Meander simulation
About this app
In time-lapse, a river winds across a flat floodplain. The cut bank is eroded and a sandbar grows on the point bar until a cutoff severs a loop and leaves an oxbow lake behind. Cross-profile with secondary flow, erodibility, valley slope and bank protection are adjustable.
How meanders form
In a river bend, centrifugal force pushes surface water outward while water near the bed flows back inward. Combined with the main flow this creates a helical secondary flow. On the outside the river runs deep and fast and erodes the bank, the cut bank. On the inside sand settles and a bar grows, the point bar. Each loop therefore migrates sideways and down-valley until two outer bends touch and the river takes the shortcut in a neck cutoff. The abandoned loop remains as an oxbow lake.
What the simulation shows
The 3D scene shows a river crossing a flat, uniform floodplain in time lapse. Loops grow out of small irregularities, cutoffs remove them, and faint lines record former channels and scroll bars. Clicking the river opens the cross-section with secondary flow, radius of curvature and lateral migration per year. The display shows model years, sinuosity and the number of cutoffs. The model follows Ikeda, Parker and Sawai and Howard and Knutson and is meant for teaching.
Experiment
Raise bank erodibility or valley slope and watch how quickly the loops develop. Click on a bend and compare its cross-section with a crossing between two bends. Place riprap as bank protection: lateral erosion stops there and migration shifts to unprotected reaches. “New seed” starts a fresh run with a different initial disturbance.
Frequently asked questions
- What is the difference between a cut bank and a point bar?
- The cut bank is on the outside of a river bend, where water flows fast and deep and erodes the bank, so it is usually steep. The point bar is on the inside, where the current is weaker and sand and gravel are deposited, so it slopes gently. Together they make the bend migrate sideways.
- How does an oxbow lake form?
- When two loops migrate towards each other, the strip of land between them, the meander neck, becomes narrower. Once the river breaks through, it takes the shorter path and the cut-off loop is left behind as an oxbow lake. Its ends silt up, and over a long time the still water turns into marsh and floodplain.
- What is river sinuosity?
- Sinuosity is the ratio of the channel length to the valley length measured along the valley axis. A straight river has a value of 1, and rivers above about 1.5 are usually called meandering. The more a river winds, the gentler its own gradient for the same valley slope.
- What can I adjust in the meander simulation?
- You control the time-lapse speed, bank erodibility, valley slope and the size of the initial disturbance, and you can place and remove bank protection. Clicking the river shows the cross-section at that point. The time axis should only be read as an order of magnitude.
Subject: Geography & earth science
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