Many have seen the recent news about residents in Fort Bend County who have decided to build their own levee to reduce their risk of flooding. Unlike the news articles, we say “reduce their risk” because if not properly designed it could end up causing more flood related problems with the levee than without. Although we do not promote levees as a great mitigation option for individual homes, it can be an acceptable alternative in certain situations. When considering options for mitigation, here are a few things to consider when it comes to a levee. Most of this information might be overkill for a small private levee, but every levee is different in size and location. Being over educated in your decision is never a bad thing.
National Flood Insurance Program
Building a levee will provide a certain level of protection; however, it will not automatically remove a structure from the floodplain. For properties within an A Zone, the downside is that the levee would not be recognized by FEMA, which means the property would remain in the floodplain. If flood insurance is required (mortgage, previous individual assistance grants, etc.) the premium would be based on the risk as if the levee did not exist. There are options for FEMA certification; however, it requires that a public agency (State, County, City, LID, Etc.) to maintain and operate the levee which is difficult for a single residential levee.
Levee Parameters (Material, Side Slope, Height, Top Width)
Many probably think building a levee is as simple as pushing some dirt into a triangle or trapezoid and they are good to go. In reality, this is probably not too far off for a private levee protecting a single home, but there are still things to consider. A good contractor would help you figure these out; however, not all contractors would take time to properly plan and construct the levee which could cause major issues in the future. If you are curious, you can learn more about the requirements for levees from FEMA 44CFR65.10, USACE EM 1110-2-1913, and the Fort Bend County Drainage District Criteria Manual.
Material
The type of material is key. You want a material that is solid enough to withstand the water pressure and prevent water from seeping through the levee while at the same time you being able to grow quality grass with a good root system to keep the surface of that material together to prevent washouts. You also want to make sure the contractor is not using material with debris, especially large organics such as tree branches. Over time these materials will break down leaving void spaces in the levee. It will add additional cost to the construction, but having a geotechnical engineer test the material during construction could reduce you risk of major structural or seepage issues in the future.
Height
FEMA requires that levees be built with a height of 3 feet above the Base Flood Elevation. Fort Bend County has added an additional 1 foot. This additional 3 to 4 feet is referred to has freeboard which is why you see levees in Fort Bend County that are 6 to 12+ feet tall. Freeboard is a factor of safety to account for things like wave action and/or future changes in the Base Flood Elevation. For a private levee, a resident might not want an additional 3 to 4 feet of freeboard on their system, but at a minimum you need to consider the following. Depending on your location, you have the potential for highwater vehicles or boats that are traveling in the area to rescue or supply residents who are in flooded homes. Those vehicles or boats can create waves that could easily be higher than the static water level during a flood. Secondly, records can and will be broken so designing to the Harvey highwater mark might not be enough. Always look to go higher than you “think” you need to go. Depending on the storm, it might be difficult to get material and/or contractors and equipment out if you need additional height during an event.
Side Slope
The side slope is a key component for two main reasons. One of the main reasons is in your ability to maintain the levee in the future. A steeper side slope is more difficult for people to mow and walk. The second reason is the stability of the levee. Depending on the material, a steeper slide slope could be less stable which will cause the levee to have a higher risk of side slope failures. It means more material, but the flatter the slope, the better the stability. In Fort Bend County, new levee systems are designed to have a 4:1 side slope, which means for every foot of elevation (vertical rise) you need to four feet of distance (horizontal run).
Top Width
The top width of a levee adds stability as well as a better ability to inspect a levee system during an event. In Fort Bend County, the minimum top width for a levee is 10 feet wide. Depending on the height, 5 feet might be enough, but 10 feet is the standard.
Internal Drainage
A levee is designed to keep water out, but it has to be designed to handle water that falls behind the levee. This means at a minimum you need to have pump(s) sized correctly to handle the water that falls at a rate that can pump the water out faster than it falls. In addition, these pumps have to maintain power, so you need to consider a generator to supply power to the pumps in the event you lose electricity at your home. Both of these are where the investment into engineering is worth the effort. An engineer experienced in drainage can assist you with evaluating your risk and working with you to properly set a minimum pump rate needed for the internal pumps. It might sound like too much, but the intensity and volume of the rainfall in relation to the surface area behind the levee can have a dramatic impact on the pumping requirements. If you have a septic system you also need consider the levee’s location with respect to your system. Floodwaters could potentially enter your home underneath the levee through your septic system. Additional backflow preventative could be required.
Maintenance
On-going maintenance is an easy thing to do; however, it can easily be forgotten about which can cause major issues in the future. For some, maintaining the levee is no different than maintaining your yard. You want quality vegetation growth with a good root base clear of ants and burring animals that can damage the levee. You want to try to maintain grass that is between 3 to 6 inches tall. Anything shorter can cause the soil to loosen up while anything taller can make it difficult to watch for ants and animals. As with most mechanical equipment, you want to maintain your pumps and generator per the manufacturers’ recommendations and test them periodically to ensure they work. For more information on maintenance and operations of a levee, take a look at the USACE Levee Owner’s Manual.
Impacts to your Neighbors
One of the biggest issues with constructing a levee is the potential impacts to your neighbors. The larger the area protected by a levee the greater the chance at impacting the area around the levee. This is an item that can be easily forgotten about but it is potentially one of the highest legal risks of constructing a levee. Although it may not be required to obtain a permit, it would be a good idea to hire an engineer to help evaluate if the levee you construct would cause any impacts to the surrounding area. It might not cause an issue, but if a neighbor claims you caused damages being proactive by having an engineer look at it before construction goes a long way rather than looking at it after the fact.
Other Mitigation Actions to Consider
As we mentioned, a levee could be an “acceptable” alternative for some, but in the end if not properly designed and maintained, you could cause more issues for you as well as your neighbors. Also, if you are still paying a higher flood insurance premium, the better option might be to proceed with a different flood mitigation technique such as demolition, elevation, or a buyout program through your NFIP community. The upfront (short term) costs could be higher, but over time, it could be cheaper than remaining in the floodplain and/or constructing your own levee.
To learn more about protecting your home from flooding, check out FEMA’s Homeowner’s Guide to Retrofitting.
Flood Risk on the Bend 