I am also an engineer and working in flood risk, going travelling in 2015 - to similar places and also wanting to volunteer. I am 26 now, 27 when I go and so will be 28 when I return. I am on a temporary contract now, but they say easier to get a job when in a job... BUT hey! I want to go travelling, and I can worry about a job on my return, just keep the contacts close to hand. This is my plan.
I work in flood risk management at the moment and as well as my engineering degree I have a Geography degree. I have also wanted to travel and see the world but never had the opportunity as I have been working to get a decent job and credibility. I believe I have that now, and if I don't go now, I wont!
So I say, just do it! You can worry about that on your return. I have taken up a pub job now, a) to save money but b) because when I come back, whilst I am settling back in to routine I can work there rather than nothing whilst applying for flood risk jobs. I am hoping to volunteer in flooding countries whilst travelling that will be valuable on my CV. I would hope I would be ok on my return and you should be too.
If it makes you guys feel better, i was in a similar position a few months ago (project controls in construction) and have been travelling since July. I've been emailed several times from my previous employer asking about when I'm returning so they can slot me in for a project at that time. I've yet to give them one, and have been gone for three months at this point. I don't regret the decision to pack up my apartment and leave on contract completion for a second. (I'm 26 as well)
I'll be a party pooper. Don't do it until you've built up a pretty good retirement nest egg. I'm in my late 30's and recently went back to work after a 10 month break. Prior to the break, I had been working in software engineering since college. I was with my previous company long enough where I got five weeks of vacation a year. Couple that with requesting time off without pay and you could make it work while taking advantage of the one thing you won't get back which is time to build your nest egg. The earlier you start and contribute, the more time you have for that money to compound and grow.
Also, the older you get, the more life outside your job consumes you - a significant other + kids and many friends accumulated over the years means you will want to spend less time working and more time with them.
I'm an avid traveler and have been since college. My job was flexible enough to allow me to work remotely or tack on vacation time to overseas work trips to explore even more.