3 Questions About Bioretention Soils and Infiltration

In March we hosted a webinar where Jonathan Page, an engineer working in Dr. Bill Hunt’s lab at North Carolina State University (NCSU), provided a stormwater engineering perspective on the design principles, treatment processes, and implementation of using Silva Cells as a stormwater control measure. Current research findings and a design case study were included in the presentation. After the webinar was over, several people reached out to us with follow up questions. Knowing that others may have had the same ones, we decided to print the questions and their answers (prepared by landscape architect Nathalie Shanstrom of The Kestrel Design Group) here. -LM


I’ve read on this blog that soil with structure (peds) can achieve similar perk rates as 85 percent sand mixes, while getting similar pollutant removal rates and being better for trees. Is that true?

Commonly accepted infiltration rates for some basic soil types are listed below in Table 1 (per Saxton and Rawls 2006).

Table 1: Saturated hydraulic conductivity of various soil textures per Saxton and Rawls 2006

Texture class Sand Clay Saturated Conductivity (mm/h) Saturated Conductivity (in/h)
Sand 88 5 108.1 4.3
Loamy Sand 80 5 96.7 3.81
Sandy Loam 65 10 50.3 1.98
Loam 40 20 15.5 0.61
Silty Loam 20 15 16.1 0.63
Silt 10 5 22 0.87

Why engineers use high sand content

The above numbers are not based on data for bioretention soils. Many engineers claim that because a lot more stormwater runs through bioretention soils than through typical soils that are not part of a best management practice [BMP], bioretention soils will receive more fines and clog more easily – thus they should have high sand content to offset  the large amounts of fines they will collect over time.

The original bioretention mixes used in the 1990’s typically consisted of sand, topsoil, and organic matter (e.g., compost or shredded hardwood mulch). Many of the bioretention cells installed with these mixes failed due to clogging. Engineers responded to these failures by lowering the fines content in the soils, often recommending 80/20 mixes. These clogged less, but were not as good for plant growth or nutrient removal, so now people are moving away from 80/20 mixes again.

Some fines content doesn’t need to be bad for bioretention – and is good for trees

Several bioretention cells with fines content higher than 15% have performed well over time. For example, Emerson and Traver (2008) monitored infiltration rates of a raingarden at Villanova University in Pennsylvania with a soil with 30% fines and found that the infiltration rate did not significantly change over 4.25 years. No maintenance was performed on this raingarden during the monitoring period. Average infiltration rate was 5” (25 mm)per hour during this time, which is higher than what would be predicted for this soil texture. They attribute the sustained high infiltration rate to dense vegetation and surface mulch. Gilbert Jenkins et al (2011) found that even after 8 years, infiltration rates of this same raingarden did not significantly decrease.

Research shows that roots and animals (e.g., invertebrates) in the soil maintain macropores in the soil and can therefore maintain natural infiltration rates, even in bioretention cells.

Emily Ayers (2009), for example, studied ten bioretention cells ranging from one to 10 years of age. Sand content varied from 46.4% to 91% at the time of her study. Nine out of ten of these sites had less than 80% sand. She found no evidence of clogging at any of these sites.

She measured infiltration rates at three of these sites, and, presumably because of biological activity in the soil (which maintains soil structure), infiltration rate greatly exceeded (> 3x) the infiltration rate that was expected based on soil texture alone per Rawls et al 1982:

Table 2: Measured vs predicted infiltration rates at bioretention cells measured by Ayers (2009)

Site name Soil Predicted infiltration rate based on Rawls et al 1982 (inches per hour) Measured infiltration rate (inches per hour) Age at time of measuring infiltration rate
BP 53% sand, 23.5 % silt, 23.4% clay 0.2 3.15 7 years
NWHS 51.4% sand, 34.2 % silt, 14.4% clay 0.6 2.165 5 years
LRH 46.4% sand, 33.3% silt, 20.3% clay 0.5 3.15 10 years

Her thesis cautions against using very high sand content in bioretention cells for the following reasons:

  • “Sand does not hold water or nutrients well, and can create an inhospitable environment for rain garden plants and soil animals.”
  • “Sand does not hold a macropore structure very well, which limits the infiltration rate of the soil to the permeability of the medium. Earthworms and plant roots may create pores, but in an unstable soil, this structure will be destroyed every time it rains.”

Fines (silt and clay) also improve pollutant removal (eg Hunt et al 2012).

Research by Skorobogatov, among others, shows that tree roots increase infiltration rates even more than roots of herbaceous plants, so sustaining infiltration rates over time should be even less of a concern in bioretention sites where trees are present compared to those where they are not.

