It would seem that, something you are going to spend so much time and effort on, should develop out of your own experiences and interests. You might be able to get better advice and suggestions if you shared a little more about your experience and interest. What have you been doing with your studies? Is there a subject of your studies where you are getting good grades because you are interested and motivated to work on? Is there something outside of school that draws you to it? do you read about it? do you research it in other ways?
You have been thinking for a whole week?? Do you have an advisor for your thesis? You would probably benefit from talking to him/her since they (hopefully) know you and your interests. In my opinion, selecting a topic should be the easiest part since only you know what topics interest you or are passionate about.
Apparently you are an engineer hoping to have a job in engineering some day. Why not check with a likely possible employer and ask them what problems they may be having with some of their jobs that might take some investigating. A hands on study to the be written up will go a long ways in your future. Don't short cut the length of the study.
Why don't you do your thesis on picking out a thesis subject? You can discuss different methods such as on line boards, like this one, talking to friends, and asking strangers.
If you go into Hydraulics, your career will be very fluid, not as stable as structures, or as dirty as soil mechanics. That is my thesis suggestion. Just expand on it.
Your interests seem to align nicely with dams. There are many current research needs. Just google, and you will find documents listing research needs from FEMA, USACE, USBUR, ASDSO, etc. What do do about aging concrete dams, how to quickly deal with unusual seepage in embankments, what is society's tolerance for risk, to name a few.
There is an ongoing problem with highway rigid pavements that may still need some research and maybe more of an education for engineers, who knows. The problem is "pumping". That is the ejection of water and materials from under the slabs at the joint in wet weather. I did a brief review of some references that came up under Google search. Not one of these references dealt with the role that base course might play in keeping the "materials" from going up and out along with the water. I do know that at least Wisconsin at one time would build base courses upside down. That is, they were designed to filter out fines from soil below as well as filtering out the fines from the lower zone from migrating up and out with the water. Since this construction is more complicated than the usual, well maybe it has been dropped. In any case, examining these cases out in the field in many states that may have some form of filtered base courses might be quite useful for educating many. It would mean some field time rather then just a lot of library time. Cooperation of highway agencies would be needed and one might be surprised at how cooperative they can be. For my Master's thesis way back in 1950's the county highway departments were great to work with. That also gave me a leg up when it came to getting a job.
