Context
- The most common, and often very costly, damage from coastal flooding is to the foundation, basement, and lower stories of a property. In the design phase of your project, carefully consider the placement of permanent building infrastructure including electrical, mechanical (e.g., HVAC), and other services and utilities. When a building is flooded at ground level and the foundation is breached, damage is exacerbated. If the project involves significant storage of valuable assets (e.g., warehouse, production facility), consider the impact on operations and potential financial loss to those building contents in the event of a coastal flood.
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Coastal flood risks are defined by water levels and water velocity. The levels determine how much of the building from the ground up is exposed to potential damage. Water velocity is often discounted or entirely ignored although it is often responsible for as much or more damage than water contact alone. As with wind, rapidly moving flood waters can carry large and heavy debris that upon impact can cause direct physical damage. Soil scour within fast moving water can penetrate the building envelope and lead to expensive clean up costs. Be mindful of these risks by evaluating surrounding land use and potential land development in the vicinity of your project area.
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An intense cyclone or other weather event can bring torrential rains and have embedded severe thunderstorms that are accompanied by strong winds from tornadoes. When combined with coastal storm surges, the impacts can be catastrophic. Buildings damaged by coastal flooding often suffer wind damage as well. Water driven by hurricane-force winds can enter a building through otherwise sealed openings, and rain entering through a damaged roof can compromise the inside of the building and in some cases expose the building to dangerous water-borne pathogens (e.g., mold). In your planning process, think holistically about all sources of damage especially those related to water penetration of the building envelope.
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The risks of water damage and the health dangers of mold go hand-in-hand. After a coastal flood event, its important to have a post-disaster response plan that requires rapid inspection and response to prevent further damage and complications. Flood waters carry contaminants such as oil, pesticides, bacteria, and heavy metals. Keep in mind certain finish materials (e.g., carpeting) tend to retain moisture and develop mold days or weeks after a flood event ends, posing a serious health hazard if the building is reoccupied prematurely. Beyond your project’s base requirements, consider choice of building materials and finishes from a risk mitigation perspective.
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Note that coastal flooding is not the only cause of direct physical damages from tropical cyclones and other weather events. For example, wind-borne debris can enhances damage to a structure already compromised by flooding at the ground level. Damage can be further induced by heavy rainfall and subsequent inland flooding especially after the roof and/or windows or other openings are compromised by wind-borne debris. Considering all weather hazards in the design and planning stages results in efficient risk management for your project.
Mitigation
- Throughout the project from design to construction, consider all available mitigation strategies to reduce your coastal flood risk from cyclones and other weather events. Balance the cost of mitigation features with the long-term savings via reduced damage and lower cost of repair and replacement.
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Further detailed information should be obtained to account for the risk from other hazards related to cyclones and other weather events. Be sure to review the ThinkHazard! assessment for related hazards in this region: cyclone wind, river flood, and landslide. Keep in mind wind forces can stress structural components including the foundation that in turn subject a building to an elevated risk of flooding.
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In terms of replacement cost, basements, lower stories, and foundations can experience significant damage and as such are as important as the materials used for roofs, walls and building connections. Foundation design strongly influences a building’s resilience to coastal flooding. Properly sealed and infilled foundations tend to be the most resilient.
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Investigate national building codes for coastal flooding in your project area. Also determine from your local authorities whether local building codes override national codes, in particular along the coast where the combination of coastal surges and strong winds from cyclones are most risky. Be responsive to national and local building code requirements for coastal flooding hazards as following these codes may considerably reduce future losses.
- Start by consulting with widely accepted building code organizations such as the ICC (International Code Council) to access commercial and residential code (ICC/IRC) standards pertaining to your project.
- Next, determine if your region has adopted international codes or has modified them locally. Consult your jurisdictional planning division for the most up-to-date information on building codes. Be mindful of recent code changes as they are often developed in response to recent adverse natural events that have caused loss to similar projects.
- If national structural design guidelines do not exist, safe design practices to mitigate coastal flooding should be obtained from other countries with a similar level of risk or from international guidelines.
- Contact other national or local organizations that may have supplemental information on coastal flooding (e.g., meteorological agency, national research council, engineering associations, etc.).
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Investigate local council and planning commission meeting minutes as they often contain useful information about best practices and emerging mitigation strategies.
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As a part of comprehensive risk management, consult the International Property Maintenance Code (IPMC) to assure your project has long-term resilience to flood damage. Be sure to plan for proper maintenance post-construction.
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In high-risk areas along the immediate coastline, carefully design around your BFE (Base Flood Elevation). A first floor raised just one foot (0.3 meters) above the BFE can result in a 50% reduction in overall flood risk. Also consider extended foundation walls and other means of reducing the risk to the main building structure and its contents.
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Carefully consider foundation construction in the design phase and select high quality materials. The most important construction measures to minimize flood damage surround the foundation and its reinforcement as well as connections to the main building structure and the planned use of basement and lower stories.
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Distinguish carefully between coastal flooding and riverine (inland) flooding. For example, a common structural mitigation for river floods is in the use of reinforced concrete footings and attached piers. However, piers are not suitable as a defense against coastal floods because they cannot withstand the pressure forces exerted by coastal storm surge. Most effective coastal flood mitigation strategies account for the predominant source and direction of the water flow.
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In very high-risk areas, consider designing for a moveable structure. When practical, temporarily relocating a building provides the greatest mitigation against coastal flood risks. In such cases, consult with your local jurisdiction for structural and safety requirements around replacing the building structure to its original location.
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Be aware of and plan for the after-effects of a coastal flood event. Over time, flooding can cause substantial beach erosion in the absence of physical damage to a building lying along that beach. In time, the erosion can lead to a substantial increase in latent (hidden) risk. Track your beach erosion rate carefully within the natural course of your project’s maintenance and upkeep schedule. In some cases, a building may be subject to imminent collapse even without a precipitating flood event.
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As a part of a long-term maintenance strategy, identify and remove small trees and branches from larger ones that could detach during a flood event and pose debris risk to the building. Identify and repair loose or damaged building components such as sheathing, soffit and fascia, shingles and roofing, brickwork, and jutting building features such as chimneys as all of these become potential debris in flood (as well as projectiles in wind).
