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Spatial Variation of Infiltration in LID practices
February 2011 (volume 6 - issue 2)
Contributed by Farzana Ahmed, St. Anthony Falls Laboratory, University of Minnesota (Advisors: John Gulliver, Department of Civil Engineering, and John Nieber, Department of Bioproducts and Biosystems Engineering.)
Saturated hydraulic conductivity (Ksat) indicates the ease at which water moves through the pore space when the soil is in a saturated condition, and is used to determine the infiltration rate of low impact development (LID) practices. Measurements of the infiltration rate applied to a design storm are helpful to determine performance, schedule maintenance and meet regulatory requirements of LID practices. The most common devices used to determine the infiltration capacity of soil are the single and double ring infiltrometers. The drawback of using these devices is that they require constant head with the associated large volumes of water, which means that it is difficult to perform more than one test simultaneously. We have found, however, that infiltration rates have great spatial variation. A number of infiltration measurements are therefore required in the infiltration practice to get a representative mean value of saturated hydraulic conductivity and an uncertainty in that mean value.
The Modified Philip Dunne (MPD) Infiltrometer has been developed by St. Anthony Falls Laboratory to measure the infiltration rate of soil at up to 20 simultaneous locations at a site, sufficient to estimate a representative infiltration capacity. The MPD has been used on an infiltration basin, eight rain gardens, one swale and non-engineered soils. Since most of these infiltration practices are constructed by filling with engineered soil, a minor variation of Ksat within that infiltration practice was expected. Table 1 indicates that the converse is true; the variations in the engineered soil as shown by the coefficient of variation (COV) were found to be of the same order as that for non-engineered soil.
Table 1: Statistics of Ksat measurements.
|Location||Soil Type||Number of Infiltration Measurements||Arithmetic Mean of Ksat (cm/hr)||Geometric Mean of Ksat (cm/hr)||Coefficient of Variation|
|Stillwater Infiltration Basin||Engineered Soil||65||11.2||3.29||1.59|
|Burnsville Rain Garden||23||11.8||6.75||0.92|
|Cottage Grove Rain Garden||20||18.5||16||0.53|
|RWMWD Rain Garden||32||3.93||1.59||1.23|
|Thompson Lake Rain Garden||30||9.56||1.63||1.19|
|U of M Duluth Rain Garden||33||7.25||1.68||1.78|
|U of M St. Paul Rain Garden||40||2.76||0.833||1.21|
|French Regional Park||Non-Engineered Soil||18||3.74||2.38||1.1|
|Maple Lake Park||31||3.09||2.73||0.56|
|Lake Minnetonka Regional park||14||7.55||2.13||1.73|
Regardless of whether the soil is engineered or not, the COV of Ksat is, in most cases, relatively high. This indicates that one infiltration measurement at each site may not represent the infiltration rate of the whole area. Thus, roughly 10 measurements of infiltration rate are required to capture this high spatial variation of Ksat and compute the mean Ksat values.
The Thompson Lake Rain Garden is one example, where 30 infiltration measurements of Ksat were made with the MPD infiltrometer, as shown in the following figure. Though this rain garden had engineered soil, the Ksat value varied from less than 10 to 194 cm/hr. One measurement of infiltration rate would likely not be representative of the infiltration in the Thompson Lake Rain Garden.
Figure 1: (Image courtesy B. Asleson & R. Nestingen)
To meet requests and provide service to the industry, the St. Anthony Falls Laboratory has begun manufacturing the MPD infiltrometer for purchase. If interested, please contact Bonnie Jean MacKay.
We want to hear from you!!!
Let us know your thoughts, experiences, and questions by posting a comment. To get you thinking, here are a few questions:
- For evaluation and design purpose which mean value of Ksat is more representative for Rain gardens, Infiltration basins and Vegetated swales; Arithmetic mean or Geometric mean?