In previous articles on this site, the roles of Flood Insurance Rate Maps (FIRM’s) and Elevation Certificates as components of the US National Flood Insurance Program (NFIP) were examined. One of the aims of providing information with regard to flooding potential is to enable property owners to determine what is required to reduce either the chances of their being affected by flooding, or their insurance premiums, or both. This series is intended to increase awareness of the NFIP and its components, but is not an exhaustive treatment of the subject. For more detailed information, either consult a local expert in the field, or visit the FEMA website www.fema.gov and follow the ’Plan, Prepare and Mitigate’ links.
Designing new structures in flood-prone areas
When a new structure is intended to be built in a location identified as a Special Flood Hazard Area (SFHA) depicted on the respective FIRM flood map, mitigating measures must be built into the design to ensure that the building will not be prone to flooding. In such a case, a building may be designed to stand on piles or have high foundation walls in order that the lowest floor level is above the indicated Base Flood Elevation (BFE).
Existing buildings below BFE
For existing buildings, however, which are situated below BFE as indicated on the applicable FIRM flood map, if the cost of repairs or improvements exceeds 50% of the market value of the building before the damage or improvement, then the entire structure must be brought up to current floodplain development standards.
This may require raising the lowest floor level of the existing building above BFE, a process known as ‘building elevation’.
According to the Southern Tier Central Regional Planning and Development Board, building elevation involves “raising a building in place so that the finished living space is located above the anticipated height of flood waters … The building can be elevated on solid perimeter foundation walls, fill, or an open foundation system that supports the structure at key points”. It adds that almost any structurally sound building can be elevated.
There are a number of methods for elevating buildings, depending on the particular circumstances that apply to the site and the building. Before examining the different methods, it is necessary to look at some of the factors affecting the choice of technique.
Riverine versus coastal flooding
The type of flooding condition needs to be considered in order to choose an elevation technique that will withstand the expected water depths, velocities, debris impacts, and scour (as well as other hazards).
Flood damage from coastal flooding is generally more severe than that from riverine flooding owing to the energy contained in coastal waves striking buildings. Riverine flooding (without waves and high velocity) in an A Zone can submerge a structure without causing substantial damage, whereas a wave crest elevation flood depth of 3 to 4 feet (1m to 1.3m) above the bottom of the floor beam, or approximately 1 to 2 feet (30 – 60cm) above the top of the floor, in the coastal V Zone is sufficient to cause substantial (more than 50%) damage to a building.
Addition of ‘freeboard’
The most common approach to reducing damage in coastal flood areas is to add ‘freeboard’ to the building, i.e. to elevate the building higher than the minimum BFE. This higher level is known as the Flood Protection Elevation (FPE), or flood protection level. If the BFE is not known to be accurate, it is recommended to raise floor level at least 3 feet (1m) above BFE; if the BFE is accurate, then it may be adequate to only elevate 1 to 2 feet (30 to 60cm) above BFE.
A State or local community’s building code may reference the minimum elevation of the building’s lowest floor, along with a requirement for additional elevation if the building is located in the Coastal A Zone, to cater for wave effects.
According to FEMA’s Fact Sheet No. 1.6 Home Builder’s Guide To Coastal Construction, the benefits of designing for flood levels above BFE include reduced building damage and maintenance, longer building life, reduced flood insurance premiums, reduced periods of time during which the building occupants need to be displaced in event of a flood, reduced need for shelter and assistance, and reduced job losses.
Methods of building elevation
Where a building, especially in the case of one with masonry or brick walls, cannot be physically elevated without the potential of threatening its structural integrity, an alternative method of raising the lowest floor level is still an option. By jacking up the roof of the building by the amount that the floor level needs to be raised, and then building the walls up to the accompanying wall-plate (roof truss) level, a new floor can be installed at the necessary elevation to be above the BFE. In addition, the window and door openings will need to be raised in the walls to match the raised floor level.
In cases where a building, especially a timber-frame dwelling, is considered structurally suitable for physical elevation, the four main methods of building elevation are: Elevation on Fill, Extending Foundation Walls, Abandoning the Lower Enclosed Areas, and Elevating on an Open Foundation.
Elevation on Fill involves raising a building a small amount. It is economical only up to 2 to 3 feet (60 – 90cm), but is not a permitted form of building elevation in coastal V Zones due to wave action. Once utilities have been switched off and lines disconnected, the area around the original foundations is excavated, holes are cut into the foundations for the installation of lifting beams. The house is then lifted with hydraulic jacks and fill material is inserted beneath it. The fill is compacted and new foundations are then constructed, the house is lowered onto the new foundations, and finally the utility lines are reconnected.
Elevation on Extended Foundation Walls is a common method of building elevation up to 4 feet (1.2m) above grade; any higher and abandonment of lower enclosed areas may be more economical. The method used is similar to that of elevation on fill, with the house being lifted with hydraulic jacks and new foundations being constructed, but, instead of inserting fill below the house, the foundation walls are built up to the new level of the house and an open ‘crawl space’ left below the lowest floor or floor slab. Openings must be left in the extended foundation walls to equalize water pressure during flooding and to allow floodwaters to escape from below the building. This method is also not permitted in coastal V Zones.
Abandonment of Lower Enclosed Areas refers to the removal of non-load-bearing walls from the lowest floor level of a multi-storey building in order to permit flood waters to flow through relatively unimpeded. The implication of this is that the lower floor level is ‘abandoned’ as a habitable space and the upper floor level becomes the new lowest floor level. This may prove less expensive than attempting to elevate the whole building more than 4 feet (1.2m). The lower level could still be used partially for garaging of vehicles.
Elevation on Open Foundations refers to the jacking up of a building and the replacement of foundation walls with posts, piles or piers. In the first option, wood, concrete or steel posts are installed with new foundations in pre-dug holes. Piles driven into the ground may be stronger than posts in coastal and high velocity zones and may need cross-bracing, but steel piles will be subject to rust corrosion in coastal areas. Piers made from concrete blocks, poured concrete or brick are only suitable for areas with low flood velocity and minimal erosive force, and are thus not suitable for coastal areas.
Most appropriate elevation method
The most appropriate elevation method for frame houses is to elevate on extended foundation walls or open foundations, depending on the location. For masonry houses, abandonment of lowest floor would be the most appropriate, where feasible, or else extending foundation walls. Houses with basements usually have furnaces and other utilities in the basement that need to be elevated or relocated.
Where substantial damage has occurred to a building during flooding, or substantial improvements are to be made to a building in a flood-prone area, the NFIP limits the choice of technique that may be used. In other cases, the NFIP is less prescriptive, but local laws, codes and ordinances may limit the owner’s choice of option.
In all cases where flood mitigation measures are being considered, the assistance of the local planning and building department officials, as well as relevant professionals such as architects, surveyors and engineers should be sought well beforehand.
Images, Sources and Further Reading:
FEMA (pdf); FEMA Flood Zones; STC Planning (PDF); Texas A&M University