PPT Slide
Notes:
Erosion of bedrock in many mixed alluvial-bedrock channels may be episodic, in that supply of alluvium to the channel bottom may occur during occasional strong storms during which debris flows deliver coarse sediment to the valley bottom which mantles the bedrock channel to varying depths. This same sediment as it is comminuted and remobilized over time acts as the tools to erode the bedrock channel. Such a scenario is illustrated in this conceptual slide. Elapsed time is shown on the horizontal axis (arbitrary scale) and the top panel (A) shows occasional large floods (spikes). When a flood occurs, sediment is deposited on the valley bottom which is gradually reworked and eroded by smaller floods that do not induce debris flows (B). If the interval between debris flow events is long, almost all alluvium can be eroded from the channel bed, leaving exposed bedrock. On the other hand, shortly after one or several debris flow events the valley bottom may be deeply mantled in alluvium. Erosion of the bedrock would be limited by both too little alluvium in the valley bottom (no tools for abrasion) and by too much alluvium (the bedrock is hidden beneath an alluvial cover). This is illustrated in (C) in which the main bedrock erosion occurs 1) during the emplacement of the debris flows in major floods, and 2) as the valley floor sediment is reworked, but the rate of erosion rate drops if the alluvial cover is too thin or too thick.
Such effects of episodic sediment supply have not yet been incorporated into landscape evolution models.