CATCHMENT MODELLING USING SWMM

T. Lockie (Hydraulic Analysis Limited)

ABSTRACT Detailed hydrological and hydraulic modelling is undertaken for stormwater and wastewater catchments to assess network perfo rmance and develop remediation options. These studies are completed using a number of propriety software products; perhaps the most underrepresented modelling software in New Zealand is the US EPA’s open source and freeware Storm Water Management Model (SWWM 5.0). This paper outlines the technical capability and functionality of SWMM. In addition the advantages, limitations and applicability of SWMM are discussed and a brief comparison between SWMM and other common hydraulic models is presented.

KEYWORDS SWMM, Hydraulic Models, Wastewater Modelling, S tormwater Modelling, Stormwater Quality Modelling

1 INTRODUCTION

Detailed hydrological and hydraulic modelling is undertaken for stormwater and wastewater catchments to assess network performance and develop remediation options. These studies are completed using a number of propriety software products; perhaps the most underrepresented modelling software in NZ is the US EPA’s open source and freeware Storm Water Management Model (SWWM 5.0).

The USEPA Storm Water Management Model (SWMM) is a large complex software package capable of simulating the movement of precipitation and pollutants from the ground surface through pipe/channel networks and storage/treatment facilities to the receiving water. The model can be used to simulate a single event or a long continuous period ( Nix, 1994).

SWMM is arguably the most widely utilized model (of its type) in the world, and has been used in some form for over 30years (it was initially developed in 1971). To date the SWMM versions have been available as freeware and have been maintained by a variety of individuals and organizations. These earlier versions SWMM were comparatively user unf riendly compare to the proprietary models of the day, as they were mainly text input / output. This resulted in the development of a number SWMM Graphical User Interface (GUI) wrappers such as MIKESWMM, XP-SWMM etc.

Recently, within the last 5 years, the SWMM engine has been completing rewritten and is now maintained by the US EPA (this version is known as SWMM 5). Simultaneously a fully functional GUI was developed. Both the SWMM engine and GUI are freeware and open source.

This paper is split into three sections; Section 2.0 outlines the SWMM software functionality and capability. Section 3.0 discusses SWMM applications, advantages, limitations and associate propr iety SWMM GUI software wrappers. The final Section 4.0 compares SWMM with two other common catchment model MOUSE / MIKE URBAN and Infoworks CS.

2 SWMM CAPABILITY

2.1 HYDROLOGICAL MODEL DESCRIPTION 2.1.1 SURFACE RUNOFF The conceptual view of the SWMM runoff is relatively simple see Figure 2-1 for a summary, where each sub catchment surface is treated as a non-linear reservoir with a single inflow (rainfall). There are several outflows (discharges) including infiltration, evaporation and sur face runoff. The capacity of the reservoir is the maximum depression storage, which is surface storage provided by ponding, surface wetting and interception.

Surface runoff occurs only when the depth of water in the reservoir exceeds the maximum depression storage (USEPA SWMM, 2007).

2.1.2 INFILTRATION The water in storage is also being depleted by infiltration and evaporation. Infiltration occurs only if the ground surface is pervious. The infiltration process can be modeled by one of three different methods:

§ Horton’s Equation § Green-Ampt Equation § Curve Number Method

Figure 2-1: Non-linear Reservoir Representation of a sub catchment in SWMM (USEPA SWMM, 2007)

2.1.3 AQUIFERS Aquifers are sub-surface groundwater areas used to model the vertical movement of w ater infiltrating from the subcatchments that lie above them. They also permit the infiltration of groundwater into the drainage system, or exfiltration of surface water from the drainage system (USEPA SWMM, 2007).

2.1.4 RDII CURVES Rainfall-derived infiltration/inflow (RDII) uses Unit Hydrographs (UHs) to estimate flow into a sewer system. A UH set contains up to three such hydrographs, one for a short-term response, one for an intermediate-term response, and one for a long-term response (USEPA SWMM, 2007).

2.2 HYDRAULIC MODEL DESCRIPTION Flow routing in SWMM is governed by the conservation of mass and momentum equ ation for gradually varied, unsteady flow in one dimension (or as they are sometimes known the Saint Venant flow equ ations). SWMM offers three different levels of sophistication to solve these equations as follows:

2.2.1 STEADY FLOW ROUTING Steady Flow routing represents the simplest type of rou ting possible by assuming that within each computational time step flow is uniform and steady. Hence it simply translates inflow hydrographs at the upstream end of the conduit to the downstream end, with no delay or change in shape. The Manning equation is used to relate flow rate to flow area (or depth) (USEPA SWMM, 2007).

