Linear dunes, also known as longitudinal dunes, are among the most striking landforms found in sandy deserts and arid regions. While various theories exist regarding their initial formation, one key model describes how their distinctive shape is maintained and how they grow and propagate over time.
According to this model (Tsoar, 1978), the primary mechanism responsible for the maintenance and downwind propagation of the linear dune's longitudinal morphology is the deflection of sand grains along the dune axes for short distances.
Understanding the Formation and Maintenance Process
The formation and continued existence of linear dunes, as described by this specific model, is not solely dependent on sediment arriving directly from upwind. Instead, the process involves the internal dynamics of sediment movement within the dune system itself.
Here are the key aspects based on the referenced model:
- Axial Grain Deflection: Wind patterns, often bidirectional or oblique to the dune axis, cause sand grains to be moved not just across the dune, but significantly along its length. This lateral movement across the wider sand sheet or inter-dune area is then deflected by the existing dune body, causing grains to move parallel to the dune crest.
- Downwind Propagation: This continuous movement of sand along the dune's axis leads to the gradual extension and propagation of the dune morphology in the downwind direction. The dune essentially grows longer by adding sediment to its downwind end or extending along its crestline.
- Independence from Upwind Sources: Unlike some dune types that rely on a constant supply of sand from directly upwind sources, this model suggests that linear dunes can maintain and propagate their form through the redistribution of sand within the dune field, without necessarily needing large amounts of new sediment continually arriving from distant upwind areas.
- Age Distribution: A key implication of this formation model is that the age of the dune sediment or the dune morphology itself would not necessarily show a steady decline in age when moving downwind along the dune's crest. This contrasts with dunes that grow primarily by accumulating sediment arriving from a single upwind source.
| Feature | Description based on Tsoar (1978) Model |
|---|---|
| Primary Mechanism | Deflection of grains along dune axes |
| Morphology Maintenance | Maintained by axial sediment transport |
| Propagation Direction | Downwind |
| Reliance on Upwind Sources | Not reliant on large upwind sediment sources |
| Expected Age Downwind | No decline in age downwind |
In essence, while various factors like wind regime, sediment availability, and underlying topography play a role in where linear dunes might initially nucleate, their characteristic linear shape and impressive length are maintained and extended through the efficient transport and deflection of sand along their central axis, allowing the dune form to essentially "walk" or propagate downwind over time (Tsoar, 1978).
