v>EPA
United States
Environmental Protection
Agency
Off ice of Water
Washington, D.C.
832-F-99-035
September 1999
Combined Sewer Overflow
Technology Fact Sheet
Inflow Reduction
DESCRIPTION
This fact sheet describes inflow reduction, a
practice designed to minimize the volume of storm
water runoff that enters a combined sewer system
(CSS). Inflow reduction can result in a lower
combined sewer overflow discharge volume and
may reduce the number of combined sewer
overflows.
CSSs are wastewater collection systems designed to
carry both sanitary sewage and storm water runoff
in a single pipe to a wastewater treatment plant.
During wet weather periods, the hydraulic capacity
of the CSS may become overloaded, causing
overflows to receiving waters at discharge points
within the CSS. These overflows are called
combined sewer overflows, or CSOs. Inflow
reduction refers to a set of control technologies that
are used to reduce the amount of storm water
entering the CSS from surface sources. Inflow
reduction can be a cost-effective way to reduce the
volume of flow entering the CSS and the volume
and/or number of CSOs. It is particularly
applicable in CSO communities where open land is
available to accommodate redirected flow for
infiltration or detention, or where storm water can
be diverted to surface waters.
By helping to reduce the overall flow volumes into
a CSS, inflow reduction technologies help to
optimize the system's storage capabilities.
Maximizing storage in the collection system is one
of the nine minimum controls that every CSO
community is expected to implement in accordance
with the Environmental Protection Agency's 1994
CSO Control Policy.
Technologies used to reduce inflow include:
• Roof drain redirection.
• Basement sump pump redirection.
• Flow restriction and flow slipping.
• Storm water infiltration sumps.
• Stream diversion.
All of these technologies have relatively low costs,
and most require little maintenance in comparison
with other CSO controls.
APPLICABILITY
Roof Drain Redirection
Roof drains often convey rainfall directly from
residential and commercial roofs into a CSS. Flow
into the CSS can be reduced by redirecting roof
drains onto lawns or into dry wells or drainfields
where flows can infiltrate into the soil. Roof drain
redirection (redirection) works best in residential
areas where homes have open yards. Telephone,
mail, or door-to-door surveys are necessary to
determine the extent of roof drain connections to
the CSS. Because the net volume reduction per
household is small, implementation needs to be
broad and consistent throughout the service area.
The cumulative reduction effects of such a program
can be substantial. Redirection can be either
voluntary or mandatory. Cash or other incentives
can be instrumental in achieving widespread
redirection.
-------
Roof drain redirection is a relatively simple task
that can be done easily by individual homeowners.
Therefore, it is essential to prepare guidance for
homeowners who elect to perform the redirection
themselves. This guidance material should also
include a list of contractors able to provide
redirection for a reasonable cost. Homeowners
should be reminded to periodically check to make
sure that redirected water is infiltrating and is not
contributing to other surface water problems. The
community should develop a schedule and check
that homeowners who have agreed to disconnect
have remained disconnected. Roof drain
redirection should be combined with basement
sump pump redirection where possible.
Basement Sump Pump Redirection
Many buildings have sump pumps to pump
floodwater from basements. Often this water is
discharged directly into the CSS. Redirecting this
flow away from the CSS and onto lawns or dry
wells or drainfields reduces the volume of storm
water entering the CSS.
Unlike roof drains, which are common to most
buildings, basement sump pumps usually exist only
in discrete areas. Therefore, surveys (preferably on-
site) are necessary to determine where sump pumps
are located, whether they discharge directly to a
CSS, and whether it is feasible to redirect them into
a yard, a dry well, or an infiltration field. It is
important to note that the feasibility of discharging
downspout and basement sump discharges into a
yard, a dry well, or an infiltration field depends on
the soil type, the slope, and the drainage conditions
around the home.
As with roof drain redirection, guidelines for
basement sump pump self-redirection should be
distributed, and the area should be checked
regularly to ensure that redirected water is
infiltrating and is not contributing to other surface
water problems.
