A.1 Units of Measurement

PARAMETER	US CUSTOMARY	SI METRIC
Area (Subcatchment)	acres	hectares
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), #/L (counts/liter)	mg/L, ug/L, #/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 Coefficient (n)	seconds/meter^(1/3)	seconds/meter^(1/3)
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

Soil Texture Class	K (in/hr)	\(\psi\) (in)	\(\phi\) (fraction)	FC (fraction)	WP (fraction)
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
\(\psi\) = suction head, in.
\(\phi\) = 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 \(\psi\) and K can be derived from this table:

\(\psi = 3.23 \, K^{-0.328} \quad (R^{2} = 0.9)\)

A.3 NRCS Hydrologic Soil Group Definitions

Group	Meaning	Saturated Hydraulic Conductivity (in/hr)
A	Low runoff potential. Water is transmitted freely through the soil. Group A soils typically have less than 10% clay and more than 90% 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% and 20% clay and 50% to 90% sand and have loamy sand or sandy loam textures.	0.57 – 1.42
C	Moderately high runoff potential. Water transmission through the soil is somewhat restricted. Group C soils typically have between 20% and 40% clay and less than 50% sand and have loam, silt loam, sandy clay loam, clay loam, and silty clay loam textures.	0.06 – 0.57
D	High runoff potential. Water movement through the soil is restricted or very restricted. Group D soils typically have greater than 40% clay, less than 50% 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 Numbers

Land Use Description	Hydrologic Soil Group	A	B	C	D
Cultivated land
- Without conservation treatment		72	81	88	91
- With conservation treatment		62	71	78	81
Pasture or range land
- Poor condition		68	79	86	89
- Good condition		39	61	74	80
Meadow	- Good condition	30	58	71	78
Wood or forest land
- Thin stand, poor cover, no mulch		45	66	77	83
- Good cover²		25	55	70	77
Open spaces, lawns, parks, golf courses, cemeteries
- Good condition: grass cover on 75% or more of area		39	61	74	80
- Fair condition: grass cover on 50-75% of 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)		77	85	90	92
- 1/4 ac (38)		61	75	83	87
- 1/3 ac (30)		57	72	81	86
- 1/2 ac (25)		54	70	80	85
- 1 ac (20)		51	68	79	84
Paved parking lots, roofs, driveways, etc.⁵		98	98	98	98
Streets and roads
- Paved with curbs and storm sewers⁵		98	98	98	98
- Gravel		76	85	89	91
- Dirt		72	82	87	89

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.

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

A.5 Depression Storage

Surface Type	Depression Storage (inches)
Impervious Surfaces	0.05 - 0.10
Lawns	0.10 - 0.20
Pasture	0.20
Forest Litter	0.30

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

A.6 Manning’s Coefficient (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	0.15
- Dense	0.24
- Bermuda grass	0.41
Woods
- Light underbrush	0.40
- Dense underbrush	0.80

Source: McCuen, R. et al. (1996), Hydrology, FHWA-SA-96-067, Federal Highway Administration, Washington, DC

A.7 Manning’s Coefficient (n) – Closed Conduits

Conduit Material	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
- 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	0.011 - 0.015
- Liner plates	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 Coefficient (n) – Open Channels

Channel Type	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 Concentration
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.

