Why Rivers Flood: Implications of the Elimination of Overbank Flows Through Channel Widening in a Laboratory Flume
Abstract
We performed a series of laboratory flume experiments to document the adjustment of straight channels carved from non-cohesive sediment to overbank flows. We first created a stable channel adjusted to a bankfull discharge. Then, the discharge was abruptly increased to create an overbank flow. In all of our experiments, the channel widened during overbank flow until the increased discharge could be accommodated within the channel, suggesting that bank erosion in the absence of deposition will tend to eliminate flooding. We propose two dimensionless numbers to evaluate these processes in nature. The first, termed the Flood Elimination Number (FLENUM), represents the ratio of the duration of flooding to the time required for bank erosion to fully capture overbank flows during a single flood. The second, termed the Channel Stability Number (CHASNUM), evaluates the maximum length of channel that can accommodate the additional sediment supplied by widening without catastrophic deposition or changes in channel planform (such as braiding). Our experiments suggest that a critical value of the FLENUM is ~0.1. For FLENUM values higher than 0.1, rivers will accommodate flooding through channel widening, likely leading to braiding. As expected, braided rivers have high FLENUM values on the order of 10. Rivers with FLENUM values lower than 0.1 should endure overbank flows without significant widening, and indeed single-thread meandering rivers with cohesive banks have FLENUM values around 0.01. In our experiments, the CHASNUM was approximately 50 river widths, approximately equal to the working length of our experimental channel, which explains our ability to successfully maintain a stable, single thread channel despite extensive channel widening. The CHASNUM may provide an interesting explanation for the observation of periodically spaced disturbance zones in some gravel-bed rivers.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.H41D0767P
- Keywords:
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- 1821 Floods;
- 1825 Geomorphology: fluvial (1625);
- 1861 Sedimentation (4863);
- 1862 Sediment transport (4558)