Coastal Structures Design Questions and Answers

Answer: C

Both are true and the reason gives the limitation, and the tidal zone is the most severely corroded. Accelerated low water corrosion is a recognised problem in many harbours. Corrosion allowance is added in the tidal band.

Answer: D

Microbiological activity in that narrow band greatly increases the rate of metal loss. Rates several times the normal immersed value have been recorded in many harbours. The zone requires specific inspection attention.

Answer: A

Sulphate reducing bacteria in the band just below low water accelerate the loss. Inspection of this zone is a routine part of harbour asset management. The band is narrow and easily overlooked. Divers are needed to inspect it properly.

Answer: B

Correct answer: viscous effects within the armour layer diminish as scale grows

Model tests are scaled on the Froude criterion, which preserves the ratio of inertial to gravitational forces but not viscous ones, so the flow within the armour voids is relatively more viscous in the model than in the prototype. The discrepancy shrinks as the model is enlarged, and a Reynolds number above about 30,000 within the armour layer is the usual requirement for the results to be transferable.

Answer: C

Both are true and the reason gives the distortion, and the effect works in the unsafe direction. Facility size is what ultimately limits how far the scale can be raised. Large facilities exist precisely to reduce this effect.

Answer: D

A flume represents one cross section and cannot reproduce any alongshore variation. Diffraction around a head and penetration into a harbour both demand the three dimensional basin. A flume answers only the trunk section question.

Answer: A

Correct answer: bathymetric and above water survey of the armour profile

Monitoring means measuring the shape of the structure and comparing it with the as-built and previous surveys, so that settlement, armour displacement and toe scour show as trends rather than as surprises. The underwater part matters as much as the visible - the toe is where damage usually begins - which is why bathymetric survey is included as a matter of course.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the necessity. Every later survey is interpreted as a difference from the one before, and the first of them must be the as-built. Without an accurate record of where the structure was when finished, settlement cannot be distinguished from construction tolerance, nor armour displacement from as-placed irregularity, and years of monitoring yield nothing conclusive.

Answer: C

The first two are correct and are the basis of asset management. The third is false economy, since damage accelerates once the armour layer is breached. Early repair is far cheaper than late repair. Deferral converts maintenance into reconstruction.

Answer: D

Correct answer: displacement of a few armour units exposing the underlayer

Failure begins locally. A few units are moved by a storm, the smaller underlayer material beneath them is exposed to direct wave attack, it washes out rapidly, and the hollow that forms undermines the units around it - so the damage spreads from the initial patch. Catching it at that stage, by survey and prompt repair, is far cheaper than the reconstruction that follows if it is left.