2D Hydrodynamic Model to Prevent Overtopping in Dam Construction

Once upon a day, I visited a dam construction in far east Indonesia, namely Way Apu, Buru Island-Maluku Province. Being part of the Dam Evaluation Team during my time at the Ministry of Public Work, Indonesia, opened my eyes to a weakness in dam design. Here is the result of my photos, taken 2 days after overtopping, and I gave a little explanation of the scenery there.

Way Apu Dam Scenery

Yes! it’s very beautiful there, one of the remote islands in Indonesia. Anyway, if you look at it, overtopping flow at some point of the cofferdam can threaten and erode its geometry. Can you guess how much time is wasted and how much loss can be incurred just because the cofferdam breaks? People sometimes forget how important cofferdams are, both temporary and main cofferdam. This is not only the case at Way Apu, but also at Meninting Dam in West Nusa Tenggara and Leuwikeris in West Java.

Then, I proposed the methodology how to choose a cofferdam design based on hydrodynamic model. I choose the 2D model instead of 1D, just because it would be complicated. HEC-RAS nowadays, can do many things such as terrain modification. This is a helpful tool, since we can generate easily in one app (GIS based and hydrodynamic module).

See? it’s so easy to generate a dam terrain. It’s like playing sandcastle by the beach, isn’t it?

You can improve as much as you can to make the model terrain looks reliable presenting the real condition. But the important key for hydraulic engineer, do not forget to talk with you geologist/geotechnical engineer! Why is this important? just because you can do anything on your HEC-RAS, doesn’t mean you forget about the aspect of where your hydraulic structure rests (on a top of soil and rock).

Improve it as much as you can

After finishing all of the set-up model, including the setting of the culvert (which represents the diversion tunnel), you can compute and see what happen. Is your dam safe during construction?

In this case, I have completed run a 2D hydrodynamic model using Kedunglanggar Dam, Central Java, dataset.

Water Surface Elevation shows +129.83 m, while Temporary Cofferdam elevation is +130 m in Q5

Water Surface Elevation shows +131.77 m, while Temporary Cofferdam elevation is +132 m in Q25 discharge

From this model, it shows there is still a problem due to the freeboard distance allowed in Indonesian standard. It should be at least 0.75 m freeboard between highest water elevation and levee or embankment. For this reason, an improvement in the design of cofferdam (both temporary and main) is increasing the height for about 0.6 m or, we can improve and try to model another schematic diversion.

Last Words From Me

I believe this can be a reflection in how dam design should be done using newest technology. You can choose to increase the height of temporary cofferdam or get a new geometry of the diversion tunnel. There is another method, perhaps you can try to figure it out how to maintain the flow, so the river could flow naturally. Please check my latest research about diversion tunnel too, case study: Sawangan Dam Project (Written in Bahasa).

Makalah KNIBB 2024 Pengelakan Bdg Sawangan.pdf

Feel free to discuss!