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Using SWMM LID Tools To Calibrate Existing Condition Wet Weather Response

Mark R. Delisio, P.E., CT Consultants, Inc.

Robert H. Greytak, P.E., CT Consultants, Inc.

Christopher P. Rybak, P.E., CT Consultants, Inc.

ABSTRACT

SWMM models are one of the tools used to simulate rainfall impacts on storm and sanitary

sewer systems. The model replicates a subcatchment’s stormwater runoff characteristics by

applying measured, calculated or estimated values for various parameters and calibrating the

model against measured sewer flow data. In urban areas the values are typically an average

representation of an overall area consisting of hundreds of individual residential lots each with

their own specific runoff characteristics. Stormwater that does not find its way to the storm

sewers may find its way to the sanitary sewers as I/I by the way of illicit connections, pipe

defects, or groundwater seepage through joints. There is not a direct method to route these

conditions to sanitary sewers in SWMM and so they are estimated through the use of RTK

values applied to a sewershed. This paper investigates utilizing SWMM LID tools as a method

for modeling a small residential sewershed, analyzing private property interaction between

sanitary and storm sewers, and evaluating alternatives for reducing I/I.

KEYWORDS: SWMM, LID Tools, Small Area Modeling, Model Calibration, I/I Reduction

INTRODUCTION

The development of SWMM models (EPA Storm Water Management Model, Ver. 5.1.009) to

analyze the interaction between storm and sanitary systems on residential property is complex

due to the high number of variables and uncertainties associated with each variable. Models

representing small residential areas must consider the individual parcel features that collect and

route rainfall to the public storm sewers with a portion of the rainfall routed to the sanitary sewer

by way of I/I (Infiltration and Inflow). The model should represent features such as:

Steep roofs with gutter, downspout, and lateral sewer routing systems.

Impervious driveways with area drains routed through connected piping.

Impervious driveways with surface routing.

Pervious rear and front yards with surface routing to pervious and impervious areas.

Impervious street flow routing.

Note that street drainage, though not technically part of a residential parcel, is a feature that

needs to be included in the analysis so that the study area output represents the entire study area

subcatchment.

444Copyright© 2017 by the Water Environment Federation

WEF Collection Systems Conference 2017

This paper explores how to develop a residential parcel-scale model using LID (Low Impact

Development) tools that allow evaluation of existing conditions, provides a valid mechanism for

calibration, and incorporates an efficient routing of the parcel features. Street drainage was not

represented by LID tools in the model but rather as individual subcatchments. The aggregate of

the parcel-scale model and street drainage output must be consistent with larger subcatchment

scale models encompassing the same sewershed.

Description Of The Study Area

The study area includes a neighborhood of 102 parcels in Lakewood, Ohio (Figure 1). A

common manhole on Eldred Avenue was isolated with sandbags for the study purposes.

Sandbags were also used to block a cross connection at the intersection of Delaware and Eldred

Avenues.

Figure 1: 102-parcel study area in Lakewood, Ohio showing sewer configuration.



The study area sewer construction consists of over/under (O/U) sewers with the storm sewer

directly above the sanitary using a single manhole to access both pipes. Figure 2 illustrates the

typical configuration of the O/U sewers. An invert plate in the “over” sewer allows access to the

“under” sewer. O/U sewers were built predominantly during the 1920s and 30s although O/U

sewers were installed until the 1950s.

Figure 2: Sketch of Over/Under Sewer Configuration

Private Property Investigations and Development Conditions

Houses within the study area were assumed to have been built using similar construction

methods. Each home was constructed with a storm lateral and a sanitary lateral (in a common

trench) connected to its respective public sewer at the street. The city led field investigations of

each of the individual 102 properties to evaluate if cross connections existed between the storm



drainage system and the sanitary laterals. Field investigation concluded that there may be cross

connections between the storm drainage system (downspouts, area drains, and foundation drains)

and the sanitary laterals as shown in Figure 3.

Figure 3: Private parcel connections based on field investigations.

Closed Circuit Televising (CCTV) investigations also revealed sanitary sewer laterals were in

disrepair with joint offsets, cracked pipe, and root intrusion. Some of the worst infiltration points

were noted to be near the foundation of the structure or at the point of connection to the sanitary

sewer at the street.



Pilot Study Flow Metering

Sanitary sewer flow metering was performed to estimate and document any I/I to provide a

benchmark for evaluation of the post construction flow.

