This document describes the formulation of a SWMM input file to model stormwater runoff and water quality in an urban watershed. Key steps include: 1) Defining pollutants, land uses, and assigning land use percentages to subcatchments 2) Specifying buildup and washoff parameters for pollutant loading 3) Adding low impact development (LID) controls to certain subcatchments 4) Comparing model results for hydrographs and pollutographs with and without LIDs to analyze their impact on peak flows and pollutant loads.

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Seoul National University of Science and Technology
Professor Eun-Sung Chung

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1. Introduction

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1. Introduction

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2. Backgrounds of SWMM

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2. Backgrounds of SWMM

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2. Backgrounds of SWMM

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1 lbs = 0.4536kg
1 acre = 4,046.86 m2

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Before
After
S1
S2
S3
S4
S5
S7
S6
O1
RainGage

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 Check the result of Flooding: J8, and J9

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Length
(m)
Manning
N
Shape
Max.
Depth
(m)
Inlet
Offset
(m)
Initial
Flow
(cms)
Outlet
Offset
(m)
C1 50 0.05
CIRCLE
1 0 0 0
C2 50 0.016 1 0 0 0
C3 30 0.016 1 0 0 0
C4 40 0.05 1 0 0 0
C5 50 0.05 1 0 0 0
C6 40 0.05 1 0 0 0
C7 30 0.016 1 0 0 0
C8 50 0.05 1 0 0 0
C9 50 0.05 1 0 0 0
C10 40 0.05 1 0 0 0
C11 30 0.016 1 0 0 0

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Junction
Invert EL.
(m)
J1 520
J2 515
J3 518
J4 517
J5 516
J6 515
J7 516
J8 514
J9 513
J10 512
J11 509.5
O1 509

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 Study area description
• Result of node flooding
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3. Formulation of SWMM input file

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 Water quality
• Pollutant data can be entered through
“Pollutants” and “Land use” from the
side menu
• Click “pollutants” and click “add object”
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3. Formulation of SWMM input file

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 Pollutant editor
• Enter the name of the pollutant: TSS
• Select the units: mg/L
• Enter the value in “Rain Concen.”: 0.01
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3. Formulation of SWMM input file

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 Land use editor
• Select “Land Uses” from the side menu
• Click “Add Object”
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3. Formulation of SWMM input file

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 Land use editor
• “Land Use Editor” will appear
• Make three land use object named as
1) Residential_1
2) Residential_2
3) Commercial
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3. Formulation of SWMM input file

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 Land use editor
• Add “Buildup” data in the “Land Use
Editor” using the following table
• Select “EXP” for Function
• Select “CURB” for normalization
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Land Use
Max. Build up
(lb/curb-area)
Rate Constant
(1/day)
Residential_1 0.11 0.5
Residential_2 0.13 0.5
Commercial 0.15 0.2
3. Formulation of SWMM input file

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 Land use editing in sub-catchments
• Open the properties of one of the sub-
catchments and click “land uses”
• “Land Use Assignment” will appear
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3. Formulation of SWMM input file

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 Land use editing in sub-catchments
• Fill in the “% of Area” for the land
use from given table
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Sub-catchment Residential_1 Residential_2 Commercial
S1 100 0 0
S2 27 73 0
S3 27 32 0
S4 9 30 26
S5 0 0 98
S6 0 0 100
S7 0 0 50
3. Formulation of SWMM input file

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 Land use editing in sub-catchments
• Enter “Curb length” value for each sub-
catchment
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Sub-catchment Curb length
S1 510
S2 510
S3 305
S4 700
S5 800
S6 350
S7 180
3. Formulation of SWMM input file

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 Land use editor
• Add “Washoff” data in the “Land Use
Editor” using the following table
• Enter C1 for (washoff) “Coefficient” and
C2 for (washoff) “Exponent”
• Or enter EMC for the coefficient in the
right figure.
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3. Formulation of SWMM input file

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3. Formulation of SWMM input file

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 Hydrograph and Pollutograph at outfall using exponential functions for
buildup and washoff
4. Results (with no LID)

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 Status report in terms of water quality
• Runoff quality continuity shows runoff-quality balance over the entire
study area
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4. Results (with no LID)

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4. Results (with no LID)

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Subcatchment LID Process
Area of
Each Unit
(m2)
Numb
of Unit
Surface
Width
per Unit
% Initially
Saturated
% of
Imperv
Area
Treated
Return
all
outflow
to perv
S4 Bio-retention cell 4,750 1 0 0 10 O
S5 Permeable pavement 2,000 12 30 0 0.3 o
S5 Green roof 1,840 8 136 0 0.184 X
S6 Rain garden 820 4 20 0 1.5 X
S7 Infiltration trench 2,320 4 133 0 40 O
S7 Rain barrel 100 100 0 0 4 X
LID planning parameters for each structure
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4. Results (with LIDs)
 Compare the difference of hydrograph, maximum flow and maximum depth
at O1 with and without the above LIDs.

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 Hydrograph and Pollutograph at outfall using exponential functions for
buildup and washoff
4. Results (with LIDs)

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 Status report in terms of water quality
• Runoff quality continuity shows runoff-quality balance over the entire
study area
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4. Results (with LIDㄴ)

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5. Exercise
 Draw both hydrograph and TSS pollutograph at the outfall with and without LID.
 Compare the TSS washoff loadings for all sub-subwatersheds with and without LID.
 Compare the peak and total loadings of TSS at the outfall without and with LID using
Table.
 Draw a scatter plot between runoff and TSS loadings at the outfall.
 Repeat the above process using the following land use information
 S5: R1 5%, R2 55%, C 38%
 S6: R1 55%, R2 5%, C 40%
 S7: R1 30%, R2 30%, C 0%

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