APPENDIX A: TABLES AND FIGURES

   (A)   Table 1 - Urban Runoff Coefficients Urban Runoff Coefficients.

Type of Surface	Runoff Coefficient “C”
Asphalt	0.82
Concrete	0.85
Roof	0.85
Lawn (sandy)
Flat (0—2% Slope)	0.07
Rolling (2—7% Slope)	0.12
Steep (greater than 7%)	0.17
Lawns (Clay)
Flat (0—2% Slope)	0.16
Rolling (2—7% Slope)	0.21
Steep (greater than 7%)	0.3

 

      1.   The coefficients of this tabulation are applicable to storms of 5 to 10 year frequencies. Coefficients for less frequent higher intensity storms shall be modified as follows:

Return Period (yrs)	Multiply “C” by   (B)   Table 1A - Rural Runoff Coefficients.
Rural Runoff Coefficients
Type of Surface	Runoff Coefficient “C”
Woodland (Sandy)
Flat (0—5% Slope)	0.1
Rolling (5—10% Slope)	0.25
Steep (greater than 10%)	0.3
Woodland (Clay)
Flat (0—5% Slope)	0.3
Rolling (2—10% Slope)	0.35
Steep (greater than 10%)	0.5
Pasture (Sandy)
Flat	0.1
Rolling	0.16
Steep	0.22
Pasture (Clay)
Flat	0.3
Rolling	0.36
Steep	0.42
Cultivated (Sandy)
Flat	0.3
Rolling	0.4
Steep	0.52
Cultivated (Clay)
Flat	0.5
Rolling	0.6
Steep	0.72

 

1.   The coefficients of this tabulation are applicable to storms of 5 to 10 year frequencies. Coefficients for less frequent higher intensity storms shall be modified as follows:

Return Period (yrs)	Multiply “C” by   (C)   Table 2 Runoff Coefficients “C” by Land Use and Typical Inlet Times
Runoff Coefficients “C” By Land Use and Typical Inlet Times
Land Use	Runoff CoefficientsInlet Times (minutes)
	Flat	RollingSteep
Commercial (CBD)	0.750.830.915
Commercial (Neighborhood)	0.540.60.66
Industrial	0.630.70.77
Garden Apartments	0.540.60.6637750
Churches	0.540.60.66
Schools	0.310.350.39
Semi Detached Residential	0.450.50.5
Detached Residential	0.40.450.537908
Quarter Acre Lots	0.360.40.44
Half Acre Lots	0.310.350.39
Parkland	0.180.20.22To be Computed

 

      (1)   Flat terrain 0 2% slopes.

      (2)   Rolling terrain 2 7% slopes.

      (3)   Steep terrain greater than 7% slopes.

      (4)   Interpolation, extrapolation and adjustment for local conditions shall be based on engineering experience and judgment. Final calculations shall be based on the actual conditions that will exist after development.

      (5)   The coefficients of this tabulation are applicable to storms of 5 to 10 year frequencies. Coefficients for less frequent higher intensity storms shall be modified as follows:

Return Period (yrs)	Multiply “C” by   (D)   Table 3 Typical Values of Manning’s n.
Typical Values of Manning’s n
Material	Desirable Manning’s nMaximum Velocities
Closed Conduits
Concrete	0.01315 f.p.s.
Vitrified Clay	0.01315 f.p.s.
Brick	0.01515 f.p.s.
Cast Iron	0.01315 f.p.s.
PVC or PE Pipe	0.01115 f.p.s.
Circular Corrugated Metal Pipe, Annular Corrugations, 2-2/3 x 1/2 in.
Unpaved	0.0247 f.p.s.
25% Paved	0.0217 f.p.s.
50% Paved	0.0187 f.p.s.
100% Paved	0.0137 f.p.s.
Concrete Culverts	0.013
Open Channels
Concrete, Trowel Finish	0.013
Concrete, Broom or Float Finish	0.015
Gunite	0.018
Riprap Placed	0.03
Riprap Dumped	0.035
Gabion	0.028
New Earth (Uniform, Sodded, Clay)	0.025
Existing Earth
(Fairly Uniform, With Some Weeds)	0.03
Dense Growth of Weeds	0.04
Dense Weeds and Brush	0.04
Swale With Grass	0.035

 

   (E)   Table 4 Maximum Permissible Velocities in Vegetal Lined Channels.

