Retrogression of the bed level of a river downstream a weir, occurs due to A. Heavy impact of water B. Increase of the bed level C. Less percentage of silt D. Soft soil strata

The correct answer is D. Soft soil strata.

Retrogression of the bed level of a river downstream a weir occurs due to the erosion of the soft soil strata by the flowing water. The

weir creates a drop in the water level, which causes the water to flow faster. This faster-moving water has more energy to erode the soil, and the soft soil strata are particularly vulnerable to erosion.

The other options are incorrect. Heavy impact of water does not cause retrogression of the bed level of a river downstream a weir. In fact, heavy impact of water can actually help to build up the bed level of a river. Increase

of the bed level would not cause retrogression of the bed level. Less percentage of silt would not cause retrogression of the bed level.

In conclusion, the correct answer is D. Soft soil strata.