Large Volume
Low Impact	D
Using LID Practices in Areas with Intense Rainfall Events
SEPA
People often think low impact development (LID) practices, which are
designed to capture and treat the so-called first flush of polluted runoff, are
not suited for areas subject to large volume storms because the practices
could become overwhelmed with excess stormwaterflow and fail. This is
not the case. Properly sited and designed practices offer cost-effective
treatment in a wide range of conditions and locations, and usually serve as
an important first line of defense against high volume storm events.
Properly designed LID practices will allow excess water to bypass or
flow through the system to avoid damage. Even in areas subject to high
volume storms, LiD practices successfully trap and filter a portion of the
stormwater runoff—which helps alleviate pressure on existing stormwater
conveyance systems and reduces downstream erosion, pollutant loadings,
and damage to habitat in streams and riparian areas.
Equipping LID Practices with
High Volume Flow Controls
The process of siting and selecting LID practices is influenced by soil
type, land use, terrain, average rainfall and many other factors. Designers
must consider a practice's hydraulic performance under both low-flow and
high-flow conditions, and incorporate elements that can manage excess
flow as needed.
An infiltration system such as a rain garden or permeable pavement is
designed to treat a particular volume of water (also known as a water
quality volume) over a prescribed period of time. Because these practices
have a limited capacity to retain or treat excess volume, they are often built
with features intended to protect the long-term function of the practices.
If the stormwater enters the practice at a low velocity and is not erosive,
the excess runoff that ponds on the surface can be spilled into drainage
systems built on the perimeter of the practice or discharged through
overflow devices (Figure 1).
Other design features can be used to direct or convey high volume flows
away from the practice and to alternative drainage systems. For example,
in Figure 2, road runoff flows through curb cuts into a series of stormwater
bioretention planters in Portland, Oregon. When all stormwater planters
reach their maximum water capacity, water is forced to continue along the
curb without entering the practice, bypassing it completely and flowing into
an existing storm drain inlet.
Frequently Asked Question
Is it true that LID
practices don't
work in areas that
receive large
volume storms?
Barrier Busted!
LID practices can be effective in
areas subject to large volume
storms if properly sited and
designed to manage anticipated
runoff volumes.
EPA's LID Barrier Busters fact sheet series...
helping to overcome misperceptions that can
block adoption of LID in your community

Figure 1. A parking lot bioretention area near Frederick,
Maryland, is equipped with an overflow drain.
Figure 2. Stormwater planters treat runoff and can
bypass flows that exceed their treatment capacity.

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Temporary
water storage
Outlet structure
Outlet to
drainage
network
Figure 3. Street runoff enters the upper end of this bioretention area and
fills the practice. The overflow volume exits through an opening on the
lower end and drops into a storm drain.
Figure 4. Overflow drain. Stormwater fills the practice and begins soaking through the
bioretention soil media and into native soil. Excess water can spill through an outlet
structure to an alternative drainage network.
Specific design elements incorporated in LID practices to convey
excess flow through or around the practice include overflow and
bypass devices, backup infiltration, and underdrains:
Overflow Devices
Overflow devices are typically used in online LID practices (i.e.,
practices that are placed within the normal stormwater runoff flow
path). Online practices treat the water quality volume from smaller
storm events and are designed to convey or partially detain flows from
larger storm events. When the level of water rises to the height of the
overflow structure within the practice, any excess runoff is discharged
by gravity flow out of the practice and into another treatment practice
or into a storm drain.
•	Overflow Channels. If the runoff volume entering an online
LID practice exceeds the water treatment design capacity, a
downgradient opening such as a gravel channel or curb cut will
allow excess flow to exit the system and proceed along the normal
stormwater runoff flow path (Figure 3).
•	Overflow Drains. A common design to control excess flow entering
online LID practices is a vertical standpipe or box drain connected
to an underground drainage system and topped by a grate or
screen to prevent objects from entering the storm drain. The inlet
of the overflow drain is set at the maximum allowable ponding
elevation. The overflow control should be set at the downstream
side, far away from the incoming flow (Figures 4 and 5).
Figure 5. Overflow drain. Runoff water flows into this roadside
bioretention channel, which is equipped with an overflow drain
to prevent street flooding during high volume storm events.
\
Figure 6. Bypass system. A curb cut allows street runoff to enter
this sidewalk planter. If the planter fills completely, additional
runoff water volume will be forced to bypass the practice and
continue flowing down the road.
Bypass Devices
Bypass devices (i.e., diverters, splitters) are typically incorporated into offline LID practices that are sited outside of the
normal runoff flow path (Figures 6 and 7). Offline practices are designed to receive and treat a specified water quality
volume (e.g., the runoff generated from a 1-inch, 24-hour storm), in the case of roadside facilities (e.g., planter, bioretention
cell), the size of the inlet opening and the depth of the LID practice controls the amount of runoff allowed to enter the
practice. As a result, flow can be bypassed in two ways. First, because the offline practices are designed with an entrance
that restricts the amount of water able to enter the practice (e.g., curb cuts, weirs), high volume flows are split so only
a controlled amount of runoff enters the practice while the rest continues on its normal flow path. Second, the system
accommodates a controlled amount of runoff until the LID practice has reached its water quality treatment design volume.

