How to reduce risk to people and property during a flood

The time to think about how a flood could affect your company’s work sites is months in advance of any such possible occurrence. An Emergency Response Plan (ERP) anticipates potential threat and damage but also outlines actions to mitigate loss long before flood waters rise. Flooding in the Northeast caused by remnants of Hurricane Ida in 2021 caught the region by surprise, leaving 45 dead and damages to businesses estimated at $3 to $5 billion, showing that even areas not flood-prone are at risk.

Actions taken now potentially can mitigate damages later. Such measures should consider the location of the site, the elevation of buildings, condition of buildings, at-risk equipment and materials, and what to do after a flood.

Assessing flood risk

Your company’s site should be above any flood plain with a probability of a flood occurring once every 100 years. Research the potential risk through FEMA or other authorities and regional planning agencies that define flood zones. Assessing flood risk and acting to mitigate damage should be part of a comprehensive risk assessment analysis and should be done several months before a potential event.

Even small streams or topographic features (for example, if the site is in a low-lying area) can lead to flooding. The closeness of the company’s site to any body of water is an important factor. Other potential problems include large areas of paved surfaces, deforestation, high development in recent years, etc.

Changing runoff from rain or snow and weather patterns also changes the occurrence of floods. In mountainous areas, flooding was usually expected in late spring with the melting of snow. Other areas experience summer monsoons with heavy precipitation, or late fall/winter storms. However, a changing climate means events can coincide and create new flood seasons.

Hazards to consider:

• The site is in a flood zone with a short return period (see explanation below), i.e., a high probability of being exceeded.
• Water bodies such as rivers, streams, creeks, lakes, bays, seas or even dry riverbeds are located nearby.
• The site elevation above the nearby water bodies is relatively low.
• High hazard is assumed if buildings or the site are less than nine feet above a body of water.
• The site or nearby area has experienced flooding in the past.
• The area or the site has experienced overwhelming of gutters and drainage systems leading to surface flooding.
• Increased development in the region since facilities were built on the site.
• A high percentage of assets located at the part of the site that would be flooded.

What is return period?

Return period is another way of expressing the probability of an event of a certain magnitude within a specific time frame. For example, a 100-year flood has a 1% probability of occurring in a given year or a 10% probability within a 10-year period. However, this does not mean such a flood will occur once in the given time frame — a 100-year flood can happen more than once in any given year, and it can occur once a year over several years in a row.

Lessen the risk

Climate change, with associated increase in rainfall levels, requires close observation of the adequacy of drainage systems and the condition of your building(s). Note that flood protection systems do not provide absolute safety and can fail in case the event is larger than the design protection level. Your company will need a flood emergency and contingency plan to ensure adequate procedures are in place.

• If no site-specific flood assessment has been carried out, consider doing one. If no flood maps are available, an approximation should be made to determine the potential water height inside the site as well as extent of flooding in the region.
• Identify and check possible watercourse restrictions and how they could be avoided. Blocked culverts and bridges are known to have increased the water depth in floods considerably, i.e., from a 50-year to a 100-year event.
• Regularly consult local authorities for changes in flood hazard and associated building zone designation. It is a good idea to proactively ask for the building regulations close to water bodies and to get involved early in hazard mapping and zoning changes.
• Check with local authorities what the flood risk is for an area before considering new building or extension projects.
• Permanent protection can include, but is not limited to, barriers, dams, dikes, drainage systems, basins, reservoirs, etc.
• Establish a maintenance and inspection plan to assess the condition of flood protection systems and equipment. The inspection plan should include post-event condition and evaluation of performance.
• Consider modifying your site layout to distribute valuable equipment, systems and/or inventory. For example, relocate high-value contents and important processes away from the potential sources of flooding or at higher levels in the company’s buildings.
• When building a new site or constructing an extension to existing facilities, consider whether the construction itself will change water tables, flow patterns or runoff from standing water. Examples include large storage or parking lots, large underground construction, culverts, etc.
• Drainage of yards and lots should consider the capacity of public drainage system to which the surface runoff will be drained.
• New development should be located outside flood-prone areas, ideally above the one in 500-year flood zone.
• If flood protection turns out to be inadequate, consider:

• Strengthening and/or increasing the height of any existing flood defense structure;
• Supporting existing flood defenses by building secondary embankments. These can be as simple as sloped grass/earth embankments or more complicated with crest walls or concrete barriers.

Inspect and prepare for flooding

Make a plan: Establish an emergency response team to implement those measures should a flood threaten your site. Factor in backup team members in case appointed staff are absent from work during the flood. Make sure the proper resources are available well in advance. This includes not only trained staff, but also any auxiliary equipment, spare parts, replacements, fuel for generators, etc.

Basements

Because basements and lower ground floors are more likely to suffer from flood damage than higher floors, this is the starting point when considering flood prevention and protection. Keep in mind that a significant amount of critical services and equipment often are located below ground level, making their protection vital to keeping operations going.

• Identify below-ground structures that could be inundated by flooding and develop flood protection measures. Factors to consider:

• Unprotected basements where high values or critical equipment/installations are located;
• Unprotected ground floors of buildings where high values or critical equipment/installations are located;
• Buildings in poor condition that might be more susceptible to inundation by rising waters;
• The lack of a flood emergency and contingency plan in case mobile protection is needed. (A good plan addresses all protection steps for basements and openings.);
• Absence or inadequate design of basement and ground floor drainage.