Flow metering occurred from March 27, 2015 to May 2, 2015 and June 4, 2015 to June 29, 2015

and captured approximately 16 wet weather events of significance. The break in metering was

due to access issues in May. Representative flow meter data was selected from the observed data

set and is shown in Figure 4 and Figure 5. The June rain events were significantly larger than

those in March and May.

Figure 4: Representative Sample Storm (March to May)

Figure 5: Representative Sample Storm (June)

The meter data suggests that the flow in the sanitary sewer is impacted by Rain Derived I/I

(RDII). The question to be answered is how much of the RDII is emanating from private

property; how much from the public right of way; and what sanitary sewer flow reduction could

result from an aggressive I/I removal program on both private and public property.

Initial Model Development

The model was developed based upon the premise that the sanitary sewer system is isolated from

the storm sewer system and only the impacts to the sanitary sewer were to be evaluated. It was

assumed that any stormwater removed from the sanitary sewer would be routed to the storm



sewer. The model also needed to quantify the impacts from private property since it is the intent

of the city to remove all sources of I/I within the study area including those sources on private

property. The data from the flow meter in the sanitary sewer at the north end of Atkins Avenue is

to be used to calibrate the model.

METHODOLOGY

The general runoff characteristics typically used for modeling subcatchments were determined to

not be sensitive enough to evaluate the magnitude of I/I from each feature on a parcel. From the

flow meter data, I/I was shown to have a direct impact on sanitary sewer flow. It became

important to develop an enhanced model that evaluated the impact of I/I from each feature within

the parcel. A runoff hydrograph for each parcel was needed representing the impact of each

incorrectly connected or deficient feature. The parcel hydrograph would recognize the

contribution of roof drainage, foundation drains, area drains, and other features.

The study area was chosen in part because of the uniform parcel sizes, comparable size and age

of the houses, and similar parcel features. Each parcel consisted of a lot of roughly 0.045

hectares (0.11 acres) with a house, detached garage (with a few exceptions), driveway, sidewalk,

and lawn. (Figure 6)

Figure 6: Typical Parcel Features

Each parcel was reviewed in the field to identify its features and probable connectivity to one of

the two sewers. It was determined that there were 19 scenarios reflecting different combinations



of features and connectivity overlaying an individual parcel. Modeling of these scenarios could

be done in two different ways.

Modeling Method 1

In method 1, each parcel is subdivided into subcatchments representing features that replicate the

runoff characteristics of the parcel. The concept was to create a feature for each impervious

surface area (house roofs, garage roofs, driveways, and walkways) and a feature that represents

foundation drains. Separate pervious features would be created for the rear yard and front yard.

Each feature would outlet to the storm sewer unless they were found to outlet to the sanitary

lateral. The exception being if a downspout from the house roof or garage roof discharged to the

surface, then that feature would outlet to the pervious lawn feature. The study area consists of

102 parcels, and each parcel has between 3 and 6 features routing either to the storm sewer,

sanitary sewer, or another feature (Figure 7).

For the study area, tracking hundreds of individual subcatchments and routing them to the

observed outlet would be prohibitively time consuming and expensive. This method was

determined to be too costly to develop and manage for the study area.

Figure 7: Modeling Method 1



Modeling Method 2

In method 2, each parcel is segregated into LIDs representing the features of the parcel. The LID

tools in SWMM have controls that overlay the typical subcatchment with their own values.

This method considers every parcel a pervious lawn (Imperv % = 0) overlaid by various LIDs

representing impervious features. Each parcel has at least 3 impervious LIDs representing: (1)

the house roof; (2) the garage roof; and (3) the driveway plus any impervious walkways. This

allows the LIDs to represent existing conditions and routing as well as the results from work to

reduce I/I. For example, an LID can be used to represent a roof connected to a sanitary lateral via

a cross connected downspout, and the same LID can represent a downspout with a surface

discharge to the lawn merely by changing the values of the LID. Controlling where an LID

discharges replicates what portion of a feature may directly drain to another feature such as a

lawn, a driveway, a storm sewer, or a sanitary sewer (Figure 8).

An added benefit is that calibration of the model is simpler since a LID type is assigned to each

feature on the parcel initially and then calibrated by adjustments to the LID values rather than

redefining each feature.

