Effect of levees on river floods — as per G. W. Pickels, confining floodwaters between levees primarily tends to increase which hydraulic outcome during floods?

Introduction / Context:
Levees (embankments) restrict a river’s floodplain width, forcing more of the flood volume into the main channel. Classic viewpoints, including those attributed to G. W. Pickels, are examined to understand how levees alter hydraulic parameters like stage (water surface elevation), velocity, and scour risk during floods. This informs flood management and levee design strategies.

Given Data / Assumptions:

• River reach fitted with levees confining flood flows.
• Flood events lead to overbank flows in an unconfined state;
• Confinement reduces effective flow area on the floodplain.

Concept / Approach:

By restricting the available cross-section for flood conveyance, levees concentrate discharge within the main channel. For a given flood discharge, a smaller effective area typically requires a higher stage (water surface elevation) and, frequently, higher average velocities inside the channel, potentially increasing local scour. Pickels’ interpretation emphasizes the rise in flood stages associated with levee confinement.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Backwater profiles computed via gradually varied flow (GVF) models show higher stages for the same flood hydrograph when floodplains are constrained.

Why Other Options Are Wrong:

“rate of flood wave translation only” — wave celerity depends mainly on channel geometry and slope; confinement effect on stage is more direct and pronounced.

“maximum discharge entering the reach” — levees do not increase incoming hydrograph peak by themselves.

“surface slope far upstream” — local effects dominate; the principal noted effect is elevated flood stage in the leveed section.

“none of these” — incorrect because stage rise is a known implication of levee confinement.

Common Pitfalls (misconceptions, mistakes):

Assuming levees always reduce flood risk everywhere; they protect the adjacent land but can raise stages and velocities within the channel, increasing scour risks.

Final Answer:

water surface elevation during floods