In most areas of Louisiana, the sewer system and rain drainage system are separate. It is not unusual, however, for floodwater to infiltrate the sewer system, causing it to back up into homes. These back-ups can occur both inside and outside the identified flood hazard areas and can be caused by things other than floodwater entering the system.
When there is more water outside than inside a building, a floodwall or a levee, water continually tries to get inside. Obvious paths of intrusion are sewer drains for the bathtub and toilet and drainage tubes in floodwalls and levees. Floor drains in some areas of buildings also could provide such a path. Any drain that has its opening (inside the home) below flood level must be blocked. The drains may not be obvious – such as air-conditioning condensate drains – so look carefully.
Using Valves, Plugs, Caps and Seepage Barriers in Flood Protection
The typical home has interior plumbing drains that converge under ground near the house into a single 4-inch sewer line. The simplest, least-expensive way to prevent backflow is with a flap valve installed on the single sewer line; it allows water from the home to flow to the sewer system but closes when water flows backward toward the house. You must provide a means of accessing the valve so it can be cleaned at least semi-annually and if something obstructs the flap.
To provide the best possible protection from a flooded sewer system, the valve should have a good seal and operate automatically. Several valve types are described below. In an emergency, if you have no valve or if the valve fails, try the bag-o-rags technique.
A ball or gate valve that is closed by hand is less likely to fail because debris is preventing its closing, but it won’t stop a backflow unless someone is home to close it. Automated or hydraulically operated backflow valves are considerably more expensive but provide reliable protection at all times. It may be beneficial to combine a flap valve (for automatic closure) with a ball or gate valve that requires manual closure but provides a more-positive seal.
Installing any of these valves in an existing sewer line is equally difficult, since it requires digging up the sewer line. The benefits far outweigh the trouble and cost of preventing unhealthy sewage backflow, however, even if you don’t keep surface floodwater out of the building.
Installing a Backflow Valve in the Sewer Line
Installation of any of these backflow valves in an existing sewer line is dirty work but not technically challenging.
1 - Expose sewer line and mark sections to be remove
2 - Remove the marked section of the sewer line
3 - PVC flap valve installed in tile sewer line
4 - Flap valve with water flowing (normal operation)
Automatic Valves & Manual Valves
Automatic valves work when you are not at home to close them. Manual valves are valves that do not work on their own; someone must be at home to close them.
Hydraulic Flap Valve - Example of an Automatic Valve. This clear-port flap valve offers the advantage of being normally open (so it doesn't trap solids) and closing with force when the sewer system floods. The valve can be submerged, can be fitted with a a by-pass pump and comes in residential and municipal sizes.
Hydraulic Gate Valve - Example of Automatic Valve. This clear-port gate valve offers the advantage of being normally open (so it doesn't trap solids) and closing with force when the sewer system floods. Since the valve may not work if the canister is submerged, it should be used in basements or on the dry side of floodwalls and levees.
Flap (Check) Valve - Example of an Automatic Valve. The simple flap valve, also called a check valve, is the most-popular and least-expensive backflow valve. Since the flao is normally closed, it must be pushed open by sewer water. Solids in the sewage may stick on the seal, causing the valve to fail. Check valves are available at home improvement stores.
Ball Valve - Example of a Manual Valve. The ball valve is open when the handle is in line with the sewer line. The valve is fully closed by turning the handle a quarter-turn so it is perpendicular to the sewer line.
Gate Valve - Example of Manual Valve. The gate is open when the gate is "up." The valve is closed using multiple clockwise turns of the handle, resembling a typical outdoor water faucet.
Preventing Backflow Using Bag-o-Rags
In an emergency, if you have no valve in the sewer line, or if the valve fails, you can use a plastic bag filled with rags to stop back-flow through the sewer.
The pictures below show the bag being placed in the sewer line at the sewer clean-out. This clean-out should be accessible in the yard, between your house and the point where your sewer line enters the municipal or private sewer system collection line.
You can also use the bag-o-rags technique inside the home. To block back-flow at the toilet, remove the toilet and stuff the bag of rags in the exposed sewer pipe opening. Do not stuff the bag of rags into the toilet through the toilet bowl. This protection is similar to what you get using gripper plugs and involves the same amount of work on your part.