|
ASLESON, NESTINGEN, GULLIVER, HOZALSKI, AND NIEI3ER
<br />TABLE 4. Estimated Number of Measurements Required
<br />to Obtain a Mean Ksat Value That Is Within 5, 10, and 15 %
<br />of the True Mean 95% of the Time and Comparison With the
<br />Actual Number of Measurements Made (N).
<br />Rain Garden
<br />N
<br />5%
<br />10%
<br />15%
<br />1 (Burnsville)
<br />24
<br />56
<br />14
<br />6
<br />2 (RWMWD #4)
<br />4
<br />16
<br />4
<br />2
<br />5 (RWMWD #5)
<br />16
<br />45
<br />11
<br />5
<br />6 (UM, St. Paul)
<br />41
<br />26
<br />6
<br />3
<br />7 (Cottage Grove)
<br />20
<br />29
<br />7
<br />3
<br />9 (RWMWD #1)
<br />12
<br />45
<br />11
<br />5
<br />11 (Thompson Lake)
<br />30
<br />59
<br />15
<br />7
<br />12 (UM, Duluth)
<br />34
<br />74
<br />19
<br />8
<br />Notes: RWMWD, Ramsey - Washington Metro Watershed District;
<br />UM, University of Minnesota.
<br />4.0
<br />3.5
<br />n
<br />3.0
<br />0
<br />2.5
<br />2.0
<br />c
<br />1.5
<br />w
<br />F 1.0
<br />0.5
<br />0.0
<br />FIGURE 9. Comparison of Measured Drain Times
<br />Obtained From Synthetic Runoff Tests With Drain Times
<br />Estimated Using the Mean, Median, and Geometric Mean
<br />Ksat Values From MPD Infiltrometer Tests.
<br />volume of water used in these tests and the ability
<br />for the water to flow laterally. The synthetic draw -
<br />down test determines drainage time for the upper
<br />40 -100 cm, depending upon the depth of water placed
<br />into the basin. The soil profile results from the visual
<br />inspection of this rain garden were useful in explain-
<br />ing the discrepancy.
<br />CONCLUSIONS
<br />Three new approaches for assessing the perfor-
<br />mance of rain gardens and other infiltration storm -
<br />water BMPs were developed and evaluated: visual
<br />inspection (Level 1), infiltration rate testing (Level 2),
<br />and synthetic drawdown testing (Level 3).
<br />All three assessment approaches provided useful
<br />information regarding the overall function of rain
<br />gardens. Visual inspection of the vegetation and soils
<br />provided a preliminary indication of the ability of the
<br />rain garden to infiltrate stormwater runoff. Infiltra-
<br />tion rate testing provided information on the spatial
<br />variability in Ksat and an estimate of the overall
<br />drain time of the rain garden. This information is
<br />useful for identifying specific locations to target for
<br />maintenance, which should improve performance and
<br />may prolong the life of the rain garden, and reduce
<br />costs overall. Infiltration rate testing can also be used
<br />to ensure that, the construction of the rain garden
<br />was done properly and allows for the identification of
<br />locations which may have been compacted during
<br />construction. The combination of visual inspection
<br />and infiltration rate testing is particularly useful for
<br />assisting in the development of maintenance tasks
<br />and schedules. While infiltration rate testing has
<br />numerous benefits, this method only provides a rough
<br />estimate of the time required for the rain garden to
<br />drain, especially when relatively permeable surface
<br />soil layers are underlain by restrictive soil layers.
<br />The synthetic drawdown test can be used to measure
<br />the drainage time quickly and with little effort when
<br />water supply is available to fill the basin sufficiently
<br />to determine a drainage time, which restricts the
<br />tests to rain gardens smaller than roughly 80 m in
<br />plan area.
<br />A multilevel assessment approach allows for the
<br />identification of problems in rain gardens, potential
<br />causes, and possible solutions. Nevertheless, when
<br />there are a large number of rain gardens to evaluate
<br />and a multilevel assessment of each rain garden is not
<br />feasible, assessment by visual inspection should be
<br />done periodically (e.g., annually) to identify potential
<br />problems that may impair rain garden performance.
<br />LITERATURE CITED
<br />Bjerg, P.L., K. H:insby, T.H. Christensen, and P. Gravesen, 1992.
<br />Spatial Variability of Hydraulic Conductivity of an Unconfined
<br />Sandy Aquifer Determined by a Mini Slug Test. Journal of
<br />Hydrology 136(1- 4):107 -122.
<br />Booth, D.B. and C.R. Jackson, 1997. Urbanization of Aquatic Sys-
<br />tems: Degradation Thresholds, Stormwater Detection, and the
<br />Limits of Mitigation. Journal of the American Water Resources
<br />Association 313(5):1077 -1090.
<br />Foth, H.D. 1990. Fundamentals of Soil Science, 8E. John Wiley &
<br />Sons, Inc., New York, NY.
<br />Gulliver, J.S. and J.L. Anderson, 2007. Assessment of Storm -
<br />water Best Management Practices. University of Minnesota,
<br />St. Paul, Minnesota. http:// wrc .umn.edu/outreach/stormwater/
<br />bmpassessment /assessmentmanual/index.html, accessed July 11,
<br />2007.
<br />Hillel, D., 1998. Environmental Soil Physics. Academic Press,
<br />Amsterdam.
<br />Jang, Chen -Shin and Chen -Wuing Liu, 2004. Geostatistical Analy-
<br />sis and Conditional Simulation for Estimating the Spatial
<br />Variability of Hydraulic Conductivity in the Choushui River
<br />Alluvial Fan, Taiwan. Hydrological Processes 18:1333 -1350.
<br />JAW RA
<br />1030 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
<br />1 (Cottage Grove) 5 (RWMWD #5) 6 (U of M, St. Paul)
<br />
|