Dear Zeregaber! Your Q is a classical Q for students of Civil and Environmental Engineering. YOur A is also along the right solution. Still, a simple formula to give you a useful correct answer, does not exist. One has to run a fairly complex mathematical model in 2D or even 3D, taking into account a lot of data.

Seeing that you come from technical university ... may I ask you something: Have you considered asking your colleagues for some guidelines?

Dam break waves are dynamic flood wave problems. Bearing in mind that you are not dealing with a steady state flow, and the maximum water surface elevation at any point along the channel will precede the maximum velocity I suggest that you google DWOPER, DAMBK, or FLDWAV or at least read http://www.nws.noaa.gov/oh/hrl/nwsrfs/users_manual/part2/_pdf/24dwoper.pdf. These are all older 1D dam beak programs, but they should give you some information about your problem. They used to be freely downloadable, but that may no longer be the case. My google produced a number of studies which may be of some use to you.

Velocities along the river channel are dependent on the channel geometry. In principle, no trend can be anticipated.

Peak discharge tends to decline along the river channel.

The peak breach outflow immediately downstream the breached embankment dam will be highest. As the floodwave is routed downstream the valley, it's peak flow will attenuate gradually, depending on the valley bathymetry. As my colleagues have mentioned, in order to determine velocities (since this is what you are interested in), you need to run two models. The first model is an embankment breach model. The most common and commercially available model is NWS-BREACH. This model does have many limitations, however. There is a multitude of research on numerical breach models which simulate the breaching process using mathematical-physical relationships (40+ years ago). The second process you need to model is the propagation of the flood wave downstream using flood routing techniques. Remember, each model has its own assumptions and limitations. I see you are using HEC-RAS. As far as I remember, HEC-RAS may solve the embankment breach problem and also route the floodwave downstream simultaneously. However, the first part of your problem may not be accurately represented by HEC-RAS, since it requires a user input of the breach development time whereas the model imposes a linear growth rate in the breach channel to determine the breach outflow, based on weir flow/orifice equations. NWS-DAMBRK will also solve your problem similar to HEC-RAS. It may be more versatile since it accounts for other effects such as Non-Newtonian flow due to mud/debris for instance. Of the top of my head, those are just two models that are freely available. Best of luck with your research.

Thank you Mr Kompare, Mr Perry, Pinho and Mr Al Riffai

Well I am working on a small study project on the 1D small dam breaching using HECRAS I have got some simulation results yet I am not satisfied. I have asked for some advice from some of my colleague of course. I have referred to some guidelines like the State of Colorado Dam Breach Analysis Guidelines, HECRAS Hydraulic Reference Manual, BOSS DAMBRK user manual and more. You are very right, the estimation of the dam breach parameters in such kind of references were very helpful. Yet it looks as if something is missing in the results.

The peak depth at a cross section was decreasing with distance which is definitely correct response with the attenuation of the flow as well as depth hydrographs. The velocity on the other side starts following the same trend as the depth but at some spot downstream it rose up and remained more or less constant.

Lately, I am considering of adapting the Preismann’s scheme for transcritical flow; though I am new to the concept.

I look forth to your advices and thank you in advance.

Best regards,

Zerit

The right question is about hydraulic model, not "hydrological modeling".

The second is the dam already built? If the channel is built you must avoid any reasons of damage of the dam! If the dam is not built you have to design it without any risks of piping failure, respectively dam breaking.

The third geometry of the channel is important, but 25 km long channel could not be uniform. The slope and may be the cross section will be different along the distance.

Finally you can predict the maximum velocity after defining the slope and the channel form - all geometry conditions. In case of the very big velocities you can build additional constructions and reduce the highest values. In the opposite if the channel is not built you can design the channel and determined the maximum velocity depending of your necessities