Modeling a canal embankment breach

[Thor]

Hello,

I’m currently writing on my master thesis, which is about the risk assessment of the breach of a canal embankment out of the perspective of emergency services. Obviously the first step is to identify the inundation zone.

Canal embankment in my case means an embankment or levee which is necessary to keep water in an inland waterway/canal used for transportation purposes. Those embankments/levees are situated in areas where the surroundings of the inland waterway are below the water-level in the canal. This being said, there are parts of the canal with embankment and parts without embankment. The embankments are located just next to the canal, so there is no real canal/river bank.

The water-level in the canal is almost constant varying about +/- 10 cm. Pump stations control the water-level, which is monitored by a remote control room. Therefor overtopping as failure mode can almost certainly be excluded. In my analysis I assume piping as failure mode.

The canal is divided into multiple reaches by locks. These are used to bridge the elevation gaps. Within the reach the velocity/flow of water is fairly small and mainly influenced by the locking of ships.

After some research I decided on using Hec-RAS, QGIS and RiverGIS for the inundation modeling since they are all open access and have good documentation.

The inundation should be modeled using a 2D-Flow area.

I have an DGM in an original resolution of a 1m-grid. I reduced it to 5m-grid for my tests for better performance. I already found out that I have to “punch out” houses in the DGM (raise the elevation at the location of houses) to model those in an 2D-Flow area.

I now have multiple approaches to develop the necessary model with some thoughts by my side and would be glad for some input/opinion by you.

1. SA/2D-Flow
Model the canal reach by a storage area. Connect it to the 2D-Flow area with the SA/2D connector which is located at the embankment position.
– Due to the characteristics of the canal reach the water-level and elevation are almost the same over the reach. Wind has an influence, but can’t be modeled within Hec-RAS (as far as I know) and assumed to be zero.
– The Canal must be “punched in” in the DGM to be able to use the elevation versus volume curve
– Other embankments than the one breached can be ignored

2. 2D-Flow/2D-Flow
Model the canal reach by a 2D-Flow area and connect it with the SA/2D connector which is located at the embankment position.
– The Canal must be “punched in” in the DGM to be able to model it as a 2D-Flow area
– Information about elevations and velocities within the canal

3. 1D/2D
Model the canal reach using the 1D-Geometry and 2D-Flow area for the inundation zone.
– Lots of additional information like Cross sections, river banks, flow paths etc. needed

In case 2 and 3 I wonder how to model the embankments/levees. I’ve read about levee alignment layers and the lateral structures. I tend to use lateral structures, since levee alignments are restricted in numbers to the banks and cross sections, says the documentation.

And I can’t really see the advantage of modelling the canal with 1D Geometry, since it seems to be a lot more work, but the water is supposed to stay in the canal as long as their is no breach and the water-level will remain constant. So are there any advantages using 1D Geometry for the canal?

I’m looking forward to your opinions and thoughts about my remarks.

Best regards from Germany
Thor

[Anonymous]

I would consider modeling it as a single 2D area. The SA-2D connector can be used inside of the 2D area for the levees and the breach.

This would generally be the most stable and accurate.

The advantage of using a 1D river each or a 1D storage area is that the computer run time might be a little shorter.

There are some other slight differences. For instance, if you use a single 2D area you have to fill the canal at the start of the run. If you use a 1D storage area or two separate 2D areas, you can specify the starting water surface for the canal more easily.

[Thor]

Thank you very much!

After solving a problem with the SA/2D connection between the two 2D areas it worked fine. After a first comparison there is almost no difference between modeling the canal as 2D or SA.

If the run time is the only advantage of using 1D modeling for the canal I don’t see any since the run time in my case still is just 1-2 minutes and the creation of the model data is way faster.

What are the “must-dos” for modeling an urban environment in the 2D area besides to reference the buildings in the terrain layer and having land use data for Manning’s n?

[David Lu]

Dear Experts,
The thing is, how to connect each 2D flow area if I divide a catchment into different 2D flow areas by Thiessen’s Polygon Method and establish the grids to enter gridded precipitation data respectively?

According to my course materials, it suggests that I connect each 2D flow area by hydraulic structure(SA/2D Area Conn). However, will it be unrealistic due to the possible influence of dam on hydraulic property? Furthermore, is hydraulic structure the only way to connect each 2D flow area? Or are there any better methods to precisely connect multiple 2D flow areas?
And every time I use SA/2D Area Conn to connect 2D areas it doesn’t work . I use a simple examlpe to try connect 2d areas and cut profile from terrian.
I would appreciate for your help.

https://drive.google.com/drive/folders/1C512DhJKOxGQ77_QEIHJN-pHu9YNYTMW?usp=sharing

[Author]

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