Follow these design, maintenance and recovery tips to avert disaster when a customer’s onsite system goes underwater


Among my frequently asked questions: "What can be done to protect a system against flooding?" And: "If the system has been flooded, what needs to be done before it can be used again?"

Before addressing some of the specifics regarding these questions, let’s take a step back and look at why we worry about systems flooding, and what kind of situations and locations create flooding problems.

There are several primary reasons to worry about flooding:

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  • The house is connected to the system by piping that under some flood events can carry sewage from the tanks in the system back into the house. This can cause significant physical damage and present a health hazard from wastewater carrying disease-causing bacteria and viruses.
  • Flooding can cause long-term damage to the system, creating the need for repair or replacement.
  • There can be environmental and water-quality damage if untreated wastewater is released into the environment.

A few types of flooding can occur. One is where the area is along a stream or river that periodically floods and is associated with an identified floodplain area. The other is where the system is in a landscape location that can be subjected to flooding or — as I prefer to think about it — ponding as a result of extreme precipitation events that occur at intervals of different frequency, depending on the exact location of the system.

Topographic clues
Maps available through local planning and zoning offices should help determine if a system was or will be installed in a floodplain area. These maps show elevations and locations for insurance and safety purposes. The other kinds of frequent or infrequent flooding that are due to local precipitation events are sometimes harder to predict.

Landscape locations, such as near the bottom of slopes and depressions or next to intermittent washes or streams, are indicators that flooding may be a problem. U.S. Department of Agriculture soil maps identify soil areas with these characteristics. While these are not at a scale to locate the exact system, they can be helpful in identifying potential problems.

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If the system is installed in the flood fringe area of the floodplain, several design and installation changes can be made to help protect the system and the residence when a flood occurs. Some of these probably seem like very commonsense requirements.

The drainfield portion of the system should be located at the highest possible elevation on the lot. Where the 10-year flood elevation is identified, the bottom of the drainfield media should be at or above this elevation. If the system is in the floodplain, there will often be a problem with vertical separation distance, requiring a mound or at-grade system.

Recommendations for mounds installed in these areas include that the bottom of the infiltration bed should be at least 6 inches above the 10-year flood elevation. There should not be any direct-installed inspection or access pipes, unless the top of the mound is above the 100-year flood elevation. Also, consider that when a mound is constructed, any fill brought in must fit within local requirements so that the flow of the floodwater is not impeded.

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Design considerations
For both floodplain and identified ponding situations, the following design and installation considerations would apply:

• If the system includes a pump, a provision should be made to prevent the pump from operating when the pump tank is inundated. There should also be the ability to monitor pump operation to determine flows. This will help the homeowner and service provider determine whether excess water from groundwater intrusion into the tank is being pumped to the drainfield as floodwaters rise. If there is additional flow, the pump should be shut down and the system should no longer be used.

• There should be no inspection pipes or other installed openings direct from the distribution media in a sewage treatment trench to the soil surface. When the drainfield and septic tank are inundated, no sewage should be discharged to the system.

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• Make provisions to close off the piping to prevent backflow from the septic tank into the residence. In a floodplain area, a holding tank can be installed as an alternative place to put the sewage until flooding subsides. Plan for a rapid method to divert the flow when the system is inundated.

Following a flood
A number of steps should be taken after the floodwaters subside and before the system is used again:

• All parts of the system should be inspected for damage. The septic tank and other tanks — such as pump tanks — should be pumped out and inspected for any structural damage. Pumps should be inspected, reinstalled and recalibrated for flow. When pumping tanks, make sure groundwater levels have gone down and be aware of tank buoyancy problems, where the tank could float if it is pumped out.

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• Drainfields under the surface should be all right if excessive water was not delivered to the drainfield during the flood even. And if there are no openings, silt and other debris should not have entered the drainfield. It should not be used, however, until the groundwater has subsided and vertical separation distance is established. Saturated soil is especially susceptible to compaction, which can lead to system failure. Avoid driving or operating equipment over the drainfield.

About the author 
Jim Anderson is connected with the University of Minnesota Onsite Wastewater Treatment Program and is an emeritus professor in the university’s Department of Soil, Water, and Climate. Send him questions about septic system maintenance and operation by sending an email to kim.peterson@colepublishing.com.


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