Assessing a Project Site for Flood Zones and Evaluating Resiliency Strategies
Sea level rise and heavy rainfall may pose flooding threats to a project site. Before developing a project, it is good to assess the site to understand whether it is located in a flood zone, and if so, what risks are associated with such flood zone.
FEMA, or the Federal Emergency Management Agency provides flood maps for certain locations. One may enter a property address on their website and obtain what is called a Flood Insurance Rate Map (FIRM), which overlays flood zones on satellite imaging. FIRM Maps are typically re-evaluated after there is a significant weather event that affects the area, or after a certain amount of time. This is done through a Flood Insurance Study (FIS), which usually yields updated temporary maps that are called PFIRMs, or Preliminary Flood Insurance Rate Maps. PFIRMs are used as the most recent data available when assessing a project, while new, updated FIRMs are published.
A FIRM will typically show two different shading types or no shading: No shading indicates that the project location is not at-risk of flooding by neither a 100-yr or 500-yr storm. A storm recurrence interval defines the probability of the storm of occurring in any given day during a year. For example, a 100-yr storm, has a 1/100 = 0.01 chance of occurring in any day during a given year, which is equivalent to saying that such storm will occur 3.65 days/year. A storm with a higher frequency is less probable, but it implies larger rainfall and flooding, which is why FIRMs will always show the 500-yr storm shaded area further from the coastline and beyond the 100-yr storm shaded area.
The image below shows the PFIRM of a portion of the City of San Francisco. As mentioned earlier, the light blue shaded area indicates ‘Special Flood Hazard Areas’, which are defined as areas that will be inundated by the flood event having a 1-percent chance of being equaled or exceeded in any given year. Areas shaded in Orange, or ‘Other Areas of Flood Hazards’, are those regions vulnerable to 0.2-percent chance of flooding (or 500-yr storm), or a 1-percent flooding chance with an average flood depth of less than 1 foot. The PFIRM also provide Base Flood Elevations (or BFEs) for each shaded area.
Once the flood zone of a project site has been determined, the BFE should be taken into consideration for the design, and it is often referred to as the Design Flood Elevation (or DFE). At the discretion of the project stakeholders and local regulations, additional height might be added to the DFE to account for future climate change such as sea level rise.
There are a certain number of strategies that can be used to ensure that the equipment and structures provided by the project account for the DFE. Electrical equipment can be installed above the DFE, if possible, or enclosed in watertight panels. It is important to consider how often the equipment will be operated, as either placing them at a higher elevation or enclosing them in panels, may cause operability difficulties. Structures shall be properly reinforced to account for standing water pressure forces, buoyancy forces, and debri impact forces. Note that however, if a room is not completely watertight, flood waters from the inside will exert an equal and opposite pressure to the exterior flood waters, thus reducing or possibly eliminating the need of reinforcement.
Implementing resilience adaptation strategies shall come with a cost-benefit analysis. Because of their increased cost, it is important to determine what the value of the assets being protected is, what the project lifecycle is, and what the cost of implementing the strategies are. It a project is located in an area with a 0.2-percent chance of flooding and the assets aren’t too costly, or too old, it might be that implementing resiliency strategies is not economically beneficial.
By: Salvador Bentolila, PE, ENV SP
Disclaimer: I do not hold any relationships or affiliations with any company or organization listed above other than my own LinkedIn profile. If you have any questions or simply want to get in touch, please feel free to reach out to me via LinkedIn.
Salvador Bentolila, PE, ENV SP is a Civil Engineer, specialized in Water Resources. He currently works as a Water/Wastewater engineer at AECOM and has experience with design and development of water supply, irrigation and wastewater systems. If you have any questions Or simply want to get in touch, please feel free to reach out to me via LinkedIn.