Developing More Accurate Floodplain Maps – Climate Change

Climate change is driving the need for new approaches to
mapping floodplains and understanding flooding risks

Climate change is causing an increasing number of dangerous and
expensive floods across the U.S. — and actions on climate
resilience are lagging far behind the pace of change. That’s a
central message of the Fifth National Climate
Assessment, the most comprehensive interagency analysis yet of
how climate change will affect the nation in the years to come.

Federal Emergency Management Agency floodplain maps play a
crucial role in evaluating the susceptibility of various regions to
flooding related to climate change and guiding the work ofcity
planners, real estate developers, and insurance companies. However,
many FEMA maps are decades old and don’t account for the repercussions of climate change on
infrastructure. They are also based on broader, generalized
data that may not represent local conditions accurately. Given the
regional nature of climate-related hazards, the need for precise
mapping and detailed flood risk assessments has escalated in
significance.

It’s critical that communities, real estate developers, and
insurance companies conduct thorough investigations of flood-prone
areas, with particular attention to regional data and trends,
before designing or constructing new buildings or issuing insurance
and setting rates. FEMA flood maps can and should be used during
these investigations, but they’re only one part of the overall
picture.

FEMA floodplain maps rely on historical data

FEMA’s flood maps show where the minimum floodplain
management regulations, for example, building and land use codes,
must be applied. The Special Flood Hazard Areas (SFHAs) defined by
FEMA have a 1% chance of being inundated in any given year, i.e.,
those areas that will be affected by a “100-year flood.”
Properties within those zones are subject to more stringent
building codes and regulations.

Since the inception of the National Flood Mapping Program in
1967, only about one-third of the nation’s streams, rivers, and
coastlines have been mapped by FEMA, and only one-quarter of these models
have been updated in the past five years. Numerous communities
throughout the U.S. haven’t been mapped for decades, and some
of the most outdated maps reflect the nation’s most flood-prone
areas.

For instance, the FEMA flood map for Calais, Vermont, which
experienced a flood exceeding 100-year flood levels in July
2023, was last updated in March 2013. In certain regions
of northern Wisconsin, where
significant events like 500-year rainfall incidents have occurred
multiple times within a few years (July 2016 and June 2018), FEMA maps date back
to 1988 and 1978,
respectively.

FEMA’s traditional floodplain modeling methods depend
heavily on historical data and implicitly assume that future
hydrologic conditions will occur with the same frequency as past
conditions. They reflect previous topography, rainfall, and
development conditions, rather than proactively accounting for
present or future conditions.

This approach comes with the risk of designing and constructing
buildings and infrastructure under outdated conditions, ultimately
heightening the vulnerability of these structures to flooding.
Additionally, many communities manage flood risks through dams and
floodways, which influence how flood waters flow and which may not
be captured in FEMA maps.

Special challenges for floodplain mapping in urban areas

Urban areas may pose specific, pressing risks. FEMA floodplain
maps primarily focus on capturing river and coastal flooding, while
often overlooking the inundation caused by intense bursts of
rainfall, referred to as pluvial flooding. This is a concern for
cities where many permeable surfaces have been replaced with impermeable
ones, like concrete and tarmac.

FEMA floodplain maps are also not well-suited to account for
urban stormwater flooding triggered by heavy rainfall and flash
floods, which are becoming more frequent and severe. FEMA reports
that 40% of claims filed through its
National Flood Insurance Program come from individuals residing
outside the boundaries of the 100-year floodplain in urban areas,
meaning in areas that were not previously considered at risk. FEMA
currently categorizes substantial portions of these regions as
having a flooding probability of less than once every 500 years.

Failing to account for the effects of climate change

FEMA has proposed regulatory amendments to implement the Federal Flood Risk Management
Standard (FFRMS) and revamp the agency’s eight-step
decision-making process for floodplain reviews, as detailed on federalregister.gov. Under FFRMS, federally funded
projects would be required to include margins of safety for flood
protection or use future climate projections to inform siting and
design decisions, e.g., a climate-informed science approach (CISA). This can help minimize
the likelihood of flooding during a project’s entire design
life.

However, until those updates are made, reliance on FEMA maps to
plan for the effects of climate change poses serious risks to
infrastructure and people. Hurricane Harvey’s impact on the
Gulf Coast of Texas in 2017 provides an illustrative example. The
Harris County Flood Control District, encompassing the city of
Houston, discovered that 50% of the 204,000 homes affected by
flooding were situated outside the federal hazard zone as
defined by FEMA. While Hurricane Harvey was a 500-to-1,000-year
event, these types of extreme climate events are becoming more common.

States and local governments are also behind when it comes to
planning for the effects of climate change. Many states and cities
aren’t yet adopting building or zoning codes that consider how
sea-level rise or flooding will affect future development. Just 35%
of states and local governments have adopted building codes that
incorporate basic flood resilience features, according to
FEMA.

While federal maps can serve as tools to communicate risk, they
frequently suffer from incompleteness or obsolescence, especially
in the context of a changing climate. In cases of map inaccuracies,
the minimum building codes may only be enforced within an area smaller than the actual
100-year floodplain, potentially exposing new construction to
increased vulnerability.

Trends in independent flood studies

To permit presently low-risk developments in areas where climate
change may heighten flood risk in the coming decades, in-depth
flood studies can help account for existing and projected climate
change. Several states and municipalities have taken proactive measures in
creating their own flood mapping initiatives, as a means of
developing a better picture of their flood hazard boundaries.

For instance, Mecklenburg County and the city of Charlotte in
North Carolina have been incorporating both
current and future floodplains into their maps since 2000.
Additionally, the Harris County Flood Control District has collaborated with FEMA to update its maps, with
the forthcoming release being the nation’s first to include
considerations for urban stormwater flooding.

Another approach involves the creation of “river corridor” maps to
determine flood pathways. These maps account for the fact that
rivers change vertically and horizontally over time. While these
river corridor maps may overlap with federal flood zones in some
cases, they often encompass larger areas. In some towns and at the
state level, these river corridor maps have been adopted into
zoning regulations. Consequently, when planning construction and
setting building standards, these river corridors must be
considered — although they may not be as widely recognized or
integrated into policies as FEMA maps.

Updated flood maps can also offer a more accurate representation
of a building or property’s vulnerability to flooding. Various
factors influence the 100-year flood event boundary. New
construction, community development, or natural phenomena like
shifts in weather patterns and terrain can cause changes in water
flow and drainage patterns.

Tips for creating in-depth floodplain studies

It’s important to address several key questions when
reviewing floodplain maps and conducting studies:

• What insights can be gained from current FEMA floodplain maps
  and associated studies, e.g., FEMA insurance studies?




• Did FEMA’s maps and methodologies accurately reflect
  current conditions?




• What distinctions arise when assessing the role of land
  subsidence in floodplain delineation (changes in topography)?




• How do alterations in watersheds, such as development and
  subsidence, influence floodplain boundaries?




• What are the relevant takeaways from the impact of climate
  change on FEMA floodplain maps and the methodologies employed?




• What methodologies can be employed to evaluate the impact of
  climate change on FEMA’s floodplain maps and assess flood
  risk?

An in-depth floodplain study can provide a more precise
assessment of flood risk tailored to the context of specific
development locations and rate zones. An in-depth study also equips
stakeholders with more reliable data for long-term planning,
empowering informed decision-making for site selection,
construction, and investment, which are crucial for sustainability
and resilience in the face of evolving flood risks and climate
change.

The content of this article is intended to provide a general
guide to the subject matter. Specialist advice should be sought
about your specific circumstances.