A.10 Culvert Code Numbers

Culvert Type	Code	Description
Circular Concrete	1	Square edge with headwall
	2	Groove end with headwall
	3	Groove end projecting
Circular Corrugated Metal Pipe	4	Headwall
	5	Mitered to slope
	6	Projecting
Circular Pipe, Beveled Ring Entrance	7	45 deg. bevels
	8	33.7 deg. bevels
Rectangular Box; Flared Wingwalls	9	30-75 deg. wingwall flares
	10	90 or 15 deg. wingwall flares
	11	0 deg. wingwall flares (straight sides)
Rectangular Box; Flared Wingwalls and Top Edge Bevel	12	45 deg flare; 0.43D top edge bevel
	13	18-33.7 deg. flare; 0.083D top edge bevel
Rectangular Box, 90-deg Headwall, Chamfered / Beveled Inlet Edges	14	Chamfered 3/4-in.
	15	Beveled 1/2-in/ft at 45 deg (1:1)
	16	Beveled 1-in/ft at 33.7 deg (1:1.5)
Rectangular Box, Skewed Headwall, Chamfered / Beveled Inlet Edges	17	3/4" chamfered edge, 45 deg skewed headwall
	18	3/4" chamfered edge, 30 deg skewed headwall
	19	3/4" chamfered edge, 15 deg skewed headwall
	20	45 deg beveled edge, 10-45 deg skewed headwall
Rectangular Box, Non-offset Flared Wingwalls, 3/4" Chamfer at Top of Inlet	21	45 deg (1:1) wingwall flare
	22	8.4 deg (3:1) wingwall flare
	23	18.4 deg (3:1) wingwall flare, 30 deg inlet skew
Rectangular Box, Offset Flared Wingwalls, Beveled Edge at Inlet Top	24	45 deg (1:1) flare, 0.042D top edge bevel
	25	33.7 deg (1.5:1) flare, 0.083D top edge bevel
	26	18.4 deg (3:1) flare, 0.083D top edge bevel
Corrugated Metal Box	27	90 deg headwall
	28	Thick wall projecting
	29	Thin wall projecting
Horizontal Ellipse Concrete	30	Square edge with headwall
	31	Grooved end with headwall
	32	Grooved end projecting
Vertical Ellipse Concrete	33	Square edge with headwall
	34	Grooved end with headwall
	35	Grooved end projecting
Pipe Arch, 18" Corner Radius, Corrugated Metal	36	90 deg headwall
	37	Mitered to slope
	38	Projecting
Pipe Arch, 18" Corner Radius, Corrugated Metal	39	Projecting
	40	No bevels
	41	33.7 deg bevels
Pipe Arch, 31" Corner Radius, Corrugated Metal	42	Projecting
	43	No bevels
	44	33.7 deg. bevels
Arch, Corrugated Metal	45	90 deg headwall
	46	Mitered to slope
	47	Thin wall projecting
Circular Culvert	48	Smooth tapered inlet throat
	49	Rough tapered inlet throat
Elliptical Inlet Face	50	Tapered inlet, beveled edges
	51	Tapered inlet, square edges
	52	Tapered inlet, thin edge projecting
Rectangular	53	Tapered inlet throat
Rectangular Concrete	54	Side tapered, less favorable edges
	55	Side tapered, more favorable edges
	56	Slope tapered, less favorable edges
	57	Slope tapered, more favorable edges

A.11 Culvert Entrance Loss Coefficients

Type of Structure and Design of Entrance	Coefficient
Pipe, Concrete
Projecting from fill, socket end (groove-end)	0.2
Projecting from fill, square cut end	0.5
Headwall or headwall and wingwalls:
- Socket end of pipe (groove-end)	0.2
- Square-edge	0.5
- Rounded (radius = D/12)	0.2
- Mitered to conform to fill slope	0.7
- End-Section conforming to fill slope	0.5
- Beveled edges, 33.7° or 45° bevels	0.2
- Side- or slope-tapered inlet	0.2
Pipe or Pipe-Arch, Corrugated Metal
Projecting from fill (no headwall)	0.9
Headwall or headwall and wingwalls, square-edge	0.5
Mitered to conform to fill slope, paved or unpaved slope	0.7
End-Section conforming to fill slope	0.5
Beveled edges, 33.7° or 45° bevels	0.2
Side- or slope-tapered inlet	0.2
Box, Reinforced Concrete
Headwall parallel to embankment (no wingwalls):
- Square-edged on 3 edges	0.5
- Rounded on 3 edges to radius of D/12 or B/12, or beveled edges	0.2
Wingwalls at 30° to 75° to barrel:
- Square-edged at crown	0.4
- Crown edge rounded to radius of D/12, or beveled top edge	0.2
Wingwalls at 10° to 25° to barrel:
- Square-edged at crown	0.5
Wingwalls parallel (extension of sides):
- Square-edged at crown	0.7
Side- or slope-tapered inlet	0.2

Note:
"End Sections conforming to fill slope," made of either metal or concrete, are the sections commonly available from manufacturers. From limited hydraulic tests, they are equivalent in operation to a headwall in both inlet and outlet control. Some end sections, incorporating a closed taper in their design, have superior hydraulic performance. These latter sections can be designed using the information given for the beveled inlet.

Source: Federal Highway Administration (2005). Hydraulic Design of Highway Culverts, Publication No. FHWA-NHI-01-020.

