Stormwater
runoff from impervious surfaces is causing devastating effects on the landscape
of our developing watersheds. We are disrupting the natural hydrological cycle
that supports our potable water supplies and natural fauna. Intentional
stormwater infiltration can restore that cycle. However, the lack of awareness
and the perceived lack of data are currently limiting its use. This article
presents monitoring data for three sites in the North Carolina Piedmont that
demonstrate the success of stormwater infiltration in clay soils.
Negative
consequences of increased impervious area within developing watersheds have been
quantified and documented worldwide. The lack of success in mitigating
impervious area has also been verified and documented. Runoff from an acre of
pavement is approximately 10 to 25 times greater than the runoff from an acre of
grass. In urban areas, 30 to 40% of the rainfall runs directly into the nearest
stream. In heavily urbanized areas, such as central business districts,
precipitation runoff can be more than 50%. Compare this to the amount of runoff
from woodlands, which is often less than 5%.
Lost
ecological functions because of impervious pavements include retention and
infiltration. Stormwater infiltration is a critical ecological function within
the hydrologic cycle. Infiltration reduces flooding, recharges our groundwater,
and generates stream base flows during periods of limited precipitation.
Infiltration is a quantifiable process that follows the basic laws of physics.
Infiltration is not a new concept. In fact, the basic principle governing flow
in homogeneous porous media, Darcy’s Law, was formulated in 1856. The model
typically used today in quantifying infiltration, the Green-Ampt equation, was
formulated in 1911.
Targeting
the composition of the pavements themselves to mitigate lost retention and
infiltration function makes the most environmental sense and, in many cases,
also the most economic sense. The degree of retention and infiltration that
naturally existed prior to development can be matched, and in many cases easily
exceeded, utilizing the readily available, tested technology of porous pavement.
Porous pavements vary in infiltration rates, aesthetic values, materials, and
cost. Porous pavements typically require a higher degree of technical
consideration for bearing capacities and longevity, and have a more limited
level of maintenance that consists primarily of vacuum
cleaning.
When
impervious pavements must be used, careful design of other infiltration
techniques can mitigate the lost retention and infiltration function. Each
infiltration technique, including bioinfiltration basins or swales and porous
pavement, has its own unique advantages and caveats. Vegetated best management
practice (BMP) applications, such as bioinfiltration, may include diverse
vegetation types that can tolerate a variety of environmental conditions, can
project different landscape aesthetics, and can be discreetly adept in removing
specific pollutants. Vegetated BMPs can require a higher degree of aesthetic
design and varying levels of maintenance, including vegetation replacement, soil
amendment, and vegetation management.
Regardless
of the infiltration technique used, the common design constant is the
infiltration rate of the native soils that underlie the
system.
Within
the geologic region of the “Charlotte Belt” in Mecklenburg County, NC, soils are
typically high in clay. The two project sites presented in this article are
underlain by typical clay soils for this region, the Cecil series, classified by
the Natural Resources Conservation Service (NRCS) as well-drained, moderately
permeable soils. The phrase “clay soil” is broadly used in the design industry,
typically in negative context, when referring to drainage or
infiltration.
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| Figure 1. A bioretention garden at Wilmore Walk |
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| Figure 2. Installation of a bioretention subdrain system at Wilmore Walk |
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| Figure 3. Native soil infiltration rate testing at Wilmore Walk |
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| Figure 4. Monitoring well in a porous concrete parking lot |
According
to the NRCS (formally the Soil Conservation Service), 64% of Mecklenburg
County’s land area is composed of soils in hydrologic group B. These soils vary
in composition of clay, silt, and sand. They have NRCS-reported infiltration
rates that vary from 0.6 inch per hour to 2.0 inches per hour. The soils
observed on the sites of the two case studies presented here are in the Cecil
and Cecil-Urban series, which compose approximately 59% of the Mecklenburg
County land area.
The
first project, Wilmore Walk, is two years old and presents monitoring data of
retention and infiltration rates over a period of one year. The second project,
Jetton Street Condominiums, presents monitoring data for a project that was
planned, designed, and modeled as a low-impact development project. Also
included in this article are monitoring data from an undeveloped site adjacent
to Six Mile Creek that contains high clay soils in the hydrologic soil groups C
and D, to illustrate natural infiltration rates in difficult
soils.
