User Forum of Software BASEMENT

Alyssa

User

From: Aix-en-Provence, France

Registered: 2019-10-28

Posts: 41

Using boundary condition sediment_grain_discharge

Hi

I tried to use the boundary condition sediment_grain_discharge but encountered some problems. As a test I ran my model (multigrain) with transport_capacity for 1 year over a hydrograph, then saved the bedload flux for each grain class for a selected number of flow rates (about 30). Something like this (Q goes up to 3500, and there are 10 grain classes)

Q (m/s)    Bedload_flux_d=0.001_[m3/s]     Bedload_flux_d=0.004_[m3/s]     Bedload_flux_d=0.008_[m3/s]
232.246    3.69E-09                                     2.07E-10                                     1.23E-09
251.582    5.04E-09                                     2.83E-10                                     1.69E-09
292.965    9.80E-09                                     5.50E-10                                     3.28E-09
358.109    2.82E-08                                     1.58E-09                                     9.44E-09

So using this data into the same system with the sediment_grain_discharge, I input the bedload flux for each grain class over time (which I can relate through Q).
My expectation is to achieve something as an equilibrium bed, however the model gives an error: "sediment level exceeds the lower dike in cross section 1 (dikemax: 141.35, z:78610)"

It is as if the sediment is not flowing, although I changed no other setting, and with transport_capacity it worked fine.
My idea is to eventually apply the same sediment_grain_discharge onto a different system (wider channel).

Any ideas why it seems not to work? Did I do something wrong maybe?

Thanks
Alyssa

Matteo Facchini

Developer

From: Trento

Registered: 2014-09-05

Posts: 281

Re: Using boundary condition sediment_grain_discharge

Hi Alyssa,

boundary conditions can be tricky at times. Plus, working with 10 grain classes...you are really stressing BASEMENT

I suggest you to use IOUp instead of transport_capacity to run your initial test. To my experience it is by far the most stable upstream condition for feeding the equilibrium transport capacity. Then you can have a look at the numbers and see if they are similar to the ones given by transport_capacity.

Cheers,
Matteo

Alyssa

User

From: Aix-en-Provence, France

Registered: 2019-10-28

Posts: 41

Re: Using boundary condition sediment_grain_discharge

Hi Matteo

Thanks for your reply. I didn't know IOUp is more stable, that's good to know! However it didn't solve my issue. I tried so many things (less grain classes, simple hydrology,smaller load, fixed first cross section...), and each time the model seems to build up all sediments at the first cross section.
I tried to use this function with only 1 grain size (so automatically, it should relate to the first diameter) and it doesn't work. But when I put the same data for this grain size using the 'sediment_discharge' instead (having a mixture 100% smallest grain size), it does run!
Has the sediment_grain_discharge the same units? m3/s?
It should run the same?
Only when I put an extremely small load it runs (e.g 5e-20 instead of 5e-10), but it doesn't make sense it should be that low. Especially since it works with sediment_discharge.

thanks

Bye
Alyssa

Matteo Facchini

Developer

From: Trento

Registered: 2014-09-05

Posts: 281

Re: Using boundary condition sediment_grain_discharge

As I said boundary conditions can be tricky at times.

Usually I solve the problem of the huge mountain of sediment piling up at the intake by adding a region (~5 times the channel width) upstream of the intake, like a short channel, where I activate (only in that region!) the gravitational transport, which should help transporting the sediment.

As an alternative you can set it on your first cells, feed only water and add sediment as a source instead of a boundary condition.

Cheers

chri_and

User

Registered: 2017-07-27

Posts: 34

Re: Using boundary condition sediment_grain_discharge

Hi

Just a quick question regarding gravitation transport, how can I activate the gravitation transport only in one region? Do I need to define an index only for that reagion and then activate it?

Thanks

Matteo Facchini

Developer

From: Trento

Registered: 2014-09-05

Posts: 281

Re: Using boundary condition sediment_grain_discharge

Hi,

yes, you activate gravitational transport using the MATID of your mesh

cheers
Matteo

Alyssa

User

From: Aix-en-Provence, France

Registered: 2019-10-28

Posts: 41

Re: Using boundary condition sediment_grain_discharge

Hi,

regarding this initial question and response. Does gravitational transport work in 1D? Because my model is in 1D and I couldn't see that option.

Thanks

Matthias Bürgler

Developer

From: Zürich

Registered: 2019-04-04

Posts: 150

Re: Using boundary condition sediment_grain_discharge

Hi Alyssa

The feature for gravitational transport for 1D does not exist so far, but will be included in the next release of BASEMENT version 2.

Best
Matthias