#INFOSWMM

Default H&H Values from SWMM5


Default H&H Values from SWMM5

A.1 Units of Measurement

Parameter US Customary SI Metric
Area (Subcatchment) acres hectares45
Area (Storage Unit) square feet square meters
Area (Ponding) square feet square meters
Capillary Suction inches millimeters
Concentration mg/L (milligrams/liter) ug/L (micrograms/liter) Count/L (counts/liter) mg/L ug/L Count/L
Decay Constant (Infiltration) 1/hours 1/hours
Decay Constant (Pollutants) 1/days 1/days
Depression Storage inches millimeters
Depth feet meters
Diameter feet meters
Discharge Coefficient:

Orifice

dimensionless dimensionless
Weir CFS/footn CMS/metern
Elevation feet meters
Evaporation inches/day millimeters/day
Flow CFS (cubic feet / second) GPM (gallons / minute)

MGD (million gallons/day)

CMS (cubic meters/second)

LPS (liters/second)

MLD (million liters/day)

Head feet meters
Hydraulic Conductivity inches/hour millimeters/hour
Infiltration Rate inches/hour millimeters/hour
Length feet meters
Manning’s n seconds/meter1/3 seconds/meter1/3
Pollutant Buildup mass/length mass/acre mass/length mass/hectare
Rainfall Intensity inches/hour millimeters/hour
Rainfall Volume inches millimeters
Slope (Subcatchments) percent percent
Slope (Cross Section) rise/run rise/run
Street Cleaning Interval days days
Volume cubic feet cubic meters
Width feet meters

A.2 Soil Characteristics

Soil Texture Class K Ψ φ FC WP
Sand 4.74 1.93 0.437 0.062 0.024
Loamy Sand 1.18 2.40 0.437 0.105 0.047
Sandy Loam 0.43 4.33 0.453 0.190 0.085
Loam 0.13 3.50 0.463 0.232 0.116
Silt Loam 0.26 6.69 0.501 0.284 0.135
Sandy Clay Loam 0.06 8.66 0.398 0.244 0.136
Clay Loam 0.04 8.27 0.464 0.310 0.187
Silty Clay Loam 0.04 10.63 0.471 0.342 0.210
Sandy Clay 0.02 9.45 0.430 0.321 0.221
Silty Clay 0.02 11.42 0.479 0.371 0.251
Clay 0.01 12.60 0.475 0.378 0.265

K = saturated hydraulic conductivity, in/hr Ψ = suction head, in. φ= porosity, fraction FC = field capacity, fraction WP = wilting point, fraction

Source: Rawls, W.J. et al., (1983). J. Hyd. Engr., 109:1316.

Note: The following relation between Ψ and K can be derived from this table:

A.3 NRCS Hydrologic Soil Group Definitions

Saturated Hydraulic
Group Meaning Conductivity

(in/hr)

A Low runoff potential.

Water is transmitted freely through the soil. Group A soils typically have less than 10 percent clay and more than 90 percent sand or gravel and have gravel or sand textures.

> 1.42
B Moderately low runoff potential.

Water transmission through the soil is unimpeded. Group B soils typically have between 10 percent and 20 percent clay and 50 percent to 90 percent sand and have loamy sand or sandy loam textures.

0.57 – 1.42
Moderately high runoff potential.
C Water transmission through the soil is somewhat restricted. Group C soils typically have between 20 percent and 40 percent clay and less than 50 percent sand and have loam, silt loam, sandy clay loam, clay loam, and silty clay loam textures. 0.06 – 0.57
High runoff potential.
D Water movement through the soil is restricted or very restricted. Group D soils typically have greater than 40 percent clay, less than 50 percent sand, and have clayey textures. < 0.06

Source: Hydrology National Engineering Handbook, Chapter 7, Natural Resources Conservation Service, U.S. Department of Agriculture, January 2009.

A.4 SCS Curve Numbers1

Land Use Description Hydrologic Soil Group
A B C D
Cultivated land

Without conservation treatment

With conservation treatment

72

62

81

71

88

78

91

81

Pasture or range land Poor condition

Good condition

68

39

79

61

86

74

89

80

Meadow

Good condition

30 58 71 78
Wood or forest land

Thin stand, poor cover, no mulch

Good cover2

45

25

66

55

77

70

83

77

Open spaces, lawns, parks, golf courses, cemeteries, etc.

