JAWRA article - Rain Gardens

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

ASLESON, NESTINGEN, GULLIVER, N OZALSKI, AND NIEBER
Burnsville Rain Garden
c
ID
• C
Inlet
Hydraulic Conductivity
[cm /s] N
+ 0.0005 - 0.005
� 0.005 -0.01 W E
C 0.01 -0.02
0.02-0.04 0 1 2 4 6Meters s
0.04-0.09
FIGURE 2. Map Showing the Range of KSat Values
Measured Using the MPD Infiltrometer at Various
Locations Within the Burnsville (1) Rain Garden.
grasses. The lowest Ksat value (5.5 x 10 -4 cm/s) was
near the inlet, which could be due to the settling out
of eroded clays or from compaction of the surface due
to the inflow of runoff. However there was no
observed evidence to indicate either scenario. The
other low values ( -6 x 10 cm/s) were randomly
located throughout the basin, such that no strong cor-
relation with location in the rain garden could be
made at this site for the cause of low infiltration
rates. MPD testing could not be performed along the
west edge of the basin due to the presence of rocks
and dense shrubs.
The Cottage Grove (7) rain garden (Figure 3) con-
tained sandy soils. The distribution of the measured
Ksat values was close to normal, as indicated by the
low CV value (57.4 %) and low skewness value (0.91).
The sandy soils allowed for rapid infiltration that
resulted in 90% of the MPD Infiltrometer tests finish-
ing within 15 min. The higher KSat values
( >_2.2 x 10 -2 cm/s) were located near the failing trees
and along the side slopes of the rain garden.
The three rain gardens assessed at the Ramsey -
Washington Metro Watershed District (RWMWD) (2,
5, and 9) office are given in Figure 4. RWMWD Rain
Garden #1 (9) is the largest of the three rain gardens
assessed at this site and had the highest variability
in Keat (CV = 94.6 %). Four measurements were made
in RWMWD Rain Garden #4 (2) due to its small size
(29.08 m RWMWD Rain Garden #5 (5) had the
highest median Ksat, with the lowest values concen-
trated near the inlet on the south east corner. Accu-
mulation of fine sediment near the inlet and around
the sandbags of RWMWD Rain Garden #5 (5) was
observed. This is presumed to be the cause of the low
infiltration rates near the inlet.
FIGURE 3. Map Showing the Range of Ksat Values
Measured Using the MPD Infiltrometer at Various
Locations Within the Cottage Grove (7) Rain Garden.
Ramsey - Washington Metro Watershed District
� 2 Rain Gardens
Inlet
Inlet
#1 J /
Hydraulic Conductivity •
[cm /s]
• 0.000009 - 0.001 Inlet 0/
0.001- 0.002 �� i #4
C 0.002 - 0.004 overflow
0.004 - 0.008 (/
• 0.008 • N
\ \\ WE
0 2 4 8 12 16 �
. Meters Inlet s
FIGURE 4. Map Showing the Range of Ksat Values
Measured Using the MPD Infiltrometer at Various
Locations Within the RWMWD (2, 5, and 9) Rain Gardens.
As the oldest rain gardens assessed, the Thompson
Lake (11) rain garden was more densely covered with
vegetation. There were two relatively small curb cut
inlets along the west portion of the rain garden
receiving stormwater runoff from a parking lot. The
highest Ksat values (Figure 5) were typically within
—0.5 feet of the perimeter of the large shrubs. The
large and dense cover of vegetation may have
contributed to the higher Ksat values measured at
this site. The low Ksat values were located in the
middle of the rain garden, and may have been caused
JAWRA 1026 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION