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Hydraulic Response and NitrogenRetention in Bioretention Mesocosms
with Regulated Outlets:Part I—Hydraulic Response
William C. Lucas1,*, Margaret Greenway1
ABSTRACT: In bioretention systems used for stormwater treatment,
runoff interception improves with increased infiltration rates. However,
nitrogen retention improves with increased retention time or decreasing
infiltration rates. These contrasting responses were analyzed in 240-L
experimental mesocosms using a variety of media treatments. The
mesocosms were vegetated, except for one barren control. Dual-stage
outlets were installed to extend retention time and equalize hydraulic
responses. One unregulated treatment was free-draining. This part 1 paper
presents the media properties and hydraulic responses. The highly
aggregated media had saturated hydraulic conductivities ranging from
20.7 to 59.6 cm/h in August 2008 (austral winter), which increased to 42.8
to 110.6 cm/h in March 2009 (austral summer). The outlet regulated
mesocosms provided retention over 8 times longer than the free-draining
mesocosms, while still being able to capture large events. The outlets
provide adaptive management for bioretention design to improve both
runoff capture and nitrogen retention. Water Environ. Res., 83, 692 (2011).
KEYWORDS: bioretention, media, particle size distribution, aggrega-
tion, hydraulic conductivity, infiltration, hydraulic controls, temperature.
doi:10.2175/106143010X12780288628697
Introduction
Bioretention systems are a widely used stormwater control
measure (SCM). Recent design guidance for bioretention media
recommends the use of a loamy sand media to provide rapid
infiltration rates to improve runoff capture and minimize bypass
flow (New Jersey Department of Environmental Protection,
2009). Column studies show that, even at high infiltration rates,
both filtration and rapid sorption processes can be very effective at
retaining contaminants, such as total suspended sediments,
phosphorus, and metal cations (Hsieh and Davis, 2005). However,
biogeochemical nitrogen transformations are less effective with
a shorter retention time (Kadlec and Wallace, 2009). Rapidly
draining bioretention systems are relatively ineffective for
nitrogen retention under field stormwater loading rates (Hatt et
al., 2008).
The hydraulic performance of bioretention SCMs can vary
considerably over time. Hatt et al. (2008) observed that non-
vegetated (barren) sandy loam column experiments with initial
hydraulic conductivities ranging from 27 to 253 cm/h eventually
declined to 21 to 75 cm/h. The decline was correlated with the
extent of compaction. The effect of compaction upon reducing
infiltration rates is well-documented in the pedotransfer functions
(PDFs) developed in the soils literature (e.g., Rawls et al., 1983).
On the other hand, plant roots penetrate confining layers and
open up soil structure, increasing infiltration rates (Bartens et al.,
2009; Gilker et al., 2002). In vegetated bioretention columns,
Culbertson and Hutchinson (2004) reported that switchgrass
(Panicum virgatum) increased infiltration rates in bare soils from
0.5 to 128 cm/h. The resultant infiltration rates in deep rooted
native grass hedges are much higher than found in adjacent
shallow rooted crops planted in the same soil (Blanco-Canqui et
al., 2004; Rachman et al., 2004; Seobi et al., 2005).
As the fine particles in urban runoff pass into bioretention
systems, they are ‘‘strained’’ by the bioretention media, so most of
these particles are captured within short distances within the
media. As a result of this particle accumulation in the media,
hydraulic conductivity declines to values in the range of several
centimeters per hour (Li and Davis, 2008). The resultant increase
in ponding time allows finer particles to settle out and accumulate
on the surface of the medium as a ‘‘cake’’ deposit (Hatt et al.,
2005; Li and Davis, 2008). This deposit can reduce overall
hydraulic conductivity from between 3 and 11 cm/h (Li and Davis,
2008) to as low as 0.1 cm/h (Hatt et al., 2005). Experiments by
both Hatt et al (2008) and Li and Davis (2008) showed that
removal of this deposit with the top 2- to 3-cm clogging layer
could restore infiltration rates. Clark and Pitt (2009) also observed
a 3-cm clogging depth; however, removal of the surface 1 to 2 cm
did not restore infiltration rates to original values. These
contrasting observations may reflect differences in media
properties, sediment loading rates, and depth of clogging layer
removed.
The effects of vegetation and clogging have been documented
in a field study of 21 bioretention systems in Australia that had
been operational from 6 months to 3 years (Le Coustumer et al.,
2009). They found that infiltration rates in systems using initially
‘‘rapid’’ media (25 cm/h median initial infiltration rates) declined
over time toward a median of 10 cm/h, presumably as a result of
clogging. On the other hand, systems with initially ‘slow’ media
(1.0 cm/h) increased to 3.0 cm/h, presumably as a result of the
presence of vegetation-induced macropores. Le Coustumer et al.
(2009) hypothesized that vegetation partially mitigates clogging
1 Griffith University, Nathan, Queensland, Australia.
* 3 Lucas Lane, Malvern, PA 19355; e-mail: wlucas@integratedland.com.