Wilmore
Walk: Infiltration in Porous Pavement
The
data from Wilmore Walk demonstrate the infiltration performance of a basin
located under a pervious concrete parking lot, as well as the infiltration
potential of bioretention rain gardens in a high-density residential project.
This example provides validation of the concept of a low-impact stormwater
project in the urban landscape, demonstrates the successful use of pervious
concrete, and quantifies design criteria for clay soil infiltration.
Located
in Charlotte, NC, Wilmore Walk is a 2.84-acre condominium development that was
constructed in the summer of 2005. It was an urban redevelopment project on a
site where an existing apartment building was removed. No regulated detention
was required for the site. However, due to sensitive watershed and environmental
impacts associated with culverting an existing stream, water-quality management
techniques were required. The site design was analyzed for opportunities to
incorporate various stormwater BMPs. The analysis indicated that the
water-quality requirements could be met by installing eight bioretention
gardens. The analysis further identified the opportunity for stormwater
infiltration in the form of a 0.14-acre porous concrete parking lot. The
resulting project treated 93% of the impervious surface
runoff.
The
existing soils are Cecil sandy clay substrate. According to the NRCS Soil
Survey, the site was mapped as soil unit CuB and CuD. This soil unit is composed
of approximately 49% clay, 25% silt, and 30% sand.
The
bioretention areas were incorporated to fit a very tight site design and are
almost indistinguishable from typical landscaped areas.
As
required by the regulatory authorities at the time, the bioretention gardens all
have subdrains that connect to the conventional stormwater drainage
infrastructure and have minimal infiltration function. It is important to note
that although these bioretention gardens do not infiltrate to their maximum
capacity, they can be easily retrofitted to do so.
Pervious
concrete was chosen as the material for the porous parking lot because of its
high infiltration rate and its durability and ease of maintenance. The
infiltration efficiency of pervious concrete depends on the consistency of the
mix and installation quality, but typically exceeds that of other manufactured
paving systems. Properly installed pervious concrete contains 15 to 30% void
space throughout the entire volume of the application. The voids increase in
size from top to bottom of the concrete’s cross section, greatly reducing the
chance of clogging. Flow rates through pervious concrete range from 140 to more
than 1,000 inches per hour. Six inches of pervious concrete can detain and store
1.5 inches of rainfall. A reservoir base course composed of single-size
aggregate with approximately 30 to 40% voids and underlain with nonwoven filter
fabric was added to the design to retain the two-year-recurrence storm event for
the parking lot and adjacent roof top drainage area. The base reservoir was
excavated into existing soil as a closed basin; there is no primary drainage
outlet except infiltration into the soil. The base’s aggregate material was
placed on the soil without further compaction. For post-construction monitoring
purposes, three PVC monitoring wells were incorporated to house water level data
loggers.
Preconstruction
infiltration testing was conducted for the proposed pervious concrete parking
lot using a double-ring infiltrometer. Test results yielded a range of 0.23 to
3.1 inches per hour, with an average infiltration rate of 0.90 inch per hour for
the existing unmodified sandy clay subgrade. The lowest rate of 0.23 inch per
hour was measured in an area of light compaction caused by a Bobcat with rubber
tracks. The highest infiltration rate, 3.1 inches per hour, was measured in an
area that was cut raked with no compaction.
Post-construction
infiltration monitoring data were collected using an Infinities USA pressure
water level data logger. The monitoring period was one year. The data presented
here are for a six-month period, from August 2006 through February
2007.
During
the post-construction monitoring period, several months of data had to be
disregarded due to malfunction of the pressure sensor. The problem was
apparently due to sediment that was present in an installed conduit directly
above the sensor.
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| Figure 7. Filling the Wilmore Walk reservoir from a watering truck |
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| Figure 8. Wilmore Walk finished porous concrete parking lot |
However,
results of the monitoring data from August through September 2006 included 11
rain events, five of which ranged from 0.98 to 2.97 inches per day. The maximum
rate that the reservoir rose during any single event was 3.4 inches per hour (±1
inch water level equivalent). The reservoir water level reflects the available
void space in the stone and additional catchments surrounding the porous
concrete lot. Infiltration rates into the existing subgrade varied depending on
the depth or head of the water in the reservoir, which is consistent with
Darcy’s Law.
