Start with the property address, not a generic “sinking city” label. Check official flood maps, expected flood levels, drainage and sewer information, and local elevation rules; then ask the municipal floodplain manager whether an elevation certificate exists. Match each measure to the water source: elevate the structure for recurring deep floods, use walls or closures only where engineered and permitted, protect utilities, and use wet-floodproofing materials where water may still enter. Rain barrels and rain gardens can reduce runoff on the lot but cannot stop river floodwater, coastal surge, or a city-wide drainage failure. Keep flood insurance and an evacuation plan even after construction work.
Why subsidence changes flood risk
“Sinking city” describes a condition, not a single flood mechanism. Subsidence can lower a building relative to nearby streets, rivers, tides and drainage outlets, but the amount and pattern of sinking vary by neighborhood and lot. A home can face several hazards at once: intense rainfall that ponds on the property, sewer backup, a river rising above its banks, coastal surge, or water pushed inland through an overloaded drainage network.
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New Orleans is a useful documented example, not a template for every city. The city describes itself as low-lying, with little elevation change across much of the city, and says subsidence is causing many areas to sink farther. It also explains that heavy rain can exceed the ground’s ability to absorb water and the drainage system’s capacity to remove it. Subsidence has damaged roads and drainage systems, contributing to greater flooding and infrastructure-maintenance needs. New Orleans reports an average of 62 inches of rain each year; the page does not state a year for that figure, so it should be treated as a New Orleans-specific average rather than a general statistic for sinking cities.
Because subsidence is uneven, a city-wide description cannot tell you the water depth at your front door. The first decision is therefore an address-level assessment.
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1. Establish the actual risk at your address
Read the official flood maps
Look up the property on your national, state or municipal flood-map service. Record the mapped flood zone, the expected flood source, the design flood elevation or equivalent local reference, and whether the map distinguishes river, coastal, rainfall or drainage hazards. A map is a planning baseline, not a guarantee: local drainage failures and changing ground levels can produce water outside a mapped boundary.
Ask the municipal floodplain manager or building department these questions before buying equipment or commissioning structural work:
- What flood sources affect this parcel: rainfall ponding, sewer backup, river flooding, coastal surge, or several?
- What water-surface elevation and flood depth should be used for design?
- What freeboard, elevation, floodproofing and utility rules apply to this lot?
- Does substantial renovation trigger the same requirements as new construction?
- Which permits are required for an elevated foundation, flood wall, closure, drainage change or utility relocation?
- Are there local requirements for maintaining access, neighboring drainage and emergency egress?
Do not infer your home’s flood depth from a city-wide subsidence statement. Two adjacent properties can have different grades, foundations, drainage paths and finished-floor elevations.
Find or commission an elevation certificate
National Flood Insurance Program guidance describes an elevation certificate as a record of a building’s height compared with expected local floodwaters. Ask the floodplain manager whether a certificate already exists in permit or insurance files. If it does not, ask which qualified professional is accepted locally. A surveyor, engineer or architect can prepare one when needed.
The certificate can document the lowest floor, machinery and finished ground relative to the mapped flood elevation. It is useful for design discussions and for providing insurers with accurate elevation information, but it does not replace a current map, an engineering design or a permit decision. If the city is subsiding, ask whether the datum, benchmark and survey date remain appropriate for the project.
Inspect the ways water can reach the building
Walk the lot during and after a heavy storm if it is safe. Note where water collects, whether it flows toward doors or vents, how quickly it drains, and whether downspouts discharge against the foundation. Check floor drains, cleanouts, sump equipment and backflow protection. Then inspect the building’s lowest openings, including garages, crawl-space vents, window wells, utility penetrations and stairwells.
For sewer risk, ask the utility or public-works department about backflow protection and whether private plumbing must be modified by a licensed professional. A device that blocks sewage does not stop rain or river water entering through doors and walls, so it is one part of a layered plan rather than a complete flood solution.
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Prepare an evacuation plan
New Orleans recommends an evacuation plan along with flood insurance and physical mitigation. Write down who leaves, when, where everyone goes, how pets and people with mobility needs travel, and which routes remain usable if bridges or low roads close. Keep fuel, medications, chargers, vital records and a way to receive official warnings ready to take.