Figure 8: Modeling Method 2



Model Development

Two of the 8 LIDs available in SWMM were chosen to represent the features on each lot:

Infiltration Trench

Rooftop Disconnection

These LIDs were selected not for their nomenclature but for the variables associated with them

and the ability to modify those variables to replicate a feature of the parcel. For example, using

an “Infiltration Trench” LID does not mean that there are infiltration trenches on the parcel, but

it represents the response associated with foundation drains and sanitary laterals that are in poor

condition. The Infiltration Trench LID variables can be adjusted such that their effect on the

system is the same as foundation drains and leaking sanitary sewer laterals.

Field investigations determined that each of the 102 parcels could be assigned a scenario that

represented the features and routing of the parcel. Multiple parcels that have common features

and routing could be assigned the same scenario. Other parcels could be assigned another

scenario representing other combinations. Table 1 identifies the profiles which reflect the routing

of rooftops.

Table 1: Rooftop LID Profiles

Name LID Type Routed To

1/2ConnectedGarageRoof Infiltration Trench Sanitary Manhole

ConnectedGarageRoof Infiltration Trench Sanitary Manhole

1/2ConnectedHouseRoof Infiltration Trench Sanitary Manhole

3/4ConnectedHouseRoof Infiltration Trench Sanitary Manhole

ConnectedHouseRoof Infiltration Trench Sanitary Manhole

1/2DisconnectedGarageRoof Rooftop Disconnection Pervious Yard

DisconnectedGarageRoof Rooftop Disconnection Pervious Yard

1/4DisconnectedHouseRoof Rooftop Disconnection Pervious Yard

1/2DisconnectedHouseRoof Rooftop Disconnection Pervious Yard

DisconnectedHouseRoof Rooftop Disconnection Pervious Yard

The term “Connected” identifies any feature cross connected to a sanitary lateral. The term

“Disconnected” refers to any feature that discharges to a pervious surface (lawn or garden) or

storm lateral. Figure 9 illustrates both types of downspout conditions.



Figure 9: Downspout Configurations

Driveways with area drains connected to the sanitary lateral assumed that only the rear portion of

the driveway (between the house and the garage) was draining to an inlet and the balance of the

driveway drained toward the street. Table 2 identifies profiles which reflect the routing of

driveways and walkways.

Table 2: Pavement LID Profiles

Name LID Type Routed To

1/2ConnectedDrive Infiltration Trench Sanitary Manhole

1/2DisconnectedDrive Rooftop Disconnection Storm Manhole

DisconnectedDrive Rooftop Disconnection Storm Manhole

Potential connected downspout

Disconnected

downspout



The Infiltration Trench LID has 11 adjustable parameters of which 6 were used in the model.

The Rooftop Disconnection LID has 4 adjustable parameters, of which 3 were used (Table 3).

Table 3: LID Parameters Used In The Model

Name Infiltration

Trench

Rooftop

Disconnection

Berm height

Vegetative cover

Surface roughness

Surface slope

Storage thickness (height)

Storage void ratio

Seepage rate

Storage clogging factor

Drain coefficient

Drain exponent

Drain offset height

Storage depth

Surface roughness

Surface slope

Flow capacity

Each parcel in the study area was assigned a combination of LID profiles that created a scenario

representing the parcel. Overall, 19 scenarios were needed to represent the combination of

features and routing on the 102 parcels. Table 4 is a summary of the scenarios created for the

model. Five scenarios represented over 70% of the properties. One of the five scenarios

(Scenario 7) represented about 30% of the parcels in the project area and was modeled using LID

profiles for 3 features; a connected house roof, a disconnected garage, and disconnected drives.



Table 4: Scenario Development

LID Profiles/Scenario 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

ConnectedHouseRoof X X X X X X X

3/4ConnectedHouseRoof X X X X

1/2ConnectedHouseRoof X X X X

ConnectedGarageRoof X X X X X

1/2ConnectedGarageRoof X X X X X X X X

1/2ConnectedDrive X X X X X X X X X X X

DisconnectedHouseRoof X X X X

1/2DisconnectedHouseRoof X X X X

1/4DisconnectedHouseRoof X X X X

DisconnectedGarageRoof X X X X X

1/2DisconnectedGarageRoof X X X X X X X X

1/2DisconnectedDrive X X X X X X X X X

DisconnectedDrive X X X X X X X X X



MODEL CALIBRATION

The flow metering showed that there was significant wet weather response in the project

indicating I/I in the sanitary sewers. The hydrograph of the wet weather flow was typical of

systems that had inflow sources as well as infiltration sources. Dye testing performed in the

project area indicated that most downspouts appeared to have some sort of connection to the

sanitary sewers (direct or indirect). Most foundation drains were assumed to be connected to the

sanitary sewers although dye testing was mostly inconclusive.