Flow Restriction and Flow Slipping
Flow restriction and flow slipping methods utilize
roadways and overland flow routes to temporarily
store storm water on the surface, or to convey storm
water away from the CSS. Flow restriction is
accomplished by installing static flow or "braking"
devices in catch basins to limit the rate at which
surface runoff can enter the CSS. Excess storm
flow is retained on the surface and enters the system
at a controlled rate, eliminating or reducing the
chance that the system will be hydraulically
overloaded and overflow. The volume of on-street
storage is governed by the capacity of the static
flow device, or orifice, used for restriction, as well
as surface drainage patterns.
As opposed to flow restriction, where flow rates
into the CSS are reduced but all storm flow
eventually flows into the storm sewer system, flow
slipping refers to the intentional blocking of storm
water from entering the CSS at catch basins for the
purpose of routing, or "slipping", it elsewhere.
Flow slipping is accomplished by partially or
completely blocking the entry of surface runoff at
catch basin inlets and letting the runoff follow
overland flow routes.
Flow restriction and flow slipping can effectively
reduce inflow during peak runoff periods and can
decrease CSO volume. Use of these methods must
be carefully planned to ensure that sufficient surface
storage or overland throughput capacity exists in
the drainage area. These methods are almost
always used in conjunction with other practices,
such as roof drain and basement sump pump
redirection.
Flow restriction works best in relatively flat areas
where temporary ponding and detention of water on
streets is acceptable. Extensive public education
and testing are required to build support and
address concerns that residents and elected officials
may initially have regarding on-street storage. Flow
slipping is an option where opportunities for
on-street storage are not available. The "slipped"
flow can be diverted along natural drainage routes
to separate receiving waters, separate storm sewer
systems, or even to more optimal locations within
the CSS.
The use of flow restriction and flow slipping
requires a detailed evaluation of the collection
system and catch basins. The community must
assess the potential for unsafe travel conditions,
-------
flood damage, and damage to roadways. Pilot
studies and monitoring are recommended to identify
impacts and confirm performance.
Storm Water Infiltration Sumps
Storm water infiltration sumps are below-ground
structures used to collect storm runoff and pass it
into the soil. The infiltration sumps collect runoff
in standard storm water inlet structures at the
ground surface, and rout it to a two-chambered
system consisting of a manhole structure and an
attached sump chamber. The manhole chamber
serves as a sedimentation basin. As this chamber
fills, flow reaches an overflow point and begins to
fill the second chamber, the perforated sump. The
perforations allow the water to percolate outward
into the soil. The sump chambers are typically 8 to
12 meters (25 to 35 feet) deep and are surrounded
with granular backfill to promote infiltration. A
typical infiltration sump assembly is shown in
Figure 1.
Street Inlet
Infiltration
Sump
Chamber
Source: Modified from city of Portland, 1997.
FIGURE 1 TYPICAL INFILTRATION SUMP
ASSEMBLY
Infiltration sumps can reduce inflow in areas where
the underlying soils are moderately to highly
permeable, and the water table is well below the
ground surface. They are generally more applicable
in residential areas that are less than 50 percent
impervious. It is important to get the surface runoff
to streets where sumps have been installed, and
implementation in conjunction with roof drain and
basement sump pump redirection is recommended.
Due to the potential for chemical contamination of
ground water, infiltration sumps are not
recommended for commercial or industrial areas.
Before starting construction of a storm water
infiltration sump, the community should develop a
sump management plan that addresses the policy,
design, construction, maintenance, and public
education aspects of the infiltration sump program,
as well as addressing spill response, well
monitoring, storm water runoff quality, and storm
water sampling protocols. Consideration should be
given to local traffic disruptions during sump
installation. Sumps will need to be cleaned every
two to three years to remove material that collects
in the sedimentation basin.
Stream Diversion
As cities grew during the nineteenth and early
twentieth centuries, many small streams were
routed into pipes to facilitate development. In
communities where streams have been routed into
CSSs, the surface runoff once conveyed in these
streams reduces capacity in the CSS and contributes
to overflows. Rerouting natural streams and
surface runoff away from the CSS and back to their
original watercourse or to other receiving waters
can have a significant impact on CSS capacity.
