BED-FORM DYNAMICS IN THE NIZHNE-SHESHUROVSKII SHALLOWS OF THE VYCHEGDA RIVER

A. Yu. Sidorchuk

Hierarchical complex of different bed forms in the Nizhne-Sheshurovskii shallows of the Vychegda River (linguoid bar 2500 m long) was formed during the low-water period on 18—31 August 1997 by megaripples 3—4 m long, and 0.1—0.2 m high with a rate of displacement of 0.7—1.1 m/h and by large third-order dunes, each 200—300 m long and 1.0—1.2 m high, which displaced at a rate of 0.12—0.3 m/h. The hydraulic characteristics of a flow being the same, a total bed-load discharge varied in time from 0.02 to 0.8 m3/h with a period of 32—42 h. Bed-load discharge by megaripples varied with a slightly smaller period (30 h) from 0.02 to 0.3 m3/h. The main volume of bed load (50—70%) moved over the bottom surface covered with megaripples in transit. Load was mainly (80—95%) accumulated in the basement of the third-order dunes which passively displaced these large bed forms downstream.