Tidal Fluctuations in Groundwater Level Normal to a Salt-marsh-fringed Shoreline, Herring River Estuary, Wellfleet

Portnoy, John; Martin, Larry

In preparation for some level of tidal restoration in Wellfleet’s diked Herring River, structures that occur within the 6-ft-NGVD contour are being individually assessed for potential impacts. This study was prompted by concerns for land-surface flooding at a house (hereafter “Nieski house”) located near Snake Creek at the edge of the Mill Creek flood plain, which is tributary to Herring River (Figure 1). The study was specifically designed to assess whether an impermeable berm placed on the wetland surface adjacent to the Nieski house would be effective in preventing flooding of the yard, ranging in elevation from 2 to 5.5 ft-NGVD, assuming that spring high tides in Herring River and Mill Creek were restored up to 6-ft-NGVD. The working hypothesis was that the river’s tidal fluctuations would be dampened in the groundwater system such that the high-tide groundwater rise behind the berm would not reach the wetland or yard surface. The hypothesis was tested by monitoring groundwater fluctuations for 15 days (a full springneap cycle) in wells at various distances from the river bank just seaward of the Herring River dike, where tidal range is about nine feet. In other respects, i.e. peat depths and hydrology, the study site is very similar to Mill Creek. With tidal restoration, the expected tidal range in Mill Creek would be much smaller; therefore, observed groundwater effects should be considered a worst-case scenario with reference to properties on Mill Creek.