Effects of Sea Level Rise on Storm Surge Flooding and Current Speeds in New Hampshire Estuaries

The effects of sea level rise on storm surge energy transformation and flood and ebb current magnitudes are examined in two distinct New Hampshire estuarine systems. The Great Bay estuary is characterized by strong tidal dissipation along a long (13 km) and deep (20-25 m) rocky channel (ebb-dominated Piscataqua River) that connects to a large (flood-dominated) estuarine bay with extensive mudflat areas, whereas the (ebb-dominated) Hampton/Seabrook estuary has minimal tidal dissipation through a short (1 km) and shallow (5-7 m) sandy inlet that connects to an extensive salt marsh with narrow tidal channels. Numerical simulations are conducted using the finite-volume coastal ocean model (FVCOM) with forcing provided by the tides, with and without 0.01 annual exceedance probability storm surge estimated by the North Atlantic Comprehensive Coastal Study (NACCS) and further with and without sea level rise scenarios for year 2060 in Hampton/Seabrook (0.73 m) and 2100 in Great Bay (1.92 m). Results for the Great Bay estuary show that although the maximum sea surface elevation is higher during storm events, upstream linear wave energy loss (about 50%) of the storm surge with and without sea level rise is similar to tidal wave energy loss under present nonstorm conditions. Corresponding depth-integrated currents increase by 10%-30% with sea level rise, 23%-52% with the storm surge, and 32%-97% for the combined event. However, results from the Hampton/Seabrook estuary show that energy loss through the inlet increases from 2% to 4% for no storm and present-day sea level to 30%-40% for storm surge with sea level rise, partially mitigating inland inundation. Depth-integrated current magnitudes in the Hampton/Seabrook inlet increase by a factor of 4 under sea level rise and storm surge. Model results suggest that sea level rise has significant impacts on current speeds in both estuaries and that the energy decay of the tidal wave and storm surge depends on the nature of the estuarine system, with greater change associated with estuaries with shallow, narrow inlets somewhat reducing the effects of inland flooding.