Flash Flood Area: 4 Considerations when Building Your Retaining Wall

What You'll Need
Construction permits for retaining walls
Flood Insurance Rate Map (FIRM)
Sump pump

In areas prone to flooding, building a retaining wall may mean the difference between a dry house and a house full of water. To prepare a retaining wall that will keep as much water as possible at bay, be sure to consider these four topics:

1. Permits

Since flood barriers of all sorts may push water onto other people’s property, most areas near floodplains have zoning regulations about the size, location, and use of retaining walls used for flood prevention. In many cases, special permits are required before construction. If unsure about a community’s regulations on flood barriers consult local officials.

2. Select the Correct Type of Retaining Wall

The two most common types of retaining walls are gravity walls and cantilever walls. A gravity wall relies solely on its massive weight to hold back the earth on one side and the water on the other. There are two advantages of using gravity walls: they are easy to build and easy to maintain.

Cantilever walls, on the other hand, are shaped like a letter “T” turned upside-down. These types of walls are reinforced with steel and are more expensive to engineer. Because it uses the leverage of the fill behind it, a cantilever wall is generally stronger than a gravity wall. It is important to note that cantilever walls can be built much higher than gravity walls (up to 18 feet tall!).

3. Height of the Wall

In areas susceptible to extreme flooding, insurance rate maps will include a measurement called the base flood elevation (BFE). The BFE is the estimated height to which flood waters are anticipated to reach in a particular area. BFE’s are based on data gathered from the topography of the area and the effects of prior flooding. Ideally, a retaining wall should be one foot higher than the BFE in order to provide the maximum amount of protection against flood waters. It is important to note that even though constructing a wall higher than the BFE will increase the odds of keeping a flood out of an area, there is always a chance that flood waters could rise above it. To find more information about the BFE of a particular area, flood insurance rate maps (FIRMs) can usually be found in the town hall of most communities.

4. Interior Drainage

The same retaining wall that keeps flood waters out can also trap water within its perimeter. Water from snow and rain can collect within the perimeter of the retaining wall, sink into the soil, and either: A) Add more pressure on the wall causing it to fail or, B) Run towards the house and possibly into the basement. To prevent the wall from failing, homeowners can create a drainage system by drilling weep holes in the front of the retaining wall. Filling the area behind these 2-inch holes with gravel will encourage water to exit through them and will also filter out any material that may clog them. If the water tends to run towards the house, a sump pump may be needed to keep water out of the basement.