Building resilience to flooding is a complex, and sometimes daunting, task. Inland cities such as Asheville, Raleigh, and Chapel Hill experience floods, but those floods have very different causes than those that impact coastal cities like Wilmington and Charleston.

The complexity of this problem is driven by two principal factors: vulnerability and risk. Many assets, including people and infrastructure, are vulnerable to the threat of flooding. Risk is increasing due to both climate and non-climate stressors; for example, there are four main causes of coastal flooding, and different climate stressors are driving different risks for each of the four.

Using examples from a series of case studies from across the Carolinas, NEMAC’s Director will discuss how using a standardized risk analysis process can quantify the problem and provide assessment results in a format that allows for affordable, easily communicated, and implementable resilience solutions.

The Church Creek drainage basin in Charleston South Carolina is a 15,000-acre area which includes residential and commercial development, several major transportation corridors, expansive wetland systems and large tracts of undeveloped property. Charleston is also a coastal community with tidally affected creeks and rivers adjacent to the referenced drainage basin. Recent storm events have caused major flooding, significant property damage and impacts to transportation. The basin is primarily located within the corporate limits of the City of Charleston who owns and maintains most of the major stormwater drainage infrastructure. Based on flooding, the City implemented a moratorium on development until the drainage issues were better understood. The City commissioned a study to evaluate the existing conditions and to develop solutions to prevent future flooding to enable the area to continue to grow while protecting transportation corridors and private property. ICPR4 was used as a modeling tool based on its ability to simulate the complex hydraulic interactions between the numerous wetland areas and the various stormwater systems, ponds and conveyance channels. A combined 1D/2D modeling approach with a flexible triangular computational mesh was utilized. A host of significant improvement projects were screened to determine their effectiveness in the prevention of future flooding from storms while accounting for outfall impacts due to tidal events. The model provided graphic visualizations that were instrumental in leading the team to effective solutions while keeping the evaluation project on schedule. This is enabling the City to develop a funding structure for those projects while also allowing the development of new policies which protect and mitigate floodplain impacts and protect the drainage capacity restored by the projects that were developed through the hydraulic modeling process.

AThe conveyance and management of stormwater runoff from developed areas are intrinsically linked to climate. Design storms, which typically specify a storm depth for a given duration and recurrence interval based on analysis of historical precipitation data, are central to the engineering analysis and design of stormwater infrastructure throughout the Carolinas. For stormwater conveyance infrastructure, larger design storms associated with climate change could require larger conveyance pipes and increase the prevalence of existing infrastructure classified as deficient. For stormwater control measures (SCMs), larger designs storms will generally require larger facilities to store more runoff and could influence the type of SCM implemented. In both cases, there are clear connections between design storm depth and construction costs. With some inherent uncertainty in climate projections, municipal officials are faced with challenges in determining the extent to which design standards should change to provide climate resiliency.

Using the SWMM Climate Adjustment Tool and similar tools within the US Climate Resilience Toolkit, an analysis of climate projections for locations throughout North and South Carolina provides perspective into the range of potential design storm changes and their impact on the sizing of stormwater infrastructure. Combining this sizing analysis with an evaluation of construction costs provides insight into the potential near term economic impact of these design standard changes. In some cases, the range of projected changes is relatively narrow, simplifying justification of near term cost increases. In cases where projected changes are significant or highly variable, more careful consideration of the costs and benefits is often required. These benefits may include improved performance under current climate conditions and avoided costs of future retrofits, as demonstrated through a hydrologic and hydraulic simulation example.

Many of the challenges associated with stormwater design standard changes lie not in the technical analysis, but in stakeholder priorities and input. To facilitate an understanding of these issues in the Carolinas, audience interaction throughout the presentation using interactive polling and direct audience input will consider the following questions:
— Does your community utilize stormwater design standards that account for climate change?
— Do you think design standards should be more conservative (larger design storms)?
— What is the most significant hurdle to revising design standards?
— What are the benefits of revising design standards?
— What could be done to better facilitate improvements to design standards?

Hurricane Matthew demonstrated once again just how vulnerable North Carolina is to flooding. Torrential rains caused severe flooding that breached levees, closed major roads and inundated entire communities. Thousands of homes in Eastern NC were destroyed. Many of these same homes had flooded before. To reduce the risks of future flooding, communities across the state have been acquiring and removing the most vulnerable homes from the floodplain. These acquisitions, known as buyouts, are funded primarily by FEMA. Since the 1990s, FEMA has funded the acquisition of some 5,000 homes in North Carolina.

Buyouts can permanently reduce a community’s vulnerability to flooding, but they can also reduce the local tax base. Plus, local governments must cover the costs of maintaining the now-vacant properties. And since buyouts are voluntary, not all eligible homeowners participate, thus limiting the use of the acquired lands. The most common use of buyout lands is vacant lots.

The UNC Institute for the Environment is carrying out a study of floodplain buyouts in NC. The goals of the project are to estimate the net financial impacts of buyouts on local governments in North Carolina and to explore the factors that might motivate people to participate in a buyout.

The study is examining the impact of buyouts in eight communities: Charlotte, Greenville, Kinston, Lumberton, Raleigh, Rocky Mount, Seven Springs and Windsor. These communities vary in size from Seven Springs (pop. 110) to Charlotte (pop. 842,051). Thus far, we have met with local officials in six communities, created GIS maps of buyout areas, and collected financial data to estimate the loss of tax base and the costs associated with maintaining the buyout properties. We are also trying to estimate the savings, or avoided losses, the buyouts will provide the next time flooding occurs. That is, what expenses will the community not incur because the acquired homes will no longer be there to flood.

Our preliminary findings suggest the financial impact of a buyout varies by community and depends on a number of factors, including:

• The nature and size of the buyout. For example, many buyouts resemble a checkerboard, with vacant homes interspersed among occupied houses. With this pattern of acquisitions, most communities simply plant grass on the vacant lots and mow regularly.

• Use of the property. Some communities have created a community amenity, such as a park, out of the acquired lands. Others simply lease the isolated, vacant lots to adjacent landowners.

• Where people go. If buyout participants leave the community, then the community suffers a loss of tax base. In many cases, participants are able to find replacement housing within the community.

• What costs get reimbursed. Federal and state grants often cover the bulk of local expenses for disaster recovery. Since our focus is on the fiscal impact of buyouts on local governments, reimbursement by state or federal government affects the calculation of avoided losses.