692 Water Environment Research, Volume 83, Number 8
because plant stems break up the surface ‘‘cake’’. Media
augmented with coarse fibers similar to coir peat do not seem to
clog as readily (Hatt et al., 2008), and the more rapidly infiltrating
the media, the longer it lasts before it clogs (Clark and Pitt, 2009).
These results indicate that facilities with rapid (coarser) media
maintain higher infiltration rates than facilities using slower
(finer) media.
As a result of these interactions between texture, compaction,
clogging, and vegetation, infiltration rates of bioretention systems
inevitably change over time. When infiltration rates decline,
nitrogen removal processes are likely to improve, as a result of
enhanced retention time; however, overall interception volumes
decrease, because less flow is intercepted (Davis et al., 2005;
Lucas and Greenway, 2008). On the other hand, when infiltration
rates are too fast, nitrogen removal is much less effective, as part
II of this paper demonstrates.
One solution to both increase nitrogen retention and prevent
clogging is to provide media with high infiltration rates and then
use an outlet to regulate flowrates. However, with low effective
infiltration rates created by an outlet, overall capture would be
reduced, because peak flows during large runoff events bypass the
facility (Davis et al., 2005). To address these conflicting
objectives, we investigate the potential for using a dual-outlet
configuration, in which the treatment capability provided by
media with high saturated hydraulic conductivity is used to
rapidly filter large events, while outlets are used to provide more
retention time for small events comprising the majority of annual
runoff. We report on the hydraulic response of various media
treatments initially evaluated for phosphorus retention (Lucas and
Greenway, 2011b) and the resulting outlet-adjusted infiltration/
percolation rates.
The objective of our experiment was to manipulate the outlet
controls to improve nitrogen retention while retaining high runoff
capture efficiency. In this paper, part 1, the resultant hydraulic
properties are evaluated in a hydraulic model to estimate retention
time responses and interception volumes in a variety of runoff
events. Part II of this paper presents the resulting nitrogen
retention performance of the outlet system described in this paper.
Methods
Experimental Setup. The experiments were conducted at the
Loganholme Water Pollution Control Centre, located 25 km south
of Brisbane, Australia. The experiment comprised the bioretention
mesocosms described in Lucas and Greenway (2011b). The
mesocosms were made from 240-L ‘‘wheelie-bin’’ containers with
a mean surface area of 0.25 m2 and 99 cm deep. They were
roughened with sandy paint to minimize preferential flow down
the sides. To provide a uniform flow path through the media, the
indented bottom 14 cm was filled with gravel and overlaid by
coarse sand as a separation layer. Each media treatment was
planted with native vegetation (Lucas and Greenway, 2011b). One
barren treatment (K20nv) was used to control for vegetation
effects. All mesocosms were covered by a gravel mulch 1.0 cm
deep to reduce evapotranspiration losses and to prevent scour from
the high application rates. Figure 1a shows the experimental
setup.
Media Properties. The media matrix consisted of turf sand
with a particle size distribution (PSD) of 81.2% between 300 and
Figure 1—(a) Experimental setup, August 2008. Vegetated and barren mesocosms with compound outlets andcollection chambers. Newer systems on right side. (b) Compound outlets, showing how flows from bottom are pipedup to crossover tee (in circle) before discharge down into collection chambers. Low-flow outlet is the 4-mm tubing (insquare) shunted between vertical piping.
Lucas and Greenway
August 2011 693
420 mm, 13.2% between 420 and 1000 mm, and 3.7% over 1 mm.
There was no silt or clay and only 0.3% organic matter (Lucas and
Greenway, 2008). The following media amendments were added
to the sand in varying proportions: Krasnozem soils, neutralized
Red Mud, and alum-based water treatment residuals (WTRs)
(Lucas and Greenway, 2011b). These different media treatments
were formulated according to the ratios presented in Table 1. Coir
peat was added at 12% by volume to all media for increased water
holding capacity and infiltration performance.
Six treatments (3 replicates each) were constructed in January
2007 (experiment 1). Four treatments incorporated Krasnozem
soils, and two treatments incorporated Red Mud. Three additional
treatments using Al-WTRs were constructed 6 months later, of
which two were blended with Krasnozems (experiment 2). In
addition to the seven media components listed in Table 1, a
K10/40 treatment comprised a 20-cm lift of 40% Krasnozem
under 40 cm of 10% Krasnozem media, in anticipation that the
bottom layer with a lower permeability would control the
saturated infiltration response, while the upper layers would
initially infiltrate rapidly (Hsieh and Davis, 2005). The resulting
seven media treatments comprised K20, K10/40, K40, RM06,
RM10, WTR-K, and WTR30. The media proportions were
measured by volume and mixed with an 80-L cement mixer, to
ensure thorough mixing, and placed in 20-cm lifts. Media was
placed in the mesocosms to a depth of 70 6 5 cm to allow for
settlement. The settled media depths ranged between 60 and
67 cm, with 18 to 25 cm of freeboard.
