The correct option is 1 and 3 only.
Explanation
Barchans are specific types of sand dunes formed by the depositional action of wind in hot deserts. They are characteristic of arid environments with limited sand supply and unidirectional wind flow.
Statement-wise Analysis:
- Statement 1 is Correct: Barchans are distinctively crescent-shaped dunes. They possess a gentle slope on the windward side (convex) and a steep slip face on the leeward side (concave).
- Statement 2 is Incorrect: The "wings" or horns of a Barchan dune point away from the wind direction (downwind). This orientation distinguishes them from parabolic dunes, where the wings point towards the wind direction (upwind).
- Statement 3 is Correct: Barchans typically form in areas where the wind direction is constant and moderate, and the supply of sand is limited. If the wind direction shifts frequently or sand supply is abundant, other dune forms (such as seifs or star dunes) are more likely to develop.
Key Takeaway:
Barchans are crescent-shaped dunes with wings pointing downwind (away from the wind source), whereas Parabolic dunes have wings pointing upwind (towards the wind source).