How Is Tidal Floodproofing Done in Portsmouth NH?

The choice starts with occupancy and flood zone. Residential buildings in high-risk flood zones must use elevation or wet floodproofing - dry floodproofing is not permitted for homes under the program rules, because a barrier that depends on human deployment cannot be trusted to protect sleeping occupants. Non-residential buildings may use dry floodproofing, but only with an engineered design and a certified floodproofing certificate. The engineer explains this fork to the owner early, because it determines whether the project budget goes toward lifting the building or armoring it. Wet floodproofing design is about controlled surrender. The engineer sizes flood openings by the standard area rule, specifies concrete, masonry, and pressure-treated materials below the flood elevation, designs electrical and mechanical systems so everything below the line is submersible or relocated, and details the enclosure so water drains out as the tide falls. Dry floodproofing design is about resistance: the engineer calculates hydrostatic pressure, buoyancy, and hydrodynamic and wave loads on every wall and the slab, designs shields and closures for each opening, and specifies backflow prevention on every drain that connects to the sewer. Both approaches end with a certificate the floodplain administrator files - and both fail if the owner later defeats them, by sealing vents or leaving shields in storage.

Portsmouth's historic building stock makes textbook floodproofing difficult. Elevating a historic masonry building risks cracking fragile walls and destroying its streetscape character; the historic commission will not approve a lift that reads as a house on stilts. The engineer's answer is usually a hybrid: keep the historic massing at its elevation, wet-floodproof the ground level with compatible materials, relocate the building's systems to an upper floor or a new rear addition above the flood elevation, and install discreet deployable barriers at the historic openings for moderate events. Structural work on these buildings demands the preservation engineer's touch. Anchoring a flood shield frame into 18th-century brick requires through-bolting details that spread load without shattering historic masonry. New interior flood walls must not trap moisture against old walls. And any excavation for drainage or utilities near historic foundations is monitored for settlement. The engineer documents existing conditions thoroughly before work begins - photographs, crack surveys, elevation benchmarks - so that any change during construction is caught immediately and the historic record is protected.