Never make a barrier, pump or elevated appliance a reason to delay evacuation. Electrical hazards, contaminated water and rapidly changing conditions can make a seemingly shallow flood dangerous.
Move contents before water arrives
When an official warning leaves enough time, move documents, photographs, electronics, medicines and other irreplaceable items to an upper floor or a location outside the flood area. Put movable furniture on blocks only when doing so does not create a lifting hazard. Keep a written inventory and photographs of rooms and equipment for insurance purposes.
Store fuel, paints, pesticides, gas cylinders and other hazardous products above expected flood levels or secure them so they cannot float away. Do not enter a flooded building to retrieve property while electricity may still be energized.
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3. Use low-cost runoff measures for the hazards they can actually address
Rain gardens
A rain garden is a planted, shallow depression that temporarily holds roof or driveway runoff and lets suitable soils absorb some of it. Place it where overflow will not be directed toward the foundation, septic system or a neighbor. Confirm soil infiltration, underground utilities, setbacks and local drainage rules before digging.
Rain gardens can reduce the volume and speed of water leaving your lot during a rainfall event. They do not protect a home from deep river flooding, coastal surge, or water that rises through streets when the public drainage system is overwhelmed.
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Rain barrels
New Orleans identifies rain barrels as a simple property-level way to reduce runoff. A barrel can capture part of a roof’s runoff for later landscape use, provided it has a screened inlet, a secure lid, an overflow routed away from the foundation and a stable base. Empty or manage the stored water before a forecast storm so the container has capacity.
A barrel’s storage is small compared with the water from a severe storm. It cannot stop floodwater arriving from outside the property, compensate for a river or coastal surge, or solve a neighborhood-wide drainage failure. Treat it as a runoff supplement, not flood protection for the building itself.
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- Keep gutters, downspouts, swales and approved inlets clear of leaves and sediment.
- Extend downspouts far enough to prevent discharge at the foundation, subject to local rules.
- Maintain pumps, alarms and backup power according to the manufacturer’s schedule.
- Do not block a public drainage path or raise a drive in a way that diverts water onto another property.
- After subsidence, recheck grades: a slope that once carried water away may now drain toward the house.
4. Choose structural protection by flood source and depth
| Measure | Best suited to | What it does | Limits and requirements |
|---|---|---|---|
| Whole-home elevation | Recurring deep rainfall, river or coastal flooding where a raised design is feasible | Raises the occupied structure above the design flood level | Major engineering and construction; permits, access, utilities, foundation and neighbor-drainage issues must be resolved |
| Flood wall or closure | Floodwater approaching from a defined direction or perimeter | Attempts to keep water outside a protected area | Must be engineered for water pressure, seepage, overtopping and emergency access; may shift water toward other properties |
| Utility protection | Floods that threaten electrical, heating, cooling, fuel or mechanical equipment | Raises, anchors or shields vulnerable systems | Does not keep water out of living areas; electrical and fuel work requires qualified trades and permits |
| Wet floodproofing | Enclosed lower areas where some water entry is expected | Uses flood-damage-resistant materials and allows water to flow in and out with limited damage | Does not make the building waterproof or reduce the chance of flooding; occupied living space and utilities need separate treatment |
| Dry floodproofing | Some nonresidential or specially designed buildings | Attempts to keep floodwater out with barriers and sealed construction | More difficult and expensive; FEMA says it is typically not used for residential buildings |
Elevate the home when the hazard is deep or recurring
Elevation is the major permanent option when flood levels repeatedly threaten the occupied floor. Depending on the building, a project may raise the entire structure on piles, piers, posts or a reinforced foundation, or replace a low foundation with a compliant elevated system. The design must account for wind, scour, lateral water pressure, soil conditions, connections, stairs, ramps, accessibility, utilities and safe access during a flood.
Begin with a floodplain manager and a licensed engineer or architect familiar with local flood rules. Ask for a written design elevation tied to the accepted survey datum, not an informal height estimate. Obtain permits before lifting or rebuilding. Plan where electrical panels, heating and cooling equipment, water heaters, fuel systems and generators will go; elevating the living floor while leaving critical equipment low can preserve a major failure point.
