Changing face of floodplains

The Changing Face of Floodplains

From Acts of Man to Mother Nature

Floodplains and Flooding

As more growth occurs and climate change worsens, a hidden threat is being exposed. New estimates suggest that the flood risk in the United States is far greater than government estimates show. These estimates include sea-level rise, rainfall, and flooding along smaller creeks — not currently mapped federally. According to the First Street Foundation Flood Model, some 14.6 million properties may be at risk of what experts call a 100-year flood, which is far more than what federal government flood maps show (8.7 million)[i]. In addition, Federal Emergency Management Agency (FEMA) maps are not designed to account for flooding caused by intense rainfall that is becoming a growing problem due to the warming atmosphere. McGill has seen the effects of intense rainfall in our Appalachian communities, where flooding is exacerbated by the mountainous terrain.

Changing Views of Floodplains

Stream relocationOver the course of the 21st century, the focus has shifted away from mitigating the impacts of flooding to more of a resiliency approach that accepts economic loss and environmental damages. Typical floodplain strategy has been to alter or confine streams and rivers to our predefined shapes and sizes, maximizing the extent of developable land for business and agriculture. Today, the focus is on creating highly functioning floodplains. McGill’s work on stream restoration helps to restore these natural floodplain dynamics.

Characterizing the Acts of Man

Floodplains serve as storage for flood water. If storage space is blocked by fill material or development, future flooding may worsen. Water resource engineers review projects to ensure that construction activities do not adversely alter the floodplain and therefore impact neighboring areas. In mapping floodplains, hydrologists identify areas at-risk by how likely it is for a flood event to happen in a certain time period. For example, a 100-year flood elevation would indicate a 1% annual chance of experiencing flood conditions, whereas a 10-year flood elevation would indicate a 10% annual chance. FEMA requires regulation of floodplain development based on existing conditions; however, future development will create more impervious cover, causing more runoff, thus increasing flood frequency.

Beyond what FEMA regulates, some communities may choose to regulate development based on modeling of future conditions. This may be wise considering the number of disincentives to protect floodplains from development, such as lack of enforcement, policies that allow for below-cost flood insurance and payouts for repetitive claims, and outdated risk assessments. FEMA is attempting to decrease these disincentives by including additional metrics, such as river overflow, storm surge, and heavy rainfall in its insurance pricing methodology, Risk Rating 2.0, effective October 1, 2021, which also includes new flood hazard information from private data sets and catastrophe models[ii].

Project Example: Loves Creek Greenway Letter of Map Revision (LOMR), Town of Siler City

Loves creek greenway siler cityEven projects with the best of intentions can impact the floodway; therefore, construction activities in the floodplain must always be handled with care. McGill provided construction engineering and inspection (CEI) services for the construction of 1.4-mile Loves Creek Greenway project, which is a 10-foot-wide paved multi-use path following the Town of Siler City’s existing Love’s Creek sewer outfall along Alston Bridge Road. The project was primarily located within the 100-year floodplain of Loves Creek. McGill’s water resources team provided hydraulic analysis of the Loves Creek floodplain to correct the effective FEMA model for the area and update the effective Flood Insurance Rate Map (FIRM) maps for post project conditions. Changes to the floodplain mapping were the both the results of an erroneous effective model and the installation of a greenway along Loves Creek, which included cut and fill sections.

Measuring the Effects of Mother Nature

Climate change is projected to drastically alter characteristics of floodplains, from rising sea levels to heavier rain events, contributing to increased flooding risk and resulting in changes to current floodplain maps. Current information on the impacts of climate change is mostly general in nature and not tailored for use in floodplain management and planning. Facts such as this — historical precipitation changes have contributed to approximately one-third of cumulative flood damages over 1988 to 2017 with the collective impact of precipitation change totaling $73 billion in damages— call us to action, but do not lead to results[iii].

Decision-makers should consider the multiple measurable characteristics in planning for future flooding. Washington state has taken an innovative approach to educating local leaders. The Climate Impacts Group in collaboration with the Nature Conservancy and Floodplains by Design has created illuminating climate change fact sheets to educate public policy makers[iv]. These fact sheets graphically display specific, easy-to-digest data that reflects the overall impact of a warming climate, from increases in storm frequency, to streamflow volume. Similarly, McGill’s engineers help local entities plan for and adapt to climate change through hazard mitigation planning and project design inclusive of future flooding risks, which educates local government officials through our work.

Improving Floodplain Mapping Accuracy

Prior to working for McGill, Michael Hanson, Director of Water Resources, directed the quality assurance and quality control contract for the North Carolina Flood Mapping Program for eight years. This mapping program is part of a formal agreement between FEMA and the State of North Carolina to modernize flood maps by designating North Carolina as a Cooperating Technical Partner. Thanks to the efforts of this state program, floodplains are mapped with greater accuracy. Michael brings his extensive experience with floodplains to McGill’s highly skilled water resources team. Each time our team supplies updated information to the State — as part of the permitting process for a project — these maps become more accurate.

More Education on Floodplain Management

Lamyaa negm, phd
Lamyaa Negm, PhD, EI

This year McGill’s Lamyaa Negm attended the North Carolina Association of Floodplain Managers Fall Floodplain Institute in Cherokee County, an area of the country hard hit by flooding. At events like this, McGill’s staff have the opportunity to continue their education, connect with regulatory authorities, and take the exam to become a Certified Floodplain Manager (CFM).

Properly managing development in floodplains and restoring storage and flow capacity will be one way municipalities can prepare for changes associated with a warming climate and increased development. Our team of consulting engineers is prepared to assist local governments and private developers in the design and construction of sustainable projects that take future flood risks into account.

Our team is currently assisting the Town of Canton to recover from the flooding impacts brought on by Tropical Storm Fred in August of 2021 through assistance with flood development permitting and recovery planning to move structures out of harm’s way, where feasible, or make existing structures more resilient to future flood events.

 

[i][i] https://floodfactor.com/methodology
[ii] https://www.fema.gov/flood-insurance/risk-rating
[iii] https://www.pnas.org/content/118/4/e2017524118
[iv]https://cig.uw.edu/our-work/decision-support/floodplains-by-design

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