Wind Waves in Reservoir ~ Wind run-up and Wind set-up

[af_231]

Hello!
As part of a project related to dam overtopping induced by wind (wave run-up and wave set-up), I am looking for information on how to calculate fetch lenghts for a reservoir.
I would appreciate any information about it.
Thanks!

 

[Bobbywhy]

af 231, You may be able to use the chart here:

http://en.wikipedia.org/wiki/Beaufort_scale

It gives wave height vs wind speed (in open ocean)

:) lengths

 

[Studiot]

Open ocean waves, on the Pierson spectrum, are calculated over a fetch of 100km for Beaufort force 5 and 1000km for Beaufort force 8.

I doubt your reservoir is that big.

Further open ocean wave fields are deep water waves, you need to be using shallow water wave theory or surge in open channel theory. The plan and cross section dimensions of the reservoir are crucial, as is the bottom profile approaching the dam.

 

[af_231]

Thanks Studiot!
Have you heard about Dutch's formula to estimate wind set-up (or tide)?
Since my goal is get the overtopping induced by wind on a reservoir (wind set-up + wind rup-up), I found on internet a formula to estimate wind set-up formulated by Dutch, but could not find more information related to that formula, like a book or paper with a proposed methodology.
Please let me know if you are familiar with these procedures.. I really appreciate your help!

 

[Studiot]

No, but do you have a link?

 

[af_231]

Yes, I found a document related to Dam Freeboard Requirements, but that's the only document I've found that make reference of Dutch's formula.
http://140.194.76.129/publications/eng-manuals/em1110-2-1420/c-15.pdf

Thanks!

 

[Studiot]

For some reason I cannot access this link.
This pc is XPpro, but I have just tried in W7 and with the same result.

 

[af_231]

Ah ok. Thanks! Its a Engineer Manual published by the US Army Corps of Engineers.
Anyway, thanks for your help!

 

[Studiot]

You might like to look up the term 'backwater curve' Google seems to have lots.

You would need to calculate the backwater curve anyway and superimpose the perturbation of the wave on it.

 

[af_231]

Ok. Thanks! I am going to search about it!
Thanks for the suggestion!

 

[Studiot]

An open reservoir is basically a case of funny shaped open channel flow.

As you have already found out in your other thread there is a balance between inflow and outflow.
The inflow tends to vary considerably, with seasonal peaks.
The outflow is more regular from the drawoff and the balance flows down the spillway or wherever.

The dam and its associated structures can be thought of as an obstruction in this (variable) flow regime.
The reservoir surface profile is given by the backwater curve.

This is quite unlike the open ocean.

When the level is low waves are not at risk of overtoppoing the dam.

When the level is high there will have been/be an inflow peak. This, of itself, can create surges and waves.

The normal practice would be to superimpose the shallow water wave solution on top of this for additional wind induced waves.

Of course this is all a very complicated proceedure so is often (best) modeled by a real hydraulic model which is compared to calculations.

I wish you well with your project.

Please keep us informed.