Spatial Variability of Relative Sea Level Rise at Embayment Scale and Impact on Nuisance Flooding

The general spatial variability of relative sea level rise (rslr) along the US shorelines has been quantified based on long term water level measurements collected by NOAA’s National  Water Level Observation Network (NWLON). Long-term rates of rslr combine the effects of several factors including vertical land motion, eustatic sea level rise and changes in regional ocean circulation. One of the benefits of having precisely measured local rates of rslr is the ability for coastal communities to better plan long-term resiliency investments. Along the Texas coast, rslr rates, as measured by the NWLON, are generally higher in the North, e.g. 6.3 +/- 0.2 mm/yr for Galveston Pier 21, and lower in South Texas, e.g. 3.8 +/- 0.4 mm/yr for Port Isabel. These rates should however not be extrapolated much beyond the station locations without additional information due to the high spatial variability of vertical land motion. Coastal geological characteristics and fluid extraction such as groundwater and oil and gas pumping can lead to changes in vertical land motion at the kilometer to few kilometers scale.

The Texas Coastal Bend has been home to over 12 tide gauges from Port Mansfield to the south to Copano Bay to the North, a large number of measuring stations for a coastline less than 200 miles long. Two of the tide gauges are part of the NWLON while most of the other stations are part of the Texas Coastal Ocean Observation Network (TCOON). Except for the NWLON tide gauges, the time series of the gauges are still shorter than the 30 years typically necessary to estimate rslr with sufficient precision. At time scales shorter than 30 years the influence of interannual and decadal variability of the water level signal can become too large. However, because of their relative proximity, the Coastal Bend tide gauges experience similar weather and ocean forcings. This allows to correlate and remove these contributions from these long-term water level time series. The resulting confidence intervals are narrower allowing to better compare nearby locations. rslr trends are estimated for each of the Coastal Bend locations. Rates range from 2.8 mm/yr to 7.2 mm/yr when computed over the 1993-2015 time span. Relative confidence intervals are estimated for these trends and rlsr differences are discussed in the context of their local coastal geological setting. The significance of a 4 mm/yr rslr difference is discussed in terms of changes in inundation frequency particularly inundations related to relatively small surges often labeled as nuisance flooding.