Whether inland or coastal, flood is a very complex hazard. No other natural hazard comes in as many varied forms, and few cause damage in so many different ways. The velocity of moving water can apply damaging forces to structures, scour and undermine their foundations, and slam them with large debris. But standing water can also cause tremendous damage-just how much damage depends on how deep the water is. Unlike wind damage, which typically occurs from the roof down, flood damage essentially occurs from the floor(or basement) up. Even a moderate amount of water above ground floor level can cause structural fasteners to rust, walls and floors to warp, and building contents to become water-logged. And the water can bring with it mud and hazardous materials, such as sharp debris, pesticides, fuel, and untreated sewage.
Floods can last from a few hours to several weeks ( the multiple continuous floods in South England that lasted the three month of December 2013 through February 2014 being exceptional). Closer to where I live, major flooding across Central Europe last June affected Germany in particular. Twelve months later some insurance claims have still not been settled. While this lag may be longer than usual, it is not unusual for there to be a lag of several weeks before a reliable estimate of the flood losses can be arrived at and claims can be paid.
After flood waters recede, insurance companies begin to estimate their losses by means of claims adjusters. Losses to contents are fairly easy to asses, since the replacement cost of goods and inventory are easily determined. The estimation of building damage and related repair costs are much more difficult.
Losses to the structure tend to "develop" as the full extent of damage becomes apparent. While the cost of replacing soaked drywall might be easily estimated, for example, the damage to the wall itself can only be determined after the drywall has been removed and additional damage becomes visible.
Humidity is a major loss driver in several ways. First through the direct damage it causes, but secondly by the fact that it will determine the length of the repair time. The time it takes to de-humidify the wall is not easily predicted. A shorter time allowed for drying might reduce losses arising from paying out additional living expenses, but it might cause additional losses if the wall is not dried out completely; mold and warped hard wood floors can result, and the total repair cost could be increased dramatically.
While the ultimate loss can only be known for certain after all repairs have been completed, the estimates usually become fairly reliable after several weeks as insurance companies can anticipate and correct for the difficulties encountered during the claims estimation process.