Bring plenty of light sources including glow sticks or chemical flares for lighting. Bring a few weapons (crowbar come in handy too... not just for opening manholes). Remember that YOU aren't the only one that will have the idea. Your group may end up sharing the same 100 ft or so of culvert with at least one or two other groups. At first, everyone will be too focused on the fallout to consider anything else. However, by days 2 and 3, attention will be on survival supplies and what YOU may have that they don't, including food or water.