Soil and the Water Table: Great Ways to Predict (Or Prevent) Basement Flooding

My neighbors suffered from frequent backyard and basement flooding. But recently they added a backyard extension to the house, along with a new drainage system, and it seems to be working out great.

How’d they figure it out?

Their contractors checked the soil by taking core samples, drilling down several feet at multiple locations to pull up a cross-section at each spot. Analyzing the samples helped them locate the water table, so they could determine the best depth, location, and design for the foundation and drainage.

Soil Core Sampling

When I first heard about soil core sampling, I imagined the process was something like drilling for oil. (‘Thar she blows, we hit water!) But analyzing soil is way more sophisticated than that.

Water leaves a trail of clues about where it’s been. For example, if your basement has ever been flooded, you may have noticed a “high water line,” or the cinder block foundation may have gotten coated in a white powder, where minerals crystallized after the water receded. Water also rusts iron, rots wooden framing, grows mold, and squeezes under floor tiles.

Not fun!

But in soil, these types of clues are helpful. Experts analyze the soil color and the composition of each layer to determine where water currently saturates the soil, and where it’s infiltrated in the past. For example, red or yellow soils indicate the presence of oxidized iron; a grey color could indicate high levels of moisture.

Soil Surveys

For a more general idea about the soil and the water table levels across a region, you can check a soil survey. Scientists collect extensive data on a region through field observations, soil sampling, and lab tests. Using this data, they classify the soil into various types, and map the information to make it more accessible.

I had fun checking the soil at the Wilmette Community Playfields, using a Web Soil Survey (WSS), provided by the USDA Natural Resources Conservation Service.

The soil types at the Wilmette Community Playfield. (Source is the USDA WSS.)

Soil types at the Wilmette Community Playfield. (Source is the USDA WSS.)

Using the map (above) and accompanying soil report, it was easy to look up a few facts about the two main soil types:

54B—Plainfield loamy sand, 1 to 6 percent slopes

  • Typical profile
    A – 0 to 8 inches: loamy sand
    B – 8 to 32 inches: sand
    C – 32 to 60 inches: sand
  • Depth to water table: More than 80 inches

571A—Whitaker loam, 0 to 2 percent slopes

  • Typical profile
    H1 – 0 to 10 inches: loam
    H2 – 10 to 47 inches: clay loam
    H3 – 47 to 54 inches: sandy loam
    H4 – 54 to 60 inches: stratified loamy sand to silt loam
  • Depth to water table: About 6 to 24 inches

(Note:This is general information, which needs to be verified by soil core sampling.)

Building Above the Seasonal High Water Table

Engineers typically recommend building above the “seasonal high water table,” which is the highest level of ground saturation during the wettest part of the year, for that specific locale.

above-water-table4

Building below the water table requires a well-planned drainage system. For flat, low-lying areas (like much of Wilmette), the system would almost certainly need to include an around-the-clock sump pump with a backup generator, in case the power goes out. But in hilly areas, if the house is on high ground, a gravity drain could clear the water.

below-water-table3-sm

Waukesha Mandates Soil Surveys

In researching this blog, I discovered that just an hour and half north of us, in Waukesha County, WI, extensive flooding led to new mandates related to soil and the water table.

For new construction on a site where there are hydric soil types, as indicated by general soil survey maps, builders must take soil core samples. The samples must be analyzed to determine how far the seasonal high water table is below the surface.

When designing the new home, the builder must use this information to ensure:

A minimum 1-foot vertical separation between seasonal high groundwater table and the basement floor surface.

(Source: Basement Wetness and Flooding Prevention Standards: Waukesha County)

Variances to build below the water table are granted only for a few special cases (for example, if the home is on a hill and the builder submits a viable drainage plan that relies on gravity–not a sump pump–to keep the basement dry).

Building Above Water Table = Happy Homeowners

The Waukesha rules apply only to rural portions of the county–urban areas have their own rules, which take precedence over the county’s. However, where applied, the new rules are reportedly making a difference for individual new homeowners.

Initially, some new homebuilders might feel disappointed when they’re told that their home cannot have a traditional finished basement, but instead must be built on a slab or crawl space. But over time, homeowners benefit from the “built-in” flood prevention, avoiding the frustration and expense caused by frequent basement flooding.


Links:

The USDA Natural Resources Conservation Service (NRCS) provides general soil surveys for any location you’d like to study. Type in your home address to check out the soil on your lot:
Web Soil Survey (WSS)

Find out more about Waukesha stormwater management:
Waukesha Land & Water Conservation – News & Events

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