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Health & Fitness

A HIGH WATER TABLE AND YOUR BASEMENT

How to deal with a high water table on your property.

 

Okay, here comes some geology. It is important to understand how the water rises around your basement and can cause basement flooding. High water tables are a nuisance that many homeowners must face. The water table lies underground and is the level at which the soil and gravel are completely saturated with water. There is often some seasonal change in the water table, due to rain or drought. A high water table is especially common in areas where the soil is not well drained due to high levels of clay.

Most people know that the water table has something to do with ground water. The word table provides

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an image of a flat surface, like the surface of a tabletop. It may be visualized as the 'surface' of the subsurface materials (sediment, gravel and clay) that are saturated with groundwater in a given geographical area. There is also a general understanding that in times of drought, water table levels may drop.

 

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Understanding sub-surface water can help explain why water tables may rise and fall. Ground water is sub-surface water, but not all sub-surface water is ground water. The upper surface of ground water is the water table. Below this surface, all the spaces and cracks (pore spaces) in clay and rocks are completely filled (saturated) with water. In the top layers of soil pore spaces may not be completely filled with water. These layers may contain water, some air, and some may only be partly filled with water. This is known as the unsaturated zone or the zone of aeration. After heavy rainfall this zone becomes saturated, while during a long dry spell or drought, it becomes almost dry. Rainwater infiltrates downwards through the unsaturated zone. This infiltrating water is known as soil water when it is still shallow enough to be used by plants. With further infiltration however, excess water will eventually reach the water table. As the rainfall continues, the water table rises.

 

 

Water tables can become elevated when they receive more water than they drain off. This can be from unusually high amounts of rain, or excess water from higher elevations. High water tables are often above the level of basement floors or crawlspaces. This almost always causes flooding in these areas.

 

The level of the water table varies greatly due to amount of rainfall, time of the year and type of soil that surface water drains through. The water table is generally higher in areas with high density soil related to clay content. The denser the soil is, the slower the movement of the water (percolation) of the water through the soil occurs. The rate at which the high water table descends is related to the percolation rate, which is related to soil density.

This high density soil follows the grade of the terrain. Therefore the water table can be just as high at the top of the hill as it is at the bottom of the same hill. The flow of surface water is based on the slope of the land but not the water table. The water table conforms to the contour of the land above.

The next time you see a puddle in the yard after a 2 day soaking rain, remember the water table. It is very likely that the puddle is the top of the water table that has temporarily exceeded the grade of the yard. When the water table rises up to the level of your basement there is nothing you can do to stop it. The water table typically rises over a very wide area, not just around your house. The water table is part of the earth and re-grading your flower beds will not alter it.

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