Step 10. Waterproof Shelter.
Using a 20 foot by 100 foot sheet of 6 mil black plastic cover the shelter and the trench. The plastic should have a slight bow or at least be flat. The plastic should not have any dips where water could pool. If needed build up low spots with granular soil. Weigh the edges of the black plastic down with granular soil to temporarily hold it in place. Eventually, the entire area of black plastic must be covered with at least 3 feet of granular soil. Around the perimeter of the plastic dig a trench and then dig a connecting trench to drain any run-off from the shelter. (I expect significant storms after the impact event. Granular soil is very permeable to rain water. This step is important to keep the shelter from being flooded).
At this point it is very important to maintain the integrity of the waterproof seal. Restrict walking on the plastic to only when necessary, being very careful not to cut the plastic. You might even walk on the plastic barefooted.
With a knife, carefully cut a penetration holes in the plastic for the two vent pipes, flange bolt holes, and for the shelter entrance.