Poor Tidal Exchange

The water quality problems in Green Hill Pond are exacerbated given the limited exchange of water between Green Hill and Ninigret Ponds and in turn the limited exchange between Ninigret Pond and Block Island Sound, through the existing tidal inlets circled in red in the figure below.

Source: Grilli et al, 2018, A Hydrodynamic Model Study to Assess the Impact of Dredging the Channels Connecting Ninigret and Green Hill Pond on Circulation and Flushing of Green Hill Pond, Charlestown RI.

The tidal range in the ocean (about 1 meter shown in black in the chart below) is strongly damped by about 80% going through the inlet into Ninigret Pond (mean tidal range of about 20 cm shown in blue). It is further damped by about 45% going through the inlet into Green Hill Pond (mean tidal range of about 11 cm shown in red). In total, the ocean tidal range is reduced by nearly 90% by the time it reaches Green Hill Pond. This means that relatively little “clean” ocean water is exchanged with Green Hill Pond during each twice-a-day tidal cycle.

Source: Grilli et al, 2018, A Hydrodynamic Model Study to Assess the Impact of Dredging the Channels Connecting Ninigret and Green Hill Pond on Circulation and Flushing of Green Hill Pond, Charlestown RI.

Little water exchange during tidal cycles means that Green Hill Pond has a long tidal exchange time. For example, compared to Ninigret, the tidal exchange time for Green Hill Pond is over twice as long. The result is that Green Hill Pond has a much slower recovery from loadings of nitrogen and bacteria than other salt ponds. Put differently, Green Hill Pond is much more sensitive to additions of nitrogen and bacteria than other salt ponds.

Source: RIDEM, 2006, Determination of Nitrogen Thresholds and Nitrogen Load Reductions for Green Hill and Ninigret Ponds”

Note: Tidal exchange time is defined as the time required to replace the entire water volume with tidal changes and freshwater additions.