Charlotte, Mecklenburg County – Flooding

After numerous floods and repetitive losses of property, the City of Charlotte, NC and Mecklenburg County decided to adjust their approach to floodplain management. The Charlotte-Mecklenburg County Storm Water Services Division (SWS) took the initiative to research and update maps used to identify flood hazards. Based on these maps, they made recommendations for regulatory oversight and mitigation activities to decrease loss throughout the county.

Project Summary

Mecklenburg County covers an area of 525 square miles in south-central North Carolina, including the city of Charlotte and six incorporated towns. The floodplain management program is run by the Mecklenburg County Storm Water Services Division. Of the 32 watersheds in Mecklenburg County, all water sources originate in the county, with the exception of the Catawba River.

Charlotte-Mecklenburg‘s approach of build-out land use conditions as the basis for floodplain modeling allowed them to use floodplain and construction data as an integral component for their planning process. Previous floodplain management efforts had not considered the changes to the landscape and flooding risks caused by building structures in the floodplain. Build-out land use conditions take into account the change in volume due to construction in the floodplain, which impacts the way water moves through the floodplain during rain events and flooding. The development of Mecklenburg County led to an increase in impervious surfaces, which greatly increased runoff. Communities in the county experienced a high level of flood damage during the late 80s and 90s. Major flooding events from that period include Hurricane Hugo in 1989, which caused $1 billion in damage, a flood in 1995 that caused $20 million in losses, and another large flood in 1997 that resulted in $60 million in losses. The 1997 flood inundated 10 homes with first floor elevations above the base flood elevation. This scenario provided evidence that the county’s current strategy for floodplain management was flawed and a new approach was needed. Charlotte-Mecklenburg adopted the Floodplain Management Guidance Document, which led the county to consider assuming ultimate build-out land use conditions for floodplain mapping. Considering the build-out land use conditions, the floodplain assumes new boundaries, accounting for space taken up by buildings. When this data is used in creating floodplain maps, the results are much more accurate than when ultimate build-out is not considered.

According to the new analysis by SWS, there are approximately 4,000 structures in the county’s regulated floodplains. SWS used a two-pronged approach to reduce loss due to flooding. The first approach is to implement floodplain regulations and the second is to incorporate flood hazard mitigation activities. In terms of regulatory oversight, the SWS provides recommendations to elected officials regarding floodplain ordinances, enforces those ordinances, issues floodplain development permits, reviews building elevation certificates, maintains accurate floodplain maps, and works with the National Flood Insurance Program to identify properties with a high risk of loss.

In addition to regulatory oversight, several flood mitigation efforts were set into motion. A Storm Water Services’ Floodplain Acquisition Program allows for the removal of the highest-risk structures located in the FEMA regulated floodplains along FEMA-regulated streams. The buyout program allows lush, restored wetlands to replace flood prone houses and buildings. Additional flood mitigation activities include: implementing capital improvement projects, encouraging owners to elevate buildings, operating a flood warning system for emergency responders, maintaining a stream and rain gage network, and educating the public about flood safety, insurance, and flood risks.

Hazard identification and mapping is a key component in the county’s floodplain management program. Before adopting the Floodplain Management Guidance Document, the county had been using maps from 1975. Updated maps were needed. SWS researched and quantified the effects of future development in the county’s floodplains and watersheds, and included stormwater runoff variables such as surface and soil conditions. New maps and regulations to address hazard areas are expensive, but the agency concluded that the costs would be offset by avoiding future damage and associated disaster costs. The research and subsequent modeling have been very important to the success of this program. Older regulations allowed development in the floodplain fringe, which would result in an increased base flood elevation of almost 2.5 ft because of the space taken up by development and the runoff it causes. The average base flood elevations based on ultimate build-out were approximately 4.3 feet higher than those on the 1975 land use maps. Setting aside stream buffers, which would improve infiltration of rain and runoff, was projected to decrease flood heights by 0.5 feet.

Overall, Mecklenburg County has used hazard identification and mapping to carry out their floodplain management program. Using updated maps that include the build-out land use conditions allow for a more accurate understanding of flooding across the county. The buyout program to replace flood prone buildings with wetlands improves the absorptive capacity of the floodplain and protects neighboring properties. Public outreach was also included in the program to educate the public on flood risk, safety, and insurance.

Plan Highlights

Environment

Mecklenburg County is located in a flood prone environment. Development within the floodplain during the 1980s and 1990s exacerbated the issue and as a response, the city-county government has implemented a program to mitigate flooding hazards.

  • Mapping floodplains inclusive of ultimate build-out has been critical to the success of the City of Charlotte and Mecklenburg County’s program to mitigate flooding hazards.

Community

The communities within this county are affected by flooding in terms of damage to their homes and personal property as well as damage to public amenities.

  • Public education and outreach has been instrumental in garnering community support. When the community understands the history of flooding and can see the options for mitigation, they are more likely to support measures such as an increase in utility fees.

Lessons Learned

Investing money into hazard research and mapping has proven critical in making informed decisions about floodplain regulations and mitigation efforts. The floodplain management program in Charlotte-Mecklenburg County, NC has relied on such data to achieve significant levels of reduction in flood impacts. For example, SWS estimated that investing $250,000 in floodplain mapping could help prevent $16 million in future flood damage. Although the upfront cost may seem high, it can more than pay for itself by preventing losses.