Flood Mitigation Options for Historic Buildings

Posted: September 3, 2018

With inadequate stormwater infrastructure, King Tides and sea level rise, many Charleston-area property owners are seeking additional protection.  Several flood damage mitigation strategies are available:

Flood Barriers

Perimeter barriers are installed at the boundary of the property, and are intended to prevent water from reaching the building. There are several types of purpose-built flood barriers suitable for historic buildings. For example, many historic properties already have masonry walls surrounding their properties. With guidance from a professional, these walls can be fortified to bear the weight of the water pressure during a flood, and the gates can be made watertight with the installation of temporary flood-proof barriers in the openings. Temporary barriers that have been successful in Charleston can be found here.

The construction of new, appropriately scaled and detailed masonry walls may also be considered for historic properties in flood-prone areas.

Water-filled temporary barriers are also available. However, a professional should be consulted for proper sizing and installation. There is a potential that these temporary barriers could be breached because salt water is denser than fresh water and the barrier may float.

Barrier System Considerations

With any barrier system, there are several important considerations:

• There may be negative impacts to neighboring properties should water be redirected into their property.
• The height of the barrier must be carefully evaluated for effectiveness using FEMA flood maps or flooding history.
• Taller walls will need reinforcement from the weight and pressure of the flood waters. An engineer should review the strength of the wall.
• If you have a historic home and the barrier will be a permanent installation or modification, the material and design must be compatible with the character of the historic property. Changes to properties within the historic district under the jurisdiction of the Board of Architectural Review or Historic Charleston Foundation (due to an easement or covenant) must be submitted for review and approval prior to construction.
• Current code in Charleston may prohibit certain types of flood barriers on residential structures. This should be investigated and confirmed before installation.

Wet Flood Proofing

Wet flood proofing, often less costly than other mitigation measures, allows water to freely enter a building during a flood. Wet flood proofing will reduce the hydrostatic pressure on the foundation walls during a flood event, and therefore will reduce structural damage. This approach is most appropriate if the living spaces of the house are elevated and it is expected that water will pass under the house.

Successful wet flood proofing involves the following:

• Ensuring that flood waters enter and exit the house. Properly designed and located flood vents can help with this.
• Ensuring that flood waters inside the house rise and fall at the same rate as flood waters outside.
• Protecting the areas of the house that are below the flood level from damage caused by contact with flood waters.
• Relocating any materials stored within the affected area, such as elevating electrical wiring, electrical service, plumbing systems, mechanical/HVAC systems.
• Protecting mechanical equipment, such as HVAC condensers or hot water heaters, to reduce exposure to flood water.
• Installation of flood resistant materials on the underside of the house.
• If you anticipate that flood waters will enter the interior spaces, consider installing water resistant materials in areas outside of public view. For example, the lower portions of sheetrock walls can be replaced with a paintable material such as PVC which will not be damaged by water.

Dry Flood Proofing

Dry flood proofing prevents floodwater from entering a building. This approach is most applicable in structures that are not elevated above base flood elevation and have finished interiors that may be impacted by flood water. Dry flood proofing is often less costly than other retrofitting measures and does not require the additional land that may be needed for levees and floodwalls.  These measures may be fundable under FEMA mitigation grant programs.

Successful Dry Flood Proofing involves the following:

• Using waterproof membranes or other sealants to prevent water from entering the structure through the walls.
• Installing watertight barriers over windows and doors.
• Ensuring the foundation walls are strong enough to withstand the hydrostatic pressure due to floods.
• Installing measures to prevent sewer backup.

Mechanical Equipment

Mechanical equipment, especially HVAC equipment, should be elevated above the anticipated level of flooding. Ductwork under the house is particularly susceptible to flood damage. Ductwork under the house should be relocated, where possible, or replaced with flood proof ductwork, such as Blue Duct, if possible.

Electrical wires under the house should also be evaluated by an electrician before and after a flooding event. Salt water will corrode exposed wires. Wiring should be elevated out of floodwaters and possibly strung through PVC pipes for further protection.

Install sump pumps in basements or crawlspaces to quickly remove water.

Install and maintain backflow valves to prevent sewage backups.

More information and practical measures may be found on the FEMA website.

4 responses to “Flood Mitigation Options for Historic Buildings”

1. Charlotte Caldwell says:

   September 7, 2018 at 5:52 pm

   Thank you so much. This was very helpful.

   Reply
   • Holland Williams says:

     September 7, 2018 at 6:45 pm

     So glad to hear it may help you!

     Reply
2. AlexPappas says:

   October 7, 2018 at 7:59 am

   Are there any local engineers or architects who can assist with flood barrier design or mitigation

   Reply
   • Holland Williams says:

     October 9, 2018 at 4:53 pm

     Hi Alex. In response to your earlier post, we’ve included a list below of some potential local resources. We hope everyone who is considering flood barrier design and mitigation will take the impact to your neighbors into consideration. Water should be diverted into storm water drains vs onto your neighbors property, etc.

     Applied Building Sciences

     Russell Rosen

     4SE

     Craig Bennett
     Bennett Preservation

     Caskie Engineering

     Glenn Keyes Architects

     Reply