During the televising investigation of the sanitary laterals, it was determined that laterals were

generally in disrepair with evidence of settling, root intrusion, and debris. It was assumed that the

adjacent storm laterals were in similar or worse condition than the sanitary laterals. It was also

assumed that debris, such as leaves and sediment, had built up in the storm lateral over time. If

enough debris builds up in the storm lateral then capacity is lost. As capacity is lost, the private

property storm lateral collections system begins to store wet weather volumes that can migrate to

the sanitary lateral and/or the cross connected foundation drains.

Impervious surfaces within a parcel were initially modeled as a Rooftop Disconnection LIDs

routed to the sanitary sewer. A limit of 500 mm/hr (20 in/hr) was set for the rooftop

disconnection LID flow capacity parameter. This assured that all rainfall to the rooftop was

routed to the sanitary lateral, rather than spilling onto the pervious area.

For the purpose of comparison, RTK unit hydrograph components are used as reference to

describe the short, medium, and long term components of a LID’s response. The aggregate

response at the flow meter from all 102 parcels modeled in this way showed an RTK hydrograph

with too high of a peak flow (R1) and with a falling leg that was too steep (K1) (Figure 10).

Figure 10: RTK Hydrograph



Analyzing the existing metered hydrograph, it appeared that there was some storage in the

sanitary sewers within each parcel. A hypothesis was developed that many of the connected

impervious surfaces had inherent storage within conveyances such as clogged gutters, clogged

downspouts, or blockage in the laterals. The Rooftop Disconnection LID does not allow

sufficient storage release control. Therefore, the approach was changed to modeling of the

connected impervious surfaces within the parcels as an Infiltration Trench LID.

Using primarily the Infiltration Trench Drain Coefficient, Drain Exponent and the Storage

Thickness (height) parameters, the values were adjusted to develop a model hydrograph that

resembled the metered hydrograph. Adjusting the Drain Coefficient allowed control of the

outflow such that smaller events passed through the LID while larger events were limited and

began to store runoff volume in the LID. This shaped the short term hydrograph for the peak

flow, time to peak, and falling leg. The Drain Exponent determines the shape of the hydrograph

recession.

Adjusting the Storage Thickness controlled how long the LID would drain for after a rain event.

It became apparent that during larger or back to back events, the R2 of the storm responses

became similar and did not become larger with increased rainfall amounts. Therefore it was

understood that the inherent storage volume had a limit. This volume was controlled by the

Storage Thickness parameter. By limiting the LID Storage Thickness, the excess volume in the

LID overflows and is routed to the pervious area of the parcel. This shaped the medium term

hydrograph to resemble the sanitary sewer hydrograph (Figure 11). Storing too much volume in

the LID allowed more volume to drain for a longer period than in the metered hydrograph.

Storage Thickness calibration resulted in a hydrograph that was a much closer match to the

metered hydrograph.

Figure 11: Hydrograph With Rooftop Disconnection

& Infiltration Trench LIDs



Further adjustment to match the metered hydrograph required inclusion of an RTK long term

response (R3, T3, and K3) for saturated ground infiltration to the sanitary sewer and the storm

sewer (Figure 12).

Figure 12: Hydrograph Illustrating Rooftop Disconnection

& Infiltration Trench LIDS + R3T3K3

These adjustments created a modeled hydrograph that closely matched the metered hydrograph.

However, the adjustments were based on theory and not actual observations. To validate the

model, evidence was needed from the field that there were conditions that created storage on the

parcels. During property corrections undertaken by the City, it was found that almost all of the

downspouts were connected to the storm lateral, not the sanitary lateral as was assumed. It was

also found that the storm lateral was constructed with a “P” trap at the right of way line and that

the majority of traps were clogged with sediment and debris (Figure 13).

The clogged traps backed up storm flow and created storage around the foundation that slowly

migrated and seeped into defects in the sanitary lateral. Interviews with residents also confirmed

that in major storm events, the crock receiving the downspouts would back up and overflow onto

the ground. This overflow would find its way to the sanitary sewer by seepage along the

foundation; overland flow across the lawn with seepage into the sanitary sewer trench; and

overland flow to the storm sewer in the street where it could seep into the sanitary sewer below

it. Seepage was found to be rapid due to lenses that have developed over time in the clay soils

from the migration of flow from the storm lateral to the sanitary lateral.



