Data Summaries Shown in Step 3

1% Annual Chance Floodplain

The 1% annual chance floodplain is defined as the area that will be inundated by the flood event having a 1% chance of flooding of being equaled or exceeded in any given year. The 1-percent annual chance flood is also referred to as the base flood or 100-year flood. This area defines the Special Flood Hazard Area (SFHA) that is delineated on Federal Emergency Management Agency Flood Insurance Rate Maps. Mortgage holders within the SFHA are required to purchase flood insurance.

Flood Extents

Changes to inundation by tides and episodic coastal flooding were estimated by increasing current day conditions by the projected changes to sea level for each scenario and year.

Inundation was defined as land with elevations less than the local mean higher high water tidal datum for present day and each sea level scenario.

Coastal flooding was defined by FEMA 1% annual chance floodplain as defined for present day. The floodplain was estimated for each scenario by increasing flood elevations by the projected values.

The extent of flooding was determined using standard flood modeling practices and high resolution/high accuracy LiDAR-based topographic data. Flood extents were summarized by area in square miles and percent of the impacted neighborhood and county geographies. Neighborhood geographies are defined by U.S. Census Block Groups.

Percent Chance of Flooding in a 30-Year Period

This metric shows the potential for flood impacts at a given building within a 30-year period of time, equivalent to the standard home mortgage. For each scenario, this value was calculated for all buildings within the 0.2% annual chance floodplain.

The value was calculated by first determining the percent annual chance of flooding for each building by utilizing the 0.2, 1%, 2%, 4%, and 10% water surface elevations provided by FEMA and their associated percent-annual chance of exceedance. The 30-yr probability was then calculated using the relationship P30 = 1 – (1-p)n, where P30 is the 30-year probability of flooding, p is the percent annual chance of flooding, and n is the time period in years (30 years).

Building Exposure and Expected Damage Assessment

For each scenario, building exposure to inundation and/or coastal flood impacts were evaluated by intersecting flood extents with building footprint data. Next, flood depths were determined by subtracting water surface elevations from the topography. Lowest and highest adjacent grade values were derived by geospatial extraction of elevation data against the building footprint, and then flood depth was estimated and attributed to each structure.

Potential damages were estimated by application depth damage functions sourced from FEMA Benefit-Cost Analysis Flood Module. All structures were assumed to be slab-on-grade construction. Structures having basements were differentiated in the analysis by application of a separated depth damage function. Damages were generalized into three categories: minor (>25% damages), moderate (>25%, <50% damages), and severe (>50%) damages. These are described below:

Minor:  Less than 25% damage is expected during a 1% annual chance flood, as defined by FEMA.

In this case, short term inundation is expected; water does not rise above the level of the bottom of the first flood of the structure. Common damage may include limited scour and erosion from low velocity floodwaters, with no noticeable cracking of masonry or displacement of foundation walls. Repairs expected to take 30 to 120 days.

Moderate:  Structural damages are expected to in the range of 25-50% during a 1% annual chance flood, as defined by FEMA.

In this case, short term inundation is expected; water rises just above the first floor level. Common damage may include limited scouring or undermining of the foundation or footings, minor cracking from settlement, and/or heaving of the structural support systems. Some missing sections or open damage to portions of the wall structure, and damage to wall studs and sheathing from debris or hydrostatic pressure. Repairs expected to take 120 to 360 days.

Severe:  Greater than 50% damage is expected during a 1% annual chance flood, as defined by FEMA. In this case, long-term inundation is expected, with water up to and over 3-feet high from the bottom of the first floor level of the structure. Common damage includes cracking to displacement up to missing masonry and/or concrete foundation walls. Structure may become unstable due to foundation damage. Missing sections or open damage to significant portions of the wall structure, with significant debris and or hydrostatic damage resulting in deformation or moderate to significant distortion of the structural frame. Repairs expected to take 540 to 720 days (structure lost).

Composite Risk Analysis

The composite risk analysis metric provides an assessment of the relative degree of exposure to inundation and flood impacts across all scenarios. This assessment takes into account the number of times a building is subject to inundation, the scenarios that the structure is exposed to, as well as the severity of expected flood impacts. Scores were weighted by scenario and time slice, meaning that lower acceleration scenarios and earlier time slices received the highest scores – a reflection of their near term risk and greater need to adapt. Vice-versa, higher acceleration scenario and later time slices received the lowest scores, reflecting their long-term risk and lower need to adapt.

Individual buildings were classified into three composite risk categories according to their exposure and consequence scores. A conceptual diagram of this classification system is shown below:

Composite Risk Classification System Diagram

LOW classification indicates that the building has a low risk to inundation and/or increased flood impacts due to sea level rise. The building is not subject to permanent inundation and is expected to experience minor impacts by significant coastal storms on towards the end of the century.

MEDIUM classification indicates that the building has moderate risk to inundation or increased flood impacts due to sea level rise. Buildings in this category can be subject to permanent inundation, and/or moderate or severe impacts due to significant coastal storms over multiple scenarios from mid- to end-of-century. Owners of buildings in this category should begin planning measures to reduce their long-term exposure to flood impacts.

HIGH classification indicates that the building is at high risk to inundation and increased flood impacts due to sea level rise. Buildings in this category are subject to permanent inundation and severe impacts due to significant coastal storms over multiple scenarios in the near term. Owners of buildings in this category should act to identify and implement measures to reduce flood exposure.