Engineers Workshop

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Engineers Workshop March 4, 2005
presents excerpts from a primer /
poster BIORETENTION in CLAY SOILS for Stormwater
Management
addressing water quality and quantity management
while enhancing the visual commonwealth of our
built environment. This Primer presents the
fundamentals of bioretention systems, designed
specifically for clay, clay/silt, and compact
urban fill.
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WCD Engineers Workshop March 4, 2005
ENGINEERING
ECOLOGY
a BIORETENTION CELL is a RAIN GARDEN
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WCD Engineers Workshop March 4, 2005
ENGINEERING FUNCTIONS
BIORETENTION PLANTS Plants in bioretention cells
are subjected to highly variable moisture and
climatic conditions. See Bioretention Plant
List for selected plants. OVERFLOW DRAIN
alleviates high levels of runoff water. Concrete,
metal or plastic. 3 MULCH Coarsely shredded
hardwood bark has proven to be the most durable.
CONNECT TO UNDERDRAIN Use Schedule 40 PVC or
SDR-35 pipe. Size to carry expected
flow. CONSTRUCTED SOIL MIX the most critical
element for long-term success of a bioretention
cell. moderately fast draining (2-4 inches /
hour) 20 organic material 30 sand, pea
gravel 50 topsoil (no rocks) no more than 10
clay. . GRAVEL ENVELOPE .Geotextile Fabric Class
1 non-woven geotextile as specified in
PennDOTs. UNDERDRAIN The underdrain is
absolutely necessary in clay soils and compacted
urban fill. Size the underdrain pipe according to
expected inflow from the overflow drain (6
minimum).
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WCD Engineers Workshop March 4, 2005
ECOLOGICAL FUNCTIONS
EVAPOTRANSPIRATION Large amounts of water are
pulled up into plants from the surrounding soil
in a process called transpiration..A growing
plant transpires 5-10x as much water as it can
hold. MULCH LAYER Nutrients and toxic chemicals
attach to sediment particles on land surfaces and
are carried into bioretention basins. The mulch
layer plays a critical role in heavy metal
uptake. SOIL BIOTA POLLUTANT REDUCTION Soils
provide a physical, chemical and biological
setting for living organisms. Clay for instance,
(due to its molecular properties) provides
adsorption sites for hydrocarbons. WATER
COOLING heated runoff from impervious surfaces
is cooled through the bioretention
facility. INFILTRATION (and EXFILTRATION) with
constructed soils and a small amount of clay, a
1/2 rainfall can be completely managed through
infiltration, absorption, and exfiltration. A
DDITIONAL WATER CLEANING some water will remain
under the drain, and create anaerobic conditions.
Though this condition is undesirable in the plant
root zone, specific pollution removal occurs
here.
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WCD Engineers Workshop March 4, 2005
DESIGN SIZING a bioretention cell
VOLUME TO BE INFILTRATED
1/2 rain runoff post - development
Shallow storage in surface depression
1/2 rain runoff pre - development
Storage and adsorption in constructed soils
Calculate the storm volumes by USDA-SCS-TR-55 or
other methods such as the Rational Method In
general, one acre of pavement will require
1,800 square feet of bioretention (3.5 deep) to
handle the 1/2 rain event.
Retention in the 35 void spaces within the
underdrain envelope
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WCD Engineers Workshop March 4, 2005
A STABLE DRAINAGE AREA no upstream erosion sources
Bioretention cells work best in drainage areas
that are either permanently vegetated, or fully
built out.
This bioretention cell designed to capture runoff
from a two-lane road. An active construction site
nearby soils were carried into the curb cut/
entry into the rain garden. Over time, the
sediments blocked the flow in the cell.
After a recent rain, this relatively new
bioretention cell, without curbs, took in runoff
from an active construction site 250 up the
road.
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WCD Engineers Workshop March 4, 2005
MAINTENANCE
BIORETENTION PLANTS will eventually become
exhausted from their work in absorbing and
transforming contaminants that occur in
stormwater runoff. They will need to be replaced
over time, before the woody plants reach
maturity. Current research shows that in a
properly functioning rain garden plants can
thrive from 5-10 years. The INLET will
need to be inspected for clogging. All debris
(such as mulch) must be removed before it moves
further down into the system.

MULCH provides essential
microflora and bacteria that work to clean water.
These active cultures depend on fresh supplies
of nutrients in order to flourish, therefor,
mulch must be renewed 1x/year, and completely
replaced every 3rd year. The
UNDERDRAIN CONNECTION must be checked for, and
cleared of debris allow access through the
inlet. CONSTRUCTED SOILS will need
to be replaced when they become exhausted or
over-concentrated with contaminants, possibly
every 5 to 10 years. Plant health will be a major
indicator for when its time to replace soil.
Maintenance of the GRAVEL ENVELOPE can be
best avoided through careful construction.
Should the bioretention cell not drain as
designed (2-4/hour), and/or if ponding times are
excessive in small rain events, the UNDERDRAIN
may be clogged or compromised and will need to
either accessed and flushed through the inlet, or
dug up and reconstructed.
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WCD Engineers Workshop March 4, 2005 Primer
Boretention in Clay Soils
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WCD Engineers Workshop March 4, 2005
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WCD Engineers Workshop March 4, 2005
A minimum of 10 trees and shrubs per 1,000
square feet of basin area, with two to three
shrubs specified for each tree, or a 21 ratio
of shrubs to trees.
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new
urban retrofit
RAIN GARDENS
county park retrofit
So, what do bioretention cells look like?
urban retrofit
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WCD Engineers Workshop March 4, 2005
Enjoy your poster!

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