Figure 13: Trap clogged with sediment

MODELING I/I REMOVAL

A residual benefit of calibrating with LIDs was the ease of making model adjustments to reflect

the effects of construction in ideal proposed conditions and actual post construction conditions.

In ideal proposed conditions, all homes associated with the 102-home pilot project would be

corrected, and removal of I/I would have been 100% successful at each property. To reflect this

in the model, any rooftop or driveway LIDs which were originally connected to the sanitary

system would be rerouted to the storm system, and the RTK would be reduced to small values

reflective of a tight system. The modeler has the luxury of splashing the downspouts (i.e. routing

the rooftop LIDs to the pervious area), or connecting downspouts to the storm lateral (i.e. routing

the rooftop LIDs to the storm system). This accurately portrays the relining of the sanitary lateral

and removal of illicit connections. Additionally, the flow capacity could be increased for the

rooftop LIDs modeled as Infiltration Trenches to represent the removal of the clogged “P” traps.

Actual post construction conditions will not likely achieve the same results as the ideal proposed

(100% correction & success) conditions. Some reasons for this could be:

Inability to get 100% participation in program (if it is not mandatory), leaving some homes

unaddressed.

Inability or failure to correct the entire property, leaving some downspouts or drains

connected to the sanitary lateral.



Partial improvement/partial completion success, meaning that the targeted results are

achieved prior to completion of the planned project and no additional construction is

necessary.

The model can be easily adjusted to reflect actual post construction conditions. If a parcel was

not addressed, that particular parcel’s LIDs could simply remain unchanged. Likewise, if

correction was found to be no longer needed, those parcels’ LIDs could remain unchanged.

For parcels where private property drainage and connections were re-piped, LIDs could be

completely rerouted and discharged to the storm system. For parcels where private property

drainage and connections were partially re-piped, the area of the rooftop for any particular LID

could be altered based upon contractor observation. For example, if only the front downspouts

which collect the front half of the roof were re-piped, the rooftop LID area could be reduced by

half. This conceptually leaves the back half of the roof as contributor to the existing Infiltration

Trench LID while the front re-piped half could be assigned to a new LID which represents the

front half of the roof being routed to the storm or to the pervious area (representing splashed

downspouts). These adjustments noted can be done using a whole scale approach so as to

perform a sensitivity study, or one parcel at a time to best replicate known conditions.

It is expected that some level of infiltration exists in all systems. The likelihood of completely

removing infiltration is low, and therefore some R3 component should remain in the model

loaded to the sanitary sewer. While a placeholder value of the modeler’s choice can be used to

complete the model, it is recommended that post construction data be analyzed to refine the R3

which remains after construction.

Once an understanding is formed regarding the different hydrograph components, the model

could also be used in a predictive sense to determine where I/I is coming from by associating

lessons learned regarding hydrograph components in completed project areas to newly obtained

flow data. For example, a very peaked response may be indicative of downspouts which are

connected to the sanitary. Alternatively, if a long drawn out tail exists, lateral relining may be

necessary. Lastly, if a bench like response is witnessed on larger and back to back events, it is

possible that some private property drainage system storage is present in the project area.

CONCLUSIONS

Utilization of LIDs for existing conditions modeling provides options not available when

utilizing a traditional subcatchment. In order to provide similar routing, control, and storage

options, a single parcel subcatchment would need to be subdivided into multiple subcatchments

with connection nodes, conduits, storage nodes, and control pipes. It is estimated that a

subdivided parcel would require an additional 6 to 9 of each subcatchment, each node, and each

conduit along with 1 storage node to control pipe outflow. This creates a large model that



becomes cumbersome and expensive to calibrate to existing conditions and to use for analysis of

sewer system improvements. LIDs provide efficient options due to their ability to route flows to

nodes outside the subcatchment, other subcatchments, and to the subcatchment’s pervious area.

The LID options exceed a subcatchment’s ability to drain portions of runoff between pervious

and impervious areas before draining to its outlet node.

The process of simulating sources of I/I using LIDs was very effective in matching the actual

metering data for the project area. The corrections instituted during the city’s sewer system

rehabilitation program were easy to apply in the model and provided a quantification of the

impact of the program on I/I.

The private property corrections will be expanded to other parts of the city and the development

of this LID modeling technique will allow the city to project the I/I removal for future projects.



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