Levees and floodwalls in flood control: identify the incorrect statement (choose the single false option).

Introduction / Context:Levees and floodwalls are structural countermeasures built along rivers and coasts to prevent overtopping and inundation during high flows or surges. Sound design requires hydraulic, geotechnical, and structural considerations.

Given Data / Assumptions:

• Levees/floodwalls resist hydrostatic and hydrodynamic loads during floods.
• Design should include safety margins for uncertainties (e.g., debris, waves, settlement).
• Freeboard is the height added above the design high water level to reduce risk of overtopping.

Concept / Approach:Flood-protection structures must safely resist unbalanced lateral water loads. Adequate cross-sections and foundations are adopted to prevent sliding, overturning, piping, and excessive deformation. Freeboard allowances are standard to accommodate waves, wind setup, and forecasting errors.

Step-by-Step Solution:Check option (a): Unbalanced water load across the riverward and landward sides is fundamental ⇒ true.Option (b): “Adequate dimensions” (crest width, side slopes, wall thickness) are essential ⇒ true.Option (c): They are indeed flood-control measures ⇒ true.Option (d): Claiming freeboard is never provided contradicts standard practice; freeboard is routinely included ⇒ false.

Verification / Alternative check:Most design manuals specify freeboard above design water levels to cover uncertainties and wave run-up; many agencies mandate minimum freeboard values.

Why Other Options Are Wrong:Options (a), (b), and (c) reflect accepted engineering practice; they are not incorrect.

Common Pitfalls:Assuming crest height equals exactly the computed design level without adding freeboard; ignoring foundation seepage/piping risks while focusing only on wall geometry.

Final Answer:They are never provided with freeboard allowances.