This project will address the problem of recurrent, shallow flooding in low-lying coastal communities. As local sea level rise (SLR), land subsidence, and heavy rainfall events increase, so does the frequency of flooding in low-lying coastal areas. The tidal cycle now takes place on higher average sea levels, resulting in “sunny-day” flooding of roadways during high tides. Sea water also infiltrates stormwater drainage systems at low tidal levels, such that ordinary rainstorms lead to flooding. While these minor floods draw less attention than catastrophic storms, their high frequency imposes a chronic stress on coastal communities and economies by disrupting critical infrastructure services. However, information on recurrent, minor floods is scarce because they are hyper-local – often affecting a city block or intersection at a time – and relatively brief in duration. Further, prediction of chronic flooding is hindered by a poor understanding of subterranean contributions, non-tidal hydrodynamic contributions, and their interactions with tides and SLR. This problem is particularly critical for (but not unique to) communities in North Carolina (NC), many of which experience chronic flooding during non-King tide events and are seeking engineered interventions for adaptation.
The proposed work integrates outreach and research activities over the two-year project period to improve our prediction and communication of chronic flood hazards to stakeholders in the Town of Carolina Beach, NC (CB), a community plagued by chronic flooding. We will couple an existing high-resolution hydrodynamic model with a stormwater management model to hindcast and test hypotheses on the drivers of chronic, and sometimes unexpected, flood events in CB. In parallel, we will deploy a real-time flood sensor network in CB to continuously measure the stormwater network capacity and fill data gaps on the incidence and causes of chronic flooding. In response to the expressed need from local officials, we will also use the in-situ data to develop an early-warning system and engage community members to co-develop flood-mitigation design scenarios for future testing using the new model framework.
K Anarde, M Hino, A Gold, JC Dietrich. “Identifying the drivers of chronic coastal flooding: a community-centric approach.” National Oceanic and Oceanic Administration, North Carolina Sea Grant, 2022/02/01 to 2024/01/31, $119,411.
With sea-level rise, flooding in coastal communities is now common during the highest high tides. Floods also occur at normal tidal levels when rainfall overcomes stormwater infrastructure that is partially submerged by tides. Data describing this type of compound flooding is scarce and, therefore, it is unclear how often these floods occur and the extent to which non-tidal factors contribute to flooding. We combine measurements of flooding on roads and within storm drains with a numerical model to examine processes that contribute to flooding in Carolina Beach, NC, USA — a community that chronically floods outside of extreme storms despite flood mitigation infrastructure to combat tidal flooding. Of the 43 non-storm floods we measured during a year-long study period, one-third were unexpected based on the tidal threshold used by the community for flood monitoring. We introduce a novel model coupling between an ocean-scale hydrodynamic model (ADCIRC) and a community-scale surface water and pipe flow model (3Di) to quantify contributions from multiple flood drivers. Accounting for the compounding effects of tides, wind, and rain increases flood water levels by up to 0.4 m compared to simulations that include only tides. Setup from sustained (non-storm) regional winds causes deeper, longer, more extensive flooding during the highest high tides and can cause floods on days when flooding would not have occurred due to tides alone. Rainfall also contributes to unexpected floods; because tides submerge stormwater outfalls on a daily basis, even minor rainstorms lead to flooding as runoff has nowhere to drain. As a particularly low-lying coastal community, Carolina Beach provides a glimpse into future challenges that coastal communities worldwide will face in predicting, preparing for, and adapting to increasingly frequent flooding from compounding tidal and non-tidal drivers atop sea-level rise.