Experiments on Gravel‐Sand Transitions: Examination of Washload Deposition
Bibliographic record
Abstract
Abstract An abrupt transition in bed grain size occurs in river systems. Over a short downstream distance, often only a few channel widths, the bed surface fines from gravel (∼10 mm) to sand (∼1 mm). This is the gravel‐sand transition (GST), and it is the only abrupt downstream reduction in grain size within fluvial systems. There are several theories for the origin of the GST, including size‐selective deposition of bimodal grain size distributions and the rapid onset of washload deposition due to changes in particle suspension properties at shear velocities of ∼0.1 m/s. Here, we present a laboratory experiment examining changes in fluid and sediment dynamics across a GST. We developed a stable gravel bed reach that was just below the threshold of motion, then fed sand. We observed sand carried as washload in the gravel reach fall out of suspension, forming a sand bed and a stable GST. Shear velocity was 0.09–0.10 m/s upstream of the GST and <0.10 m/s downstream, consistent with the washload deposition hypothesis. We were then able to perturb the position of the GST by systematically varying discharge and/or sand supply, shifting it downstream with an increase in discharge or a reduction in sediment supply. A decrease in discharge or increase in sand supply caused upstream migration. Our observations support an abrupt change in washload transport conditions across a narrow range of shear velocities, consistent with the washload deposition theory and measurements taken across GSTs in natural river systems.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| 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 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.001 |
| 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 source (direct Gemma or distilled Codex), 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".