The
maximum recorded water depth within the reservoir was 13.28 inches (±4 inches
water equivalent). During this event, the soil infiltration rate at maximum head
was 0.29 inch per hour. During the following 6.3-day period between storm
events, the water level in the reservoir dropped 13.24 inches. As the water
level declined with time, the infiltration rate also declined, which is
consistent with the role of head in Darcy’s Law. The average rate of
infiltration was 0.09 inch per hour, or 2.1 inches per day. During the majority
of the recorded events, infiltration was occurring in saturated soil
conditions.
In
February 2007, the reservoir was artificially filled using a water truck. Before
the reservoir was filled, the water level was measured at 2.28 inches (±0.7 inch
water equivalent). The final water level depth after filling was 13.74 inches.
From the time the reservoir was artificially filled and the next rain event 5.57
days later, the average infiltration rate into the subgrade was 0.06 inch per
hour, or 1.34 inches per day, with a total water level drop of 7.42
inches.
Reduction
in soil infiltration rate of approximately one order of magnitude was observed
between pre-construction and post-construction infiltration rates. Possible
reasons for this variation include the introduction of the nonwoven filter
fabric; inadvertent compaction during construction, such as from the
introduction of the reservoir aggregate material; seasonally higher water table
and extended saturated soil conditions due to consecutive storms; and subsurface
flow from an adjacent uphill property.
Another
observation worth mentioning is that during the winter of 2005, Charlotte
experienced several freezing rain and snow events. On January 29, 2005, and
February 28, 2005, the project site received 0.44 inch and 0.56 inch of snow,
respectively. Although snow accumulated on an adjacent impervious asphalt
pavement, there was no accumulation on the pervious concrete surface. This is a
significant secondary benefit to typical vehicular applications, as well as
pedestrian applications that require nonslip surfaces.
Jetton
Street: Infiltration in Rain Gardens
A
detailed hydrological model was performed during the design phase of Jetton
Street Condominiums, which illustrates the effectiveness of planned bioretention
and infiltration BMPs to reduce the required detention volume. This article
focuses on the monitoring data collected for two infiltration BMPs in clay
soils. Monitoring data presented for this project spans a period of 4.5 months,
from February 8, 2008, to June 25, 2008.
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| Figure 9. Jetton Street bioretention gardens |
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| Figure 10. Jetton Street 15-inch-diameter monitoring well |
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| Figure 11. Jetton Street bioinfiltration garden |
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| Figure 12. Jetton Street bioinfiltration garden 2 (infiltration pond) |
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The
site is a 5.75-acre condominium project developed within the protected water
supply watershed of Lake Norman. The project site is underlain with sandy clay
soils of the Cecil series. This project was not a retrofit design and
incorporated the proposed BMPs into the stormwater infrastructure planning
phase.
This
site was a new development and required regulated detention. Because there was
an opportunity for preliminary planning, the proposed BMPs were incorporated
into a hydrological model utilizing HydroCAD software. The modeling allowed for
an accounting of the proposed hydraulic mechanisms within the BMP structures and
quantified the reduction in required detention volume. This project treats 83%
of the 5.75-acre parcel, of which 31% is impervious.
According
to the NRCS Soil Survey the site was mapped as Cecil soil unit CeB2. This soil
unit is composed of approximately 37% clay, 27% silt, and 24% sand. Additional
soil borings were conducted during the geotechnical site investigation.
Preconstruction infiltration testing using a double-ring infiltrometer was
conducted on the site. Infiltration rates for the existing soils at two proposed
BMP locations and grades varied from 1.13 inches per hour to 1.5 inches per
hour, with an average of 1.3 inches per hour.
This
project incorporates three bioretention gardens with subdrains and two
bioinfiltration gardens that rely entirely on exfiltration into the native soil
for drainage. The two bioinfiltration gardens are designed to retain and
infiltrate the two-year-recurrence design volume (3.12 inches, per the
Charlotte-Mecklenburg Storm Water Design Manual). Two other bioretention gardens
with subdrains detain the two-year-recurrence design storm. The remaining
bioretention garden is the smallest and is designed to detain the 1-inch storm
event, which is then routed to the first bioinfiltration
garden.
The
model predicted a 45% reduction in required detention volume for the
10-year-recurrence storm event utilizing the gardens.
The
monitoring devices for this project site differ from those used at the Wilmore
Walk site. The monitoring devices used here are capacitance water level probes
placed in 6.5-foot-deep wells that extend from the surface of the installed soil
mix to approximately 1.5 feet to 3.3 feet below the bottom of the constructed
infiltration gardens. These monitoring wells are perforated pipe and also act as
infiltration wells.
