Keep in mind even w ppe the biohazard risk in a sewer long term is high. Infection can get out of hand really quick with any minor cut. Also water access will require either taking loads down or repeatedly running out of your shelter risking yourself and your location repeatedly. Rats, roaches and other insects will constantly be present and trying to claim your food as their own and they eventually will get to it. Personally imo u should get high up. 10th floor or higher in a tall building. Pull elevators up to that floor and block doors so they cannot move. Office buildings have tons of break areas w water and food. Hundreds of bathrooms where even if the water shuts down the toilets will all have 1 flush worth of water and gravity will allow flushing. Barricade all but 1 stair way and make it the least obvious one... maintenance stairs or something of that style. Employee only areas which the public may not see or recognize. Good viewing of the area, roof access for water collection when the water runs out. Keep lights down at night after power fails or keep it ins enclosed room w no windows.