediment of the seafloor shallower than 2 m outside both the ASYS and PSLB becomes increasingly coarse from sea toward the land. Grain-size analysis results from 207 surficial sediment samples collected in 2007 from below the 5 m isobath east of the ASYS (Zhang et al., 2014) show that sand accounts for less than 10%, silt from more than 70%, and clay for more than 20% of the total deposited sediment. The sand content increases, but silt and clay contents decrease significantly, from the 5 to 2 m isobaths. Therefore, a narrow zone of coarse particles parallel to the coastline, with the characteristics of high sand and low mud content, develops between the outer edge of the tidal zone and the 2 m isobath. After the sub-delta of ASYS was abandoned, the sediment source disappeared, resulting in a relatively enhanced geomorphic effect of waves, and a relatively weakening of the geomorphic effect of tidal currents. Part of the clay components in the original tidal flat deposits were also transported away from the intertidal zone, so the sediments of tidal flats eventually became coarse.

As sediment coarsening caused by transverse sediment exchange between tidal flat and the submarine slope, the morphology of ASYS coast has remained relatively stable in recent decades compared to rapidly retreat in the ANYS (Figure 7). During the 50-year period from 1959 to 2009, although the shoreline of ASYS has advanced and retreated, but the magnitude and rate of change are more than an order of magnitude lower than that of other sub-deltas in YRD. During the period from 2002 to 2009, the shoreline of ASYS retreats relatively obviously than before, which may be related to the rapid erosion and retreat of the Qingshuigou sand spit after 1996, which led to a weakening of its barrier function and ultimately an increase in the direct wave action.

According to historical and geological records, the Yellow River delta has repeatedly been subject to the alternation of both sandy and muddy coasts and the marine transgression and regression with time scales of centuries and millennia over the last 2500 years (Chen, 1980; Chen, 1982; Saito et al., 2000). On a short time scale, many researchers have shown that the abandoned northern Yellow River sub-deltas retreated rapidly because of the river estuary migration and disappearance of sediment supply in recent decades (Guo, 1980; Wang et al., 2006). However, even under the background of the significant reduction of sediment into the sea of the Yellow River in recent decades, the coastline of ASYS has not retreated significantly. Thus, we conclude that marine sediments from the Laizhou Bay give a substantial contribution to the maintenance of tidal flats and that there can be a long-time lag between coastal erosion and the deltas response to the decline in sediment supply, as the study of the Yangtze River Delta shows (Dai et al., 2018; Wei et al., 2018; Leonardi et al., 2021). Understanding the geomorphic response to sediment change is not only relevant to the relationship between surficial tidal flat sedimentation and a decrease in sediment flux, it may contribute to the understanding of morphodynamic development along muddy coasts around the world.