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At that time, the system will redirect all excess stormwater back into s
the normal runoff flow path, which is often a conventional curb-and-
gutter stormwater conveyance system.
Backup infiltration
Backup infiltration approaches can be used when adjacent surface
areas are available to provide additional infiltration capacity. For
example, overflows from permeable pavements can be managed by
placing a strip of exposed gravel downslope of the pavement (Figure 8).
Excess runoff will flow into this gravel strip, which will discharge a
non-erosive flow stream to nearby vegetated areas.
Underdrains
LJnderdrains are needed where soils don't percolate/drain well or
where there's a high groundwater table or frequent inundation of
the practice. An underdrain is installed near the bottom of the LID
feature. Some practices are designed with underdrains in tandem
with overflow and bypass systems to ensure acceptable dewatering
times and to protect the long-term functioning of the practice. Avoiding
long-term ponding within the system prevents mosquitos from
breeding and helps protect the health of plants that treat the runoff.
Where infiltration could deteriorate overtime and drainage become
compromised due to sediment deposition, underdrains can be
designed with removable caps to allow access for cleaning.
If water retention is a performance requirement, underdrains can be
installed above the bottom extent of the practice or designed with a
90-degree upturned pipe so that the system begins to drain only after
the required water volume is retained (Figure 9). The water percolates
down through the soil into the internal water storage (IWS) layer and is
slowly released into the soil underneath the practice.
Figure 8. Back up infiltration system. A gravel aggregate strip
adjacent to a permeable pavement parking lot collects overflow
and directs it into a storm drain or alternative drainage practice.
Figure 7. Bypass and overflow system. Runoff enters a planter
box through a curb cut and collects until the practice's storage
capacity is reached; additional water will either overflow
through the second curb cut or will bypass the practice.
Slotted underdrain
Temporary
water storage
Outlet structure
irnal water
rage (IWS)
levation
Gravel layer
Capped cleanout/
observation well
Capped "tee"
connector for
maintenance
Outlet to
drainage
network
Figure 9. In this underdrain cross-section image, an upturned pipe design allows excess water to drain to an alternative
drainage network while also ensuring a permanent internal water storage layer within the practice. An outlet structure can
be included in the design to provide added protection against high volume flows.

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Case Studies: Successfully Treating anil Controlling Flow from large and Small
Storms with Diverse LID Practices
LID practices typically are designed to manage small- and medium-sized runoff events. Large storms will generally be
diverted using bypass or overflow design features. The following case studies show how communities are balancing the
need to convey and control peak stormwater flows with the need to treat and retain stormwater runoff.
Michigan Avenue Bioretention Planter Boxes, Lansing, Michigan
In 2006 bioretention planter boxes were installed along four blocks of Michigan Avenue, a busy five-lane street in Lansing,
Michigan (Figure 10). The planters can treat the runoff from 1 to 4 inches of rain falling on the adjacent street and sidewalk.
Water held in the soil is used by the plants, infiltrates to groundwater, or is released slowly through an underdrain. If the
planter reaches its maximum volumetric capacity, the extra stormwater flows to the conventional curb-and-gutter street
drainage system and into a storm drain.
Results
•	Flow meters were used to monitor the system; model results show that about
90 percent of the total annual stormwater volume was treated by the planter box.
•	The planters absorb/retain 16 percent and ultimately discharge 84 percent of the
total volume of stormwater received. By collecting and filtering the stormwater,
the planters delay discharge of the excess water into the local water body. The
peak flow rate of the water released through the underdrain is lowered by 87
percent, thereby reducing the overall impact of the stormwater runoff.
Christian, D. and Novaes, V. 2011. Michigan Avenue Bioretention: Monitoring the Results Three Years Later.
In Proceedings of the Michigan Water Environment Association's 86th Annual Conference, Beilaire, Michigan,
June 26-29, 2011.
Sterncrest Drive Bioswale and Rain Gardens, Cuyahoga County, Ohio
In 2007 the Chagrin River Watershed Partners received a U.S. Environmental Protection Agency grant to install nine
rain gardens and replace 1.400 feet of roadside ditch with grassed bioswale (Figure 11). The U.S. Geological Survey
monitored the site from 2008 to 2010 to assess the effect of green infrastructure on stormwater runoff. The rain gardens
and bioswales were designed to handle a 0.75-inch rain falling on the adjacent roadway. Rainfall and runoff data were
collected, along with overflow data, to determine how well the system
performed. A 2-foot-square elevated grate (6 inches above the land
surface) in the center of each rain garden allows excess stormwater
runoff to overflow into to the storm sewer (an example of an overflow
system installed in an online LID practice). A perforated underdrain
prevents long-term saturation of the LID system.
Results
•	Numerous rainfall events greater than 0.75-inch were absorbed by the
bioswales and rain gardens.
•	The bioswales and rain gardens performed better than expected.
During three years of monitoring, the system overflowed only 19 times
during 47 rain events when more than 0.75 inches of rain fell within a
96-hour span.
Source: Darner, R.A., and Dumouchelle, D. H. 2011. Hydraulic characteristics of low-impact development practices in northeastern Ohio, 2008-2010.
U.S. Geological Survey Scientific Investigations Report 2011-5165, 19 p.
United States Environmental Protection Agency • Office of Wetlands, Oceans, and Watersheds
1200 Pennsylvania Avenue, NW, Washington, DC 20460
EPA 841-F-16-009 • October 2017
Figure 10. Bioretention planter boxes in Lansing
capture and treat much of the stormwater from
roads and sidewalks.
Figure 11. Roadside rain garden in Cuyahoga County is
monitored for its effectiveness in absorbing stormwater

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