Nuisance Flooding and Relative Sea-Level Rise: the Importance of Present-Day Land Motion

Sea-level rise is beginning to cause increased inundation of many low-lying coastal areas. While most of Earth’s coastal areas are at risk, areas that will be affected first are characterized by several additional factors. These include regional oceanographic and meteorological effects and/or land subsidence that cause relative sea level to rise faster than the global average. For catastrophic coastal flooding, when wind-driven storm surge inundates large areas, the relative contribution of sea-level rise to the frequency of these events is difficult to evaluate. For small scale “nuisance flooding,” often associated with high tides, recent increases in frequency are more clearly linked to sea-level rise and global warming. While both types of flooding are likely to increase in the future, only nuisance flooding is an early indicator of areas that will eventually experience increased catastrophic flooding and land loss. Here we assess the frequency and location of nuisance flooding along the eastern seaboard of North America. We show that vertical land motion induced by recent anthropogenic activity and glacial isostatic adjustment are contributing factors for increased nuisance flooding. Our results have implications for flood susceptibility, forecasting and mitigation, including management of groundwater extraction from coastal aquifers.

. Map showing the location of GPS sites and 18 regions (boxes) for which the geologic rate is known. Circle color indicates decadal average vertical land motion in IGb08 reference frame. Black triangles are locations of tide gauges for which nuisance flooding level data and nuisance flooding frequency are available. The GPS rates and nuisance flooding level data shown in Figure 2 and Figure S2-S5 are average values for all stations in the boxes. Map is 3 generated using GMT software version 5.1.0 (http://gmt.soest.hawaii.edu/) (Wessel et al. 2013). Modified from Karegar et al. (2016). Figure S2. Comparison of spatially averaged GPS (gray triangles), geologic data (red circles), and GIA model ICE6G-VM5a (green circles) for eighteen coastal sites in the US and southern Canada ( Figure S1). Error bars are 1σ.
4 Figure S3. Comparison of nuisance flood level (red circles) as standardized by tidal range (MHHW-MLLW), GPS rate (blue triangles) and geologic rate (green circles) as a function of latitude along the US eastern seaboard. Figure S3 is an alternative plot to Figure 2. Here the possible effects of tidal range variations are isolated by dividing the nuisance flood level (measured from MHHW) and tidal range. As in Figure 2, the same relationships are seen between nuisance flood level and GPS rate. Note that the GPS rates and standardized nuisance flooding level data are averaged for all stations and tide gauges in the boxes shown in Figure S1 where geologic data are available. Figure S4. Comparison of GIA-corrected GPS-derived vertical rate (red dots) and average trend in groundwater-level changes (gray dots). The black and red solid curves are quadratic polynomials fit to the groundwater and vertical rate data, respectively. Figure S5. Map of Lake-Dam system in Quebec, Canada. Lakes with water-level data (virtual gauges) are numbered. For time series see Figure S6. Lake area and annual rate of water-level change are listed in Table S2. Map is generated using GMT software version 5.1.0 (http://gmt.soest.hawaii.edu/) (Wessel et al. 2013).
8 Figure S6. Time series of water-level change from satellite altimetry measurements produced by different processing centers. Lake level products are courtesy of (a) USDA/NASA G-

REALM. (b) database for Hydrological Time Series of Inland Waters (DAHITI) (c)
HYDROWEB database from Legos and THEIA platform. A least squares model fit was used to 9 define the rate. Model parameters include an initial offset, a constant rate and fixed amplitude annual variation.   (Wessel et al. 2013). Table S1. Statistics of GRACE TWS trends (in equivalent water height) based on three GRACE solutions (CSR, GFZ, and JPL) and two post-processing products including Tellus gridded GRACE TWS data (isotropic filter: Gaussian + de-stripping filters) and non-isotropic filter (DDK2 filter). Unit: mm yr -1 . 1 G: Gaussian filter, De: de-stripping filter

CSR
-6.7 ∕ -9.9 13.1 ∕ 17.9 4.3 ∕ 6.1 6.0 ∕ 8.5 GFZ -6.6 ∕ -11.0 13.9 ∕ 18.5 4.7 ∕ 6.3 6.4 ∕ 8.8 JPL -6.2 ∕ -10.8 12.0 ∕ 17.5 4.7 ∕ 6.1 6.3 ∕ 8.5 Table S2. Characteristics of lakes and annual rate of water-level change from different satellite altimetry missions and processing centers. The non-parametric Mann-Kendall trend test was applied to the water-level change time series. The P-value of the two-tailed test at the significance level of 0.05 is listed. The trends are considered statistically significant when the p value falls below a critical value (P < 0.05). It is sufficient to conclude that there is a positive trend in the water-level variations for different periods. The total rate of change of water volume from the nine reservoirs listed in Table S2