Levees in 

Risk Rating 2.0 

February 2022
Levees in Risk Rating 2.0

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Table of Contents

1. Introduction ................................................................................................................................ 1

2. Background ................................................................................................................................ 1

3. Levees in Risk Rating ................................................................................................................ 1

3.1. Risk Associated with Levee Systems ............................................................................. 2

3.2. Anatomy of a Levee......................................................................................................... 3

3.3. Key Levee Data Overview ............................................................................................... 5

3.4. Data Sources ................................................................................................................... 6

3.4.1. National Levee Database and Levee Screening Tool .................................................. 6

3.4.2. Understanding Data Availability .................................................................................... 6

3.4.3. Other Data Sources ....................................................................................................... 7

3.4.4. Data Source Hierarchy................................................................................................... 8

3.4.5. Data Pull ......................................................................................................................... 8

3.5. Levee Centerline and Crest Profile................................................................................. 9

3.5.1. Methodology................................................................................................................. 10

3.6. Leveed Area.................................................................................................................. 10

3.6.1. Methodology................................................................................................................. 10

3.7. Levee Overtopping Estimates...................................................................................... 11

3.7.1. Methodology................................................................................................................. 11

3.7.2. Data Development....................................................................................................... 12

3.7.3. Assumptions................................................................................................................. 13

3.8. Levee Performance (System Response Curve) .......................................................... 13

3.8.1. Methodology................................................................................................................. 14

4. Further Study .......................................................................................................................... 15

Figures

Figure 1. Risk Rating 2.0 simplified process graphic, including levee inputs..........................................2

Figure 2. Risk Rating 2.0 data sources for key levee data ....................................................................... 3

Figure 3. Generalized levee system labeled with common levee terminology ........................................4

Figure 4. The levee crest profile and water surface profile, plotted along the levee in a profile view ..4

Figure 5. A levee system composed of a series of levee segments ......................................................... 5

Figure 6. Centerline adjustment procedure ............................................................................................ 10

Figure 7. Example of Fathom water surface profiles for recurrence intervals (shown: 5 of the 10 

available) compared to the levee crest profile ................................................................................. 12

Figure 8. Examples of system response curves with different evaluated levee performance ............ 14
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Tables

Table 1: Risk Rating 2.0 key levee data source hierarchy ........................................................................ 8

Table 2: Risk Rating 2.0 data sources for key levee data......................................................................... 9
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1. Introduction

This document provides an overview of the levee data that supports Risk Rating 2.0. It offers a 

baseline understanding of the sources and methodology used to develop key levee data input for the 

catastrophic modeling and actuarial rating process. Computing the average annualized loss (AAL) 

from catastrophic loss models and setting insurance rates are not described in this document. Refer 

to https://www.fema.gov/flood-insurance/work-with-nfip/risk-rating# for additional information. 

2. Background

FEMA is updating the National Flood Insurance Program’s (NFIP) risk rating methodology through the 

implementation of a new pricing methodology called Risk Rating 2.0 to better serve the nation. By 

leveraging “industry best practices” and current technology, FEMA aims to deliver rates that are 

actuarily sound, more equitable, and easier to understand and that better reflect a property’s 

individual flood risk. Risk Rating 2.0 allows FEMA to calculate premiums more consistently for all 

policyholders, based on the value of their home and their property’s individual flood risk, by moving 

toward a graduated risk approach that expands beyond the binary depiction of flood risk centered 

around the 1%-annual-chance (100-year) flood event. 

The graduated risk approach for Risk Rating 2.0 accounts for flood hazards larger and smaller than 

the 1%-annual-chance flood by leveraging additional data, beyond that depicted on Flood Insurance 

Rate Maps (FIRMs). The resulting rates more clearly and accurately identify and communicate the 

flood risk exposure of an individual building by accounting for multiple flood hazard sources (e.g., 

river, rainfall, coastal storms, and tsunami) and flood risk reduction measures such as levees. 

It is important to note that not all aspects of the NFIP are changing. Community officials and lenders 

will continue to use effective FIRMs to determine if a building is located within a Special Flood 

Hazard Area (SFHA), an area that is subject to inundation by the base (1%-annual-chance) flood, to 

inform floodplain management and mandatory flood insurance purchase requirements. Lenders will 

also retain the prerogative to require flood insurance, even in the absence of the federal requirement 

to purchase coverage.

