Reddit mentions of Backpacking Tarp. Camping, Picnic & Beach Blanket/Mat. X-Large 85"X58"- Ultralight, Waterproof, Sand Resistant For Hostel, Festival, Park, Outdoor. Includes loops. Multiple colors available.By Meerkat

For this scenario, I'm going to assume that you did not evacuate early, in which case you'd have had plenty of time to pack the car. So we'll say that you stuck around until getting out was no longer really an option and are now being evacced to a shelter. I'm going to start this off assuming that this only entails you and other adults, so if there are kids or pets please include specifics on those (including ages) and I will update for that.

Your BOB should include:

• A backpack, obviously. Large enough to do the job, small enough so that you don't over pack.
  
  
• Food: Three Mainstay 2400 bars which are available at Walmart for $5 each. Maybe pre-purchase one and give it a whirl before getting all three. If the taste isn't right for you there are other varieties like the Datrex bar. These are sealed water tight, last for up to 5 years, and will put up with extreme heat and cold really well. Make sure to put each bar in its own gallon ziploc freezer bag, as once opened the bars inside are not individually wrapped. The purpose here is to not be completely reliant on the shelter providing you food while also not having to worry about cooking/heating anything.
  
  
• Water: Rotate flats of bottled water in your house. In the event that you need to be rescued or bail out to the shelter, get a minimum of 3 liters into your pack. One per day. This way you're not completely reliant on the shelter to provide for you. It would also be a good idea to include a Sawyer Water Filter in case there are problems obtaining water. This is light weight and will allow you to get good drinking water for yourself from the bad if the fresh water situation does not dissipate quickly.
  
  
• Clothing: Have a change of clothes. Specifically, maybe even a couple of changes of underwear and socks. You'll really appreciate both. If you can, get it all in waterproof bags. If not, make sure the socks and underwear at least are dry. Preferably wool socks if you have them.
  
  
• Sanitation: Have your own roll of TP in another gallon sized freezer ziploc baggie. You don't know how much or if the place you go will have any. Also, tote along an appropriate amount of hand sanitizer. Not just for the bathroom, but for any time you interact with flood waters. I have disposable food gloves in my kitchen, and I'd probably consider tossing 3-4 pair in my bag. They're low weight and keep your hands clean if you end up doing some dirty work. Then... throw a stick of deoderant in your bag. That's not a necessity, but there's no reason to add more stink to what will likely already be a stinky environment. Consider also having some wet wipes in the kit to freshen up a bit.
  
  
• Health: You don't need a full med kit exactly, but have some ibuprofen or a pain reliever of your choice in fairly good amounts (others will almost definitely ask for some), some pepto tablets to either stop upset stomachs or to just back things up for a while if you don't want to use the facilities where you're staying, a small tube of antiseptic cream and band-aids for small wounds. With flood waters being filthy, you'll want to be able to treat minor cuts and scratches before they can become a real problem. Larger wounds you'll likely be able to get real medical treatment for, so you can just plan to take care of the small stuff yourself. Add in any medications that are specific to your situation as well.
  
  
• Documents: Have copies of documents that prove who you are and what you own. Have copies of insurance papers. Most importantly, have a list of phone numbers to people and companies that you would want to have in this situation. There is no guarantee that your cell phone will either survive or have a charge, so having a paper copy is crucial. Obviously you'll want to protect the more sensitive bits of info.
  
  
• Light: Have your own means of generating light. Invest in a decent 'waterproof' flashlight of some type. Here's a decent pair that won't break the bank but will give you some redundancy.
  
  Then there's comfort stuff. This one is a biggie and parts of it are largely optional, so I'll break down into its own section and you can pick what sounds best.
  
  
• First, make sure to have a charger for your phone, but if you want to be a really clever monkey then also include something that will turn one electrical outlet into many. There's near a 100% chance that someone else will have their charger plugged into an outlet with a line behind them. If you want to be sure of charging yours, bring an electrical splitter of some sort. You'll be a mini-hero to a bunch of folks, but more importantly you'll get to charge your phone. Write your name or initials on the splitter as well, just so you can prove it to be yours when you eventually need to take it back.
  
  
• Then also, you'll want to plan to take care of yourself for sleeping. A compact, light weight, and water proof blanket may well be useful for keeping you dry. Then you'll also likely want a compact sleeping bag, or maybe even one for two if you're with someone. Something small to port around, but that will give you a real blanket to sleep with. Then... there's no guarantee of a cot, so having a compact sleeping mat would probably be a smart call as well. You don't want to sleep directly on concrete if you can avoid it. For extra fanciness, don't forget a pillow, though you could probably make do without one.
  
  
• You might also want some ear plugs, as any shelter will be noisy... and people snore. Obviously sleeping with these in presents some dangers of not hearing what's going on around you, so use your judgement based on how safe you feel.
  
  
• Have cash, in small bills. Nothing higher than a 20, though plenty in smaller. Chances are atm's and credit card machines will not work, so you'll have what you have.
  
  
• Have some non digital means of entertainment. Maybe that's a book or two (though a water proofed kindle would probably last plenty long as well), or a deck of cards, or a compact chess board, or whatever tickles your fancy. Just don't rely completely on your phone for entertainment. It may be damaged or need a charge.
  
  
• Consider grabbing a camping tooth brush and a small tube of toothpaste. Sometimes it's the small things that make you feel human again. Might as well add in a small bar of soap as well in case you get a chance to take a 'sink bath'.
  
  
• Lastly, if you want to be a king of comfort in your new domain, you'll need a throne. Nothing special. Just a small back packable chair, but in a crowded shelter there might not be a place to sit other than the floor, so if you really want to splurge on this bag it's worth considering. Again, initial it so that you can later prove ownership.
  
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  That's the list off of the top of my head. You can easily stop at just the necessities, or go all in and plan to be comfortable in the situation you think you're most likely to face. Hope that helps.