In THE PERFECT STORM: The Science Behind Subrogating Catastrophic Flood Losses, hydrology expert witness Richard Van Bruggen writes on hydrologic simulations and modeling:
Frequently, even a storm event of historic proportions might not have caused damage to your insured’s property had it not been for a specific existing condition, such as a levy in disrepair, clogged sewer drains, culverts in need of maintenance, malfunctioning flap valves, etc. While it is easy to show that a drain was not kept clean or that a culvert was left in a clogged condition, it is another thing altogether to prove to a jury that the condition actually caused the flood damage for which the insurance company has paid and you are now subrogating. A flood level of two feet might require some cosmetic cleanup and minor repairs to a fleet of stored vehicles. Six inches higher and you could be looking at crushing all of the cars. This is where modeling becomes indispensable.
There are computer models that use rainfall depth-duration-frequency data and watershed characteristics, such as the Time of Concentration, in order to develop peak flows (Qs). It is usually the case that stream flow gage data is either of a short time record or is unavailable altogether, whereas rain gages are more plentiful and typically have longer periods of record. It is by way of a Hydrologic Analysis that we determine what the design flows are for storm drain systems, bridges and culverts.