As part of Risk Rating 2.0, FEMA has not proposed any changes to the levee accreditation 

requirements established in the Code of Federal Regulations (CFR) at Title 44 – Emergency 

Management and Assistance, Section 65.10 (44 CFR 65.10), nor to the levee analysis and mapping 

procedures for non-accredited levees, which are documented in FEMA’s guidelines and standards for 

the analysis and identification of the 1%-annual-chance flood hazard on a FIRM.

3. Levees in Risk Rating

FEMA included the impact of levee systems on flood risk reduction in Risk Rating 2.0. To do so, it 

partnered with the U.S. Army Corps of Engineers (USACE) to identify and use credible and 

consistently available information and methods to account for the level of risk reduction levees
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provide to the buildings located behind them. FEMA partnered with USACE to determine the key

levee data needed to evaluate the impact of residual flood risk at the building level, identify readily 

available sources of levee information, and implement best practices to develop or refine data when 

possible. 

Risk Rating 2.0 estimates the likelihood that a building will experience damage from flooding (and 

how much damage would occur), to set insurance policy rates. Levee data helps to inform the 

benefits and risks that a levee provides to businesses, communities, and the public for purposes of 

modeling the losses due to floods. A levee reduces – but does not eliminate – flood risk. The 

limitations of a levee system, in both its capacity to reduce flooding in leveed areas (informed by 

overtopping frequency) and its ability to perform adequately during flood events (levee performance) 

are fundamental to developing a graduated risk approach for Risk Rating 2.0. As such, more robust 

and better performing levees yield a quantifiable benefit to insurance policy holders. 

In general terms, Risk Rating 2.0 follows three primary steps for areas with levees, as depicted in the 

simplified process graphic (Figure 1). The first step is the key levee data input, the second step uses 

catastrophic models that calculate average annualized loss at the building level, and the third step 

involves the actuarial rating process to set a base rate and rating factors. Levee data has an impact 

on the catastrophic modeling and the rating process (e.g. Levee Quality factor). This document 

focuses on the key levee data required to support Risk Rating 2.0 and explains the sources and/or 

methodology for developing the needed data.

Figure 1. Risk Rating 2.0 simplified process graphic, including levee inputs

To identify the risk for communities behind levee systems, FEMA considers the likelihood of a flood 

hazard occurring, how a levee is expected to perform when exposed to that hazard, and the related

consequences of a levee overtopping or a levee breach (Figure 2). For this document, hazard is 

defined as flooding from riverine sources and consequences are defined as damages a building may

sustain from flooding. To incorporate the impact of levees in Risk Rating 2.0 one must understand

the hazard, the anticipated performance of the levee, and the number of buildings that would be

3.1. Risk Associated with Levee Systems
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impacted if the levee is overtopped or does not perform as intended. Levees serve as a structural 

risk reduction measure to decrease the likelihood of consequences being realized. How well a levee 

reduces the consequences of flooding is an important factor for catastrophic loss modeling and rate 

setting in Risk Rating 2.0. 

Figure 2. Risk Rating 2.0 data sources for key levee data 

3.2. Anatomy of a Levee 

The following section defines and describes the anatomy of a levee to provide a common 

understanding of the physical features used to evaluate the residual flood risk for buildings behind 

levees. Figure 3 illustrates common levee-related terms used in this document. 

A levee is a man-made structural feature, usually an earthen embankment or concrete floodwall,

designed and constructed in accordance with sound engineering practices to contain, control, or

divert the flow of water so as to provide risk reduction from temporary flooding. The levee’s function

is to exclude water from a portion of the floodplain – referred to as a leveed area – where houses, 

businesses, and assets may have a reduced flood risk because the levee exists. The levee crest is

the top of the levee. The levee centerline is a line representing the highest points along the levee

crest. The elevations of this centerline, when viewed in profile, yield a levee crest profile (Figure 4). 

Finally, the water surface profile represents the elevation of the water against the levee for a flood

event of a particular frequency.
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The overhead (or plan) view in Figure 5 introduces two other important concepts: levee systems can 

be made up of discrete segments. The levee system as a whole creates a leveed area that 

represents the areas of reduced flood risk due to the presence of the system. While the 

segmentation of levees is typically used to designate different ownership or responsibilities, each 

segment is crucial for the entire levee system to function and reduce flood risk in the leveed area. 

