Migration status

  • Home Page: Activity map.

DELWAQ

DELWAQ is the engine of the D-Water Quality and D-Ecology programmes of the Delft3D suite. It is based on a rich library from which relevant substances and processes can be selected to quickly put water and sediment quality models together.

The processes library covers many aspects of water quality and ecology, from basic tracers, dissolved oxygen, nutrients, organic matter, inorganic suspended matter, heavy metals, bacteria and organic micro-pollutants, to complex algae and macrophyte dynamics. High performance solvers enable the simulation of long periods, often required to capture the full cycles of the processes being modelled.

The finite volume approach underlying DELWAQ allows it to be coupled to both the structured grid hydrodynamics of the current Delft3D-FLOW engine and the upcoming D-Flow Flexible Mesh engine (1D-2D-3D) of the Delft3D Flexible Mesh Suite (or even other models such as TELEMAC).

'DELWAQ in open source' is our invitation to all leading experts to collaborate in further development and research in the field of water quality, ecology and morphology using Delft3D. Feel free to post your DELWAQ related questions or comments in this dedicated forum space. If you are new to DELWAQ, the tutorial (in the user manual) is a good place to start. A list of DELWAQ related publications is available here.

** PLEASE TAG YOUR POST! **

 

 

Sub groups
D-Flow Flexible Mesh
DELWAQ

Cohesive sediments & muddy systems

 


Back

delwaq time-step relative to telemac timestep

SR
Shivanesh Rao, modified 1 Month ago.

delwaq time-step relative to telemac timestep

Youngling Posts: 3 Join Date: 12/10/18 Recent Posts

Hi all,

I've been handed a delwaq model (forced by 1-hourly hydrodynamics from telemac3D via the respective *.dwq files) that runs at 5 min time-step. The model domain is a lake with a tidal inlet and several freshwater inflow locations.

I noticed that the continuity of the model output (in *.ada) varies between 0.7-1.1. Some areas of the domain drop to 0.7, while most of the domain remains around 0.9, and peaks to 1.1 at the tidal inlet of the lake. The model uses integration scheme, 15.70.

**The reason of checking this is: a tracer plume (dTr) appears to advect at ~1m/s, while the telemac hydro output (from the *.slf) only shows maximum magnitudes of 0.25m/s.***

 

I reduced the delwaq time-step from 5mins to 30s, in an attempt to see if the continuity improved, but there is no improvement.

I'm just wondering if there is a limit on the ratio between the delwaq:telemac timesteps, beyond which the continuity will not be equal to one. Right now, my ratio is 5:60 (1/12) and I have done 0.5:60 (1/120).

Is it best to remain close to 1/4 (or something like that)? i.e., have finer temporal hydrodynamic input (15 mins instead of hourly).

Any insight would be appreciated.

Cheers,

Shiv

PS: I ask here because each run takes 2 weeks to finish, so I figured to tap into the user-knowledge base first to save time.

AM
Arjen Markus, modified 1 Month ago.

RE: delwaq time-step relative to telemac timestep

Jedi Knight Posts: 181 Join Date: 1/26/11 Recent Posts

The schemes used by DELWAQ are only mass-conserving when the underlying hydrodynamics is volume-conserving. The deviations you report are far too large to be comfortable. Something is wrong here and that cannot be helped by a smaller DELWAQ timestep. (In case you are wondering: the procedure used by DELWAQ is volume-conserving - the flow is kept constant between hydrodynamic timesteps and the volume is interpolated linearly.)

You mention that the run takes two weeks. This indicates a very large model. Could you describe it in some more detail? How many grid cells, how large are the grid cells? That sort of things. I am also surprised by the difference in advection/flow velocity - I have no idea where that is coming from. How did you find that out?

 

SR
Shivanesh Rao, modified 1 Month ago.

