Flotation calcs. 1

I think that your starting point is the design life of your structure. If your structure is designed to operate for (say) 60 years, you don't necessarily need to design for a 1 in 60 year flood event. You need to perform some form of risk analysis to this in conjunction with your client and ask the client what risks he/she is prepared to accept in terms of the structure becoming buoyant.

Once the flood return period is set (say 1 in 30 year flood) then you will have your design water level from your hydrograph. The minimum factor of safety you need to prevent buoyancy at this floodwater level is 1.1. This may seem low, but water is a known fluid of specific density (which changes very little). Also, note that 1.1 is a MINIMUM factor of safety.

Regards Andy Machon

If there is no water expected, design for water 1 m above bottom of structure, or 1/3 of depth below ground surface, which ever is worse.
If there is any water expected, design for water 1/4 of depth of structure below ground surface.
Water may come from burst mains, climatic change etc., so design for the "water expected" situation, or provide external drainage system. Alternatively advise the client against having a basement.

If designing a bridge structure in a rural situation please keep in mind that it is acceptable for the structure to be overtopped, especially if the grade of the existing is not to be improved much (i.e., improving the grade of the roadway will cost much, much more than is available). In designing for this case provisions must be made to vent the structure and resist uplift.

Gglass,

Flood reccurence of say 1:5 is okay. The water table fluctuation will allow u to fix the maximum level. Piezometer can be installed on site for monitoring this.

Depending on the location where u intend/propose to carry out construction, your local meteorolgical dept. will give u sufficient information on flooding propensity.

Water table during any flood allows for 100% saturation of the soil. But water above the ground level assists.
Factor of safety for buoyancy check should be taken as 2.
This takes into account probable rise in gwt.

Resisting forces against vertical upthrust is the weight of the building. For small structures this is sometimes a problem. The upthrust is actually surface area of the building that interfaces with the ground X the depth of maximum water table X 10kN/m3.

For basements, provision of relief drains become important if it has slab on grade with joints.

(Also look for articles or reference on pore water dissipation.)