Maximum Permissible Velocities in Vegetal Lined Channels
			Permissible Velocity
Cover	Slope	Erosion Resistant	Easily
Soils	Range	Soils	Eroded
	(percent)	(f.p.s.)	(f.p.s.)
Bermuda grass	37625	8	6
	over 10	6	4
Bahia
Buffalo grass
Kentucky Bluegrass	37625	7	5
Smooth Bromegrass	37750	6	4
Blue Grama	over 10	5	3
Grass Mixture	37625	5	4
Reed Canary grass	37750	4	3
Sericea Lespedeza
Weeping Lovegrass	37625	3.4	2.5
Yellow Bluestem
Redtop
Alfalfa
Red Fescue
Common Lespedeza		5	6
Sudangrass	37625	3.5	2.5

 

      (1)   From Soil Conservation Service, SCS TP 61, Handbook of Channel Design for Soil & Water Conservation.

      (2)   Use velocities exceeding 5 feet per second only where good covers and proper maintenance can be obtained.

      (3)   Do not use on slopes steeper than 10 percent except for vegetated side slopes in combination with a stone, concrete, or highly resistant vegetative center section.

      (4)   Do not use on slopes steeper than 5 percent except for vegetated side slopes in combination with a stone, concrete, or highly resistant vegetative center section. 5

      (5)   Annuals B use on mild slopes or as temporary protection until permanent covers are established.

      (6)   Use on slopes steeper than 5 percent is not recommend.

   (F)   Table 5 Rainfall Depths for Various Return Periods and Storm Durations.

Rainfall Depths For Various Return Periods and Storm Durations
Duration	Depth Return (inches) Period (years)
5 min.	0.38	0.48	0.55	0.69	0.81	0.93
10 min.	0.7	0.88	1.02	1.27	1.48	1.7
15 min.	0.85	1.08	1.25	1.56	1.81	2.09
30 min.	1.17	1.47	1.73	2.14	2.48	2.86
60 min.	1.49	1.88	2.2	2.72	3.15	3.63
2 hrs.	1.85	2.36	2.72	3.41	3.96	4.56
3 hrs.	2.02	2.56	2.96	3.71	4.29	4.95
6 hrs.	2.37	3	3.47	4.34	5.03	5.8
12 hrs.	2.75	3.48	4.02	5.04	5.84	6.72
18 hrs.	2.91	3.68	4.25	5.33	6.17	7.11
24 hrs.	3.16	4	4.62	5.79	6.71	7.73
48 hrs.	3.6	4.52	5.28	6.48	7.58	8.62
72 hrs.	3.92	4.92	5.75	7.05	8.23	9.4
5 days	4.48	.57	6.5	7.91	9.16	10.57
10 days	5.74	7.09	8.07	9.54	10.68	11.79

 

   (G)   Table 5A Rainfall Intensities for Various Return Periods and Storm Durations.

Rainfall Intensities For Various Return Periods and Storm Durations
Duration Intensities (inches/hour) Return Period (years)
5 min.	4.56	5.76	6.6	8.28	9.72	11.16
10 min.	4.2	5.28	6.12	7.62	8.88	10.2
15 min.	3.4	4.32	5	6.24	7.24	8.36
30 min.	2.34	2.94	3.46	4.28	4.96	5.72
60 min.	1.49	1.88	2.2	2.72	3.15	3.63
2 hrs.	0.92	1.18	1.36	1.7	1.98	2.28
3 hrs.	0.67	0.85	0.99	1.24	1.43	1.65
6 hrs.	0.4	0.5	0.58	0.72	0.84	0.97
12 hrs.	0.23	0.29	0.34	0.42	0.49	0.56
18 hrs.	0.16	0.2	0.24	0.3	0.34	0.4
48 hrs.	0.08	0.09	0.11	0.14	0.16	0.18
72 hrs.	0.05	0.07	0.08	0.1	0.11	0.13
5 days	0.04	0.05	0.05	0.07	0.08	0.09
10 days	0.02	0.03	0.03	0.04	0.04	0.05

 

   (H)   Table 6 Huff 50% Rainfall Distribution, Median Time Distributions of Heavy Storm Rainfall at a Point.

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