intro story Coast / Estuary

Coast / Estuary

Coastal systems are among the most dynamic physical systems on earth and are subject to a large variety of forces. The morphodynamic changes occurring to coastlines worldwide are of great interest and importance. These changes occur as a result of the erosion of sediments, its subsequent transport as bed load or suspended load, and eventual deposition. 
 
Estuaries are partly enclosed water bodies that have an open connection to the coast. Estuaries generally have one or more branching channels, intertidal mudflats and/or salt marshes. Intertidal areas are of high ecological importance and trap sediments (sands, silts, clays and organic matter).
Within the Delft3D modelling package a large variation of coastal and estuarine physical and chemical processes can be simulated. These include waves, tidal propagation, wind- or wave-induced water level setup, flow induced by salinity or temperature gradients, sand and mud transport, water quality and changing bathymetry (morphology). Delft3D can also be used operationally e.g. storm, surge and algal bloom forecasting. 
 
On this discussion page you can post questions, research discussions or just share your experience about modelling coastal and/or estuarine systems with Delft3D FM. 
 

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Sub groups
D-Flow Flexible Mesh
DELWAQ
Cohesive sediments & muddy systems

 

 

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Sediment flux at inflow not matching imposed value in bcc

GS
Gerard Salter, modified 11 Months ago.

Sediment flux at inflow not matching imposed value in bcc

Youngling Posts: 3 Join Date: 10/16/14 Recent Posts

Hello,

I am trying to set the upstream sediment flux in conjunction with a discharge per cell boundary condition. However, I am finding that the sediment flux does not match what I'm imposing. The sediment concentration, which I calculate as total sediment load (m^3/s) times sediment density (2650 kg/m^3) divided by total water discharge (m^3/s) should match what I set in the bcc file, but it does not. 

Interestingly, changing the concentration in the bcc file does change the sediment transport rate, but not by the correct factor (i.e., doubling the concentration should double the total transport given that the water inflow is fixed). I found that changing the initial bathymetry but keeping the bcc the same also changes the calculated sediment flux.

I'm running the simulations long enough that this isn't some transient effect. The sediment flux has clearly reached a steady value, which it remains at for the duration of the simulation. 

 In the mor file, EqmBC is set to false, and for good measure I also tried adding NeuBCSand = false just in case, although they should do the same thing. I also discovered adding the keyword UpdInf = true to the .mor file changes the results, but it actually became worse than before. I'm just using a single grain size. I'm running Delft3D 4.02.02 on windows. 

 

Any ideas? Is this a bug in Delft3D?

Thanks,

Gerard

GS
Gerard Salter, modified 11 Months ago.

RE: Sediment flux at inflow not matching imposed value in bcc

Youngling Posts: 3 Join Date: 10/16/14 Recent Posts

I figured out the solution, and thought I'd post it for anyone looking at this in the future. The manual is not clear on this.

 

The .bcc specifies the suspended sediment, but bed load is set independently, even when EqmBC=false. Setting bed load is a bit complicated. 

Append to your .mor file the following:

   
   BcFil            = #mybcm.bcm#                   file containing mophological boundary conditions.
[Boundary]
   Name             = #upstream#
   IBedCond         = 5                             bed load or bed level boundary condition.

Then write a .bcm file with this template, replacing names and values as needed.

 

table-name           'Boundary Section : 1'
contents             'Uniform   '
location             'upstream            '
time-function        'non-equidistant'
reference-time       20180711
time-unit            'minutes'
interpolation        'linear'
parameter            'time                '  unit '[min]'
parameter            'transport excl pores Sediment_sand end A'       unit '[m2/s]'
parameter            'transport excl pores Sediment_sand end B'       unit '[m2/s]'
records-in-table     2
 0.0000000e+000  3.1400000e-004  3.1400000e-004
 3.0960000e+005  3.1400000e-004  3.1400000e-004

 

Notice that this specifies the sediment transport per unit width, but the manual incorrectly says the units are m3/s. 

 

Hope this helps someone!
 

BY
Bing Yuan, modified 3 Months ago.

RE: Sediment flux at inflow not matching imposed value in bcc

Youngling Posts: 5 Join Date: 9/22/17 Recent Posts

Hi Gerard, thanks for sharing! 

For a 2D study domain I did try your method, i.e., set sediment concentration to 0 in bcc file, and use nonzero values for sediment flux per unit width (as you mentioned) in bcm file. However, the resulting total sediment flux at the inflow boundary (remain nearly constant though) did not agree with the specified values at the boundary. May I ask did you get agreed total sediment flux (in time) at the boundary in the end? Thanks! 

ps. I used Engelund-Hansen formulation. 

pps. I did a few 1D simulations using bcc only, the resulting total sediment flux agrees well with the specified value. 

SS
Samuel Schroers, modified 5 Months ago.

RE: Sediment flux at inflow not matching imposed value in bcc

Youngling Posts: 7 Join Date: 9/29/17 Recent Posts

Hi Gerard,

Thx for the work-around! I was wondering about a similar question...

In general- anybody could tell me what is the difference between the .bcc file (which imposes supposedly transport at the boundaries but the results are something different) and the .bcm file?

Which combination would i use is if i know the sediment concentration (kg/s) for a certain inflow (m3/s) and I would like to model suspended and bed load transport?

Thx!

SS
Samuel Schroers, modified 5 Months ago.

RE: Sediment flux at inflow not matching imposed value in bcc

Youngling Posts: 7 Join Date: 9/29/17 Recent Posts

Somehow i managed as you laid out to implement the .bcm file- i changed the values of the time series but the result stays the same...

( i have put

    EqmBc            = false                
   NeuBcSand        = false  )

GS
Gerard Salter, modified 5 Months ago.

RE: Sediment flux at inflow not matching imposed value in bcc

Youngling Posts: 3 Join Date: 10/16/14 Recent Posts

My understanding is that with those two conditions set, the bedload should be set by the .bcm file (make sure you make the above modifications to the .mor file as well) and the suspended load is set by the .bcc. If you know the inflow concentration you want to set, I would do that through the .bcc file, and set the bed load sed flux to 0 in the .bcm. The exact way you set it shouldn't matter (hopefully!) as long as your domain of interest is sufficiently far from your boundaries. If you've checked all of this and are still getting unexpected results, maybe someone with more knowledge of the inner workings of Delft3D could explain why we are getting different results. 

SS
Samuel Schroers, modified 5 Months ago.

RE: Sediment flux at inflow not matching imposed value in bcc

Youngling Posts: 7 Join Date: 9/29/17 Recent Posts

Thank you for your answer-

I have for testing 2 sediment types (specified in .sed file). In the .bcm file i set one sediment inflow to 0 the other some concentration.

Still i get results for both sediments at the inflow - it seems like no matter what i set as inflow sediment / bedload boundary i get always the equilibrium result for the transport formula chosen.

Anyone knows where is the trick?