All of the amendments contain a high proportion of clay
minerals. Krasnozem soils comprise 50 to 70% clay minerals
(Sills et al., 1974), and Red Mud comprises 40% clay, 50% silt,
and 10% sand (Snars, 2003). Coagulated from dissolved organic
compounds and clay colloids, WTRs could be classified as
entirely clay. If dispersed throughout the media, the high clay
content of these amendments would result in very low
permeability. However, Krasnozem soils are noted for their high
aggregate structure, which provides very good drainage properties
for soils with such high clay content (Isbell, 1994; Sills et al.,
1974). The dry aggregate percentage ranges from 45 to 80%, and
most are stable under wet-sieving, which is indicative of high
aggregate stability (Carter et al., 2002). The clay fraction of Red
Mud also is aggregated entirely into silt-sized particles (Snars,
2003). Likewise, WTRs form stable aggregates that have to be
ground to form sand-sized particles (Babatunde et al., 2008).
The presence and persistence of these media aggregates has a
significant effect on hydraulic responses. To examine the extent of
media aggregation over the 18-month duration of experiment 2
and 24-month duration of experiment 1, core samples were taken
from 10-cm depths from each replicate of the RM10 and K40
treatments, and from 50 cm depth of the WTR30 treatment, and
blended together for textural analysis. The PSD was evaluated
with a Beckman Coulter Multisizer (Brea, California) for particle
sizes in the range from 1 to 600 mm. Because our focus was on
hydraulic responses, aggregated clays were not dispersed
physically or chemically. Such minimally dispersed measure-
ments more accurately represent the PSD, which determines
hydraulic properties (Leys et al., 2005). Table 2 and Figure 2 give
the PSDs in the original media, as compared with that observed in
the sampled cores after the experiment.
Hydraulic Dosing. An average of 56 cm of tertiary treated
effluent was applied on a weekly basis to accelerate nutrient loads
for the experiments reported in Lucas and Greenway (2011b) and
part 2 of this paper (Lucas and Greenway, 2011a). The small
amount of total suspended solids (TSS) occasionally observed in
the clarified effluent comprised readily biodegradable organic
material. As such, TSS accumulation was not anticipated to be a
factor affecting hydraulic responses in this experiment. The
hydraulic measurements used synthetic stormwater comprising
nutrients dissolved in tap water. Experimental applications were
distributed at a rate of 90 64 L/h through 25-mm tubing by use of
0.6-mm diameter sprayers, supplied by a large-diameter, dual-
manifold distribution system to diminish differential pressure
losses. Dosing rates were manipulated by a pressure regulating
valve on the supply line. The dosing used for hydraulic
measurements applied from 133 to 155 L (56 to 62 cm) to the
Table 1—Media composition of various blends. K = Krasnozem, RM = Red Mud, and WTR = water treatment residuals.Mineral proportions are by weight, while coir peat proportion is by volume.
Media Label Turf Sand Topsoil Krasnozem Red Mud WTR Coir peat
K10 93% - 7% - - 12%
K20 86% - 14% - - 12%
K40 70% - 30% - - 12%
RM06 75% 20% - 5% - 12%
RM10 71% 20% - 9% - 12%
WTR30 80% - - - 20% 12%
WTR-K 71% - 20% - 9% 12%
Table 2—Media PSDs (mm) between 1 and 600 mm, minimally dispersed; initial media and as of March 2009 (after24 months for RM10 and K40 in experiment 1 and 18 months for WTR30 in experiment 2).
Period As installed March 2009
Particle size
Clay
,2 mm
Silt 2 to
63 mm d10 d50 d90 Clay ,2 mm
Silt 2 to
63 mm d10 d50 d90
RM10 0.1% 2.4% 181 308 481 0.2% 12.6% 50 241 419
K40 0.0% 8.8% 70 280 446 0.2% 15.5% 35 281 443
WTR30 0.0% 1.6% 166 309 496 0.0% 2.4% 164 332 513
Lucas and Greenway
694 Water Environment Research, Volume 83, Number 8
mesocosms over periods ranging from 90 to 110 minutes. For
comparison to bioretention facilities designed with a 25:1
impervious-source-to-treatment-area-capture ratio, the 56-cm
application is equivalent to 22 mm of runoff. Accounting for
3 mm of initial abstraction, this would be expected from a 25-mm
rainfall event at an average rainfall intensity of 17 mm/h for
90 minutes. This rate is nearly two-thirds of the 90-minute
intensity of 28 mm/h allocated for an annual recurrence interval
event for Brisbane (Queensland Ministry for Natural Resources
and Water, 2000). Assuming this quite high capture ratio, our
stormwater dosing regime represents a large event that occurs
perhaps twice per year. If the capture ratio were less, this relative
event size would be that much greater. It was our intention to test
the performance of our mesocosms under relatively intense events
where bypass flow is likely to occur.