While engineers fear that bioretention infiltration rates will decrease over time due to influx of fines from the surrounding watershed, research indicates that correctly designed and installed bioretention cells often have higher infiltration rates than expected due to macropores created and maintained by plant roots and animals.


 I’m confused about soil selection. As I understand it, we should aim for more sandy soils to allow for the best percolation/filtration possible. Is that right?

Infiltration rates of 1” to 2” (25mm – 50mm) per hour are optimal for most stormwater goals (Hunt et al 2012). This corresponds to predicted infiltration rates for a sandy loam, with around 60-65% sand.

Infiltration rates must be high enough to be able to drain the stormwater control measure within the required draindown time (typically 72 hours, and as low as 24 hours in some jurisdictions), but if the infiltration rates are too high, plant health and pollutant removal will decrease.

So: there must be a balance between sand and fines. Including fines in the soil improves pollutant removal both by increasing cation exchange capacity and increasing contact time. It is also important to have some fines in the soil for optimal tree health. The healthier the tree, the greater the stormwater benefits it will provide. This is a design area where we still have a lot to learn as far as which mixes are best for the tree while performing a stormwater management function over time.


In your webinar earlier this year, Jonathan Page discussed storage volumes in the Silva Cell system – only he did not count the 20% holding capacity of the soil that is contained in the system. How was he defining storage? 

We asked JP and his answer was “I was assuming all runoff was stored above the soil in the air space and/or aggregate layer. This is because most permitting entities that I have interacted with will not allow soil water storage to be included in the calculation, and I would have to assume the soil was at the field capacity. Obviously there is a substantial amount of storage available in the soil and could certainly be included in other designs.”

When we asked him if he accounted for evapotranspiration (ET) he said, “It is important to remember that there is very little appreciable ET and/or nutrient uptake that takes place on a storm event basis which may last for 1 – 3 hours and occurs under cloudy/wet conditions. I do not know a lot about tree physiology, but it seems unlikely that any vegetation would be generating an appreciable ET/uptake volume on a storm event basis, thus we focus primarily on the storage volume. Long term continuous simulation models can account for this as DRAINMOD does, but it is still not more than 5-8% of the annual water balance.”

However, opinions on this do differ. At Kestrel, for example, we think that evapotranspiration can provide a real impact on stormwater management, and Minnesota is an example of a state that actually provides a credit for trees on the basis of ET, in addition to infiltration and interception; you can read more here.

For Further information

This article just skims the surface of the literature on this topic, and there are many factors that affect clogging, and many other ways to increase water retention besides increasing clay content. Please contact us if you would like to work with us or learn more about anything discussed here!


Ayers, Emily Mitchell. 2009. Pedogenesis in Rain Gardens: The Role of Earthworms and Other Organisms in Long-Term Soil Development. Dissertation submitted to the Faculty of the Graduate School of the University of Maryland, College Park in partial fulfillment of the requirements for the degree of Doctor of Philosophy.

Emerson, C. and Traver, R. (2008). ”Multiyear and Seasonal Variation of Infiltration from StormWater Best Management Practices.” J. Irrig. Drain Eng. 134, SPECIAL ISSUE: Urban StormWater Management, 598 –605.

Hunt, W., Davis, A., and Traver, R. 2012. Meeting Hydrologic and Water Quality Goals through Targeted Bioretention Design. J. Environ. Eng., 138(6), 698–707. doi: 10.1061/(ASCE)`EE.1943-7870.0000504

Jenkins, J., Wadzuk, B., and Welker, A. (2010). ”Fines Accumulation and Distribution in a StormWater Rain Garden Nine Years Postconstruction.” J. Irrig. Drain Eng., 136(12), 862–869.

Loh, F. C. W.; Grabosky, J. C.; Bassuk, N. L. (2003) Growth Response of Ficus Benjamina to Limited Soil Volume and Soil Dilution in a Skeletal Soil Container Study. In Urban For. Urban Gree. 2 (1) 53-62.

Rawls, W. J., D. L. Brakensiek and K. E. Saxton. 1982. Estimation of soil waterproperties. Transactions of the ASAE 25(5):1316{1320.

Saxton, K. E. and W. J. Rawls. 2006. Soil Water Characteristic Estimates by Texture and Organic Matter for Hydrologic Solutions. Soil Sci. Soc. Am. J. 70:1569–1578.

Skorobogatov, Anton. Wendy Thorne, and Bernard Amell. 2013. Biological Elements in Rain Garden Design. Presentation given at 2013 International Low Impact Development Symposium, August 18-21, 2013, Saint Paul, Minnesota.


Nathalie Shanstrom is a sustainable landscape architect with The Kestrel Design Group.

Aaron Volkening / CC BY 2.0

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