The location where overtopping occurs most frequently sets the elevation at which water begins to 

flow over the levee and into the leveed area. 

Figure 3. Generalized levee system labeled with common levee terminology

Figure 4. The levee crest profile and water surface profile, 

plotted along the levee in a profile view 
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3.3. Key Levee Data Overview

FEMA and USACE have a long history of partnering and developing complementary procedures 

related to levees. For Risk Rating 2.0, FEMA partnered with USACE to obtain credible and 

consistently available data and methods to determine a levee’s level of risk reduction. Five key 

levee data elements were needed to support Risk Rating 2.0: 

1. Levee Centerline: Defines the location of a levee in plan view. For Risk Rating 2.0, the levee

centerline represents the top of the levee.

2. Levee Crest Profile: The elevation of the top of the levee along a levee centerline. For Risk Rating

2.0, the levee crest informs the overtopping frequency.

3. Leveed Area: A polygon representing the lands (and buildings) excluded from the floodplain by

the presence of the levee (i.e. the area with a reduced flood risk due to a levee).

4. Levee Overtopping Frequency: The likelihood that a flood event will exceed the height of a levee,

allowing flood water over the top of the levee (i.e. overtopping).

Figure 5. A levee system composed of a series of levee segments
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5. Levee Performance: A system response curve representing the estimated likelihood of breach 

under various flood levels. For Risk Rating 2.0, the system response curve was limited to levee 

performance prior to overtopping.

3.4. Data Sources

The long-term management of Risk Rating relies on key information from comprehensive data 

sources, including those related to levees. This allows FEMA to efficiently process and update rates 

at an individual building level. Risk Rating 2.0 used data that was either readily available or could be 

generated in a consistent and timely manner. Therefore, key sources of levee information, such as 

the USACE-maintained National Levee Database (NLD), were used to identify available levee data. 

USACE and FEMA are both committed to using the NLD as the authoritative and primary national 

resource for levee information. 

3.4.1. NATIONAL LEVEE DATABASE AND LEVEE SCREENING TOOL 

The NLD, developed and maintained by USACE, is the focal point for comprehensive information 

about all the nation's levees, as authorized by Congress. The NLD is a dynamic database that is 

continually updated to add or refine levee data from federal agencies, states, tribes, territories, and 

local sources. 

The Levee Screening Tool (LST) is a web-based risk assessment tool that USACE uses to efficiently 

characterize levee risk. Users combine readily available information on hazards, conditions, 

performance, and consequences and apply engineering judgment to describe the performance of a 

levee and the potential consequences of flooding. At the time of the data pull for Risk Rating 2.0 

(Section 3.4.5), USACE had applied the LST risk assessment process to more than 1,700 of the 

levee systems in the NLD. This subset of levees is referred to in this document as “screened 

levees.” 

Levee information is shared between the NLD and LST. For screened levees, the LST is the primary 

source of levee performance data, and the NLD is the primary source for levee location information

such as the centerline, crest profile, and leveed area. Estimates of levee overtopping frequencies for 

screened levees are contained in both the NLD and LST, with the estimate typically originating from 

the LST. 

3.4.2. UNDERSTANDING DATA AVAILABILITY

The quantity and quality of levee information in the NLD and LST vary. FEMA used the data available 

from these sources as described in the sections that follow. Levees that USACE routinely inspects 

generally have more high-quality and detailed information available; that data was used for Risk 

Rating 2.0. For other levees where detailed information was not readily available, data from the NLD 

was used and enhanced using consistent methods. For some key levee data fields, a lack of readily 

available information required assumptions to be made. 
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The NLD is a dynamic database that is continually updated, so it may not yet contain any information 

for some of the nation’s levees. USACE and FEMA are working together to learn more about these 

levees and will continue to collaborate to update levee information in the NLD as appropriate. 

For Risk Rating 2.0, most of the screened levees (approximately 20% of the levee systems in the 

NLD) had the information to fulfill all five key levee data needs (levee centerline, levee crest profile, 

leveed area, overtopping frequency, and levee performance). The NLD reports that these screened 

levees mitigate the risk for approximately 60% of all buildings behind levees. Approximately 80% of 

the levee systems in the NLD had not been screened at the time of the data pull for Risk Rating 2.0 

(Section 3.4.5). 