RE: delwaq time-step relative to telemac timestep

Youngling Posts: 3 Join Date: 12/10/18 Recent Posts

Dear Arjen,

 

**What the model is ***

The hydrodynamic model is 32734 nodes x 10 sigma layers. The smallest grid size in the shallower areas is ~30 m expanding to ~300 m in the center of the lake. The model simulates Lake Macquarie in Australia. The shallow areas are around 1 m deep (we have wetting and drying) and the deep areas are in the range of ~20-30m. The original time-step of the DELWAQ was 5 mins. The model runs for 3 months (using dt=5 mins) in wall-time of 6 days. Once the dt drops to 30s, the wall-time blows up to 2+weeks. This is done using 12 cores in a older HP Z800 desktop.The DELWAQ model doesn't aggregate in time or space in this case.

 

**How we figured the advection overestimate**

In DELWAQ (or is it D-water), we have 35 variables being simulated (NO3, etc), and 5 of them are tracers (decaying). When we plotted 1 of the tracers, we found that the tracer plume advects a long distance (3km) (See Fig 1a & 1b) in an hour (~1m/s). Half hourly outputs also show the same behaviour. This is a relatively large speed and much larger than usually observed for this region. Similar behaviour happens with other tracers (but not as intense, i.e., small fast moving plume).

I plotted the surface velocity from the telemac output (*.slf) and the speeds at these times are in the range of 0.2-0.3 m/s. Of course, DELWAQ doesn't use the *.slf hydro files as input, but rather their equivalent *.dwq. I haven't been able to open and read this file yet to confirm that the behaviour is not hydro forced (but I checked the *.slf and *.dwq were created the same time, so created by the same telemac run).

Do you have a matlab/python/fortran script that would read the *.dwq files?

 

The other behaviour is the tracer plume advects almost exclusively through the surface layer. See Fig 1b & 2, which are the top and the layer below the top layer respectively. Each layer is ~0.1 fraction of depth. The surface plume covers ~3km while the layer just below barely moves 600m...this region is not known for strong stratification.

 

These odd behaviours led me to check the continuity output, which were !=1 for most of the domain. I've attached the *.inp file for reference. The scheme is 15.70.

 

I appreciate the help.

Shiv

AM
Arjen Markus, modified 1 Month ago.

RE: delwaq time-step relative to telemac timestep

Jedi Knight Posts: 181 Join Date: 1/26/11 Recent Posts

Let me start with the problem that the tracer is mainly present in the surface layer:

The vertical diffusion is set to 1.0e-7 m2/s - if there is no vertical flow, then the timescale this diffusion causes is around 10000 days - depth**2/ diffusion. So vertical mixing is a matter of vertical flow. You may want to put in a larger diffusion coefficient.

The integration method, 15, is an implicit method. This means that - in principle - substance can travel from one side of the model area to the other side in one single step. Of course the concentrations will be very small if it is controlled by flow (instead of diffusion/dispersion) but still. It simply means that the fast transport you see may be due to this and you need a different method to estimate the travel time. Try a calculation with cTr1 and dTr1 and the Age process turned on. You only need a few time steps and study the "age" output parameter for this.

The continuity is a mystery to me. Quite odd :(.

As for the .dwq files:

These files are "unformatted, big-endian" files, the record structure is quite simple: the first number is an integer that represents the time in seconds (since the reference time - T0) and the rest are the N volumes or the M flows or areas - N is the total number of segments, M is the total number of exchanges.

Fortran code (using Intel Fortran, because of the big-endianness):

open( 10, file = '...', form = 'unformatted', convert = 'big_endian' )

do

     read( 10, iostat = ierr ) itime, flow  ! or volume

     if ( ierr /= 0 ) exit

enddo

 

SR
Shivanesh Rao, modified 1 Month ago.

RE: delwaq time-step relative to telemac timestep

Youngling Posts: 3 Join Date: 12/10/18 Recent Posts

Thanks Arjen. I'll do these over the break and get back to the forum to close the discussion loop.

 

Cheers,

Shiv