Landslides risk in New Hampshire

Landslide Risk in New Hampshire: How This Hazard Behaves Here

New Hampshire is a small, steep, heavily forested state, and its landslide risk grows out of that terrain. Glaciers scoured the White Mountains and left behind steep valley walls, thin rocky soils and slopes that sit close to the angle at which they can no longer hold. Landslides are far less frequent here than in places such as California or parts of Appalachia, which is why New Hampshire rates at a relatively low level for this hazard. Where they do occur, however, they can be sudden and locally destructive.

The most exposed ground includes the White Mountains and Presidential Range, the steep walls of Franconia Notch and Crawford Notch, and the valley and ravine communities that sit beneath them. Steep bedrock slopes, glacial till and highway road cuts are all settings where rockfalls, debris slides and debris avalanches are possible. In southeastern New Hampshire, parts of the coastal lowland are underlain by glaciomarine clay that can lose strength when saturated and slump or flow. Steep bluffs along lakes such as Winnipesaukee and Squam, and along the Connecticut and Merrimack river valleys, can also fail when water erodes their base or saturates their soils from above.

Timing follows the seasons:

• Late winter and early spring: snowmelt and rain-on-snow can saturate slopes and raise stream banks, triggering slumps on steep embankments and road cuts.
• Summer: slow-moving, high-intensity thunderstorms can drop several inches of rain in a short time, producing debris flows in steep ravines.
• Late summer and fall: the remnants of tropical storms and nor'easters deliver prolonged heavy rain, the pattern most often linked with larger slope failures in New England. In October 2005, extremely heavy rain around Alstead in southwestern New Hampshire produced numerous debris flows and slope failures.
• Late fall and winter: repeated freeze-thaw cycles loosen rock on steep faces, increasing the chance of rockfall onto roads, driveways and homes below.

Landslides are also sometimes human-influenced. Excavating at the toe of a slope, adding fill or structures at the top, concentrating roof and driveway runoff, or letting a leaking water line or septic system saturate a hillside can all tip a marginal slope toward failure.

How It Affects Homes

Damage to New Hampshire homes typically comes in one of two ways: the ground moving beneath a structure, or material moving into it. A slope failure below a house can undermine a foundation, crack basement walls, break water, sewer and septic connections, and destroy driveways, retaining walls and septic leach fields. Rockfall and debris flows can strike a building directly, breaking windows, punching through walls, damaging roofs and siding, and burying vehicles, decks and outbuildings. Even relatively small movements may leave doors and windows sticking, floors out of level or new cracks appearing in masonry.

Common damage patterns include:

• Cracked or bowed foundation walls and offsets in concrete slabs
• Separation of a house from an attached garage, deck or chimney
• Failed retaining walls, driveway washouts and undermined footings
• Broken utility lines, wells and septic systems
• Impact damage from boulders, trees and debris carried in a flow

Insurance is the part many owners are surprised by. Standard homeowners policies generally exclude damage from earth movement, including landslides, mudslides, settlement and sinkhole collapse. Federal flood insurance through the NFIP covers mudflow, which it defines as a river of liquid and flowing mud, but it does not cover landslides or slope failure. Some private insurers offer limited endorsements or surplus-lines options for earth movement, and coverage wording varies widely. Read your policy, ask your agent directly whether landslide, mudflow and debris flow are covered, and photograph your property before and after any event.

How to Prepare Your Home

Preparation is mostly about slope stability and water management. Practical steps include:

1. Know your slope. Watch for new cracks in the ground, soil pulling away from foundations, tilted trees or utility poles, springs appearing on a hillside, or a bulge at the base of a slope. These are warning signs worth investigating.
2. Move water away from slopes. Keep gutters and downspouts clean, and extend discharge well away from foundations and hillsides. Never dump roof, driveway or sump water onto a steep slope.
3. Fix leaks quickly. Repair leaking water lines, irrigation and septic systems, and seal cracks in driveways and patios so water does not soak into a hillside.
4. Keep deep-rooted vegetation. Native trees and shrubs help hold soil in place. Remove dead or leaning trees that could fall, but avoid clearing an entire slope or stripping it of ground cover.
5. Do not alter a steep slope without expert advice. Adding fill at the top, cutting into the toe, or building a wall can increase risk. Have a licensed geotechnical engineer or engineering geologist evaluate the site and follow local permitting requirements.
6. Maintain retaining walls and drainage. Engineered walls need working drain pipes and weep holes. A saturated wall is a failing wall.
7. Plan for heavy rain. During and after intense storms, stay off steep slopes and out of ravines and their outlets. Listen for cracking trees, knocking boulders or a low rumbling sound.
8. Evacuate when advised. If local officials issue an evacuation order, leave immediately. Follow guidance from the New Hampshire Department of Safety and your town emergency management office.

Frequently Asked Questions

Are landslides common in New Hampshire?

Large landslides are uncommon, which is why the state rates at a relatively low level for this hazard compared with mountainous western states. Small rockfalls, debris flows and streambank slumps do occur, especially in the White Mountains, in steep road cuts and along river and lake bluffs. Risk is very site-specific: a home on flat ground may face almost none, while a home below or on a steep, saturated slope faces considerably more.

Does homeowners insurance cover landslide damage in New Hampshire?

Usually not. Most standard homeowners policies exclude damage caused by earth movement, including landslides, mudslides and settlement, and federal flood insurance covers mudflow but not landslides. That means repairs are often paid out of pocket unless you have a specialty policy or endorsement. Ask your agent in writing what your policy does and does not cover, and keep dated photos and receipts of your property and any improvements.

What should I do if I see signs of a landslide on my property?

Move people and pets away from the area first, and call your town or city emergency management office if there is an immediate threat to life or property. Then have a licensed geotechnical engineer or engineering geologist assess the slope before you repair, regrade or rebuild. Document the damage with photos and contact your insurance agent promptly. Do not try to stop an active slide with equipment or by piling material at the toe of the slope, as this can make the situation worse.

Official Resources

• New Hampshire Homeland Security and Emergency Management — state emergency preparedness, alerts and disaster information.
• USGS Landslide Hazards Program — science, maps and landslide preparedness guidance.
• Ready.gov – Landslides and Debris Flow — federal guidance on what to do before, during and after a landslide.