DIFFERENT BEHAVIORS OF DELTA DISTRIBUTARY CHANNELS BY DIFFERENT BASIN WATER DEPTHS: THE GRADE INDEX MODEL AS A RATIONALE

The behavior of river deltas and their distributary channels is affected by upstream conditions, including variations in sediment and water discharges which would reflect the geology, climate and tectonics of the hinterlands. Recent research suggests that thebasin water depth (h) as a downstream condition can also govern the long-term morphodynamics of river deltas. Evidence suggests that distributary channels tend to be more stable on deltas that face deeper water basins and more prone to avulse in the opposite case. A possible rationale for these different behaviors of the channels is provided by the grade index model. The grade index (G_index), a volume-in-unit-time ratio of subaerial sediment allocation to the entire supplied sediment, can be given as a function of the dimensionless basin water depth (h_*: hnormalized with the height of the delta’s apex above base level). A series of tank experiments conducted by ourselves reveal that when the G_index value is lower (i.e., when the basin water is deeper; h_* >> 1), delta progradation and deltaplain aggradation are suppressed and distributary channel migration and avulsion take place less frequently. If the G_index ~ 0 (i.e., extremely deep basin water; h_* ~ +∞), the delta can neither prograde nor aggrade, and the distributary channels tend to stabilize. The grade index model helps explain the contrasting morphodynamics of the Liwu Delta (east Taiwan) and Yellow River Delta (China), as natural examples of deep-water and shallow-water deltas, respectively. Such a process can be quantitatively described by the grade index model. The grade index model is thus applicable to natural deltas and might help in the evaluation of the stability of delta distributary channels.