Good condition: grass cover on

75% or more of the area

39 61 74 80
Fair condition: grass cover on 50-75% of the area 49 69 79 84
Commercial and business areas (85% impervious) 89 92 94 95
Industrial districts (72% impervious) 81 88 91 93
Residential3

Average lot size (% Impervious4)

1/8 ac or less (65) 1/4 ac (38)

1/3 ac (30)

1/2 ac (25)

1 ac (20)

77

61

57

54

51

85

75

72

70

68

90

83

81

80

79

92

87

86

85

84

Paved parking lots, roofs, driveways, etc.5 98 98 98 98
Streets and roads

Paved with curbs and storm sewers5

Gravel

Dirt

98

76

72

98

85

82

98

89

87

98

91

89

Source: SCS Urban Hydrology for Small Watersheds, 2nd Ed., (TR-55), June 1986.

Footnotes:

1. Antecedent moisture condition II.

2. Good cover is protected from grazing and litter and brush cover soil.

3. Curve numbers are computed assuming that the runoff from the house and driveway is directed toward the street with a minimum of roof water directed to lawns where additional infiltration could occur.

4. The remaining pervious areas (lawn) are considered to be in good pasture condition for these curve numbers.

5. In some warmer climates of the country a curve number of 95 may be used.

A.5 Depression Storage

Impervious surfaces 0.05 – 0.10 inches
Lawns 0.10 – 0.20 inches
Pasture 0.20 inches
Forest litter 0.30 inches

Source: ASCE, (1992). Design & Construction of Urban Stormwater Management Systems, New York, NY.

A.6 Manning’s n – Overland Flow

Surface n
Smooth asphalt 0.011
Smooth concrete 0.012
Ordinary concrete lining 0.013
Good wood 0.014
Brick with cement mortar 0.014
Vitrified clay 0.015
Cast iron 0.015
Corrugated metal pipes 0.024
Cement rubble surface 0.024
Fallow soils (no residue) 0.05
Cultivated soils

Residue cover < 20%

0.06
Residue cover > 20% 0.17
Range (natural) 0.13
Grass

Short, prairie Dense

0.15

0.24

Bermuda grass 0.41
Woods

Light underbrush

Dense underbrush

0.40

0.80

Source: McCuen, R. et al. (1996), Hydrology, FHWA-SA96-067, Federal Highway Administration, Washington, DC

A.7 Manning’s n – Closed Conduits

Conduit Material Manning n
Asbestos-cement pipe 0.011 – 0.015
Brick 0.013 – 0.017
Cast iron pipe

– Cement-lined & seal coated

0.011 – 0.015
Concrete (monolithic) – Smooth forms 0.012 – 0.014
– Rough forms 0.015 – 0.017
Concrete pipe 0.011 – 0.015
Corrugated-metal pipe

(1/2-in. x 2-2/3-in. corrugations) – Plain

0.022 – 0.026
– Paved invert 0.018 – 0.022
– Spun asphalt lined 0.011 – 0.015
Plastic pipe (smooth) 0.011 – 0.015
Vitrified clay

– Pipes

– Liner plates

0.011 – 0.015

0.013 – 0.017

Source: ASCE (1982). Gravity Sanitary Sewer Design and Construction, ASCE Manual of Practice No. 60, New York, NY.

A.8 Manning’s n – Open Channels

Channel Type Manning n
Lined Channels
– Asphalt 0.013 – 0.017
– Brick 0.012 – 0.018
– Concrete 0.011 – 0.020
– Rubble or riprap 0.020 – 0.035
– Vegetal 0.030 – 0.40
Excavated or dredged
– Earth, straight and uniform 0.020 – 0.030
– Earth, winding, fairly uniform 0.025 – 0.040
– Rock 0.030 – 0.045
– Unmaintained 0.050 – 0.140
Natural channels (minor streams, top width at flood stage < 100 ft)
– Fairly regular section 0.030 – 0.070
– Irregular section with pools 0.040 – 0.100

Source: ASCE (1982). Gravity Sanitary Sewer Design and Construction, ASCE Manual of Practice No. 60, New York, NY.

A.9 Water Quality Characteristics of Urban Runoff

Constituent Event Mean Concentrations
TSS (mg/L) 180 – 548
BOD (mg/L) 12 – 19
COD (mg/L) 82 – 178
Total P (mg/L) 0.42 – 0.88
Soluble P (mg/L) 0.15 – 0.28
TKN (mg/L) 1.90 – 4.18
NO2/NO3-N (mg/L) 0.86 – 2.2
Total Cu (ug/L) 43 – 118
Total Pb (ug/L) 182 – 443
Total Zn (ug/L) 202 – 633

Source: U.S. Environmental Protection Agency. (1983). Results of the Nationwide Urban Runoff Program (NURP), Vol. 1, NTIS PB 84-185552), Water Planning Division, Washington, DC.

Source: http://nepis.epa.gov/Exe/ZyPDF

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