Outlet Controls. Figure 1b shows the experimental outlet
configuration. The outlet control system regulates outflows by
using a lower outlet sized to extend retention time, with a larger
diameter outlet placed above it to convey flows ponded during
simulated runoff. The 2-mm-diameter lower outlet is set 18 cm
above the bottom of the mesocosm to provide a permanent
internal saturated zone intended to promote denitrification, while
the 10-cm-diameter upper outlet is set at or slightly below the
surface of the media. The lower outlet restricted flow to a rate of
8.1 cm/h when the ponding depth was 10 cm, which occurred at a
head of 72 6 5 cm. This flowrate provided identical percolation
responses once the surface ponding had drained.
The upper outlet controlled flows as a function of head across
the media according to Darcy’s Law. Generally close to the media
surface, the upper outlet regulated outflow when the systems were
Figure 2—Coulter multisizer PSDs of (a) RM10 (at 10 cm), (b) K40 (at 10 cm), and (c) WTR30 (at 50 cm) treatments.Initial media before experiment and media as of March 2009.
Lucas and Greenway
August 2011 695
saturated and ponding had occurred. This outlet comprised a 10-
mm riser pipe extending upward to a tee, where flows crossed
over at the level, passing into another tee before descending to the
final discharge, as shown in Figure 1b. The tees provided vacuum
breaks to prevent siphon flows. The outlet elevation controlling
flow through the media was measured at the top of the crossover
pipe, as the crossover pipe typically flowed full. By adjusting its
elevation to provide more or less head in response to media
hydraulic conductivity, this arrangement was intended to provide
similar ponding and drainage responses in replicates containing
media with varying permeabilities. This outlet configuration also
allows for adaptive management as flowrates change over time.
One treatment (WTR-Knr) was left as free discharge to compare
the effect of outlet controls on nitrogen retention. Including the
barren control, nine different treatments were examined.
Hydraulic Measurements. Hydraulic measurements were
conducted during the stormwater dosing applications in the
austral winter of August 2008 (n 5 2) and austral summer of
March 2009 (n 5 5). Measurements of ponding depth were taken
during the filling phase in August 2008 runs. The resulting
infiltration rate Q was computed as follows:
Q~Qi{(p2A2{p1A1)
tð1Þ
Where
Qi 5 inflow rate (m3/h),
A1 5 average ponding area at first interval (m2),
A2 5 average ponding area at second interval (m2),
p1 5 initial ponding depth (m),
p2 5 second ponding depth (m), and
t 5 elapsed time (hours).
Until ponding occurs, eq 1 reduces to Qi, as the infiltration rate is
limited by the application rate.
Saturated hydraulic conductivity (Ksat) was determined by
falling head measurements after the stormwater applications
ended. This was accomplished by closing off the lower outlet so
that flows were constrained to passing through the upper outlet.
Ponding depth to the nearest millimeter was observed at 10-
minute intervals for at least 40 minutes. Saturated hydraulic
conductivity (Ksat) then is computed as adapted from Bedient and
Huber (1988):
Ksat~AtL
Amtln
h1
h2
� �ð2Þ
Where
At 5 average ponding area (m2),
Am 5 average media cross-sectional area (m2),
L 5 media depth (m),
h1 5 initial head (m),
h2 5 final head (m), and
t 5 elapsed time (hours).
The area adjustments allow for rates to be corrected for the surface
area variations resulting from the tapered geometry of the
mesocosms. These corrections were based on rating curves
developed to determine surface area as a function of elevation.
Based on Darcy’s Law, infiltration rate Q at a single point in
time during the falling head measurements is as follows:
Q~Ksat|(pzd)
Lð3Þ
Where
p 5 ponding depth (m), and
d 5 outlet depth (m) below the media surface (m).
Head h is the sum of p and d. Equation 3 was used to determine
infiltration rates after inflow ceased and the lower outlet was
closed. Because falling head measurements were not possible
without an elevated upper outlet to control flows, no Ksat
measurements were made on the free-discharge WTR-Knr
replicates (numbers 25 to 27) until outlets were installed for the
last two measurements.
Results and Discussion
Media Aggregation. The PSDs for minimally dispersed R10,
K40, and WTR30 media are given in Table 2. Respectively, these
media would be classified as 9, 30, and 21% clay if they had been
dispersed fully. However, the clay fraction in the RM10 media
and WTR30 was only 0.1%, and no clay was observed in the K40
treatment. Instead, the majority of the amendments comprised
sand-sized aggregates with only a minor amount of fines,
dominated by silt particles. The median particle size was no
more than 310 mm in all treatments. Compared with the sand
matrix with a larger median PSD, this indicates that the majority
of media aggregates in the sand fraction were less than 310 mm.
