The Dirt on Soil Infiltration

Accurate determination of the soil infiltration rate beneath a stormwater infiltration practice is critical to properly designing the practice. Soil borings are often used to determine soil texture, which is then used as a surrogate for infiltration rate. For example, the Minnesota Stormwater Manual provides design infiltration rates for different soils (e.g. 0.8 inches per hour for loamy sand, 0.45 inches per hour for silty sand, and so on). Field measurement of infiltration rate is preferred. Previous posts provide information on collecting and interpreting soil borings and measuring infiltration rate in the field.

Whether using soil borings or field measurement to determine soil infiltration rate, it is important to understand factors that affect the accuracy of the determination. This post provides some basic information about soil infiltration and factors that affect infiltration.

Infiltration of water into soil is a complex process. Factors affecting infiltration include soil texture, heterogeneity, initial water content, presence of secondary porosity (e.g. macropores), surface roughness, and vegetation (USDA-NRCS, 2014). While soil texture is typically the most important factor affecting the soil infiltration rate, using only soil texture fails to consider the other factors affecting the infiltration rate. Field measurement accounts for some of the factors discussed below, but even field measurements are not entirely accurate, particularly if there are an insufficient number of tests.

Infiltration rates over time in initially dry soils

Steady-state infiltration rates are typically used to design infiltration practices. The schematic at the right shows that infiltration rate into an unsaturated soil is initially high and eventually reaches steady state. Steady-state rates will underestimate the actual infiltration rate taken over the timeframe when water is infiltrating, particularly if the soil is initially dry.

Infiltration rates used to design infiltration practices assume one-dimensional movement of water. For rainfall infiltrating a typical soil this assumption is reasonable. Infiltration practices capture and store water below the land surface. Infiltration is therefore three-dimensional away from the stored water, at least in the early stages of infiltration when the surface wetting front has not penetrated deeply into the soil. The schematic below illustrates this, where the soil wetting front proceeds vertically from the land surface but the wetting front from the ponded area proceeds in three dimensions. There are few models of infiltration beneath infiltration practices and models often assume one- or two-dimensional flow (Aravena and Dussaillant-Jones, 2009; Lee, 2011). Assuming one-dimensional infiltration beneath the infiltration practice underestimates the actual infiltration rate.

3-dimensional infiltration from an infiltration practice

Standard infiltration rates associated with different soil types do not consider the effect of ponding. While ponded water creates a positive head that increases the infiltration rate, the effect is small in most soils, where ponded depths are very shallow. However, ponded depths in an infiltration practice may be 2 feet or more. There have not been sufficient studies to describe the effect of ponding in infiltration practices.

Infiltration into a sand with a clay layer

The above factors may result in underestimation of infiltration rates, but there are factors that may result in overestimation of infiltration. The most important of these is the presence of a low permeability layer in the soil profile. Even a very thin clay layer in a sandy soil can severely restrict infiltration, as shown in the schematic on the right. The wetting front slows in the vertical direction as water accumulates above the clay layer. Eventually the wetting front will break through the restricting layer, but failure to identify the restricting layer will result in significantly overestimating infiltration.

Soil compaction reduces soil porosity and can lead to overestimating infiltration rates. Soil porosity naturally decreases with depth, resulting in decreased infiltration capacity with depth. Surface compaction may occur during construction of the practice or during maintenance activities. Compaction may also occur from ponded water compressing the underlying soil over time. Compaction can significantly decrease infiltration rates over time. Vegetation and biological activity may offset effects of compaction by development of secondary porosity (macropores) from plant roots and burrowing by organisms such as earthworms. Pitt provides a discussion of compaction in urban soils, including methods for alleviating compaction.

Infiltration rates also decrease over time due to sedimentation and clogging within the BMP. Without properly maintaining or providing adequate pretreatment for infiltration practices, these decreases can be significant. 2NDNATURE and Northwest Hydraulic Consultants (2013) provide a good discussion of reduced infiltration rates in infiltration practices. In poorly maintained practices, infiltration rates can decrease by an order of magnitude due to clogging. The decrease in infiltration occurs at the bottom of the practice where sediment accumulates, while infiltration into the sides of the practice is maintained over time. Thus deeper practices help maintain infiltration function.

Mounding beneath an infiltration practice

Another factor that can reduce infiltration rates is a shallow groundwater table. A mound develops beneath an infiltration practice following a rainfall event. If the mound intersects the BMP, vertical infiltration stops. If the minimum 3 foot separation distance is observed, mounding will generally not be problematic unless the groundwater occurs within fine-textured material. For more information, including a simple method for calculating the mound height beneath an infiltration practice, see the Minnesota Stormwater Manual.

This post does not present all factors affecting soil infiltration rates in soil, but identifies some of the more important factors affecting hydrologic performance of an infiltration BMP. General guidance on the design, construction and maintenance of BMPs, such as in the Minnesota Stormwater Manual, provides a sound basis for proper BMP selection and implementation. However, understanding site characteristics is critical in final design of infiltration practices.

References

Aravena, J.E.; and Dussaillant-Jones, A. 2009. StormWater Infiltration and Focused Recharge Modeling with Finite-Volume Two-Dimensional Richards Equation: Application to an Experimental Rain Garden. Journal of Hydraulic Engineering. 135:12.

Lee, R. 2011. Modeling Infiltration in a Stormwater Control Measure using Modified Green and Ampt. M.S. Thesis. Villanova University.

Pitt, Robert. Compacted Urban Soils and their Remediation. University of Alabama, Department of Civil, Construction, and Environmental Engineering. 14 pp.

2NDNATURE and Northwest Hydraulic Consultants (2013). Infiltration BMP Design and Maintenance Study. Prepared for Tahoe Regional Planning Agency. March 2013.

United States Department of Agriculture – Natural Resources Conservation Service. 2014. Soil Infiltration, Soil Quality Kit – Guides for Educators.