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# Sub-critical vs Super-critical flow

## Sub-critical vs Super-critical flow

(OP)
Hi!

I was wondering, when a dam breaks is the flow in the river sub-critical or super-critical?

I am trying to determine the height of the water at different sections, when the flow in the river is equal to the flow after a rupture.

Any tips or ideas would be useful,

Thanks

### RE: Sub-critical vs Super-critical flow

it depends on the slope and cross section of the "river" valley.  Can't answer this any better than that unless more details are given.  Hydraulic height, breach width, downstream slope, location of breach, channel/valley cross section, n-values etc.

### RE: Sub-critical vs Super-critical flow

Wow, you are asking a very difficult question. Not only is this an unsteady flow, it is also a varied flow, which is the most difficult kind of one-dimensional flow problem to solve. It's also probably beyond the scope of this forum.

I would recommend a local hydraulics expert for this one.

Good luck,

Robert Weber, EI
Maurer-Stutz, Inc
www.maurerstutzinc.com
www.geocities.com/robntweber

### RE: Sub-critical vs Super-critical flow

If you wish to get a quick and dirty answer, get the program SMDBK ( Simplified Dam Break ).  It will model a dam break for you with very little input information required.  It makes a number of simplifying assumptions and is based in part on hydrauic theory and experience with actual dambreaks.

### RE: Sub-critical vs Super-critical flow

To be synthetic, I would say that a dambreak wave knows a significant attenuation even on short distance, due to the steepness of the hydrograph: the "flow after a rupture" can only be locally defined, in a single section.
Thus to evaluate the highest(s) water elevation(s), you should model the whole reach downstream from the dam, in an unsteady way. (not the easiest thing...)
(I speak with my theorical background rather than experience)

-Regards-

### RE: Sub-critical vs Super-critical flow

Hey RWF7437:

I couldn't find SMDBK or any mention of it in the search index.  Simplified Dam Break is apparently called DAMBRK a an acronym, but I couldn't find a download option, just mentions of it in documents...any further info?  Thanks a lot.

--Steve

### RE: Sub-critical vs Super-critical flow

Here's a more complete reference:

AVAILABILITY  LETTER

The Simplified Dam Break (SMPDBK) Flood Forecasting Model is a simplified procedure for predicting downstream flooding produced by a dam failure. This procedure produces information needed for delineating areas endangered by dam-break floodwaters while substantially reducing the amount of time, data, computer facilities, and technical expertise required in employing more highly sophisticated unsteady flow routing models such as the NWS DAMBRK or FLDWAV models. The SMPDBK model can easily be processed on an inexpensive microcomputer; and with a minimal amount of data, the user may within minutes predict the dam-break floodwave peak flows, peak flood elevations, and peak travel times at selected downstream points. This capacity for providing results quickly and efficiently makes the SMPDBK model a useful forecasting tool in a dam failure emergency when warning response time is short, data are sparse, or large computer facilities are inaccessible. The SMPDBK model is also useful for pre-event dam failure analysis by emergency management personnel engaged in preparing disaster contingency plans when the use of other flood routing models is precluded by limited resources.
The SMPDBK model was written in Fortran and C. The documentation describes the theory behind the model, describes the input parameters and lists their defaults, and shows a hand computation of an example problem. The latest version (1991) of the SMPDBK model including application, source code, example files, and documentation are available over the Internet through NTIS at http://www.ntis.gov.

River Mechanics
Hydrology Laboratory
Office of Hydrologic Development
National Weather Service

Webmaster: Michael.Richardson@noaa.gov

Other Models
DAMBRK
BREACH
SMPDBK
FLDWAV

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