A.12 Standard Elliptical Pipe Sizes

Code	Minor Axis (in)	Major Axis (in)	Minor Axis (mm)	Major Axis (mm)
1	14	23	356	584
2	19	30	483	762
3	22	34	559	864
4	24	38	610	965
5	27	42	686	1067
6	29	45	737	1143
7	32	49	813	1245
8	34	53	864	1346
9	38	60	965	1524
10	43	68	1092	1727
11	48	76	1219	1930
12	53	83	1346	2108
13	58	91	1473	2311
14	63	98	1600	2489
15	68	106	1727	2692
16	72	113	1829	2870
17	77	121	1956	3073
18	82	128	2083	3251
19	87	136	2210	3454
20	92	143	2337	3632
21	97	151	2464	3835
22	106	166	2692	4216
23	116	180	2946	4572

Note: The Minor Axis is the maximum width for a vertical ellipse and the full depth for a horizontal ellipse, while the Major Axis is the maximum width for a horizontal ellipse and the full depth for a vertical ellipse.

Source: Concrete Pipe Design Manual, American Concrete Pipe Association, 2011 (www.concrete-pipe.org).

A.13 Standard Arch Pipe Sizes

Concrete Arch Pipes

Code	Rise (in)	Span (in)	Rise (mm)	Span (mm)
1	11	18	279	457
2	13.5	22	343	559
3	15.5	26	394	660
4	18	28.5	457	724
5	22.5	36.25	572	921
6	26.625	43.75	676	1111
7	31.3125	51.125	795	1299
8	36	58.5	914	1486
9	40	65	1016	1651
10	45	73	1143	1854
11	54	88	1372	2235
12	62	102	1575	2591
13	72	115	1829	2921
14	77.5	122	1969	3099
15	87.125	138	2213	3505
16	96.875	154	2461	3912
17	106.5	168.75	2705	4286

Corrugated Steel, 2-2/3 x 1/2" Corrugation

Code	Rise (in)	Span (in)	Rise (mm)	Span (mm)
18	13	17	330	432
19	15	21	381	533
20	18	24	457	610
21	20	28	508	711
22	24	35	610	889
23	29	42	737	1067
24	33	49	838	1245
25	38	57	965	1448
26	43	64	1092	1626
27	47	71	1194	1803
28	52	77	1321	1956
29	57	83	1448	2108

Corrugated Steel, 3 x 1" Corrugation

Code	Rise (in)	Span (in)	Rise (mm)	Span (mm)
30	31	40	787	1016
31	36	46	914	1168
32	41	53	1041	1346
33	46	60	1168	1524
34	51	66	1295	1676
35	55	73	1397	1854
36	59	81	1499	2057
37	63	87	1600	2210
38	67	95	1702	2413
39	71	103	1803	2616
40	75	112	1905	2845
41	79	117	2007	2972
42	83	128	2108	3251
43	87	137	2210	3480
44	91	142	2311	3607

Structural Plate, 18" Corner Radius

Code	Rise (in)	Span (in)	Rise (mm)	Span (mm)
45	55	73	1397	1854
46	57	76	1448	1930
47	59	81	1499	2057
48	61	84	1549	2134
49	63	87	1600	2210
50	65	92	1651	2337
51	67	95	1702	2413
52	69	98	1753	2489
53	71	103	1803	2616
54	73	106	1854	2692
55	75	112	1905	2845
56	77	114	1956	2896
57	79	117	2007	2972
58	81	123	2057	3124
59	83	128	2108	3251
60	85	131	2159	3327
61	87	137	2210	3480
62	89	139	2261	3531
63	91	142	2311	3607
64	93	148	2362	3759
65	95	150	2413	3810
66	97	152	2464	3861
67	100	154	2540	3912
68	101	161	2565	4089
69	103	167	2616	4242
70	105	169	2667	4293
71	107	171	2718	4343
72	109	178	2769	4521
73	111	184	2819	4674
74	113	186	2870	4724
75	115	188	2921	4775
76	118	190	2997	4826
77	119	197	3023	5004
78	121	199	3073	5055

Structural Plate, 31" Corner Radius

Code	Rise (in)	Span (in)	Rise (mm)	Span (mm)
79	112	159	2845	4039
80	114	162	2896	4115
81	116	168	2946	4267
82	118	170	2997	4318
83	120	173	3048	4394
84	122	179	3099	4547
85	124	184	3150	4674
86	126	187	3200	4750
87	128	190	3251	4826
88	130	195	3302	4953
89	132	198	3353	5029
90	134	204	3404	5182
91	136	206	3454	5232
92	138	209	3505	5309
93	140	215	3556	5461
94	142	217	3607	5512
95	144	223	3658	5664
96	146	225	3708	5715
97	148	231	3759	5867
98	150	234	3810	5944
99	152	236	3861	5994
100	154	239	3912	6071
101	156	245	3962	6223
102	158	247	4013	6274

Source: Modern Sewer Design (Fourth Edition), American Iron and Steel Institute, Washington, DC, 1999.

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