In the lower fluid system, the bed sand morphology (also called bottom morphology) is formed by fluid flow. There are common sand patterns (Figure 2-4 and Figure 2-5), sand waves (Figure 2-6) and sand dunes (Figure 2-7 and Figure 2-8). The movement of sand patterns, sand waves and sand dunes forms staggered layers with their own characteristics.

Figure 2-4 Sand Table (Weihe River, Shaanxi Province)

Figure 2-5 Sand Layer Model (near the border between Oman and UAE)

Figure 2-6 Shalang (Weihe River, Shaanxi)

Figure 2-7 Dune (Weihe River, Shaanxi Province)

Figure 2-8 Dune (near the border between Oman and UAE)

(1) geometry

The formation of small cross bedding is sometimes attributed to the movement of sand grains, and most medium or large cross bedding is also formed by the movement of large corrugated sediments (sand dunes or large sand grains) (Blatter et al., 1978).

The common geometric shapes of sand patterns, sand waves and sand dunes are linear, curved, chain, tongue and crescent. Plate-like cross bedding is formed when bed sand with straight ridges or slightly curved ridges moves. Trough cross bedding is mainly formed by bed sand movement with straight or slightly curved ridges, or by irregular bed sand movement. Most large-scale trough cross bedding is formed by the movement of curved or crescent dunes.

(2) hydrodynamic characteristics

All sediments are suspended in the form of bedding or transported to the breaking point under the bottom load, where they are separated from the river bed to form wave tracks or reverse circulation zones.

The separation of water flow leads to the separation of transported sediments into two parts: bottom load and suspended load.

(3) The ripple movement and the formation of cross bedding.

The type of deposition structure formed by ripple movement depends not only on the geometry of ripple, but also on the relative importance of scouring and deposition during ripple movement. Any movement in the form of sediment usually includes local scouring and deposition. Only when the deposition rate is greater than the movement rate of riverbed sediment morphology can the riverbed sediment morphology itself or the sediments originally contained in the riverbed sediment morphology be preserved. Sand migration forms sand cross bedding (Figure 2-9), and sand dunes with straight or curved ridges migrate to form plate cross bedding (Figure 2- 10A, b).

Figure 2-9 Cross Bedding of Sand Layer

Fig. 2- 10 plate-like cross bedding