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

 

 

Message Boards

Mass balance problem

IP
Irene Palazzoli, modified 3 Years ago.

Mass balance problem

Youngling Posts: 3 Join Date: 2/16/16 Recent Posts
Hello everyone,

I am using Delft3D Flow to simulate sediment concentration under the influence of tide in a system of shallow bays.
The boundary conditions in the model are water level, while in the initial conditions I set a constant concentration of mud inside the lagoon. Moreover, I am considering only the sediment deposition, so I imposed no erosion in the model (a critical bed shear stress for erosion equal to 100 and the erosion parameter equal to 0).

Once the simulation was done, I checked the mass balance inside the entire domain, but it doesn't work. The difference between the initial mass of sediment and the sum of suspended and deposited sediment is not equal to 0, it increases right after the first time step and at a certain point it becomes negative. I made a lot of attempts, changing some parameters in the model, such as the Foster filter, the morphological factor (equal to 1 and greater than 1), but it didn't change the result.

I evaluated the mass of the suspended sediment as the product of the suspended concentration and the volume of water (= water depth times the grid cell area). The mass of deposited sediment was given by the product of the volume of deposited sediment (the height of sediment at the bottom and the grid cell area) and the dry bed density.

I don't understand what is missing in my bass balance and why I get large differences in these values.

Thank you in advance for your help!

Irene
Richard Measures, modified 3 Years ago.

RE: Mass balance problem

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

I had one thought regarding this: Are you including sediment which leaves or enters through your boundary condition in your mass balance. I.e. if water containing sediment is leaving through your tidal boundary then this will represent a loss to the system and vice versa. If your mass balance is tidally varying then this could be the cause?

Cheers,
Richard
IP
Irene Palazzoli, modified 3 Years ago.

RE: Mass balance problem

Youngling Posts: 3 Join Date: 2/16/16 Recent Posts
Hi Richard,

thank you for your answer. You are right, there was a loss of sediment through the open boundaries of the domain. In order to reduce the amount of sediment loss, I increased the dimension of the grid. Now the loss of sediment is very low, so this issue is negligible.
However, now the sum of the suspended and deposited cohesive sediment exceeds the initial concentration inside the lagoon. What could be the cause of this result?

Thank you for your support,

Irene