The following steps were taken to develop data for unscreened levees in the NLD: 

 Secondary data sources were identified and used.

 Existing data in the NLD was improved using consistent methodology. 

 Assumptions were made in the absence of readily available data. 

Not all levees have buildings behind them; thus, an absence of data for these systems is 

inconsequential for setting rates. Data refinement and improvements were prioritized for levees 

where high-quality Digital Elevation Model (DEM) data was readily available and where 50 or more 

buildings were in the leveed area. 

3.4.3. OTHER DATA SOURCES

This section describes other national datasets used to develop supplemental levee data to support 

Risk Rating 2.0. FEMA used the Fathom dataset to calculate levee overtopping frequency where that 

data did not exist. Also, publicly available DEM from the USGS 3D Elevation Program (3DEP) were 

used to develop geospatial features such as levee profiles and leveed areas. 

Fathom

FEMA used the national Fathom-US 2.0 dataset to estimate the overtopping frequency for levees 

that did not have data available from a previously discussed source. Fathom produces 10 

frequency-based water surface elevation grids, based on nationwide regional frequency analysis, to 

establish stage relationships at critical sites throughout the nation. FEMA leveraged the Fathom data 

to provide expedient and consistent estimates of levee overtopping frequencies. The Fathom data 

was converted and associated to the levee centerline to compare the levee crest profile to the 

frequency-based water surface elevations. Water surface elevations were available for these 10 

frequencies: 5-year, 10-year, 20-year, 50-year, 75-year, 100-year, 200-year, 250-year, 500-year, and 

1,000-year floods. Section 3.7.1 describes the general methodology used for estimating overtopping 

frequencies from Fathom data, quality control, and validation.
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3.4.4. DATA SOURCE HIERARCHY

The data source hierarchy indicates the key sources of levee data that were used and which sources 

were considered to have a higher quality than others. When data was available from multiple 

sources, the highest quality data (as determined by the hierarchy) was used in Risk Rating 2.0. 

However, if Fathom estimated levee overtopping to be more frequent than the 1% annual 

exceedance probability (AEP) for an accredited levee on a FIRM, the overtopping frequency was 

assumed to be the 1% AEP. Levee centerlines, crest profiles, and leveed areas were sourced from 

the NLD. In some cases, the levee centerline and leveed area were improved and updated in the 

NLD. 

Table 1: Risk Rating 2.0 key levee data source hierarchy 

Key Levee Data Hierarchy

Levee Centerline National Levee Database

Levee Crest Profile National Levee Database

Refinement Process (Section 3.5) 

Leveed Area National Levee Database

Refinement Process (Section 3.6) 

Levee Overtopping Frequency Levee Screening Tool

National Levee Database

Fathom 

Assumed Values

Levee Performance Levee Screening Tool

Assumed Values

3.4.5. DATA PULL 

Data to support Risk Rating is pulled annually from its data sources to coincide with the planned 

annual update cycle. Each iteration of Risk Rating represents a snapshot in time of the information 

available from the key levee data sources. For Risk Rating 2.0, levee risk reduction benefits were 

considered for every feature identified as a levee system in the NLD in early March 2020. This is 

important to note, as the NLD is a dynamic database that is continually updated. Current NLD 

information may reflect changes made since the data pull for Risk Rating 2.0. FEMA will continue 

working with USACE to leverage the highest quality levee data readily available in support of future 

rate updates and a risk-informed NFIP.

Table 2 provides a summary of the data sources and methods used for each key levee data field for 

Risk Rating 2.0. It also summarizes the approximate percentage of levee systems and buildings 

associated with each data source or method. These percentages will change over time as more levee 

data becomes readily available in the future. 
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Table 2: Risk Rating 2.0 data sources for key levee data 

3.5. Levee Centerline and Crest Profile

The levee centerline and crest profile are required to derive additional key levee data. The levee 

centerline is needed to develop the crest profile, and the levee crest is needed to assess the 

likelihood of levee overtopping and to derive a leveed area. 

