River-bed retrogression (degradation) downstream of a weir — identify the principal cause that promotes lowering of the bed level after construction.

Introduction / Context:
When a weir or barrage traps part of the incoming sediment load, the water released downstream can become “clearer” (sediment-deficient). Such flows have excess transport capacity relative to available bed material and therefore pick up sediment from the downstream reach, causing degradation (retrogression).

Given Data / Assumptions:

• Weir or barrage with ponding and sediment interception upstream.
• Downstream reach initially alluvial and movable.
• Observation: bed level lowers with time.

Concept / Approach:

Clear-water scour is a well-known phenomenon: the smaller the percentage of silt in the released flow (relative to its transport capacity), the greater the tendency to erode the bed and banks until a new graded regime is achieved. While strata properties matter, sediment deficit is the primary driver of systematic retrogression.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Compare pre- and post-project bed profiles; use regime or sediment routing models to show declining bed elevations correlated with reduced suspended load concentration.

Why Other Options Are Wrong:

Impact/jet action is local near structures; increased bed level implies aggradation (opposite); soil softness influences rate but is not the principal cause without sediment deficit.

Common Pitfalls (misconceptions, mistakes):

Attributing long-reach degradation only to foundation type; ignoring watershed sediment yield changes; mistaking local scour for systemic retrogression.

Final Answer:

less percentage of silt (clear-water flow)