INTRODUCTION TO EPA SWMM 5.0

Rodrigo Concha Jopia FLUMEN Research Institute

Technical University of Catalonia UPC

Introduction: What is EPA SWMM 5.0?

EPA SWMM

Environmental Protection Agency Storm Water Management Model

SWMM is a distributed dynamic rainfall-runoff simulation model used for single event or long-term (continuous) simulation of runoff quantity and

quality from primarily urban areas

SWMM’s Process Models Precipitation

Snowmelt

Surface Runoff

Evaporation/ Infiltration

Groundwater

Overland Flow

Channel, Pipe & Storage Routing

Washoff

Sanitary Flows

RDII

Treatment / Diversion

Buildup

Key Hydrological Features

User-defined subcatchment areas Spatial Representation

Heat Balance/Degree Day Model Snowmelt

Localized Two-Zone Flux Model Groundwater

Nonlinear Reservoir Overland Flow

Horton Method Green-Ampt Method

SCS Method

Infiltration

User supplied Interception/Evaporation

User supplied Rainfall

Process In SWMM 5

Key Hydraulic Features

Overflow or Ponding Flooding

Rule-Based Controls Modulated Controls (including PID)

Variable Speed Gate Opening

Controls

Steady Flow Kinematic Wave (nonlinear form)

Dynamic Wave (semi-implicit)

Flow Routing

20 common shapes + irregular open channels + custom closed conduits

Conduit Shapes

Nodes (Junction, Storage, Outfall) Links (Conduits, Pumps, Regulators)

Drainage Elements

Key Water Quality Features

User-defined functions Drainage System Treatment

CSTR model Drainage System Routing

User-defined, Sanitary DWF, RDII inflow Non-Runoff Loads

User-assigned percent reduction BMP Removal

Rate proportional to runoff and buildup or can use an EMC

Pollutant Washoff

Power, exponential or saturation function of time Pollutant Buildup

Process In SWMM 5

Typical Applications of SWMM • Design and sizing of drainage system components

including detention facilities • Flood plain mapping of natural channel systems • Control of combined and sanitary sewer overflows • Generating non-point source pollutant loadings for waste

load allocation studies • Evaluating BMPs and LIDs for sustainability goals

SWMM version timeline

1971 - SWMM 1 (M&E, UF, WRE) 1975 - SWMM 2 (UF) 1981 - SWMM 3 (UF & CDM) 1983 - SWMM 3.3 (PC Version) 1988 - SWMM 4 (UF & CDM & OSU) 2004 – SWMM 5 (EPA & CDM)

∗ No sediment transport and erosion routines ∗ No pollutants routing in receiving waters and in the

sub-surface flow ∗ It is a hidrological-hydraulics analysis tool, not an

automatic design tool ∗ No direct linkage to GIS

SWMM 5.0 Limitations

Program structure

Example of .INP file

SWMM 5 Objects

∗ Visual Objects: elements that constitute the drainage system

∗ Non visual Objects: several data (tables, timeseries, etc.) neccesary in order to peform simulations

Conceptual modeling scheme used by SWMM 5

Atmosferic compartment Precipitation falls on the

Land Surface compartment Land Surface

compartment Important hydrological

process are modeled Rainfall losses Surface runoff

Conceptual modeling scheme used by SWMM 5.0

Groundwater compartment Receives infiltration from

Land Surface compartment Transport compartment Network of conveyance

elements: channels, pipes, manholes, etc.

Use of Nodes and Links in order to represent this network

Key parameters for subcatchment objetcs

Node – Link network representation(from Roesner et al.,1992)

Nodes

Links

Non visual object categories

Hydrology Climatology Aquifers Snow packs

Hydraulic Transects Unit Hydrographs Control Rules External Inflows

Water Quality Pollutants Land Uses Treatments

General Curves Time Series Time Patterns

Basic steps developing a new SWMM 5 project from scratch

Specify a set of options and common object properties (Measurement units, offsets, etc.) Draw a scheme of your catchment (or

network) using Visual Objects Edit the properties of Visual Objetcs that

make up your project Select a set of simulation options Run a simulation View the results of the simulation

Precipitation in SWMM 5.0: Rain Gage object

∗ Using a user-defined external datafile (Data File) ∗ Using a time series (Time Series): ∗ entering “by hand” both the rainfall and time values ∗ importing data from an external file ∗ Copying and pasting from a spreadsheet

Rainfall input data: two options

Rain Gage: mininal data requiered

∗ Rain Gage name ∗ Rain data format ∗ Time interval between each rain

data ∗ Way to feed Rain Gage with the

rain data: Timeseries or External File

20

Rain data format in SWMM 5

21

Rainfall losses in SWMM 5

∗ Three types of rainfall losses can be modeled: ∗ Evaporation ∗ Depression storage ∗ Infiltration

∗ All subcatchments contained in a project use the same infiltration model

∗ User should select these models according to his/her knowledge of the catchment (types of soils, land uses, measured data, etc.)

