Bank discharge is 103 times greater than that from a sand bank at a corresponding time as follows from the difference in hydraulic conductivity. Bank discharge from a clay bank is 10-3 less than that from a silt bank at corresponding times. These low discharges would continue for many decades were channel water levels to remain low. This scenario is very unlikely except perhaps where nearly all channel flow was permanently diverted. The percentage of the total bank storage that has drained at various points in time from floodplains composed of gravel, sand, silt and clay in valleys of varying width is shown in Figure 3.
Gravel banks have such high hydraulic conductivity that they drain completely in a few days.
Water is retained in floodplains composed of silt or clay far longer than floodplains composed of sand or gravel. These wider valleys would continue to drain for more than 50 years after the water level dropped. Summary The amount of water stored in the banks and thus potentially available for release is greater in valleys that are wide, with channels that are deep, and where the material is coarse. Wide valleys have more area available for storage. Valleys with deep channels have a greater thickness of floodplain material in which storage can occur.