Structural Drying >> Structural Drying Courses

Hurricanes Ivan (2004) and Katrina (2005) have demonstrated that constructing a building to the minimum National Flood Insurance Program (NFIP) requirements or constructing a building outside the SFHA shown on the FIRMs is no guarantee that the building will not be Structural Drying Courses damaged by flooding. 

This is due to two factors: 1) flooding more severe than the base flood occurs, and 2) some FIRMs, particularly older FIRMs, Structural Drying Courses may no longer depict the true base flood level and SFHA boundary.

The black line in Figure 3 shows the probability that the level of the flood will exceed the 100-year flood level during time periods between 1 year and 100 years; there is an 18-percent chance that the 100-year flood level will be exceeded in 20 years, Structural Drying Courses a 39-percent chance it will be exceeded in 50 years, and a 51-percent chance it will be exceeded in 70 years. 

As the time period increases, Structural Drying Courses the likelihood that the 100-year flood will be exceeded also increases. Figure 3 caption. Probability that a flood will exceed the n-year flood level over a given period of time (Note: this analysis assumes no shoreline erosion, and no increase in sea level or storm frequency/severity over time). 

Figure 3 also shows the probabilities that floods of other severities will be exceeded. For example, taking a 30-year time period where there is a 26-percent chance that the 100-year flood level will be exceeded, there is an 18-percent chance that the 150-year flood will be exceeded, Structural Drying Courses a 14-percent chance that the 200-year flood will be exceeded, and a 6-percent chance that a flood more severe than the 500-year flood will occur.

FIRMs depict the limits of flooding, flood elevations, Structural Drying Courses and flood hazard zones during the base flood. As seen in Figure 3, buildings elevated only to the BFEs shown on the FIRMs have a significant chance of being flooded over a period of decades. 

Users should also be aware that the flood limits, flood elevations, and flood hazard zones shown on the FIRM reflect ground elevations, Structural Drying Courses development, and flood conditions at the time of the Flood Insurance Study (FIS).  Coastal Construction Manual (FEMA 55, 2000 ed.) provide guidance on evaluating a FIRM to determine whether it still provides an accurate depiction of base flood conditions, or whether it is obsolete. 

Consequences of Flood Levels Exceeding the BFE Buildings are designed to resist most environmental hazards (e.g., wind, seismic, snow, etc.), Structural Drying Courses but are generally designed to avoid flooding by elevating the building above the anticipated flood elevation. The difference in design approach is a result of the sudden onset of damage when a flood exceeds the lowest floor elevation of a building. 

Unlike wind where exposure to a wind speed slightly above the design speed does not generally lead to severe building damage occurrence of a flood level even a few inches above the lowest floor elevation generally leads to significant flood damage, Structural Drying Courses therefore, the recommendation to add freeboard.[Begin text box]FIRMs do not account for the following: 

Shoreline erosion, wetland loss, subsidence, and relative sea level rise Upland development or topographic changes Degradation or settlement of levees and floodwalls Changes in storm climatology (frequency and severity) The effects of multiple storm events Thus, Structural Drying Courses what was once an accurate depiction of the 100-year floodplain and flood elevations may no longer be so.

This is especially true in cases where waves accompany coastal flooding. Figure 4 illustrates the expected flood damage (expressed as a percent of a building's pre-damage market value) versus flood depth above the bottom of the lowest horizontal structural member supporting the lowest floor (e.g., bottom of the floor beam), Structural Drying Courses for a V Zone building and for a riverine A Zone building. [footnote 2]Figure 4 caption. 

Flood depth versus building damage curves for V Zones and riverine A Zones (Source: FEMA 55, Coastal Construction Manual). [end of caption]Footnote 2. Since the normal floor reference for A Zone buildings is the top of the lowest floor, Structural Drying Courses the A Zone curve was shifted for comparison with the V Zone curve. 

One striking difference between the two curves is that a V Zone flood depth (wave crest elevation) 3 to 4 feet above the bottom of the floor beam (or approximately 1 to 2 feet above the top of the floor) is sufficient to cause substantial (>50 percent) damage to a building. In contrast, Structural Drying Courses A Zone riverine flooding (without waves and high velocity) can submerge a structure without causing substantial damage. 

This difference in building damage is a direct result of the energy Structural Drying Courses contained in coastal waves striking buildings something obvious to those who saw the wave damage that Hurricane Katrina caused in Mississippi and Louisiana (see Figure 5).Figure 5 caption. Hurricane Katrina damage to buildings in coastal Mississippi. 

The upper left, upper right, and lower left photos are of buildings that were close to the Gulf shoreline and subjected to storm surge above the floor and large waves striking the building walls. The lower right photo is of a building almost 1 mile from the Gulf, Structural Drying Courses and subjected to storm surge flooding only. [end of caption]In cases where buildings are situated behind levees, a levee failure can result in rapid flooding of the area. 

Buildings near a levee breach may be exposed to high velocity flows, Structural Drying Courses and damages to those buildings will likely be characterized by the V Zone damage curve in Figure 4. Damages to buildings farther away from the breach will be a result of inundation by floodwaters, and will likely resemble the A Zone curve in Figure 4.General Recommendations 

The goal of this Advisory is to provide methods to minimize damage to buildings in the event that coastal flood levels rise above the BFE. Achieving this goal will Structural Drying Courses require adherence to one or more of the following general recommendations: In all areas where flooding is a concern, inside and outside the SFHA, elevate the lowest floor so that the bottom of the lowest horizontal structural member is at or above the Design Flood Elevation (DFE). 

Do not place the top of the lowest floor at the DFE, since this guarantees flood damage to wood floor systems, wood floors, floor coverings, Structural Drying Courses and lower walls during the design flood, and may lead to mold/contamination damage (see Figure 6).

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