Urban stream diversion is one of the more
expensive inflow reduction options since it typically
requires design and construction of new storm drain
lines. Stream diversion resembles CSO separation
in that new alternative flow routes are required for
surface runoff. It is typically employed in situations
where less expensive and less disruptive options for
inflow reduction are not feasible, or do not provide
sufficient inflow reduction. The potential amount
of inflow to be diverted from the CSS needs to be
well documented in order to assess its
cost-effectiveness.
The first step in implementing a stream diversion
program is to identify and evaluate alternative
routes from the stream to nearby receiving waters.
It may be possible to "daylight" streams by allowing
them to flow in an open channel in some semblance
-------
of their natural condition. Daylighting can provide
greenspace and can serve as an amenity to the
community. However, daylighting opportunities
are limited in most CSS communities because the
streams were originally piped in order to support
property development. Diversion through separate
pipe systems is also an option, and may be more
practical where open space is limited and the
potential for flooding is high.
ADVANTAGES AND DISADVANTAGES
Inflow reduction can be a very important part of a
CSO management program. By reducing the
amount of extraneous inflow to a system, CSSs may
avoid CSO problems. Specific advantages and
disadvantages of each of the inflow reduction
methodologies discussed in this fact sheet are
presented below.
Roof Drain Redirection
Roof drain redirection is a relatively simple and low
cost (per unit) option for reducing storm water
inflow to a CSS. However, in order for a roof drain
redirection program to be successful, the public
must be educated as to the benefits and methods for
implementing such a program. This can be time-
consuming and will most likely require some sort of
rebate program or other incentive for compliance.
In addition, because the effect per individual roof
drain redirection is small, this program must be
implemented in a wide service area to be effective.
Basement Sump Pump Redirection
Advantages and disadvantages of a basement sump
pump redirection program are similar to those of
roof drain redirection programs. A basement sump
pump redirection program can be a relatively
inexpensive and easy way to reduce flow to CSSs.
However, unlike roof drains, basement sump pumps
may occur less frequently and their locations may
be harder to determine. And as with roof drain
redirection, a basement sump pump redirection
program can be time-consuming and will most
likely require some sort of rebate program or other
incentive for compliance. And similarly to roof
drain redirection, because the effect per individual
sump pump redirection is small, this program must
be implemented in a wide service area to be
effective.
Flow Restriction and Flow Slipping
Flow restriction and flow slipping methods can be
effective ways to manage flow in specific parts of
the CSS. They are relatively inexpensive and easy
to implement. However, implementation will
require a public awareness campaign to inform the
public of the purpose of the flow diversions. In
addition, these inflow reduction methods require
large surface areas to temporarily store flow before
it enters the CSS.
Storm Water Infiltration Sumps
While more expensive than roof drain or basement
sump pump redirection, storm water infiltration
sumps can be very effective in areas where there is
highly permeable soil and little chance of
groundwater contamination. However, infiltration
sumps are not recommended for use in areas with
high groundwater tables or in soils with low
permeability.
Stream Diversion
Stream diversion can be an expensive method for
reducing storm water inflow into the CSS.
Changing a stream channel may also require
numerous permits and may not meet with public
approval. However, stream diversion will often
cause a significant decrease in the amount of storm
water flowing into a CSS.
PERFORMANCE
The performance of inflow reduction programs is
usually measured by the reduction in CSS volume.
As sources of inflow are identified and eliminated,
the overall flow volume in the CSS decreases, and
the likelihood of CSOs decreases as well. Several
specific cases studies for inflow reduction are
presented below.
Roof Drain Redirection
In St. Paul, Minnesota, an estimated 20 percent of
CSO volume came from roof drains. As a result of
-------
a $40 rebate for voluntary redirection and other
innovative outreach efforts, approximately 18,000
homes redirected their roof drains over a three year
period. Presently, 99 percent of all residential
properties are disconnected. In addition, the city is
evaluating creative funding options to assist
commercial and industrial property owners to
disconnect roof drains.
