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.

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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,

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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

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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

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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.

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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.

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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:

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          U.S. EPA
          Mail Code 4204
          401 M St., S.W.
          Washington, D.C. 20460
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