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Guest Daniel Botelho

Bed sediment profile warm-up

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Guest Daniel Botelho

How can I generate initial conditions for my bed profile without measurements?

 

Problem:  Sometimes we need to model suspended sediments (TSS) but do not have any information about the sediment bed profile.  When we initialise the model with a zero mass in the bed, more often than not we cannot reproduce the required amount of re-suspension to obtain the TSS concentrations given by the field data.  On the other hand, if we set the bed mass to too high values, we may have too much re-suspension, resulting in excessive TSS concentrations.  So, how can we set up a bed profile that can reproduce realistic TSS concentrations in the water column? 

 

Solution:  We have obtained some success following the procedure below.  It has been working for tidal estuarine flows, and coastal areas, but it should work for rivers, lakes and reservoirs as well.  The procedure aims at establishing zones of erosion and accretion taking into consideration the model calculations alone.

 

1-      Preparing a warmup bed:

a.       First we assume that you will be familiar with the commands used in the suspended sediment simulations. We also assume you will be simulating only one fine sediment fraction.  If not, just extend the method to the number of sediment fractions you intend to model;

 

b.      You will be running the model just for salinity (if part of your set-up), temperature (if part of your set-up), and suspended sediments for at least two years and assume you have at least some six months of TSS data to calibrate the model against.

 

c.       In your sediment control file ensure you add the following lines:

 

morfac == 10.0

morphological coupling == 0

 

In a nutshell, the first line tries to scale up the sediment dynamics 10 times.  So, simulating two years is effectively running the model over 20 years with the hydrodynamic conditions of the 2-year simulation period.  Morphological coupling set to zero does not impose changes in bed elevation.

 

d.      Assign a rough initial distribution of the initial bed mass of sediments throughout the domain.  This is easily done with the use of materials.  In areas that would generally be more susceptible to erosion and are known to have a rocky bed set the initial mass to zero. Others areas will be prone to accretion and will have softer sediments, set the initial mass to say 1.0 m thickness for these areas.  Work out the thickness by dividing mass by rhodry.

 

e.      There are other ways (using matlab scripts) to create a hot bed if you have data to do so but  It is most likely you will not.  Please contact TUFLOW support if you believe you could benefit from the matlab scripts.

 

f.        Play with your erosion rate pick-up parameter (the first parameter in the line "erosion rate params == 0.15, 1.0") until you start obtaining reasonably high TSS levels in the water column (i.e. high values during spring tides).

 

g.       Ensure tauce is larger than taucd’s.  These values for critical shear stresses are between 0.10 and 0.15 N/m2, not much less or more than these.

 

h.      Once you are happy with the high TSS levels, play with the settling rate to adjust the lower values in your TSS signal.  For example, if your TSS does not drop to background levels, increase the setlling velocity.  If they drop too much, reduce the settling velocity.

 

i.         Do not worry about results in the first stages of the simulation.  In fact, look only at the final six months as these would have been warmed-up sufficiently.  Also, do not worry too much about getting a dot-by-dot agreement.  You are just warming up the bed.

 

j.        Once you are sufficiently happy with the results, set aside the *_bed.rst file at the end of the simulation.

 

2-      Run your calibration, starting from the warmed up bed *.rst file.

 

a.      First, ensure the following lines below are in your sediment command file:

         morfac == 0.0

                     morphological coupling == 0

                     restart == .\log\”MY_WARM_BED”_bed.rst

 

b.      Adjust your parameters slightly from what you did in the generation of the warm bed.  If you need a lot of adjustment, I suggest you carry out the procedure in 1- again.

 

Once again, we have been successful using this procedure, obtaining good agreement between simulations and field data. Hope this helps!

 

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