Basement Sump Pump Redirection
South Portland, Maine, conducted a visual survey
of the 6,000 residential buildings in the city and
found 380 homes with roof drains and 300 homes
with sump pumps discharging directly into the CSS.
The city mailed letters offering to reimburse each
property owner for redirecting their roof drains and
sump pumps. After the work was completed and
inspected by the city, homeowners were reimbursed
$75 for each redirected roof gutter and $400 for
each redirected sump pump. The program has
redirected more than 379 roof drains and 304
basement sump pumps. The resulting reduction is
approximately 58 million gallons of water per year.
Because of the variables involved (rainfall patterns,
differing drainage areas, etc.), the city has not
determined a direct correlation between these
programs and CSO events; however, overall flow to
the city's wastewater treatment plant has been
reduced by 2 percent through these efforts.
Flow Restriction and Flow Slipping
Gently graded berms can be added to roads and
used in conjunction with flow restriction to
maximize on-street temporary storage. In Skokie,
Illinois, a fully developed suburb of Chicago served
by a CSS that covers 8.6 square miles, an integrated
program was developed that emphasized berms and
flow restrictors to both increase on-street storage
and to reduce the peak rate of flow entering the
CSS. In order to accomplish this, 2,900 flow
restricting devices were installed at catch basins,
and 871 berms were constructed on streets. In
addition, 100 percent of the roof drains previously
connected to the CSS were disconnected.
South Portland, Maine, conducted a detailed
evaluation of 750 catch basins within the CSS and
determined that 30 catch basins could be eliminated
without any adverse safety or flood damage
consequences. Solid covers were placed on these
catch basins to direct flow to separate storm sewers
or natural drainage. Flow slipping in this manner
has reduced flow in the CSS by approximately 12
million gallons per year.
In Hartford, Connecticut, a pilot flow slipping
project was implemented as part of the city's CSO
abatement plan. The method employed catch basins
that completely crossed the streets, incorporating
sumps that could be located anywhere in the
roadway. This strategy allowed the city to work
around the myriad of existing subsurface utility
lines. To ensure safe pedestrian crossings, the catch
basins were located upgradient of pedestrian
crosswalks.
Storm Water Infiltration Sumps
Infiltration sumps can be retrofitted within
combined sewer areas, usually beneath the street
system. In Portland, Oregon, much of the
combined sewer area has highly permeable soils
with a high hydraulic capacity. Portland is
currently wrapping up a program that installed
approximately 4,000 infiltration sumps between
1994 and 1998. Combined with other CSO control
programs, including a successful roof drain
disconnection program, sewer separation, and
stream diversion, it is predicted that the total CSO
volume will be reduced by 3 billion gallons per year
(approximately half of the total CSO volume).
Portland is initiating another program to maximize
storage capacity in the CSS to further reduce the
overflow volume.
Stream Diversion
Portland, Oregon's Ramsey Lake Wetland Project
involved completely separating an urban stream
from the CSS, creating a wetland to treat the
separated storm water flow prior to discharge into
a receiving stream. The project also provides
community involvement opportunities such as
school field trips for the planting of wetland plants,
and there are future plans to use the facility as a
nature interpretive center. Portland has six
additional stream diversion projects planned.
-------
COSTS
Cost comparisons for the wide range of inflow
reduction techniques are difficult to make without
consideration of site specific factors. The following
section discusses the range of costs representative
of inflow reduction programs. Findings are
summarized in Table 1:
Roof Drain Redirection
Some programs have offered rebates or incentives
of from $40 to $75 to homeowners and businesses
participating in voluntary redirection programs to
offset typical costs for redirection.
Basement Sump Pump Redirection
Typical costs for basement sump pump regulations
are $300 to $500 per home. Full rebates can be
used to encourage homeowners to participate.
Flow Restriction and Flow Slipping
Flow restricting orifice devices for catch basins are
priced from $500 to $1,200 each depending on the
size and number ordered. In contrast, simple steel
plates to cover catch basins can be fabricated and
installed for approximately $100 each. Detailed
engineering studies to evaluate the collection
system and catch basins require substantial in-house
expertise or consultant assistance.