Actions to take to reduce the risks above should include:

• Move valuable or production-critical equipment to higher levels.
• Identify which material and portable equipment must be relocated if flooding is expected, in accordance with flood monitoring and warning systems.
• Define septic tanks, sewage lines, etc., through which flood water can backflow into the buildings or site and install backflow prevention valves.
• Utilize mobile protection features and actions for basement and ground floor areas at risk from flooding. These may include backflow prevention valves, gates and pumps, bulkheads, locating key control and electrical systems one foot above expected water levels, raising or protecting stock and machinery, tank anchors, etc.
• Use more resilient building materials and good general condition and maintenance of building elements to render property more resilient to flooding. Consider tightening the fittings and checking the seals of doors and windows.
• Design new buildings with flood prevention or protection in mind. Options include:
• Wet protection: Basement and openings are water resilient and equipment is protected or located above the designed water level.
• Dry protection: This means that water will not ingress into the basement because the building is watertight. Although the premises flood, no water will get into the building due to sealing. This protection measure is ideal in areas with low flood levels. Alternatively, the building is shielded from water due to defenses and barriers or is raised above water level (stilts, walls, hills).
• Complete an asbestos survey if the building is constructed of materials containing asbestos. While asbestos may be well-contained during normal conditions, a flood could damage the asbestos, releasing asbestos fibers. Not only would the building material require expensive replacement, but also the released fibers could cause contamination of standing flood water and surroundings.

Building exteriors

Inspect roof panels, gutters, waterproofing systems, roof-mounted equipment anchorages, conditions of eaves, etc., as part of the building’s regular maintenance. Consider these factors:

• Architectural and topographic features can result in high variations in wind forces on different parts of a building. Identify these critical areas.
• Wind damage of these elements can lead to tear-off of roof panels and exposure of building contents to rain damage. Regular inspection and maintenance will reduce the likelihood of damage.
• Engage a structural engineer to conduct an analysis of wind resistance of these components, especially for older buildings.
• Ensure that building contents are protected when conducting any activities, such as pressure testing of drainage pipes.
• Ensure that roof and site drainage systems have been designed according to local design codes, with special attention to local rainfall intensity-duration-frequency parameters.

Any expansion of an existing site (especially older ones) should factor in the rainfall parameters. Note that code-defined rainfall characteristics, drainage system requirements, safety factors, etc., may have changed since the site was developed. As it is also highly likely that the drainage is not an "engineered" system (i.e., it has not been designed to code nor with consideration of local rainfall intensities) but rather merely been procured off-the-shelf, the design should be conducted by a qualified specialist.

Ensure utility supply

In the event of flooding, a continued supply of utilities will be necessary to rehabilitate the company’s site and achieve a resumption of business as quickly as possible. Therefore, it’s vital that your flood plan includes details of utility suppliers (gas, power, water etc.).

Contact utility suppliers in advance and become familiar with their response plans, including definitions of the critical flood levels and corresponding actions.

Lock down contractors

Your flood plan should consider post-event rehabilitation of the site. You’ll want to establish relationships with contractors for sprinkler systems, power transformers and heat exchange rooms, as well as plumbers, decorators, etc.

Also make sure to sign contracts with subcontractors for post-event recovery work, including debris removal, repair of damaged infrastructure, etc.

Stock materials

Identify construction material that could potentially be damaged by flood water, such as composite panels for facades, and prepare a stock of replacement material.

Scarcity of construction materials is common after any natural hazard. Providing a stock of replacement material, as well as onsite expertise to conduct necessary repairs, ensures quick restoration of operations after the flood event.

Consider replacement of water-sensitive construction materials, for example, replace composite panels for facade elements with precast concrete at critical buildings.

Protecting equipment and goods

Building contents exposed to floodwater have varying degrees of vulnerability. Often, it is not the water alone that damages or destroys the inventory or machinery, but also the debris and the silt that come with the water. Flood water is far from clean — it typically will include sand, silt, dirt or worse still, contamination from other sites that have flooded. Even after a flood, risk lurks. Moisture may linger long-term, ultimately damaging equipment that had escaped the flood itself undamaged.

Potential danger zones:

• Industries that have medium or high sensitivity to water/moisture contamination, such as printing, car manufacturing, power generation, chemical and food manufacturing/storage, pharmaceutical, textiles and communications.
• Contents that are highly sensitive such as computers, scientific equipment, food products, clothing, etc.
• Any contents that have a long replacement time, such as specialized machinery or specifically designed equipment.

If sensitive equipment is identified in an area prone to flooding, consider:

• Replacing sensitive equipment with flood-resilient counterparts, if available.
• Adding protection measures for sensitive equipment to the flood contingency plan.
• Moving the equipment to safe locations temporarily or if possible, permanently.
• Improving the flood resilience of buildings, or parts of buildings, that contain sensitive goods or critical equipment, especially the layout and the organization of critical production areas and warehousing operations.

Find guidance on the other phases in a Flood Emergency Response at Zurich’s Flood Resource Hub.

The guidance in this article was provided by the Zurich Resilience Solutions Risk Engineering team.