Passage VI

Water flowing over the sediment in a river channel creates bedforms (3-dimensional structures formed in the sediment). The processes that produce bedforms in presentday river channels produced the same types of bedforms in ancient river channels. Ancient bedforms are often found preserved in sedimentary rock layers. Figure 1 shows a longitudinal cross section of 3 types of bedforms: ripples, dunes, and flat beds.

A geologist conducted 3 studies using an artificial river channel 15 m long, 1 m wide, and 0.6 m deep.Water velocity through the channel and water flow depth could be controlled. At the start of each study, a flat, 15 cm thick layer of sand particles of uniform diameter was placed in the bottom of the channel. Various combinations of water velocity, from 0.2 m/sec to 2.0 m/sec, and of water flow depth, from 0.1 m to 0.4 m, were tested in each of the 3 studies.

Study 1

The bottom of the channel was covered with fine sand particles, all having a diameter of 0.19 mm. Water was passed over the sediment bed. The resulting bedform stability fields (water velocities and water flow depths at which various types of bedforms were present) are shown in Figure 2.

Study 2

Study 1 was repeated using medium sand particles, all having a diameter of 0.45 mm.The resulting bedform stability fields are shown in Figure 3.

Study 3

Study 1 was repeated using coarse sand particles, all having a diameter of 0.93 mm.The resulting bedform stability fields are shown in Figure 4.