Cyclic steps and roll waves in shallow water flow over an erodible bed
Bibliographic record
Abstract
Abstract The St Venant equations in conjunction with a phenomenological law for erosion are used to explore the nonlinear dynamics of cyclic steps – linearly unstable bedform patterns which emerge when uniform flow over an erodible bed becomes supercritical. The instability saturates by blocking the overlying flow and creating hydraulic jumps just downstream of the steepest part of the steps. Near onset, steadily migrating, nonlinear step patterns are constructed and shown to suffer a short-wavelength secondary instability that ‘roughens’ the bed and renders the staircase patterns less regular and time-dependent. An eddy viscosity is needed to regularize both the onset of the primary steps and the secondary instabilities. Further beyond the critical Froude number, the steps block the flow sufficiently to arrest erosion significantly, creating complicated patterns mixing migrating steps and stationary bedforms. The reduction in flux also stabilizes roll waves – a second, hydrodynamic instability of uniform supercritical flow. It is further shown that roll waves are purely convective instabilities, whereas cyclic steps can be absolute. Thus, in the finite geometries of the laboratory or field, it may be difficult to excite roll waves. On the other hand, the complicated spatiotemporal patterns associated with the cyclic-step instability should develop naturally. The complicated patterns resulting from the secondary instability do not appear to have been observed experimentally, calling into question the validity of the model.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".