I believe the best bet would be swapping rather small cache boxes with friends around the city or/and renting some storage space for that (could be shared with friends as well), let's say along the pre-selected bug out route just out of the city. Everything else will be eventually digged out. In California, we have a lot of national parks and general wilderness closeby enough. That's the natural bug out destination for many. I know some people making cashes in the woods (Tahoe National Forest, north from I-80) using large sealed plastic barrels. However such caches are periodically being found due to the erosion caused by rivers exposing and/or destroying the buried stuff.