2.2.2 KINEMATIC WAVE ROUTING This routing method solves the continuity equation along with a simplified form of the momentum equation in each conduit. The latter requires that the slope of the water surface equal the slope of the conduit (USEPA SWMM, 2007).

2.2.3 DYNAMIC WAVE ROUTING Dynamic Wave routing solves the complete one-dimensional Saint Venant flow equations and therefore produces the most theoretically accurate results. These equations consist of the continuity and momentum equations for conduits and a volume continuity equation at nodes (USEPA SWMM, 2007).

2.3 WATER QUALITY MODEL DESCRIPTION 2.3.1 POLLUTANTS SWMM can simulate the generation, inflow and transp ort of any number of user-defined pollutants. Pollutant buildup and washoff from subcatchment areas are determined by the land uses assigned to those areas (USEPA SWMM, 2007).

2.3.2 LAND USES Land Uses are categories of development activities or land surface characteristics assigned to subcatchments. Land uses are used solely to account for spatial variation in pollutant buildup and washoff rates within subcatchments (USEPA SWMM, 2007).

The following processes can be defined for each land use category:

§ Pollutant Buildup § Pollutant Washoff § Street Cleaning

2.3.3 TREATMENT Removal of pollutants from the flow streams entering any drainage system node is modeled by assigning a set of user defined treatment functions to the node.

2.4 ADDITIONAL MODEL FEATURES 2.4.1 TIME SERIES DATA Time Series objects are used to describe how certain object properties vary with time. Time series can be used to describe: § temperature data § evaporation data § rainfall data § water stage at outfall nodes § external inflow hydrographs at drainage system nodes § external inflow pollutographs at drainage system nodes § control settings for pumps and flow regulators

2.4.2 CURVES Curve objects are used to describe a functional relationship between two quantities. The following types of curves are available in SWMM 5: § Storage - describes how the surface area of a Storage Unit node varies with water depth. § Shape - describes how the width of a customized cross-sectional shape varies with height for a Conduit

link. § Diversion - relates diverted outflow to total inflow for a Flow Divider node. § Tidal -describes how the stage at an Outfall node changes by hour of the day. § Pump - relates flow through a Pump link to the depth or volume at the upstream node or to the head

delivered by the pump. § Rating - relates flow through an Outlet link to the head difference across the outlet. § Control -determines how the control setting of a pump or flow regulator varies as a function of some

control variable (such as water level at a particular node) as specified in a Modulated Control rule. 2.4.3 REAL TIME CONTROL Real Time Controls can be set up to determine how pumps, orifices and weirs in the drainage system will be adjusted over the course of a simulation.

2.4.4 PUMP STATIONS Pumps are links used to lift water to higher elevations. A pump curve describes the relation between a pump's flow rate and conditions at its inlet and outlet nodes. Four different types of pumps are supported. 2.4.5 DRY WEATHER FLOW (DWF) SWMM 5 utilizes a Time Pattern to simulate DWF profiles which allow external dry weather flow (DWF) to vary in a periodic fashion. They consist of a set of adjustment factors applied as multipliers to a baseline DWF flow rate or pollutant concentration. 2.5 SWMM USER INTERFACE The EPA SWMM main window (Figure 2-2) is pictured below. It consists of the following user interface elements: a Main Menu, several Toolbars, a Status Bar, the Study Area Map window, a Browser panel, and a Property Editor window.

Figure 2-2: EPA SWMM Main Window (USEPA SWMM, 2007)

2.5.1 RESULT DISPLAY AND ANALYSIS Results can be displayed in the following ways:

§ Plan View (see Figure 2-2) § Hydraulic Profile (see Figure 2-3) § Time Series Plot (see Figure 2-4) § Scatter Graph (see Figure 2-5) § Tabular Data

In addition to the above a Statistics Report can be generated from the time series of simulation results. For a given object and variable this report will do the following: § Segregate the simulation period into a sequence of non-overlapping events. § Compute summary statistics for the entire set of event values (mean, standard dev iation and skewness), § Perform a frequency analysis on the set of event values.