Figure 2 presents the PSD of the original RM10, K40, and
WTR30 media, as compared with the media after 2 years
(experiment 1) and 18 months (experiment 2) after hydraulic
measurements were taken in March 2009. In this case, it can be
seen that there was a substantial shift from sand-sized aggregates
into the silt fraction in the RM10 treatment. There was a less
pronounced disaggregation trend in the Krasnozem treatment. On
the other hand, the WTR30 treatment suggests an opposite trend
of aggregation into larger particle sizes. These changes are
represented by the respective changes in clay and silt fractions
shown in Table 2. While there was only a slight increase in the
clay fraction, there was substantial increase in the silt content of
the RM10 and K40 treatments. This also is confirmed by a
corresponding shift in the d10, d50, and d90 particle sizes in the
RM10 and in the d10 size in the K40 treatment. In contrast, there
was no change in the d10 of the WTR30 treatment, while the d50
and d90 sizes increased slightly. These results indicate that most of
the amendments originally were, and still remained as, aggregates,
albeit more dispersed in the case of Krasnozem and Red Mud
amendments.
In addition to the structure provided by the aggregates, fibers in
the coir peat shrink and swell in response to moisture status, also
opening pores. It would be expected that the presence of such
pores with fissures between aggregates would promote Ksat. Note
that the effects of aggregates on soil structure are not addressed by
conventional approaches to determine hydraulic conductivity
using PDFs based on fully dispersed analyses that do not capture
the effects of aggregation and coarse organic matter (Rawls et al.,
1983).
Hydraulic Conductivity Observations. Table 3 presents the
means and coefficient of variation (CV) of saturated hydraulic
conductivities across the treatments at the end of August 2008 and
6 months later in February to March 2009. The mean Ksat values for
Lucas and Greenway
696 Water Environment Research, Volume 83, Number 8
these treatments were remarkably high for media comprising such a
high proportion of clay materials. The unvegetated K20nv treatment
was the slowest media in August 2008, with a mean Ksat of 21 cm/h.
The most rapidly draining media was K10/40, with an average Ksat
of 60 cm/h. The anticipated slower drainage rates expected from an
impeding layer comprising nearly 30% clay did not occur. The
average Ksat for the other media ranged from 28 to 46 cm/h. The
K20nv and WTR treatments were the most consistent between
replicates, with the coefficient of variation at or below 21%. The
K10/40, K20, RM06, and RM10 treatments had coefficients of
variability from 33 to 59%, highlighting the variability in infiltration
rates, even with ostensibly identical plants and media.
To evaluate differences in Ksat between the K20 treatment and
the corresponding barren K20nv treatment, log-transformed
observations were analyzed with the t-test with unequal variances.
The difference between the two observations in August 2008 was
significant (p 5 0.034), while the difference between the four
observations in March 2009 was significant (p 5 0.0002). These
data suggest that plants significantly improve Ksat.
The K40 treatment had an extremely high coefficient of
variation approaching 120%. Bin 6 had a very high Ksat over
109 cm/h, bin 5 had a Ksat of 19 cm/h, and bin 4 had a Ksat of only
10.3 cm/h. We believe that the low rate of bin 4 was the result of
accidentally dousing the first lift during construction with flows of
such high pressure that the media was partly homogenized,
dispersed into the underlying stone, and then puddled. The very
high rate in replicate 6 is remarkable for the same media
comprising soil with nearly 30% clay minerals.
Further confirmation of the variability in treatment response is
evident in both the high coefficient of variation between replicates
in most treatments and the marked changes in means shown in
Table 3. Figure 3 graphically illustrates the changes in Ksat in the
replicates between the two measurement periods. Figure 3b shows
that, just 6 months later, Ksat markedly increased in all replicates
except K10/40 in bin 17. Excluding this replicate, Ksat increased
in the other two replicates by over 110%. All other treatments
increased by at least 54%, with an average increase of over 74%
and a maximum of 107%. Note that the unregulated free discharge
control treatments (bins 25 to 27) had a very high Ksat of
approximately 110 cm/h.
The paired two-sample t-test for means was used to compare the
individual replicates over the two runs in August 2008 and March
2009. The increase in Ksat was highly significant, with probability
well below 0.01 for all treatments except for the WTR30 and RM06
treatments. The increase in these treatments was still significant at a
probability ,0.05. The narrow standard deviation displayed in
Figure 3 indicates that the infiltration rates were quite consistent
over each observation period. The Ksat in the barren K20nv
treatment also increased over time. Given the absence of vegetation
in the K20nv treatment, it appears that plant-associated processes
are not responsible for this increase in infiltration rates.
As a result of decreases in viscosity, Ksat increases approxi-
mately 2% for each degree increase in temperature. Applying the
mean daily temperature to the period of observations, the mean
temperature would be 13.5uC in August 2008 (austral winter) and
26.3uC in February to March 2009 (austral summer). Using
standard temperature correction factors relative to 20uC, these
temperatures result in adjustment factors of 1.135 and 0.850,
respectively. The relative difference between these two values is
33%. Compared with observed increases from 54 to 107%,
temperature effects on viscosity would explain less than half of
the difference observed in most treatments. Measurements of the
WTR30 replicates taken in April 2010 showed an average
decrease of 9% from the March 2009 observations, which was
not significant. This suggests that the trend of increasing Ksat does
not persist. It may be that the initially low measurements had been
biased by an accumulation of TSS in the form of organic matter
flocs in the tertiary effluent at the time of measurement.
