Modelling a breakwater on bouss2d

[Benja]

Hello everyone,

I've been trying to run a model with a breakwater on it, but the results I have make me suspicious.

I represented the breakwater as a part of the bathymetry and I used a porous layer around it (n=0.4), in the interface between the upper part of the breakwater and the submerged one. The width of this porous layer is equal to that used by the slope of the breakwater. The average water depth in this region is 20 meters, and the wave field has an Hs=1.0m and Tp=15s.

I've used the model in natural bathymetries and have had good and reasonable results, but in the case of this breakwater an standing wave appears in the side of the breakwater that faces the waves, that have 4 or 5 meters of amplitude and therefore couldn't be physically possible.

My questions, finally, are:

1. is this the right way to represent an emerging breakwater, or is there any other? because I couldn't find the answer neither on the documentation of the software, or in the examples, or in this forum

2. could this standing waves be there for any other boundary condition? In fact the wave maker is in the grid boundary and I don't have a damping region behind it... should I use one? is that the real cause or should I pay more attention to the way of representing the breakwater?

that was a lot more than 2 questions...

thanks in advance to whoever could (and wanted) to help me

regards,

Benjamín

Edited August 10, 2009 by Benja

[BCGhydraulics]

I'm having the same problem. Have you ever figured out anything regarding this issue? Simulating a submerged porous structure is OK, but whenever I need to have a surface-piercing structure, I can't get reasonable results. - John

Hello everyone,

I've been trying to run a model with a breakwater on it, but the results I have make me suspicious.

I represented the breakwater as a part of the bathymetry and I used a porous layer around it (n=0.4), in the interface between the upper part of the breakwater and the submerged one. The width of this porous layer is equal to that used by the slope of the breakwater. The average water depth in this region is 20 meters, and the wave field has an Hs=1.0m and Tp=15s.

I've used the model in natural bathymetries and have had good and reasonable results, but in the case of this breakwater an standing wave appears in the side of the breakwater that faces the waves, that have 4 or 5 meters of amplitude and therefore couldn't be physically possible.

My questions, finally, are:

1. is this the right way to represent an emerging breakwater, or is there any other? because I couldn't find the answer neither on the documentation of the software, or in the examples, or in this forum

2. could this standing waves be there for any other boundary condition? In fact the wave maker is in the grid boundary and I don't have a damping region behind it... should I use one? is that the real cause or should I pay more attention to the way of representing the breakwater?

that was a lot more than 2 questions...

thanks in advance to whoever could (and wanted) to help me

regards,

Benjamín

[Joel]

Hi John,

We work extensively with Breakwaters in Bouss2D. I'm not clear where your problem lies, so I will respond to the original queries from Benjamin:

1) The BOUSS2D manual and associated technical notes indicate that porosity layers can be used to simulate breakwater effects, however we have had limited success with this - note our work is predominantly with rubble mound breakwaters. We generally simulate breakwater effects using damping strings - the manual and Bouss2D page on the SMS Wiki (http://www.xmswiki.com/xms/SMS:BOUSS-2D) provide a chart which can be used to estimate the required damping parameters based on a desired reflection co-efficient for the breawater. Be careful using damping strings in smale scale applications (e.g. entrances to small harbours/inlets where damping strings have the potential to reduce the effective entrance width).

2) These standing wave effects occur when waves reflected form the structure interact with the incoming wave train. It is important to note that these standing wave effects do occur in reality (this effect is known as clapotis http://en.wikipedia.org/wiki/Clapotis). If your wave maker is too close to the breakwater, or if your reflection coefficient for the breakwater is too high (see number 1 above), these effects may become amplified in a non-realistic manner (this may also cause the model to crash). Some methods to avoid this include:

- use realistic reflection co-efficients

- keep your wavemaker at least 4 wavelengths (rule of thumb) from any structures in your domain

- consider using an internal wavemaker with damping on the seaward boundary

- if you are not concerned with the region in between the wave maker and the seaward side of the breakwater, consider applying some strong damping to stop the formation of standing waves.

As noted above, there is some good info on the Wiki regarding these issues (http://www.xmswiki.com/xms/SMS:BOUSS-2D). As a word of caution, remember that these standing wave effects are real before trying to eliminate them. In our work, we sometimes have to carry out additional post processing and analysis to separate the incoming and reflected wave trains.

Regards

Joel

JFA Consultants

[Benja]

Hi everyone,

Despite it's been a while since I started this topic, I think it can always be helpful to share experiences.

Finally, after a long try and error process, I think we have achieved a reasonable technique.

To my understanding, the best way to represent a breakwater is as part of the bathymetry, i.e. as an abrupt change of bottoms elevation. Nevertheless, this may lead to numerical problems, since the boussinesq equations that SMS solves, were derived using the assumption of a mild slope bottom. Anyway, this problem can be addressed by putting a typical coastline damping layer.

With this, I don't take into account the effects of porosity from the wall of the breakwater, since they are one order of magnitude less than thos due to bathymetric change.

The reason why I, personally, prefer this approach instead of putting a purely damping layer, lies in the numerical tuning that is required to set it up, tuning that can affect greatly the results.

I hope this can be helpful.

Saludos,

Benjamin Carrion

Ingeniero Hidraulico

Hi John,

We work extensively with Breakwaters in Bouss2D. I'm not clear where your problem lies, so I will respond to the original queries from Benjamin:

1) The BOUSS2D manual and associated technical notes indicate that porosity layers can be used to simulate breakwater effects, however we have had limited success with this - note our work is predominantly with rubble mound breakwaters. We generally simulate breakwater effects using damping strings - the manual and Bouss2D page on the SMS Wiki (http://www.xmswiki.com/xms/SMS:BOUSS-2D) provide a chart which can be used to estimate the required damping parameters based on a desired reflection co-efficient for the breawater. Be careful using damping strings in smale scale applications (e.g. entrances to small harbours/inlets where damping strings have the potential to reduce the effective entrance width).

2) These standing wave effects occur when waves reflected form the structure interact with the incoming wave train. It is important to note that these standing wave effects do occur in reality (this effect is known as clapotis http://en.wikipedia.org/wiki/Clapotis). If your wave maker is too close to the breakwater, or if your reflection coefficient for the breakwater is too high (see number 1 above), these effects may become amplified in a non-realistic manner (this may also cause the model to crash). Some methods to avoid this include:

- use realistic reflection co-efficients

- keep your wavemaker at least 4 wavelengths (rule of thumb) from any structures in your domain

- consider using an internal wavemaker with damping on the seaward boundary

- if you are not concerned with the region in between the wave maker and the seaward side of the breakwater, consider applying some strong damping to stop the formation of standing waves.

As noted above, there is some good info on the Wiki regarding these issues (http://www.xmswiki.com/xms/SMS:BOUSS-2D). As a word of caution, remember that these standing wave effects are real before trying to eliminate them. In our work, we sometimes have to carry out additional post processing and analysis to separate the incoming and reflected wave trains.

Regards

Joel

JFA Consultants