Modelling Rivers

River

Rivers are natural water courses. They have vital contributions to the social and economical welfare of societies living along their course. They provide continuous, usually dependable, flow and supply of water. Accordingly, centralised civilisation began emerging independently along number of river valleys; e.g. the Nile, Tigris and Euphrates, the Indus and the Yellow Rivers. Since then, mankind has been in continuous struggle to understand and live in harmony with this gift/force of nature. Modelling rivers is the modern way to understand their behaviour and is an essential tool to tackle several river problems.

Delft3D is the state-of-the-art modelling framework for hydrodynamics, water quality, ecology, waves and morphology.
 
Here, in this space, we share knowledge & experiences, and discuss issues related to modelling of rivers.
 
 

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Flow Distribution at Total Discharge Boundary

Richard Measures, modified 8 Years ago.

Flow Distribution at Total Discharge Boundary

Jedi Knight Posts: 178 Join Date: 3/23/11 Recent Posts
Hello,

I am working with shallow braided rivers and am having ongoing difficulties reliably representing the flow distribution across the upstream boundaries of my models. Depending on the flow rate and braid configuration (they are morphological models so this changes) the distribution of flow across the upstream boundary needs to change. Unfortunately there are no structures or permanently channelised sections where I can set the upstream limit of the model.

I have been using the total discharge boundary (time series) but have found several problems with they way it distributes the flow:
  • Roughness effects - If the roughness varies across the channel at the upstream boundary then the boundary pushes too much flow into the rough areas, this can result in much higher water levels in the floodplain than in the channel at the boundary.
  • Downstream flow resistance - if part of the upstream boundary has a comparatively low slope, or is affected by a backwater effect from downstream then this results in a much higher water level at this part of the boundary.

I think the effects i have seen relate to the fact the boundary distributes the flow solely based on flow depth (equation 4.4 in the manual). Does anyone have any suggestions for ways I can improve the representation of flow distribution at the boundary? Has anyone else had similar problems? I guess it is so significant in my models because of the shallow flow depths and wide boundary.

At the moment I am having to trim the boundary so that no flow enters in the floodplain (even during large floods), and modify the roughness and topography at the upstream part of the models to get a sensible distribution of flow. Even after doing this I find that I have to disregard the results from the upstream most part of the model as it does not calibrate.

Ideally I would like a boundary where water levels are horizontal across the boundary but increase or decrease to achieve the specified flow rate in the time series.

All suggestions and comments welcome!

Richard
Bert Jagers, modified 8 Years ago.

RE: Flow Distribution at Total Discharge Boundary (Answer)

Jedi Knight Posts: 201 Join Date: 12/22/10 Recent Posts
Hi Richard,

First of all, many thanks for your answers to a couple of the other questions on the forum!

The total discharge boundary is indeed giving problems from time to time; even for a simple straight channel lateral oscillations may occur -- however, the total discharge formulaturns out to work well in many other cases. Although the idea seems simple, it's difficult to get right. The code for distributing the flow is located in
src/engines_gpl/flow2d3d/packages/kernel/src/timedep/incbc.f90
At the total discharge boundary nob(3,n) == 7. The equation 4.4 should be adjusted to read H_i^{3/2}*B_i*C_i instead of just H_i^{3/2}; the slope_i^{1/2} is ignored because it's difficult to determine. That's what line 366 in incbc computes. So, roughness is already included (high roughness, low Chézy, will get less discharge). Btw., unless you're using trachytope formula 154 for modeling vegetation roughness (see Section 9.11.2 of the flow manual), you can ignore the difference between czeff and czbed.

I have thought about a procedure in which a constant water level is assumed, but how to implement it best has eluded me so far.

Best regards,

Bert
Richard Measures, modified 8 Years ago.

RE: Flow Distribution at Total Discharge Boundary

Jedi Knight Posts: 178 Join Date: 3/23/11 Recent Posts
Hi Bert,

Thanks a lot for your reply- I understand a bit better now what's going on. Thanks also for pointing me towards the right bit of the code as I hadn't managed to find it. Good to see that the cell width is in there as well as roughness - I was wondering why that was missing from eqn 4.4 in the manual...

I was using trachytope formula 154 in one model and was having problems with vegetation on/close to the boundary (before I removed it all from the upstream part of the model!) but this might have been due to areas where the boundary did not lie on vegetation but there was some close to it causing a backwater effect at the boundary.

I'll have a closer look at this tomorrow morning (6pm in New Zealand now) and see if I can think of an improvement. It would be nice if any improvement also enabled 'turning on' a dry total discharge boundary but this is a nice to have rather than a necessity.

I'm thinking along the lines of calculating the flow distribution using the existing formula but based on flow depths given by a horizontal (averaged??) water level at the boundary rather than the actual water depths - this would have no effect if the water level at the boundary was pretty level anyway but would help stop the feedback loop where the flow depth increases at one part of the model because of a backwater effect, then the model forces more water into that part of the boundary, so the depth/flow increases further etc etc. I can think of a couple of possible pitfalls with this approach but they might not be important.

cheers,

Richard
JR
John Roseberry, modified 8 Years ago.

RE: Flow Distribution at Total Discharge Boundary

Youngling Posts: 1 Join Date: 8/24/11 Recent Posts
Hi, Bert,

I am experiencing a similar problem as Richard was with the distribution of flow across a total discharge boundary. I have a single thread channel with complex bed morphology at the upstream boundary. This stretch of the channel is a high resolution grid of a portion of a larger reach-scale model that is used in part for supplying boundary conditions to this model. (So DD may be an option, but I would also like to be able to run it independent of the larger model).

I am running a recent trunk version in Linux, and we are also working on a licensed version 3.28.03 in XP. Do you know of any ways to take boundary conditions from within a larger model that have a more realistic distribution of flow than just doing a total discharge boundary?

Thanks,
John Roseberry