Storm Water Infiltration Sumps
Costs are closely related to the type of soil, the
density of the sumps, and the desired amount of
inflow reduction. Depending upon site specific
conditions, total costs are expected to range from $2
to $8 per 1,000 gallons per year.
Stream Diversion
Costs for the Ramsey Lake Wetland Project
included $500,000 for purchasing the 29.75 acre
wetland area; $2.6 million for wetland planning and
construction costs; $ 10 million for sewer separation
and $3 million for new storm sewer trunklines; and
$1.3 million for sump disconnection. This wetland
drains 640 acres.
REFERENCES
1. City of Hartford, Connecticut, 1999.
Denise Horan, Hartford Metropolitan
District Commission, personal
communication with Parsons Engineering
Science, Inc.
2. Hides, S., 1997. "Quantity is the Key to
TABLE 1 COSTS OF INFLOW REDUCTION ACTIVITIES
Technology
Cost
Comments
Roof Drain Redirection
Basement Sump Pump Redirection
Flow Restriction and Flow/slipping
Storm Water Infiltration Sumps
Stream Diversion
Costs range from $45-$75 for
individual homeowners to disconnect
Disconnection costs approximately
$300-$500 per home
Flow restrictors can cost between
$500 and $1,200; covers for catch
basins to encourage flow slipping can
cost as little as $100
$2-$8 per 1,000 gallons inflow
removed
$17.4 million for 29.75 acre stream
diversion into a created wetland in
Portland, OR
Rebates for individual homeowners to
disconnect
Incentive programs usually used to
reimburse homeowners
Indirect additional costs may be
incurred because of additional water
pooling on surface streets
Costs are closely related to the type
of soil, the density of the sumps, and
the desired amount of inflow reduction
Costs included land acquisition, storm
sewer separation and new trunk lines,
and wetland design and construction.
Source: Compiled by Parsons Engineering Science, Inc., 1999.
-------
Improving Quality: a Common Sense Approach for
Reducing Wet Weather Impacts." Presented at the
26th Annual WEAO Technical Symposium and
OPCEA Exhibitions, London, Ontario, Canada.
3. McKelvie, S., 1996. Flow slipping: An
Effective Management Technique for Urban
Runoff. Parsons Brinckerhoff Quade &
Douglas, Inc. Proceedings from Urban Wet
Weather Pollution Conference, Quebec
City.
4. City of Portland, Oregon, 1994. CSO
Facilities Plan. Prepared by CH2M HILL.
5. City of Portland, Oregon, 1999. Lester Lee,
City of Portland Bureau of Environmental
Services, personal communication with
Parsons Engineering Science, Inc.
6. City of South Portland, Maine, 1999. Jay
Reynolds, City of South Portland
Engineering Department, personal
communication with Parsons Engineering
Science, Inc.
ADDITIONAL INFORMATION
9050 Gross Point
Skokie, IL 60077
City of South Portland, Maine
Jay Reynolds
City of South Portland Engineering Department
25 Cottage Road
South Portland, ME 04106
The mention of trade names or commercial
products does not constitute endorsement or
recommendation for the use by the U.S.
Environmental Protection Agency.
City of Hartford, Connecticut
Denise Horan
Metropolitan District Commission, Engineering and
Planning Department
555 Main Street, P.O. Box 800
Hartford, CT 06142-0800
City of Portland, Oregon
Lester Lee
City of Portland Bureau of Environmental Services
1211 Southwest 5th Avenue, Suite 800
Portland, OR 97204
City of St. Paul, Minnesota
Mike Kassan
Sewer Utility, Department of Public Works
1000 City Hall Annex
St. Paul, MN 55102
City of Skokie, Illinois
Eddie Nikai
Village of Skokie Department of Public Works
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
Washington, D.C. 20460
IMTB
Excellence fn complfance through optimal technfcal solutfons
MUNICIPAL TECHNOLOGY B R A N*
------- |