The
monitoring period was from February 8, 2008, to June 25, 2008. The precipitation
from 37 precipitation events ranged from a trace to 1.5 inches per day, and the
corresponding infiltration rates were recorded.
For
infiltration garden 1, the maximum rate the well water level rose during this
period was 46.24 inches per hour (±15 inches per hour water equivalent). The
maximum water level recorded during this period was 74.06 inches above the
bottom of the well. Consistent with Darcy’s Law, infiltration rates through the
installed soil mix varied with depth or head of the water. Once the water level
infiltrated past the bottom of the installed soil mix and filter fabric, the
infiltration rate increased by a factor of 3.7. The average rate increased from
0.26 inch per hour through the installed soil mix to 0.97 inch per hour into the
native soil matrix.
For
infiltration garden 2, noted as “infiltration pond” on the data sheets, the
maximum rate the reservoir water level rose during the period was 5.6 inches per
hour. The maximum water level measured in the field during this period was 111
inches (33 inches above the top of the well). Again, consistent with Darcy’s
Law, infiltration rates through the installed soil mix varied with depth or head
of the water from 0.15 to 0.31 inch per hour. A consistent and distinct rate
increase was recorded once the water level reached native soils below the
geotextile fabric.
The
reasons for variation in infiltration rates between the infiltration gardens may
be the elevation differences, affecting proximity to the existing water table,
and the introduction of irrigation runoff. Infiltration garden 2 is lower in
elevation and maintains a shallow vadose zone. Irrigation water is introduced to
infiltration garden 2 through the storm system.
Six
Mile Creek: Infiltration in Natural Preserve
The
data collected for the Six Mile Creek project provide an example of natural
infiltration in difficult clay soils at an undeveloped site. The intent of this
data is to illustrate natural infiltration conditions in a worst-case
scenario.
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| Figure 13. Six Mile Creek site monitoring well |
The
site is located within 36.2 acres of formerly agricultural floodplain adjacent
to Six Mile Creek in southwest Mecklenburg County. The site was previously used
as cattle pasture and is dominated by fescue turf. The Mecklenburg County Soil
Survey indicates that soil series at the site are Monacan (MO) and Iredell (IrA)
fine, sandy loams with 0 to 1% slopes. MO soils (hydrologic soil group C) are
somewhat poorly drained, nearly level soils found on flood plains along streams
and drainage ways. IrA soils (hydrological soils group D) are moderately well
drained soils found on broad flat areas on the uplands.
Monacan
soils are classified as fine-loamy, mixed thermic Fluvaquentic Eutrochrepts. The
Monacan series consists of somewhat poorly drained, moderately permeable soils
that formed from recent alluvium. The organic content is low in the surface
layer of this Lignum soil. The permeability is slow and runoff is medium. The
water table is below 5 feet with the exception of a perched water table at 1 to
2.5 feet during wet seasons (apparent water table November through May). The
depth to bedrock ranges from 48 to 72 inches, according to the 1980 Mecklenburg
County soil survey. This Monacan series is listed on the Hydric Soils of North
Carolina list.
Iredell
is classified as a fine, montmorillonitic, thermic Typic Hapludalfs. The Iredell
series consists of moderately well-drained slowly permeable soils that formed in
residuum from basic crystalline rock. The organic matter content is low in the
surface layer. A seasonally perched water table is only 1 to 2 feet below the
surface. Depth to bedrock is greater than 60 inches. This Iredell series is
listed on the Hydric Soils of North Carolina list.
The
monitoring period was from March 2006 to March 2007. Two Infinities USA pressure
water level data loggers were installed 41.5 inches below the ground surface to
document the existing hydrology prior to restoration activities. The monitoring
data were compared to rainfall data to gauge infiltration rates into the
existing subgrade.
The
12 months of data were analyzed, and the wettest months were used as the
sampling average. From January 1, 2007, to March 7, 2007, 19 precipitation
events occurred, ranging from 0.01 inch to 2.17 inches, with a total rainfall of
9.58 inches for the period.
The
measured infiltration rates ranged from 0.354 inch to 0.439 inch, with an
average infiltration rate of 0.42 inch per hour.
Conclusion
With
the number of proven BMP methods available to designers, stormwater runoff can
be reduced and managed effectively with a great range of benefits both
economically and environmentally. Correctly implemented stormwater infiltration
can be the most effective and important of all the stormwater strategies
available.