Braga et al. (2007) reported that permeability varied several-fold
as a function of temperature. As a result of both viscosity and
presumed permeability changes, the difference between maximum
and minimum Ksat varied over 300%. The overall temperature
response was fitted by the following relationship (Braga et al., 2007):
K~0:0072Tz0:0196 ð4Þ
Where
T 5 temperature (uC).
Comparing the results from eq 4 in relative terms using the
temperatures for each season in our experiment leads to an
Table 3—Mean and variation in saturated hydraulic conductivity (cm/h), as derived from falling head saturatedhydraulic conductivity measurements. Measurements from August 2008 compared with February to March 2009.Percent increase is comparison of means. WTR-Knr is the unregulated system, which was measured only in March2009. Significance of K20 means compared with K20nv indicated by asterisks: *(p,0.05) and **(p,0.001).
Treatment
August 2008 February to March 2009Treatment
percent increase
Paired sample
P-statisticMean CV Mean CV
RM06 36.2 52% 55.6 28% 54% 0.01023
RM10 26.4 27% 48.4 28% 83% 0.00030
K20 42.6* 59% 78.0** 44% 83% 0.00081
K20nv 20.7 9% 42.8 19% 107% 0.00062
K10/40 59.6 33% 85.6 33% 44% 0.00003*
K40 46.1 119% 72.9 70% 58% 0.00002
WTR30 42.4 20% 69.7 43% 64% 0.01303
WTR-K 27.7 21% 56.5 36% 104% 0.00593
WTR-Knr Not available Not available 110.6 5% Not available Not available
* Excludes replicate 17, which was the only replicate in which conductivity was reduced. Including this replicate, the P-statistic increased to
0.12008.
Lucas and Greenway
August 2011 697
increase of 78%—a value quite similar to the average increase
noted in our observations.
Outlet Regulated Observations. Regardless of the extent of
disaggregation during the experiment (Table 2 and Figure 2), the
high aggregate content in the media provided remarkably high
Ksat values in February to March 2009. The Ksat exceeded 50 cm/h
in almost 75% of the replicates and exceeded 95 cm/h in almost
30% of the replicates. Unregulated, such a high Ksat value
provides only a very short retention time. While seasonal
variations in Ksat may amplify variability in outflow responses,
the retention time afforded by even the slowest replicate is
measured in minutes, as discussed below.
Using regulated outlets to equalize flow responses, Figure 4a
shows the infiltration rates of all 24 replicates, while Figure 4c
shows the corresponding ponding depths during the rising and
falling head regimes on August 17, 2008. Figures 4b and 4d
display the corresponding responses of the K40 mesocosms. This
treatment had the greatest variation between replicates. Even with
more than a 10:1 difference between the lowest and highest Ksat,
the dual outlet configuration generated quite similar ponding and
drainage responses. The initial infiltration rate is understated,
because observations are limited by the application rate.
Infiltration rates then decreased over the first hour, as the media
became saturated. Once inflow rates exceeded outflows through
the lower outlet at saturation, ponding then increased after the first
hour, but more slowly as the upper outlets began to convey more
flow. As a result of the contribution of flows through the upper
outlet and the increasing head on both outlets, infiltration rates
increased to a maximum, until inflow ceased.
Notwithstanding this variability, infiltration rates were similar
through the majority of the observation period, as shown in
Figure 4a. Figure 4c presents the corresponding ponding eleva-
tions that resulted in the infiltration rates displayed in Figure 4a.
Because of differences between the highest and lowest Ksat values,
the ponding depths were higher for the lower Ksat replicates, while
they were lower for the highest Ksat replicate, as would be
expected by Darcy’s Law. This is more clearly displayed in
Figure 4d.
Figure 5a displays the ponding depth during the final
measurements on March 15, 2009. After several iterations of
upper outlet elevation adjustments to reduce variability in ponding
depth, the outlets provided nearly identical drainage responses. As
a result, infiltration rates displayed in Figure 5b show less
coefficient of variation than the results from 6 months earlier
shown in Figure 4a. Note that, because the draining limb does not
include the lower outlet contribution, the overall infiltration rate is
reduced by approximately 8 cm/h. Added to the upper outlet flows
observed, the resulting coefficient of variation would be reduced
even more. Because the lower outlet rate is slower than media
Ksat, this phase of the drainage response is also identical for all
replicates. As such, these outlet regulated systems can provide
similar retention times throughout the entire drainage response,
even in media with widely varying Ksat values. This is significant
in reducing effect of retention time upon nitrogen processes.