Water Elevation Profile: Node OC0054 - SWOUT0091

01/02/2006 17:10:00

Distance (m)2602502402302202102001901801701601501401301201101009080706050403020100

OC

0054

OC

0055

OC

0729

OC

0730

OC

0058

SW

IN00

34

SW

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0330

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T009

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vatio

n (m

)

12

11

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9

8

7

6

5

4

3

2

1

0

-1

Figure 2-3: Hydraulic Profile

Link SWOUT0045_SWIN0032 Flow

Elapsed Time (hours)2520151050

Flow

(CM

S)

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Figure 2-4: Time Series Plot

Link SWSTOR003_SWIN0014 Flow v. Node SWSTOR003 Depth

Node SWSTOR003 Depth (m)10

Link

SW

STO

R00

3_S

WIN

0014

Flo

w (C

MS

)

0.08

0.06

0.04

0.02

0.0

-0.02

Figure 2-5: Scatter Graph

3 SWMM APPLICATIONS, LIMITATIONS AND ADVANTAGES

3.1 SWMM APPLICATIONS

SWMM has been used in thousands of wastewater and stormwater studies throughout the world. Typical applications include: § Design and sizing of drainage system components for flood contro l § Sizing of detention facilities and their appurtenances for f lood control and water quality protection § Flood plain mapping of natural channel systems (SWMM 5 is a FEMA-approved model for NFPI

studies) § Designing control strategies for minimizing combined sewer ov erflows § Evaluating the impact of inflow and infiltration on sanitary sewer overflows § Generating non-point source pollutant loadings for waste load allocation studies § Evaluating the effectiveness of BMPs for reducing wet weather pollutant loadings.

3.2 SWMM LIMITATIONS SWMM has a number of limitations, these include:

• GIS Linkages - No GIS linkages have to either rely on propriety GUI’s or develop in house GIS linkages.

• Numerical Instability – The SWMM hydraulic engine is slightly more unstable then other common hydraulic engines. However the engine is constantly undergoing upgrades which will gradually improve performance.

• Simulation Speed – The SWMM hydraulic engine is slightly slower then other common hydraulic engines. Again the engine is constantly undergoing upgrades which will gradually improve performance.

• Support – There is no formal support offered for EPA SWMM. A SWMM Users List serve, established by the University of Guelph, allows subscribers to ask questions and exchange information.

• NZ Experience – Only limited use of SWMM within NZ, hence there is limited experience with SWMM. 3.3 SWMM ADVANTAGES The perceived main advantages of the SWMM software for catchment analysis include:

• The hydraulic engine of SWMM has been proven and tested since the early 1970’s and is reported to be the most widely applied stormwater and wastewater model in North America. The number of alternative software packages available on the market that utilize the SWMM hydraulic model as their basis also evidences this. The SWMM software is maintained by the US EPA. Due to these factors SWMM is considered very unlikely to become out of date or superseded in the near to medium future.

• SWMM is a free downloadable software package which offers the flexibility to TLA’s to release their developed SWMM models to local land developers and consultants for independent use as required i.e. the developers and/or their consultants do not need to pur chase expensive software to complete their analyses.

• While SWMM has a more basic interface and fewer in-built data management capabilities compared to the other 3 packages assessed, these aspects are considered to be of f-set by the free-availability of the package and robust hydraulic performance.

• A SWMM model constructed using the US EPA user interface can be imported into a number more comprehensive user interfaces such as MIKE URBAN, XP-SWMM, and H20MAP SWMM etc. if more complex data management / result display features are required.

• SWMM is Open Source which has three main advantages: o Firstly the modeler / engineer can read the code see exactly how the software simulates a certain

process. o Being open source this allows the modelling community to contribute and develop the engine

and incorporate additional features, for example a specific BMP device functionality could be coded and added as required.

o Again being open source allows the code to be reviewed by a large number of people – far more than a proprietary code this reviewing community allows the rapid identification bugs.

3.4 PROPRIETARY SWMM GUI WRAPPERS There are a number of common catchment models that provide a GUI wrapper for the SWMM engine, these products include: § MIKEURBAN – Uses the SWMM 5.0 engine § H20MAP SWMM – Uses the SWMM 5.0 engine § MikeSWMM (freew are – uses the SWMM 4.4x engine) § XP SWMM (Utilizes a modified 4.4x engine) § PC SWMM

While Infoworks cannot be considered a SWMM wrapper as it utilizes its own propriety hydraulic and hydrological engine. However many of the catchment objects have a one to one relationship with SWMM, for example Infoworks implements the following SWMM process:

• The SWMM non-linear runoff model • Utilizes RDII curves • The SWMM aquifer object, • And implements the SWMM snowmelt model.

Hence, prov iding the same functionality are utilized in the Infowo rks model, conversion between the two models should b e relatively straightforward.