The NLD contains geospatial levee data, including the levee centerline and crest profiles. All levee 

entries in the NLD have a levee centerline that locates the levee geographically, but not all levees in 

the NLD currently have a crest profile associated with the centerline. This can hinder users’ ability to 

assess the likelihood of levee overtopping. The NLD centerline and profile data may be derived from 

survey information (as is the case for most screened levees) or terrain-based methods. Risk Rating 

2.0 used terrain-based methods when centerline and profile data were not available from the NLD.
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3.5.1. METHODOLOGY 

In the absence of surveyed levee crest elevations, existing levee centerlines in the NLD were 

topographically adjusted to improve the spatial location and ultimately derive a levee crest profile. 

The terrain-based method uses DEM with a spatial resolution of 3 meters or better to improve the 

location of the levee centerline and the crest elevation profile. The primary source of DEM was USGS 

3DEP, supplemented by data from state repositories as needed. The levee centerline was refined by 

cutting cross-sections along the existing centerline and shifting the centerline vertices to the highest 

point at each levee cross section (Figure 6). The crest profile was developed from the elevations 

pulled from the DEM at each new vertex along the updated centerline. The quality of resultant levee 

profiles was reviewed and rejected if anomalies were present in the data. Levee centerlines were 

improved using the assumption that all levee segments in the system were identified and the levee 

extents (the start and stop points) did not require adjustment. A total of 130 levee system centerline 

and profiles were updated with this method.

Figure 6. Centerline adjustment procedure

3.6. Leveed Area

The leveed area represents the lands from which the levee system excludes flood water. Geospatial 

data was used to identify buildings with a reduced flood risk due to the presence of a levee. The 

leveed area is separate and distinct from the regulatory Special Flood Hazard Area, because it is 

based on flood loading to the levee crest and not a discrete flood frequency event (i.e. 1%-annualchance flood). Similar to levee centerlines, all levee systems in the NLD have a leveed area. The 

leveed area in the NLD may be developed by various methods. 

3.6.1. METHODOLOGY 

The Risk Rating 2.0 effort included refining leveed areas where sufficient data was available. This 

required a levee crest profile and a high-quality DEM. Leveed areas were developed using a “flood 

fill” method that creates a polygon around all areas in the DEM below a target elevation. The target 

elevation is typically the lowest elevation in the levee crest profile, which represents the extent of 
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flooding for a flood event at the capacity of the levee. This leveed area method only applies to levee 

systems with a relatively flat levee crest profile in relatively flat terrain. Engineering judgment was 

applied to determine if the modified leveed area was an improvement over the existing leveed area 

in the NLD. Ninety leveed areas were modified using this method. 

3.7. Levee Overtopping Estimates 

A levee overtopping estimate is required to support Risk Rating 2.0. This estimate was made by 

comparing the levee crest profile elevations with the frequency-based flood water surface elevations. 

For screened levees, an overtopping frequency was estimated following USACE’s levee screening 

process and entered in the LST. For non-screened levees, overtopping frequency was estimated 

using the methodology and assumptions described in the following paragraphs

3.7.1. METHODOLOGY 

Levee overtopping estimates are sourced and selected for use based on a hierarchy (discussed in 

Section 3.4.4) that uses the most precise method for which data is readily available. If a levee 

segment or levee system did not have an overtopping estimate from an available source, such as the 

LST or the NLD, the overtopping estimate was derived by comparing Fathom data to the levee crest 

profile (Figure 7). The selected overtopping frequency was the most infrequent Fathom flood 

frequency water surface profile that is wholly below the levee crest profile. For example, in Figure 7, 

the levee centerline crest profile can wholly contain the 100-year (1%-annual-exceedance probability) 

flood event, but not a greater Fathom profile, without overtopping. 
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Figure 7. Example of Fathom water surface profiles for recurrence intervals (shown: 5 of the 10 

available) compared to the levee crest profile

Before using Fathom data, FEMA conducted a series of quality checks and then validation. 

Quantitative and qualitative data analysis checks were used to verify the quality of the data. Data 

that did not conform to expected patterns, such as predictable increasing river stage with less 

frequent flood events and generally monotonic water surface profiles, and data having excessive 

data anomalies, were discarded. Next, Fathom-derived levee overtopping estimates were validated 

to give confidence that the data source could reasonably predict overtopping frequency when there 

is no LST or NLD data. The validation compared levee systems where both Fathom and NLD or LST 

data were available, to understand the potential bias. No significant data bias was identified; 

however, the Fathom data generally skewed to overstate the risk reduction (estimating a less 

frequent overtopping value than established LST or NLD estimates).

