1st or 3rd type BC using time varying data for coastline boundary?

Hi Users,

Q1: I want to build a 2D (horizontal) and 3D model. One boundary of my model connects with Coastline. The purpose of my research is to assess the changes between the variations of Sea Water Level (Tidal) on ground water level and salt intrusion. Therefore, I suppose that I may have to set up 1st or 3rd type BC for the boundary connecting with the Coastline by using Time varying water level (Series of Tidal level). Is that correct if I use 3rd BC for Coastline boundary condition? How is the type of input data (constant value or Time varying water level values)?
Q2: There is a mountain in the area of my model, but I want to exclude simulating groundwater in that mountainous area. This is because I don’t know precisely stratigraphic information beneath the mountain (lithological components may consist of sedimentary rock or laterite). How can I do so that the Model does not simulate GW flow in the area of mountain?

Thanks for your time,
Lamm

The main difference between a Type 1 and Type 3 boundary is this:
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[li]At a Type 1 boundary, head is specified for those nodes and the modelled heads will comply with specified values exactly. Constant or time varying head values can be specified. A Type 1 boundary could also be called a constant or variable specified head boundary.  This boundary type is typically used where a strong hydraulic control on groundwater levels exists, such as from the sea or a large lake or river.[/li]
[li]At a Type 3 boundary, a target head is specified along with a conductance term which is used to calculate flows to/from the boundary.  Modelled heads at the boundary will not necessarily comply with the BC head values.  Note also the conductance terms in FEFLOW are set as material properties to elements.  Note that a type 3 boundary must be defined across element faces in order to function.  Type 3 boundaries are commonly used to represent streams or small rivers which have weaker hydraulic controls on groundwater and have a limited recharge or discharge capacity (e.g. due to channel dimensions, bed sediment characteristics, water availability).  I suggest referring to the FEFLOW material for a proper description of this BC and its maths.[/li]
[/list]

A general rule of thumb is that Type 1 (specified head) should be used to represent the coastline.

For Q2, could you just limit your model domain to exclude the mountainous area?

Hi Blair,
Thanks you so much.
Q1: I understood that you suggested using 1st BC with constant or time varying values for Coastline. For 2D (Vertical), the head can be set for BC at sea level (h=0) at the top of the model and then be corrected with the depth because of the overlaying denser salt water. In term of 2D (horizontal) and 3D, I suppose that the way to set BC may be different from that of 2D (vertical). Basing on Tidal level, I will create time varying data and input them to every note along the Coastline BC. Do you thing is that possible?

Q2. The mountain is located inside Domain, so I can not limit the Domain of model.  Do you think that I could delete every notes both  inside and boundary of the mountain? By doing this How are the influences on  model results?

Lam

To clarify, I assume you are NOT considering density dependent flow in your 2D (horizontal) and 3D models.  If this is true, then yes, you can simply assign a constant or variable head (1st type BC) to the coastline nodes is appropriate.

Is your mountain completely enclosed by the aquifers to be modelled?  If so, you would need to create a "hole" in your mesh, which I believe is not possible until the next feflow release. Otherwise, you can create an irregular boundary which excludes the region from your domain.

what i he is considering density dependent condition?