## Forum

### Sill simulation

U

#### Sill simulation

Posts: 1

I am simulating a sill downstream bridge piers in 2DH simulation with crest level of the undisturbed bed.

I simulated this sill in three different ways

1- by fixing the cells in the cross-section of the weir (no erosion to the cell) 2- using a weir (2D weir model) 3- using a local weir

Most of this sill is under the bed in the cross-section of the river as I simulated it in a cross-section where a central bar in the middle and scour on both sides to study its effect in mitigating the bars formed and redistribute the bed.

But there is something strange to me. In some simulations most part of the weir is covered under the bed and however that there is a water head of around 3 cm away from the bridge piers effect. I notice this when I simulated it as a 2D weir and a local weir as well.Only local weir simulation gives me satisfying results in 2D but when I fixed the bed or using a local weir it did not give me good results was thinking about the shortcoming of using these structures because they mentioned in the manual they are simulated by overall influence by the energy losses due to the weir. These energy losses are described as an additional quadratic friction term in the momentum equations.

Any recommendations?

#### RE: Sill simulation

Famous Fortune teller Posts: 48 Join Date: 4/24/20
Hi, I have a few questions that would help me understand what the problem is. First, I guess you are using Delft3D-4 (curvilinear grid). I understand that your intention is to simulate morphodynamic changes. I understand that you intend to model the kind of structures constructed dowsntream from bridges for the sake of preventing erosion, in this manner protecting the bridge foundation. How do you fix the bed? (option 1). You mention that you find "something strange", but what is strange is not clear to me. Part of the weir is below the bed level (but part is not?) and you find an effect of the weir upstream. When you find strange results is unclear to me. Using local weir seems to give you both satisfaying and unsatisfying results. The first thing to realize is that hydraulic structures do not prevent passage of sediment. Worded differently, structures do not stop sediment from passing it. Bed level changes depend on the divergence of the sediment transport rate (i.e., on the divergence of the velocity field), which is certainly affected by the presence of structures, but structures will not stop sediment (if they do not stop the flow). Regards, V
U

#### RE: Sill simulation

Posts: 1
I used Delft 3D-4 as you mentioned. I fixed the bed by selecting the cells where the sill exists and give it a sediment layer thickness with a value of zero.Thank you for your elaboration at the end of your comment i think it is helpful to me.

#### RE: Sill simulation

Famous Fortune teller Posts: 48 Join Date: 4/24/20
Hi, One important thing. By setting the thickness of sediment to 0 you prevent erosion but not deposition. Regards, V