What U.S. Wet Floodproofing Practice Can Contribute to Ghana's Flood-Resilience Discussion

Eric Paddy Boso's recent white paper, "A National Engineering Policy Framework for Urban Flood Resilience for Ghana," raises an interesting question about the role individual properties can play in the movement and management of urban floodwater.

One of the paper's proposals is particularly familiar from a U.S. floodplain-management perspective: the use of engineered flood openings to relieve trapped water and reduce damaging hydrostatic pressure.

The concept deserves further study in Ghana. It is also important to understand how wet floodproofing and flood openings are actually used in the United States, and where that experience may, and may not, translate to Ghana's built environment.

Wet Floodproofing in the United States

In the United States, flood-resistant construction generally begins with a simple objective: keep vulnerable occupied portions of a building above the design flood elevation whenever possible.

For residential buildings in regulated flood hazard areas, this often means elevating the lowest occupied floor. The area below may sometimes remain enclosed for limited uses such as parking, building access, or storage.

When those lower areas are permitted to flood, the goal changes. Instead of trying to keep all floodwater out, the enclosure is designed to allow water to enter and exit in a controlled manner.

This is known as wet floodproofing.
The reason is hydrostatic pressure. If floodwater rises several feet higher on one side of a wall than on the other, the resulting pressure can damage or collapse the wall. If water levels are allowed to rise and fall on both sides, that differential pressure is substantially reduced.

Flood openings are one way of accomplishing that equalization.

Engineered and Non-Engineered Flood Openings

U.S. floodplain regulations recognize two general approaches.

The first uses prescriptive, non-engineered openings. These openings must provide a specified amount of net open area and meet requirements governing their number, location, and ability to permit the automatic entry and exit of floodwater.

The second approach uses engineered flood openings.

Rather than relying solely on a prescribed opening-area ratio, an engineered opening is designed and certified to provide the hydraulic performance necessary to equalize flood forces. Engineered products with moving components typically remain closed during normal conditions and open automatically when flooding occurs.

The important principles are that the opening works automatically, permits water to move in both directions as flood levels change, and performs as part of the overall flood-resistant design.

These requirements are addressed through the U.S. National Flood Insurance Program, Federal Emergency Management Agency (FEMA) Technical Bulletin 1, building codes, and standards such as the American Society of Civil Engineers (ASCE) 24.

What This Means for Ghana
The Ghanaian application proposed by Boso is different from the traditional U.S. application.

FEMA Technical Bulletin 1 primarily addresses foundation walls and the walls of enclosed areas below elevated buildings. It was not written as a standard for residential compound or perimeter walls.

However, the underlying engineering principle is relevant.

A solid wall with water accumulating on only one side experiences a hydrostatic pressure differential. Providing an adequately designed path for water to move through the wall can reduce that differential and, in some circumstances, reduce the amount of time water remains trapped behind the wall.

That does not mean flood openings should simply be installed in every compound wall.

Boso makes an important distinction in his framework. Some properties may primarily be trapping rainfall and runoff generated within the compound. In those situations, controlled low-level openings could potentially help release that water.

Other properties may be exposed primarily to floodwater arriving from streets, drainage channels, or neighboring properties. In those situations, a solid perimeter wall may actually be providing useful flood protection. Adding openings could increase rather than reduce the property's exposure.

That distinction is critical.
Another approach may also warrant evaluation where the primary problem is rainfall generated within an enclosed compound. If a perimeter wall is intended to limit incoming street floodwater but traps runoff generated inside the property, the desired hydraulic function may not be bidirectional equalization at all. A properly designed one-way drainage outlet or backflow-prevention device could allow internal stormwater to discharge while limiting reverse flow when exterior water levels rise.

However, this approach transfers flood loads to the wall itself and may leave the property without gravity drainage when exterior flood levels are high. Any pilot would therefore need to evaluate wall structural capacity, outlet sizing, debris blockage, internal storage, overflow routes, and pumping requirements before determining whether a one-way or bidirectional opening is the better solution.

Pilot Should Test Performance, Not Copy U.S. Standards

If Ghana evaluates this approach, the objective should not be to copy FEMA requirements or U.S. flood-vent installations verbatim.

Instead, U.S. experience can provide a starting point for developing a Ghana-specific performance standard.

A pilot program could evaluate questions including:

• How quickly must water pass through a compound wall to prevent damaging impoundment or pressure differentials?
• What opening area and placement are appropriate for typical Ghanaian compound sizes and wall configurations?
• How will openings perform when exposed to sediment, vegetation, plastic waste, and other debris carried by urban floodwater?
• Can openings maintain the security and privacy functions that make solid compound walls important to homeowners?
• What happens to water after it leaves an individual property?
• Could increased parcel-level drainage transfer additional flood risk to downstream properties or already overloaded drainage systems?
• Should openings be permanently open, one-way, screened, mechanically operated, or automatically activated?
• What inspection and maintenance requirements would be necessary to ensure that the openings remain functional over time?

These questions should be answered through site surveys, hydraulic analysis, controlled testing, and pilot installations before any broad regulatory requirement is considered.

One Tool in a Larger Flood-Resilience System

Wet floodproofing is not a substitute for drainage improvements, floodplain management, land-use planning, elevation, flood barriers, detention, early warning, or other flood-mitigation strategies.

It is one tool.
The United States has decades of experience using flood openings to address a specific structural problem: hydrostatic pressure on walls surrounding areas that are intentionally allowed to flood.

Ghana may have a different but related problem in communities where solid property boundaries unintentionally trap stormwater and interrupt natural overland drainage.

That makes Boso's proposal worthy of investigation.

The next step would not be widespread installation of flood vents. It would be a carefully designed Ghanaian pilot that combines local hydraulic expertise with lessons learned from established wet-floodproofing standards elsewhere.

If the testing demonstrates that engineered openings, one-way drainage devices, or a combination of strategies can safely reduce trapped water and damaging hydraulic pressure while preserving security, structural integrity, and downstream drainage capacity, Ghana would then have the evidence necessary to develop requirements suited specifically to its cities and communities.

That evidence-based approach could turn an established flood-resilience principle into a solution designed for Ghana's unique conditions.

Brian Shaw, CFM, is Senior Vice President of Strategy & Flood Policy at Floodproofing.com. He works on flood-resistant design, building codes, standards, and flood-mitigation strategies in the United States.