Evaporation and Depression storage

∗ Used at daily scale modeling (it is a slow process)

∗ Useful for continous modeling studies

∗ Not applied for a single storm event

It corresponds to a volume that must be fill prior to the ocurrence of any runoff

It represents initial abstractions such as surface ponding, interceptation by vegetation and surface wetting

Evaporation Depression storage

Infiltration in SWMM 5

∗ Process applied only on the pervious area of each subcatchment

∗ Three infiltration models

∗ User should select the model according to the degree of knowledge of the catchment

∗ While better it is the knowledge of the catchment, it is possible to use models of greater number of parameters

∗ Data input in each Subcatchment editor

∗ Empirical method ∗ Model of 3 parameters

∗ Drying Time: number of days for a fully saturated soil to dry completely

∗ Max Volume: Maximum infiltration volume possible

∗ Two last parameters are used in continuos modeling

Horton infiltration method in SWMM 5

∗ Physically-based method ∗ 3 parameters although the

last one is the difference between soil porosity and initial moisture content. So, 4 parameters are necessary

∗ G-A is not a popular method used in urban hydrology studies

Green–Ampt infiltration method in SWMM 5.0

Curve Number (CN) infiltration method in SWMM 5

∗ Derived from (but not the same as) the well-known SCS Curve Number method used in simplified runoff methods

∗ A derived equation from the classical SCS method is used:

where ∗ P, precipitation; R, potential runoff ; S, maximum soil

potential moisture retention, and CN, Curve number ∗ Total infiltration (F) can be computed as

2PRP S

=+

F P R= −

1000 10SCN

= −

∗ One parameter model: CN ∗ The parameter called

Conductivity is not used in computations anymore

∗ User should use tables to get CN values according to type of soil, land uses, etc.

Curve Number (CN) infiltration method in SWMM 5 (II)

Surface runoff model in SWMM 5

I(t): Inflows

O(t): Outflows

S: Storage volume

Q: Surface runoff

W: Subcatchment width

dp: Depression storage

d: Water depth

So: Subcatchment slope

n: Surface roughness coefficient

5 03

( ) ( )

( )p

dSI t O tdt

SQ W d d

n

− =

= ⋅ − ⋅

Each Subcatchment is treated as a Nonlinear Reservoir

H

ho

i (t)

H

ho

i (t)

H

ho

i (t)

i(t) = Rainfall – (Infiltration + Evaporation)

Surface runoff model in SWMM 5 (II)

Subcatchment width in SWMM 5

Subcatchment is conceptualized as a rectangular surface that has a uniform slope and a width W that drains to a single outlet channel

Initial estimate is given by

where, A: Subcatchment area Lfp: length of the longest overland flow path Maximum Lfp in rural areas: 150 m For urban catchments Lfp could be the

length from the back of a representative lot to the center of the street

If the overland flow length varies greatly within the subcatchment, then an area-weighted average should be used

W is often used as a calibration parameter due to it is not always easy to determine

Another way to determine W: subcatchment contribution width to the main closer conduit

𝑊𝑊 = 𝐴𝐴𝐿𝐿𝑓𝑓𝑓𝑓

Subcatchment width in SWMM 5

• DiGiano et al. (1976) • Subbasin=subcatchment

Hydraulic routing models used by SWMM 5 1. Steady Flow:

instantaneous traslation of a hydrograph from the upstream end of a conduit to the downstream end with no time delay or change in shape

2. Kinematic Wave: uniform unsteady flow, using the continuity equation and the normal flow condition

3. Dynamic Wave: this method solves the complete 1D Saint Venant equations for the entire conveyance network, allowing to the user simulate all gradually-varied flow conditions (backwater, surcharged flow and flooding)

0

0

SSxQ

tA

f =

=∂∂

+∂∂

0

0

2

=⋅⋅+⋅⋅+∂∂⋅⋅+

∂

∂

+∂∂

=∂∂

+∂∂

Lf hAgSAgxHAg

x

AQ

tQ

xQ

tA

0SS f =

Steady Flow model in SWMM 5

∗ Estimation just for some cases

∗ Steady Uniform flow ∗ Useful only to pre-design

the conveyance network, not to make the final design of the network

∗ Only applied to dentritic conveyance networks

Kinematic Wave flow model in SWMM 5

∗ Appropiate for steep slope conduits,

where there are supercritical flows ∗ It should not change the shape of the

hydrograph (if do, this is because of numerical reasons)

∗ It not take in account the downstream boundary conditions

∗ From a numerical point of view, more stable than Dynamic Wave method

Dynamic Flow model in SWMM 5

SWMM 5.0 uses a explicit finite difference numerical scheme in order to solve…

… the complete 1D the Saint Venant equations at each Conduit and…

… a continuity relationship at each Node

This model requires small time steps (Δt between 30 and 1 second usually)

Nodal flooding options in SWMM 5: Ponding Off and Ponding On

All excess inflow to node is lost from the system

All excess inflow to node is ponding on it. When adjacent conduits recover its conveyance capacity, then ponded volumen will be reintroduce to them

SWMM 5 web

http://www.epa.gov/nrmrl/wswrd/wq/models/swmm/

Information and uselful help (manuals, source codes, updates) for downloading

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