What general specifications are there in building a storm shelter in the American Midwest? I believe the major considerations are tornados and severe thunderstorms, as well as flooding along river valleys.
FEMA (Federal Emergency Management Agency) publishes some specifications for the design of safe rooms.
Per their guidance:
And according to their FAQ, there are restrictions regarding potential flood risks.
Per FEMA P-361, flood hazards should be considered when designing a residential safe room. Flood loads acting on a structure containing a safe room are strongly influenced by the structure’s location relative to the flood source. Tornado or hurricane residential safe rooms should be located outside of the following high-risk flood hazard areas:
- Flood hazard areas subject to high-velocity wave action (Zone V areas) and Coastal A Zones
- Any areas subject to storm surge inundation associated with any modeled hurricane category, including coastal wave effects
More information on these siting restrictions can be found in FEMA P-361.
You should also look at their answer regarding other applicable standards:
FEMA P-361 provides the design criteria used with common building codes and standards to design a building. This means that the underlying building code (such as the International Building Code or International Residential Code) applies and building construction must comply with the many items that are regularly governed by code requirements. Standards such as ASCE 7 also apply but are used in conjunction with the safe room design criteria described in FEMA P-361 to produce a structure capable of resisting loads much higher than those for normal buildings.
So you are correct that the major considerations are tornadoes and severe thunderstorms. Their considerations for flooding are fairly common sense as they can be summarized with "don't build where it will flood", but they certainly provide more guidance than that.
It's also worth noting that the guidelines are applicable outside of the North American Midwest.