JAWRA article - Rain Gardens

• Project Type: General OWC
• Title: Performance Assessment on Rain Gardens
• Date: 12/15/2009
• Water Conservation - Doc Type: Reports

PERFORMANCE ASSESSMENT OF RAIN GARDENS
Thompson Lake Rain Garden
0
0
•
•
Inlet
0
Hydraulic Conductivity" +�
[cm /s]
• 0.000001 -0.003 Inlet
0 0.003 - 0.006 0 O
0.006 - 0.012
0.012 - 0.024 "
• 0.024 -0.06 W E
0 2 4 8 12 16
Meters s
FIGURE 5. Map Showing the Range of Ksat Values
Measured Using the MPD Infiltrometer Locations Within
the Thompson Lake (11) Rain Garden.
by fine solid deposition clogging the macropores
connected to the surface; however, this effect was not
measured.
The UM — Duluth (12) rain garden (Figure 6) had
the greatest variability in the measured Ksat values.
The rain garden contained two distinct types of soil:
coarse sand located directly over an underdrain sys-
tem and sandy loam overlying clay beneath the vege-
tated portions of the rain garden. The overall size of
this rain garden was 1,350 m which consisted of
both upland or woodland zones and rain garden
zones. No MPD measurements were made in the
woodland zones as the area was at a higher elevation
than the rain garden portion. Measurements were
concentrated near the inlet of the rain garden and
along the pathways of the underdrains. The highest
measured Ksat values were primarily located on the
coarse sand trenches where the underdrains were
located. As stormwater enters the rain garden via the
sediment forebay, it flows directly to the areas con-
taining the coarse sand and much of it flows down-
ward into the tile drains. The low Ksat values
University of Minnesota, Duluth
Rain Garden
Hydraulic Conductivity
[curls]
Inlet
Sediment Torbay
• 0.0600006 - 0.003
F
O 0.003 - 0.006
0.006 - 0.012
• 0.012 -0.024
` s
0.024 -0.07
0 4 8 16 24
32
Meters
FIGURE 6. Map Showing the Range of Ksat Values
Measured Using the MPD Infiltrometer at Various
Locations Within the UM — Duluth (12) Rain Garden.
( <2.6 x 10 -3 cm/s) were primarily in the vicinity of
the sediment forebay (northwest corner) and lime-
stone rock riprap inlet in the southwest corner of the
rain garden.
At the UM — St. Paul (6) campus rain garden (Fig-
ure 7), all of the low Ksat values ( <2.9 x 10 -3 cm/s)
were located near the center of the basin. Two of the
MPD tests conducted in the center of the basin
required more than 3 h to drain. Overall, 58% of the
MPD tests were completed in less than 1 h at this
site. The low Ksat values found here could be a combi-
nation of the restrictive soil layer found at the
20 -48 cm depth and clogging of surface soils due to
the settling of particles from the stormwater runoff.
This rain garden receives stormwater from both the
storm sewer system and from the street via a curb
cut. The inlets occur at the north and northwest por-
tion of the rain garden, which is also where one MPD
test (located just below the north inlet) was termi-
nated because there was no change in the water level
over a time period of 3 h.
The distributions of Ksat data for each rain garden
are shown in Figure 8. Statistical analyses were per-
formed on the measured Ksat values and the descrip-
tive statistics including arithmetic mean, geometric
mean, median, SD, and CV of Ksat for each rain gar-
den were computed and are summarized in Table 3.
The variability of Ksat was large as indicated by the
CV range (57- 174 %) (Hillel, 1998). Based on the com-
bination of statistical tools utilized, it was determined
that a log- normal distribution fit the data best in all
cases. It has been observed that field hydraulic
conductivity data are often best described by a log-
normal distribution (Bjerg et al., 1992; Vauclin et al.,
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1027 JAWRA