COASTAL FLOODING & SOLUTIONS, Workshop July 2020
iFlood: A citizen-science approach to understanding groundwater contributions to flooding on barrier islands
Dare County, North Carolina
MIT/WHOI Joint Program, PhD Candidate
Nearly 1.5 million people inhabit barrier islands along the U.S. Atlantic and Gulf Coasts with population densities approximately three times greater than their state averages. Flooding of these low-lying islands, which often is associated with coastal storms, has negative societal and economic impacts. Here, barrier island flooding owing to the combined effects of ocean surge and waves, groundwater levels, and rainfall is examined using citizen-science reports and an analytical model based on in-situ observations.
A citizen-science phone application (app), iFlood, was developed in partnership with the Town of Duck and the Town of Nags Head to collect locations, depths, and photographs of flooding on the North Carolina Outer Banks. Between Sept 2019 and Feb 2020, 34 reports associated with 7 different storms (including 15 reports following Hurricane Dorian in 2019) showed flooding on natural (permeable) land surfaces between Corolla and Rodanthe, NC. Flooding occurred after storms with large ocean surge and swell, with and without significant rainfall.
The iFlood reports are being used to evaluate an analytical model for groundwater contributions to flooding on the Outer Banks. The model was developed using groundwater level measurements at 8 locations spanning the 550-m-wide barrier island near Duck NC, as well as observations of ocean water levels, wave heights, and precipitation collected by the U.S Army Corps of Engineers Field Research Facility (USACE FRF). The observations were obtained from Oct 2014 – 2018 and include 27 storm events (including Hurricanes Joaquin (2015), Hermine and Matthew (2016), Jose and Maria (2017), and Michael (2018)). Infiltration of precipitation results in approximately a threefold increase in the groundwater level relative to the amount of rainfall (porosity ≈ 0.3). Infiltration from tides, surge, and waves causes up to 2 m increases in the groundwater level under the dune about 5 m onshore of the upper beach. The resulting bulge of high groundwater propagates inland at roughly 60 m/d, attenuating with distance from the shore, roughly consistent with linear pulse theory. Owing to the slow movement of the bulge, inland flooding can occur several days after a storm has passed. High groundwater levels can slow infiltration of surface water (e.g., rain, runoff), extending the duration of ponded water.
The analytical model is driven with estimated ocean shoreline water levels (based on the 36-hr-averaged offshore tide, surge, and wave height) and measured precipitation. Drainage and ponding on impermeable surfaces (e.g., roadways, 3 reports) and flooding driven by changes in the water level on the inland (sound) side of the islands (6 reports) are neglected. The model predicts a flooding event that is consistent with the timing and location for 19 of the remaining 25 oceanside iFlood reports. The model will be used to examine vulnerability of the Outer Banks to coastal groundwater flooding resulting from future extreme events, and to provide town managers with information about the processes contributing to flooding in their regions.
Funded by USCRP and an NSF Graduate Research Fellowship. We thank Levi Gorrell and USACE FRF personnel for help maintaining groundwater wells.
The predictions of the simple analytical model for coastal groundwater flooding are consistent with the timing and location of flood reports submitted through the iFlood app. We received 25 reports through the iFlood app that we could use to validate our flooding model. These reports enabled us to expand the geographic scope of our model beyond the site of our field measurements. We received a lot of engagement and support from the local community who sent us the flood reports.
Improved characterization of the geologic structure of the OBX would be useful to identify regions where the flooding model can or cannot be applied.
Sound water-level measurements or simulations might enable the model to be extended to include sound-side flooding owing to overtopping or groundwater processes.