Elevation does not eliminate every hazard. Roads, bridges, yards, septic systems and utility networks can still flood, and evacuation may remain necessary. A raised structure also needs durable access: stairs, ramps, landings and handrails should be designed for normal use and inspected after major storms.
Use flood walls and closures only as engineered systems
Flood walls, deployable barriers and door closures can reduce water entry where the source and direction are well understood. They must resist hydrostatic and hydrodynamic pressure, seepage beneath or around the wall, floating debris and overtopping. A gate that is not installed, latched or maintained before the water arrives provides no protection.
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New Orleans lists flood walls or closures as permanent measures but does not present them as universal substitutes for elevation. The right choice depends on expected depth, duration, soil and the building’s layout.
Protect utilities and service connections
Have qualified professionals evaluate the electrical panel, outlets, heating and cooling equipment, water heater, fuel tanks, generator, pumps and communications equipment. Possible measures include relocating equipment above the design flood level, anchoring tanks, installing flood-resistant materials and providing protected shutoffs. Keep generators outdoors and away from doors and windows to prevent carbon-monoxide poisoning; follow local fire and electrical rules.
Utility protection limits repair time and reduces contamination or fire risk, but it does not prevent water from entering through walls, floors or openings. Coordinate private equipment changes with utility providers and the building department so that service disconnects, meters and emergency access remain compliant.
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5. Understand wet and dry floodproofing
Wet floodproofing: limit damage when water enters
FEMA defines wet floodproofing as using flood-damage-resistant materials and techniques that allow water to flow into and out of a home without damage beyond cosmetic repairs. In practice, that can mean nonabsorbent or easily cleanable finishes, flood openings that equalize pressure, raised or protected utilities, and avoiding finished living space in areas expected to flood.
Wet floodproofing is a damage-reduction strategy, not a waterproofing claim. FEMA explicitly says it does not make a home waterproof and does not reduce the chance of flooding. Water can still be contaminated, destructive to belongings and dangerous to occupants. Design details, allowable uses and flood-opening rules are jurisdiction-specific; obtain approval before converting a crawl space, garage or lower enclosure.
Dry floodproofing: a limited residential option
Dry floodproofing attempts to keep water outside with sealed walls, doors, windows, penetrations and barriers. FEMA says it may be possible for some buildings but is more difficult and expensive, and these measures typically are not used for residential buildings. Residential walls and foundations may not be designed to withstand the pressure from deep water on one side, making an apparently sealed room unsafe.
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Do not improvise with caulk, plywood or sandbags as a substitute for an engineered design. Ask a qualified professional to verify structural capacity, flood depth, duration, seepage, emergency egress and maintenance before considering any dry system.
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6. Sandbags and temporary defenses: use them carefully
Temporary barriers can buy time for shallow, short-duration water at specific openings, but they are labor-intensive and have failure points. New Orleans cautions that wet sandbags should not be stacked directly against exterior walls because the added pressure can affect the foundation. Place any temporary defense according to local emergency guidance, keep doors and exits usable, and never let it delay evacuation.
A barrier must have a planned end point and a way to remove or pump out water safely. If the surrounding water rises above the barrier, overtopping can be sudden. Sandbags also do little against water entering through floors, sewer lines, walls or utility penetrations.
7. Check rules, permits and maintenance before construction
Requirements depend on jurisdiction, flood zone, project scope and building type. New Orleans provides one clear local example: new construction or substantial renovation generally must be elevated one foot above the Base Flood Elevation (BFE) or three feet above the highest adjacent curb, whichever is higher. That rule is specific to New Orleans guidance and must not be applied to another city without verification.
Before signing a contract, obtain written confirmation of:
- the design flood elevation and required freeboard;
- whether the work is classified as new construction, substantial improvement or a repair;
- foundation, flood-opening, utility and materials standards;
- permits for structural work, barriers, grading, drainage and plumbing;
- inspection milestones, as-built documentation and any required elevation certificate update;
- maintenance duties for pumps, closures, generators, drainage features and access routes.
