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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