By Adam Reeder and Capt. Jay Manik, P.E., F.SAME, USCG (Ret.)
Many federal facilities are located in areas that are vulnerable to flooding. Much of this is by necessity, as many military installations are strategically located on coastline, for instance. However, designs to address flood vulnerability are often done by looking at flood data in a report or on a map and looking for a single piece of data such as the 100-year flood (a 1 percent annual chance). An entire project could be defeated by not understanding the conditions prior to starting.
To devise a better flood protection design, investigation should include not only maps and reports, but also putting boots on the ground and evaluating the area. Ideally, the depth of flooding, flood velocity, rate of rise, duration, and potential sources of debris all should be considered.
DETERMINING FLOOD PROTECTION
Once the conditions are understood, it is necessary to look at the flood data and determine the height of flood protection. Building codes and engineering standards are a great starting point; it also is important to consider that they provide minimum levels of protection. Designers too frequently mistake the minimum level of protection for the required level of protection. This can result in significant losses when floods overtop a protection measure.
The output of hydraulics and hydrology modeling is often the median value of the analysis. When evaluating a 100-year flood, for instance, this means that approximately half of the 100-year floods exceed that elevation. While incorporating freeboard in the codes account for some unknowns, risk could still be underestimated. Consider other factors such as the age of the flood information (date of the model, not the date of the report) and what changes might increase flood heights over the life of the building. Since building lifecycles often are 75 years or more, the future should be accounted for.
The final part of the analysis is trying to understand what recurrence interval or annual probability of protection has been achieved by the selected level of flood protection over the life of the building. Consider the probability of an event exceeding the flood protection level. What are the consequences to the building if it were to flood?
The primary approach to selecting a flood protection measure should be understanding which measure can be omitted and what happens if the measure is not maintained or fails.
Elevation. Elevation should always be considered the ideal measure for new buildings and buildings that will remain in the floodplain. The value of elevation is that damages only begin to accumulate when floodwaters reach the flood protection level. In coastal areas, it may provide the best option when storm surge includes high velocities and wave action. However, elevation may require long ramps or lifts to provide access. As a retrofit measure, it becomes difficult and expensive for large buildings.
Dry Floodproofing. Dry floodproofing is waterproofing a building and covering or sealing openings prior to flooding. While a common retrofit measure, it is also considered for new buildings, particularly where rain either coincides or is the cause of flooding, since the only pumping necessary is to address leakage through the system. Unlike elevation, the biggest drawback of dry floodproofing is that when it is overtopped, the building quickly fills with water and it is rarely possible to pump out the flooding fast enough to prevent major damage. The technique may not be ideal for buildings that would require extensive updates to the façade or the foundation if they need to be used during the renovations. Post-disaster experience indicates that poorly designed and/or poorly maintained dry floodproofed buildings often fail long before they are overtopped. Some systems take extensive time to install, so lead time before floodwaters reach the building is key. Dry floodproofing may be difficult with existing buildings since waterproofing older foundations, walls, and slabs can be difficult and you may not find an oversight until there is a flood.
Wet Floodproofing. Wet floodproofing consists of using flood damage resistant materials below a flood protection elevation and allowing the building to flood with a design approach to minimize damage and cleanup. For precautionary reasons, mechanical, electrical, and plumbing systems are either elevated or absent. Openings are used in walls to allow floodwaters to freely flow in. This approach is often applied to the lowest floor areas used for parking, building access, or storage. Existing buildings with low level flooding may be candidates for this approach, provided the contents can be quickly moved and temporary loss of the building is not a large impact to operations.
Floodwalls and Levees. Utilizing floodwalls and levees are options that may be better suited to buildings that need to be used during the construction of the flood protection or older facilities that may never be possible to waterproof using dry floodproofing. Floodwalls and levees require extensive pumping systems to keep the area within the protected perimeter free of water. This may not be feasible during heavy rains, however. Additionally, as soon as water reaches the lowest opening point, those gates need to be closed. This could cut off access, even in a low-level flooding event.
Once overtopped, floodwalls and levees will result in the same level of damage as if they were not in place. Floodwalls and levees reduce storage capacity within the floodplain and could make flooding worse in surrounding areas. Moreover, these systems also can be extensive and require completion of the entire project in order to obtain flood protection. On the other hand, wet and dry floodproofing and elevation measures can be done incrementally based on available capital.
Relocation or retreat is another available approach. However, this is likely contingent upon available land and the impacts associated with moving a building or buildings.
CONSIDERING ALL FACTORS
Other considerations prior to making a final selection and beginning design include conducting research on the individual components of a mitigation measure. For example, the time necessary to install dry floodproofing or floodwalls should be evaluated prior to selecting them. Carefully consider the normal use of the facility when planning the location of components like shields and opening protection. Will it be vulnerable to damage from a truck backing up to a loading door? What are the power requirements for any pump systems? How much fuel is necessary, and is a larger generator necessary?
As designers develop the options, they should also develop a plan of special inspections to make sure they can verify that the protection measures were constructed as designed and will not fail prematurely. The final component to be considered prior to plan development is whether there are any adaptive approaches that might allow the elevation of the flood protection to be enhanced at a later date, such as constructing a larger foundation than is currently necessary and allowing for wall height to be increased. The reliability of flood protection can be greatly increased with proper planning, appropriate selection of the right measure for the project, and most importantly, determining a sufficient height for implementation. If designers plan for construction verification, maintenance, and an achievable operations plan, then owners can expect more reliability from their flood protection and an approach that will better suit their needs.
Adam Reeder is Principal, and Capt. Jay Manik, P.E., F.SAME, USCG (Ret.), is Vice President, CDM Smith. They can be reached at firstname.lastname@example.org; and email@example.com.
[This article first published in the July-August 2021 issue of The Military Engineer.]