Keep plans, permits, survey records, photographs and inspection reports. Recheck systems annually and after a major storm, settlement event or nearby road and drainage project. Subsidence can change grades and clearances, so a measure that worked when installed may need adjustment.
8. Compare options without treating them as substitutes
Each measure answers a different question. Use this decision framework with your floodplain manager and design professional:
- Is the main problem water generated on the lot? Start with grading, downspout control, a rain garden or rain barrel, while recognizing their limited storage and depth.
- Does sewage back up? Investigate approved backflow protection and plumbing changes; this does not address surface or river water.
- Does deep water surround the property? Evaluate elevation first, then engineered perimeter protection where permitted.
- Can water enter a lower enclosure? Consider wet floodproofing, flood-resistant materials, flood openings and utility relocation instead of trying to seal an unsuitable residential space.
- Could occupants be trapped? Prioritize evacuation triggers, routes, communications and accessible transport over property-saving measures.
- Would a project redirect water? Require an engineering review of neighboring lots, public drainage, easements and emergency access.
Compare proposals by the hazard addressed, whether they prevent entry or reduce damage after entry, the depth and duration they can handle, permanence, permits, maintenance, professional-engineering needs and consequences if the system fails. A rain barrel, flood insurance policy, evacuation plan and elevated structure are not interchangeable products.
9. Keep flood insurance after mitigation
Physical work can reduce damage without eliminating flood risk. New Orleans recommends flood insurance, and elevation information may help an insurer assess the property. Ask an insurance professional exactly what is covered, what exclusions and deductibles apply, whether contents and temporary living expenses are included, and how the policy treats below-grade areas, sewer backup and mitigation equipment.
Do not assume a standard homeowners policy covers flood. Do not assume a map change, elevated floor or barrier automatically lowers a premium; any rating effect depends on the insurer, policy, documentation and current rules. Keep the elevation certificate, permits and invoices available for underwriting and claims.
10. A practical order of work
- Map the address. Save the official flood map, design flood information and local contact details.
- Meet the floodplain manager. Confirm hazards, permits, substantial-improvement triggers, freeboard and accepted survey data.
- Document the building. Locate or commission an elevation certificate and record lowest floors, utilities, openings and drainage paths.
- Protect people and records. Set evacuation triggers, routes, contacts, medications, documents and pet arrangements.
- Apply low-cost runoff controls. Maintain drainage, redirect downspouts legally and add a rain garden or rain barrel only where site conditions support it.
- Obtain professional designs. For elevation, walls, closures, utility relocation or wet floodproofing, use licensed professionals and written calculations.
- Permit and build. Confirm structural, electrical, plumbing, grading and floodplain approvals before work starts.
- Test and maintain. Operate closures, pumps, alarms and generators; inspect after storms and update the plan when grades or rules change.
- Insure and review. Maintain flood coverage, inventory contents and revisit limits after renovations or a new elevation certificate.
What to do if the home is already below the flood level
Do not guess the required raise from a street view or a neighbor’s project. Obtain the mapped design elevation, verify the home’s lowest-floor elevation and ask the floodplain manager which options are legal. A lower floor may be suitable for compliant wet floodproofing or storage, while occupied rooms and critical utilities may need to be elevated. In some cases, the safest decision is a major elevation or relocation rather than repeated repairs.
Until a permanent plan is approved, keep valuables and essential equipment above the expected water level, maintain evacuation readiness, use only approved temporary measures and avoid entering floodwater. A product that slows shallow runoff cannot make a below-grade home safe from deep or fast-moving water.
Quick Recap
Common mistakes that make flooding worse
- Buying a pump, barrier or barrel before identifying the flood source and design depth.
- Using a city-wide subsidence description as if it were a survey of the individual lot.
- Sealing a lower room without checking whether walls and foundations can resist water pressure.
- Putting wet sandbags directly against exterior walls.
- Raising a driveway, wall or landscape feature in a way that diverts water to neighbors or blocks public drainage.
- Elevating living space while leaving electrical, heating, fuel or communications equipment at the old level.
- Assuming floodproofing removes the need for evacuation or insurance.
- Starting substantial renovation before confirming whether local elevation rules are triggered.
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