This document discusses the number of infiltration measurements needed to characterize the performance of rain gardens for stormwater control. It finds that 5-6 single ring infiltration tests, spaced evenly throughout the rain garden, are sufficient. The tests aim to account for spatial variability in soils. Comparing geometric means of test combinations to measured ponding recession rates, the standard deviation falls to within 10% of the recession rate after 5-6 tests.

2. HYDRAULIC CONDUCTIVITY:
HOW MANY MEASUREMENTS
DO YOU NEED?
Dr. Andrea Welker, PE, F.ASCE, ENV SP
Dr. Kristin Sample-Lord, PE
Villanova University

3. Outline
3
Rain gardens as stormwater control measures
Need to quantify infiltration rates
The research goals
Ponded recession rate
Spot infiltration testing program
Results from five rain gardens

4. Rain gardens – stormwater control
4
PA BMP Manual
Actual (Beautiful!) Rain Garden on the
Jenkintown Creek in Abington, PA

5. April 6th, 2017: 10:10 AM to 6:30 PM (1.10”)
5

6. Infiltration is an important mechanism
Infiltration and evapotranspiration are the two key mechanisms that control
stormwater in rain gardens
Quantifying infiltration is important in design phase and is often part of
inspections – a minimum value is often set by regulations
6

7. Research goals
7
• Time spent testing
• Volume of water used
• Number of tests required
Optimize efficiency in measurement of infiltration rate
• Accurately estimate performance of rain garden
• Err on the side of conservativism
Obtain a reliable and representative value

8. k Soil Suction
Soil Thickness
Moisture Content
Piezometric Head
Factors influencing infiltration
8

9. Ponded recession rate
• An indirect measurement of infiltration (Emerson 2008)
• A very useful performance indicator
• Simply measured using bubblers, pressure transducers
• Provide infiltration rate over the entire rain garden bowl, so this is
considered the “true” value
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10. Ponded recession rate
10

11. 11

12. Summary of sites
12
Site Surface Area
(m2)
Hydraulic
Loading Ratio
Size of Design
Storm (cm)
USCS
Classification
USDA
Classification
VURG1 235 10:1 2.54 SM Silty sand Loamy sand
VURG2 92 0.87:1 2.54
SP Poorly graded
sand
Sand
USRG 80 8.6:1 3.61 SM Silty sand Sandy loam
DSRG 62 13:1 4.24 SM Silty sand Sandy loam
SMP A 429 19:1 8.5
SW-SM Well
graded sand
with silt
Loamy
sand/sandy
loam

13. Field infiltration tests
13
Double ring is still considered
the “gold” standard by many
Temperature correction and
seasonal consistency
Relate infiltration test results
to ponding recession rate
Site
Number of
Infiltration
Tests
Performed
Method
Used
Geometric
Mean
Average k
(m/s)
Measured
Ponding
Recession
Rate (m/s)
VURG1 7
Single-
Ring
2.61x10-6 1.90x10-6
VURG2 10 MPD 1.06x10-6 1.06x10-6
USRG 10 SATURO 2.48x10-5 4.72x10-5
DSRG 8 SATURO 1.87x10-5 4.22x10-5
SMP A 8 SATURO 1.06x10-5 3.70x10-5

14. Single-Ring
Infiltrometer
Modified Philip-
Dunne
SATURO
Method of
Measurement
Continuous refill,
constant head
Full to empty, falling
head
“DualHead”
approach, completely
automated
Precision (Range of k)
7.1x10-5 cm/s to
7.1x10-3 cm/s
1x10-6 cm/s to
1x10-2 cm/s
1.0x10-6 cm/s to
3.2x10-2 cm/s
Dimensions (cm) H = 15.2, d = 15.2 H = 61 cm, d = 10 cm 5 cm insertion ring
Additional Notes
Driven halfway into
soil
Sealed at top to
prevent evaporation
5 gallon water supply
tank
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15. Testing rationale
15
Spatial variability within the soil
media and over the surface area will
result in different test results
Perform a number of tests, evenly
spaced, same season
Analyze data in the context of
ponding recession data, i.e. what is
actually happening

16. Data analysis
Organize Test
Data
Generate All
Possible
Combinations
up to r = n
Plot the
Geometric
Mean of Each
Combination
Relate to
Actual
Recession
Rate
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17. SMP A
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Measured Recession Rate (m/s) 3.70x10-5
Number of Tests 1 2 3 4 5 6 7 8
Average Value (m/s) 2.31x10-5 1.56x10-5 1.32x10-5 1.21x10-5 1.14x10-5 1.10x10-5 1.08x10-5 1.06x10-5
% Difference from Recession Rate 38 58 64 67 69 70 71 71
Standard Deviation (m/s) 2.49x10-5 1.47x10-5 9.58x10-6 6.65x10-6 4.77x10-6 3.39x10-6 2.23x10-6 -
Coefficient of Variation 1.08 0.94 0.73 0.55 0.42 0.31 0.21 -
Standard Deviation as % of Recession
Rate
67 40 26 18 13 9 6 -

18. SMP A
18

19. All sites
19
Standard deviation as a % of
recession rate occurs around 5 to 6
tests

20. Bottom line
Five to six single ring infiltrometer
tests (e.g. Saturo) should be able to
adequately characterize the
infiltration capacity of a rain garden
20

21. Acknowledgments
21
The views expressed here are the authors’ and
not those of the sponsors.
Thanks to Zach Zukowski, James Press, Virginia
Smith, and Billy Nichols.

22. QUESTIONS?