Hydraulic Responses. The orifice equation controls outlet
rate in the lower outlets as a function of coefficient C (typically
0.6), orifice area A (m2), and head h (m), as follows:
Q~CA|ffiffiffiffiffiffiffiffi2gh
pð5Þ
While the lower orifice should be treated as ‘‘culvert’’, inlet
controlled headloss equations are also proportional to h1/2. For
systems regulated by a single orifice at the bottom of the media, Q
is thus a function h1/2. Given that h 5 L + p 2 5 (or 72 6 5 cm), if
flows are to be constricted to approximately 8 cm/h when h 5 L
(e.g., saturated, but not ponded, conditions), increases in ponding
Figure 3—Saturated hydraulic conductivity from (a) two events in August 12–17, 2008, and (b) 5 events from February24–March 15, 2009. Means and standard deviation shown.
Lucas and Greenway
698 Water Environment Research, Volume 83, Number 8
depth p represent only a relatively small increase in total head, the
effect of which is reduced even further by the half-power
relationship. Therefore, when flows through highly permeable
media are constricted enough to extend retention time, the
capacity to handle large flowrates without excessive ponding will
be restricted. As a result, substantial amounts of annual runoff
could be bypassed without any treatment in large events.
Measures that treat larger events by infiltration through the
media are far preferable to letting untreated flows bypass the
systems. Given its shallow outlet depth d, the head through the
upper outlet is much more responsive to ponding depth p, because
it responds as a function of h1 according to Darcy’s Law, and the
relative head increases rapidly. Compared with the effects of p on
a constricted lower outlet as function of h1/2, this ability to treat
high flows through the upper outlet is particularly important in
systems with limited ponding depth, such as tree planters.
Modeled Responses. Retention time is a parameter of
considerable importance for nitrogen removal processes (Kadlec
and Wallace, 2009). Plug flow is the most appropriate metric for
retention time, as it represents the average of individual retention
times for each increment of inflow, taking into account antecedent
storage provided by the 18-cm-deep internal saturated zone. The
computer program HydroCAD (HydroCAD Software Solutions
LLC, Chocorua, New Hampshire) offers explicit computations of
retention time for design storm events. HydroCAD has been used
successfully for the design of bioretention systems for urban
Figure 4—Infiltration rates (cm/h) and ponding depths (cm), August 17, 2008: (a) infiltration rates—all replicates, withstandard deviation; (b) infiltration rates—K40 replicates; (c) ponding depths—all replicates, with standard deviation;and (d) ponding depths—K40 replicates. Elapsed time from beginning of dosing.
Figure 5—(a) Ponding depth (cm) and (b) infiltration rates (cm/h) with lower outlets closed during falling head Ksat
measurements. All replicates during March 15, 2009, run. Elapsed time from beginning of falling head measurements.
Lucas and Greenway
August 2011 699
retrofits with outlets similar to that described in this paper (Lucas,
2010).
HydroCAD was used to simulate the setup used in this
experiment to reproduce observed results and compute the
resulting retention time. The system is differentiated into nodes
that represent pond, media, and stone layers, with the latter
incorporating the outlet controls. Each node has a narrow
‘‘dummy’’ column extending into the adjacent layer. This
arrangement permits flows to be modeled as if flowing
horizontally according to the head between nodes, even though
flows are actually vertical. While this simplification results in
instantaneous outflow from the media, it occurs at very low rates.
Given that outflows from the free discharge control treatment
began far sooner than the advance of wetting front would be
expected, such simplification may not be unrealistic.
Darcy’s Law then controls flow through the media as a function
of head from the surface pond to that created by outlet array. It is
represented by a coefficient based on Ksat times area divided by
media depth, applied as a rating curve. Flows between these nodes
then are routed according to standard hydraulic algorithms. Refer
to Lucas (2010) for more detailed discussion of the procedures
used to model the system. Even with a time step of 0.4 seconds,
oscillations occur in such systems as a result of the small size of
the dummy columns, so the modeled system was scaled up by a
factor of 10. Given the observations of March 15, 2009, the model
was based on media with a Ksat of 50 cm/h. Table 4 presents the
input parameters used in the model.
Figure 6a presents the response of an outlet regulated system
compared with a free discharge system in Figure 6b. Figure 6a
displays the media hydraulic grade (equivalent to pond elevation at
saturation) and the corresponding infiltration rate to the media.
Initial infiltration rates are as fast as application rates, but, once the
media is saturated, as evidenced by its hydraulic grade meeting the
surface, infiltration rates decline rapidly, because they are now
regulated by the outlet configuration. Compared with the infiltration
response displayed in Figure 5a, these ponding and infiltration
trends are similar, indicating that the model provides a reasonable
representation of actual responses. Figure 6a also displays the
resulting partition of flows through the outlets, showing how the
upper outlet starts to flow after half an hour and stops once ponding
is over. In this manner, the peak flows that would otherwise bypass
the system also are treated. Meanwhile, the lower outlet continues to
run until the media and stone are drained completely to field
capacity, or roughly 8 hours in our mesocosms.
Figure 6b displays the same responses for the free-discharge
system. Absent regulation by the outlet, infiltration rates exceed
the application rate for the entire event, so no ponding occurs.