4 SWMM FUNCTIONALITY COMPARISON WITH OTHER COMMON CATCHMENT MODELS

The below Table 4-1 summarizes and compares current model functionality for SWMM, MOUSE / MIKE URBAN and Infoworks CS. As this table shows SWMM provides almost all of the functionality provided by the more the common propriety models, with the main weakness of SWMM being the more basic interface. Hence as this table shows all three models provide a similar level of technical performance, capability and reliability, with the main difference being the perfo rmance of the user interface and data management tools.

Table 4-1: Model Functionality Comparison

Model Topic Item

SWMM 5.0 MOUSE / MIKE URBAN INFOWORKS CS Flow Routing Dynamic Wave Dynamic Wave Dynamic Wave

Routing Engine Explicit numerical engine can have stability issues if the model not constructed and reviewed carefully.

Implicit numerical engine, a stable and fast hydraulic engine. Though it is considered to be a slightly slower engine than

InfoWorks.

Implicit numerical engine. Generally considered the fastest and most stable fully dynamic

engine.

Inlet Control No No Yes

Detention Storage Yes Yes Yes

RTC Yes Yes Yes

Pumps Yes Yes Yes

Hydraulics

Irregular XS Yes Yes Yes

Surface Runoff Utilizes a non-linear reservoir model to simulate runoff.

Provides a number of surface runoff models, such as a time area method and a Kinematic wave model (Non Linear

Reservoir Model). This model behaves exactly the same as the SWMM non-linear reservoir model

Provides a number of surface runoff models, including the SWMM non-linear reservoir

model.

Infiltration Provides three infiltration options, Cur ve Number, Horton’s Equation and Green Ampt.

In addition to the RDII model (see below) MOUSE utilizes the Horton’s Equation or SCS Cur ve Number to simulate

infiltration.

Fixed PR Model (simple percentage) Green Ampt Model

Horton Infiltration Model New UK PR Model

Wallingford Procedure Model Constant Infiltration Model

US SCS Model

RDII Provides either unit hydrographs to simulate RDII or a

groundwater infiltration module to simulate the influence of groundwater table on infiltration flow.

MOUSE employs a complex RDII model.

Provides either unit hydrographs to simulate RDII or a groundwater infi ltration module to

simulate the influence of groundwater table on infiltration flow. As per SWMM.

Hydrology

Continuous Simulation Yes Yes Yes

Pollutant Build Up / Washoff Yes Yes Yes

Pollutants Modeled Yes Yes Yes Water Quality

Treatment Yes Yes Yes

LTS - Job List No Yes - MOUSE provides a job list file which allows a selected number of events to be run by the HD model. No

Miscellaneous Statistics Yes Yes Yes

User Interface Basic user interface. Good user interface. Sophisticated user interface.

Data Management None Reasonable data management with the scenar io manager. Excellent data management.

Result Display Reasonable Good Excellent Use Ability

Support No formal support, A SWMM Users List server, allows subscribers to ask questions and exchange information. Comprehensive Comprehensive

Purchase Cost Free ~$15k to $40k dependant on pipe limitation and modules selected.

~$30k to $60k dependant on node limitation selected. Price

Maintenance Cost Free ~10% of the purchase price ~15% of the purchase price

5 CONCLUSIONS

SWMM 5.0 is a freeware and open source software maintained by the US EPA. SWMM is a proven and tested model since the early 1970’s where SWMM provides a similar level of technical performance, capability and reliability to the common propriety catchment models, with the main difference being the perfor mance of the user interface and data management tools. However this limitation can be overcome if required through model customization or the purchase / use of one of the number of SWMM wrapper user interfaces which provide additional functionality. The below Table 5-1 summarizes the SWMM 5.0 software pros an d cons.

Table 5-1: SWMM Use Pro’s and Con’s

Pro's Con's Freeware and Open Source No Support

Large user base Hydraulic engine is slightly slower and suffers from more instabilities the other common propri ety engines

Has been used in some form since 1971 Basic user interface, with limited data management tools.

Provides a similar level of technical performance,

capability and reliability of other common catchment

models.

Limited NZ experience

REFERENCES Nix, S. 1994. Urban Stormwater Modelling and Simulation , Boca Raton, Lewis Publishers.

USEPA SWMM. June 2007. Storm Water Management Model User’s Manual Version 5.0 , EPA/600/R-05/040

Websites: DHI Ltd: http://www.dhigroup.com/Software/Urban/MIKEURBAN.aspx

Wallingford Ltd: http://www.wallingfordsoftware.com/products/infoworks_cs/

SWWM: http://www.epa.gov/ednnrmrl/models/swmm/index.htm