3.7.2. DATA DEVELOPMENT

Fathom data was used to calculate overtopping frequency estimates for 17% of levee systems, all of 

which were unscreened. As a result, 37% of levee systems considered in Risk Rating 2.0 have an 

overtopping estimate derived from either the LST, NLD, or Fathom data, and they cover 75% of the 

buildings behind levees. 

178

178.2

178.4

178.6

178.8

179

179.2

179.4

179.6

179.8

180

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

Elevation (feet)

Distance (feet)

Levee Crest Profile & Fathom Water Surface Profiles 

Comparison (Example)

1000-year 500-year 250-year

200-year 100-year Levee Crest Profile
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3.7.3. ASSUMPTIONS

Assumptions for overtopping frequency were required for levee systems that did not have available 

or reliable information from the LST, NLD, or Fathom. One of two assumptions was applied in these 

cases. First, if a levee system was identified as accredited on a FIRM, the assumed overtopping 

frequency was a 1% annual exceedance probability (AEP) flood event. In situations where Fathom 

data estimated a more frequent overtopping for an accredited levee system, the 1% AEP overtopping 

frequency was still assumed. However, if Fathom estimated levee overtopping to be less frequent 

than the 1% AEP for an accredited levee, then the overtopping frequency derived from Fathom was 

used. Second, FEMA assumed the levee overtopping frequency as a 2% AEP flood event for the 

remaining 57% of levees (and associated 10% of buildings in leveed areas). 

3.8. Levee Performance (System Response Curve)

A measure of the anticipated levee performance during flood loading is represented by a system 

response curve. The system response curve is a curve denoting the likelihood of a levee breach 

under various flood heights. Generally, a levee has an increased likelihood of breach as flood water 

surface elevations increase. Figure 8 illustrates examples of system response curves for three 

categories of levee quality or performance. For Risk Rating 2.0, the levee overtopping frequency, 

levee toe loading frequency, and levee performance factors are needed to create a system response 

curve. The shape of the curve varies based on available levee incident data. System response curves 

and overtopping frequencies are used in FEMA’s catastrophic modeling and during the rate-setting 

process. The “Levee Quality” factor used in rate setting is derived from the system response curve. 

Additional information on the application of the “Levee Quality” factor in “Risk Rating 2.0 

Methodology and Data Source” (April 16, 2021) is available at https://www.fema.gov/floodinsurance/work-with-nfip/risk-rating#. 
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Figure 8. Examples of system response curves with different evaluated levee 

performance 

3.8.1. METHODOLOGY

For all screened levees, system response curves were developed from risk assessment information 

in the USACE LST. The probability of a breach with water at the top of the levee (P-Top) is calculated 

for each potential failure mode evaluated in the LST and then combined to estimate a P-Top for the 

levee system. With the levee crest and levee toe loading frequencies, the P-Top value is used to 

estimate the levee system response curve. Once the system response curve is developed, a rating 

variable indicating levee performance is calculated from the area under the curve. The performance 

indicator value is a probability weighted average. The system response curve is also adjusted for the 

likelihood of flood-fighting efforts to intervene and mitigate levee performance issues during a flood 

event. 

In the absence of a LST risk assessment-based system response curve, the USACE incident data 

statistics on average performance of a typical levee were used to derive an “average” levee system 

response curve for the remaining levees. 

For Risk Rating 2.0, all segments in a system were evaluated together to identify residual risk at the 

system level. The segment with the highest likelihood to overtop and most likely to experience poor

performance governs system performance and was used to derive the system response curve. The 

curve was applied to the entire system’s leveed area.
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4. Further Study

A key element in understanding the level of risk reduction provided by levees is having credible and 

readily available data that describes the location, leveed area, likelihood of overtopping, and the 

performance of levees. FEMA will continue working with USACE to improve levee data and to refine 

risk assessment methodologies in support of future iterations of Risk Rating and a risk-informed 

NFIP. FEMA and USACE are committed to further collaboration and alignment around a shared 

methodology for assessing levee risk across the nation, communicating flood risk in a consistent and 

comprehensive manner, and equipping the public to make risk-informed decisions.