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
Columbus, Ohio), Modified Philip -Dunne (MPD) Infil-
trometer (Nestingen, 2007), Minidisk Infiltrometer
(Decagon Devices, Inc., Pullman, Washington), and
Tension Infiltrometer] for determining the Ksat of sur-
face soils were evaluated by Nestingen (2007). The
MPD Infiltrometer developed in our laboratory was
selected due to the minimal volume of water necessary
(to run the test), ease of use in the field, low cost of the
device, and transportability of the equipment. With the
MPD Infiltrometer (Figure 1), 30 -40 individual mea-
surements in one rain garden can be completed in one
day, with additional time required to perform the data
analyses. The Ksat results from such an investigation
can be used to predict the drain time of the basin, which
is useful for routine evaluation and determining
whether a rain garden was properly installed. Also,
tracking infiltration performance over time via infiltra-
tion rate testing at the same locations throughout the
FIGURE 1. Important Parameters of the Modified Philip -Dunne
Infiltrometer (Nestingen, 2007). H is the initial height of water,
H(t) is the height of water at time t, L,,, is the depth of insertion
into the soil, r is the equivalent source radius, r is the radius of
the cylinder, r is any radius within the wetted front, and R(t) is the
radius to the sharp wetted front at time t.
rain garden provides long -term data that can be used to
develop a maintenance schedule when changes in infil-
tration rates are seen. Furthermore, the spatial distri-
bution of the measured Ksat values can be used to
identify low permeability areas within a rain garden
that require rehabilitation.
Synthetic Drawdown Testing (Level 3)
As with any soil system, infiltration rates can vary
substantially at small distances. This test is an averag-
ing approach for events sufficient to fill the entire sur-
face volume of the rain garden. Synthetic drawdown
tests, like the infiltration rate testing, provide the time
required for the stormwater BMP to drain when the
rain garden encounters a storm. Once permission to
use a fire hydrant has been obtained or arrangements
to use a tanker truck have been made, a single test will
require less than one day to complete. A large tanker
truck contains —27 m and most fire hydrants may be
used with permission to supply between 0.07 and
0.15 l/min for 20 min, to give 80 -160 m After filling
the basin to the desired level, the water level is mea-
sured over time and the time required to completely
empty the basin (i.e., the drain time) is recorded. The
main advantages of a synthetic drawdown test (in
comparison to infiltration rate testing) are that it pro-
vides a direct measure of the rain garden drain time
and it potentially can be used to assess pollutant
removal efficiency. The main disadvantages are that
the size of a rain garden that can be tested is limited
because of typical water supply constraints and no
information on spatial variability in infiltration
performance within a rain garden is obtained.
The above - mentioned assessment techniques, along
with continuous monitoring by placing automatic
monitors upstream and downstream of the device
(Level 4), can be used individually or in combination,
depending on the goals of the specific assessment pro-
gram. This has been termed a four -level BMP assess-
ment approach (Gulliver and Anderson, 2007). For
example, if the goal is to calculate the runoff volume
reductions provided by the rain gardens in a
watershed, the Levels 2 and/or 3 (infiltration rate
testing and synthetic drawdown testing), should pro-
vide adequate information. If the goal is to calculate
pollutant load reductions for a Total Maximum Daily
Load study, then synthetic drawdown testing and
water quality monitoring may be required. If the goal
of the assessment is to simply satisfy state permit
requirements then a routine visual inspection may be
all that is required. For developing maintenance
schedules, and checking functionality over the longer
period, a combination of visual inspection and infil-
tration rate testing is recommended.
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1021 JAWRA
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FIGURE 1. Important Parameters of the Modified Philip -Dunne
Infiltrometer (Nestingen, 2007). H is the initial height of water,
H(t) is the height of water at time t, L,,, is the depth of insertion
into the soil, r is the equivalent source radius, r is the radius of
the cylinder, r is any radius within the wetted front, and R(t) is the
radius to the sharp wetted front at time t.
rain garden provides long -term data that can be used to
develop a maintenance schedule when changes in infil-
tration rates are seen. Furthermore, the spatial distri-
bution of the measured Ksat values can be used to
identify low permeability areas within a rain garden
that require rehabilitation.
Synthetic Drawdown Testing (Level 3)
As with any soil system, infiltration rates can vary
substantially at small distances. This test is an averag-
ing approach for events sufficient to fill the entire sur-
face volume of the rain garden. Synthetic drawdown
tests, like the infiltration rate testing, provide the time
required for the stormwater BMP to drain when the
rain garden encounters a storm. Once permission to
use a fire hydrant has been obtained or arrangements
to use a tanker truck have been made, a single test will
require less than one day to complete. A large tanker
truck contains —27 m and most fire hydrants may be
used with permission to supply between 0.07 and
0.15 l/min for 20 min, to give 80 -160 m After filling
the basin to the desired level, the water level is mea-
sured over time and the time required to completely
empty the basin (i.e., the drain time) is recorded. The
main advantages of a synthetic drawdown test (in
comparison to infiltration rate testing) are that it pro-
vides a direct measure of the rain garden drain time
and it potentially can be used to assess pollutant
removal efficiency. The main disadvantages are that
the size of a rain garden that can be tested is limited
because of typical water supply constraints and no
information on spatial variability in infiltration
performance within a rain garden is obtained.
The above - mentioned assessment techniques, along
with continuous monitoring by placing automatic
monitors upstream and downstream of the device
(Level 4), can be used individually or in combination,
depending on the goals of the specific assessment pro-
gram. This has been termed a four -level BMP assess-
ment approach (Gulliver and Anderson, 2007). For
example, if the goal is to calculate the runoff volume
reductions provided by the rain gardens in a
watershed, the Levels 2 and/or 3 (infiltration rate
testing and synthetic drawdown testing), should pro-
vide adequate information. If the goal is to calculate
pollutant load reductions for a Total Maximum Daily
Load study, then synthetic drawdown testing and
water quality monitoring may be required. If the goal
of the assessment is to simply satisfy state permit
requirements then a routine visual inspection may be
all that is required. For developing maintenance
schedules, and checking functionality over the longer
period, a combination of visual inspection and infil-
tration rate testing is recommended.
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1021 JAWRA