Storm Surge is an abnormal rise of water generated by offshore storms, over and above the predicted astronomical tides. This rise in water level can cause extreme flooding in coastal areas particularly when storm surge coincides with normal high tide.

Sea Level Rise is the increase in the average height of the ocean's surface.

This illustration shows the 100-year coastal storm surge elevation as modeled in the study. The lower gray area represents a seawall, which protects against daily high tides. The highest tide of each day over a 19-year period is known as Mean Higher High Water (MHHW). In the case of a 100-year coastal storm or a 6.8 foot tidal surge, the house in this illustration would have water nearly to the top of the first floor. Portsmouth residents are well accustomed to the effects of the tides and coastal storms. However, thus far we have not experienced the direct impacts from a 100-year coastal storm, which would add nearly 7 feet to the water elevations along the Piscataqua River and Sagamore Creek.

The Coastal Resilience study modeled 4 sea level elevations to representing various combinations of sea level rise and storm surge. The scenarios are based on two scenarios of global greenhouse gas emissions: a low emissions scenario ("best case") and a high emissions scenario ("worst case"), projected forward to the years 2050 and 2100. Even in the best-case emissions scenario our sea level could rise 2.5 feet by 2100. The report considers adaptation strategies including flood walls, tide gates, culvert replacements, and elevating roadways and properties. Putting in flood protection measures and elevating infrastructure in advance of flooding would protect low lying properties and result in a net savings.

This Illustration shows the modeled scenarios (all elevations are relative to current mean sea level).

• 7.5 feet is about 3 feet higher than today's normal high tide and approximates the Mean Higher High Water (MHHW) in 2100 under the best case scenario
• 11.5 feet is close to the present day 100-year coastal flood at high tide, and also corresponds to the normal high tide in 2100 under the worst case scenario
• 13.5 feet represents the 2050 100-year storm surge at high tide under the worst case scenario, and the 2100 100-year storm surge at high tide under the best case scenario
• 18 feet corresponds to the 2100 100-year storm surge at high tide under the worst case scenario