Drag and Wave loading against a cofferdam

[structuresOSU07]

Hello all.

The construction company I am working for is building a 380' longways by 160' wide cofferdam off the coast in a confluence funneling storm surge towards a city. Currently a surge barrier has been built to block future storm surge from the city. The part that we are doing is the final piece and is to be a gate to let barge traffic through in either direction. I am now trying to explore what wave and tidal loads should be expected against the cofferdam. The interesting part about all of this is that the surge barrier is blocking almost all of the brackish lake except a portion 150' wide which is directly next to our cofferdam, so any past tidal flow data is basically useless to me.

Would tidal flows even be an issue since there is that opening right next the cofferdam?

If so, then wave loading is still an issue but the owner requires a 100 Kip barge impact loading. I am almost positive that this will be the limiting factor of design anyways but I just do not know if that will be good enough for the owner. Should I do calcs and explore tidal, wave and drag forces against the cofferdam or just say what my limiting factor of design is and move on from there?

Any help would be appreciated. Thank you in advance.

 

[SlideRuleEra]

Tidal flow through the 150' wide opening next to the cofferdam will be a major issue. Consider that the same volume of water will flow (or attempt to flow) through a much smaller crossectional (since the cofferdam will block much, but not all, of the opening). Velocity of the flowing water may increase substantially, compared to what it was before. This could cause scow adjacent to the cofferdam.

The daily tidal range will be a significant factor. If the range is only a foot or so, probably not a big problem. If it is several feet, will become progressively more important. Be sure to consider cyclical tidal variations (neap & spring tides).

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[structuresOSU07]

I agree with the scour issue & do believe that this will have to be considered. I am just not sure about how to model the effects of the increased flow and the tidal action against the cofferdam. Any gathered data from past tidal action is really not similar enought to apply to the current situation. That is why I would like to just say that the barge impact load is the limiting factor of design.

 

[SlideRuleEra]

structuresOSU07 - I'll offer some constructive criticism, that you may not like; just disregard it if that is the case:

Consider this possibility: When is a high-impact barge collision most probable? IMHO, it would be when tidal forces are at a high level (making a barge difficult to control). You MAY need to consider both, simultaneously.

Perhaps the tidal force can be ignored, but why make an arbitrary guess based on no calculations? The excuse of not having accurate data is not a valid reason to say "Tidal Force = 0". Do the best you can, with what you've got.

Why not TEMPORARILY assume that the available data has SOME value to your situation. Model those numbers and see what the results are; this is probably a "best case" - what you ACTUALLY have is worse. At this point, now that you have a minimum bound for the tidal forces, you can start to make assumptions.

Maybe you can ignore the tidal force, maybe not. One (old) way to treat an unknown (but estimated) dynamic force is to assume a static force equivalent. This is what is being done for the (100 kip) barge impact. With some basic (approximate) calculations, you could do the same for the tidal force.

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[structuresOSU07]

Well believe it or not I love criticism. Thank you again, that is great advice.