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Longitudinal and transverse dispersion coefficients in braided rivers
Li Gu1, Zinan Jiao2, Zulin Hua1
1Key Laboratory of Integrated Regulation and Resource Development on Shallow Lake of Ministry of Education, College of Environment, Hohai University, Nanjing 210098, China E-mail: zulinhua@hhu.edu.cn; National Engineering Research Center of Water Resources Efficient Utilization and Engineering Safety, Hohai University, Nanjing 210098, China.
Summary
Dispersion in braided rivers is complex. This study found higher dispersion coefficients in separation zones and wider anabranches, with natural rivers showing larger values than laboratory models.
Area of Science:
- Environmental science
- Fluid dynamics
- River morphology
Background:
- Dispersion characteristics in braided rivers remain poorly understood, impacting environmental modeling.
- Accurate estimation of dispersion is crucial for predicting pollutant transport in river systems.
Purpose of the Study:
- To investigate the dispersion characteristics of braided rivers using a physical model.
- To quantify the dispersion coefficient tensor and analyze its relationship with flow structure and river geometry.
Main Methods:
- A physical model of a braided river was constructed and instrumented to measure flow structure.
- The dispersion coefficient tensor was estimated from the measured flow data.
- Laboratory results were compared with data from a natural braided river.
Main Results:
- The largest longitudinal and transverse dispersion coefficients were observed in the separation zones of anabranches.
- Smaller coefficients were found in models with smaller diversion angles, where separation zones were absent.
- Dispersion coefficients increased with upstream flow rates and were higher in wider anabranches.
- A negative correlation between longitudinal and transverse dispersion coefficients was noted in several sections.
- Coefficients from laboratory tests were generally smaller than those measured in natural braided rivers.
Conclusions:
- River geometry, particularly anabranch separation zones and width, significantly influences dispersion.
- Flow rate is a key factor affecting dispersion coefficients in braided rivers.
- Natural braided rivers exhibit higher dispersion than laboratory models, suggesting scale effects are important for pollutant transport prediction.
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