This is indicated by the media hydraulic grade line remaining
below the media surface. Media outflows still are regulated by
Darcy’s Law, so outflows increase as the media partially fills.
Because Ksat is high enough that outflows match inflows at this
hydraulic grade, ponding would not occur, unless the application
rate was increased. The media then drains down within 45 minutes
after inflow ceases. This response corresponds to what we have
observed for the unregulated control treatment.
In the modeled representation, plug-flow retention time in the
outlet-regulated media was 108.4 minutes, with another 49.1 min-
utes required to pass through the stone. This provides a total
media/stone retention time of 157.5 minutes. Compared with the
same mesocosm treatments without outlets, the corresponding
retention times were 17.3 and 1.5 minutes, respectively, for a total
retention time of 18.8 minutes. Thus, the outlet-regulated
mesocosms provided over 8 times as long a retention time. This
considerably improved nitrogen retention performance in the
outlet-regulated mesocosms is discussed in part II of this paper
(Lucas and Greenway, 2010, 2011a).
Conclusions
To provide rapid infiltration performance in bioretention
systems, typical recommendations for media composition suggest
that no more than 2 to 5% be in the clay fraction (New Jersey
Department of Environmental Protection, 2009). In determining
the clay fraction, typical PSD analyses use both mechanical and
chemical dispersant methods to dissolve clay aggregates. While
this method may represent the hydraulic implications of easily
dispersed clays, this method does not characterize the aggregation
found in the media that we selected for phosphorus sorption
properties (Lucas and Greenway, 2011b). The extent of aggrega-
tion noted in all three media amendments resulted in very high
Ksat rates, even with quite high clay fractions. While there was
some disaggregation as the silt fraction increased over time, very
few clay-sized particles were observed.
Comparison of the planted treatment to its corresponding barren
treatment showed that a significant increase in Ksat was associated
with the presence of plants. There was also a substantial increase
in Ksat in summer compared with winter. This temperature effect
exceeded that attributable to viscosity alone. Furthermore,
because Ksat also increased in the barren treatment, this increase
cannot be attributable to macropores associated with plant root
formation. It is possible that other processes may be involved,
such as bias by accumulation of bacterial flocs or other responses
resulting in changes in intrinsic permeability. Further research into
this phenomenon is warranted.
Within many treatments, there was a very high coefficient of
variation in Ksat of the replicates. To minimize artifacts in
retention performance introduced by such variations in flow
responses, an innovative dual-stage outlet configuration was
Table 4—Routing parameters used in the HydroCADmodel.
Pond area m2 2.53
Pond elevation m 0.76
Media area m2 2.24
Media Ksat m/h 0.50
Flowrate L/s 0.31111
Media depth m 0.62
Darcy flow coefficient 0.00050
Stone depth m 0.14
Stone area m2 2.00
Free discharge flow
Orifice elevation m 0.00
Orifice diameter cm 1
Number of orifices 10
Controlled discharge flow
Upper elevation m 0.70
Upper orifice cm 1.0
Number of orifices 10
Lower elevation m 0.18
Lower orifice cm 0.18
Number of orifices 10
Lucas and Greenway
700 Water Environment Research, Volume 83, Number 8
developed to manage flows. This configuration provides an
opportunity for each SCM system to be engineered more precisely
for its particular distribution of flow volumes, durations, and
frequencies. With judicious selection of design parameters, the
outlets can be configured to detain all small flow events, while
still treating large events by filtration through the media.
Our experiment demonstrates that media with high infiltration
rates offer the potential to effectively intercept even quite large
events, given the high hydraulic conductivities shown in Table 3.
Combined with the extended retention time afforded by the low
flow controls, this arrangement offers a highly effective treatment
system for runoff events of all magnitudes when Ksat is high, as is
the case with the present media.
The ability to lower the upper outlet and increase the
diameter of the lower outlet provides a consistent infiltration
rate in the face of inevitable clogging as the systems
accumulate sediments. Eventually, the media will have to be
renovated by removal of the clogging layer, after which, the
outlets can be reconfigured. This capability for adaptive
management is an emerging trend in the design of stormwater
controls. This also has important implications for the design of
bioretention systems to reduce nitrogen loadings to surface and
groundwater.
Credits
We extend our thanks to the staff of the Loganholme Water
Pollution Control Centre (Logan, Queensland, Australia) for
providing the meteorological data, space, and infrastructure to run
the experiments. We also thank the following students of Griffith
University who so ably assisted in the hydraulic observations:
Alana Scott, Jennifer-Leigh Campbell, Alison Atkinson, and
Wendy Tang. Craig Strong provided key assistance in the
interpretation of the particle size distribution results.
Submitted for publication April 7, 2010; revised manuscript
submitted September 10, 2010; accepted for publication Septem-
ber 15, 2010.
Figure 6—Flow (L/s) and elevation (m) responses: (a) outlet controlled system, and (b) free discharge system.Elapsed time from beginning of dosing.
Lucas and Greenway
August 2011 701
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