Appendix B Stormwater Management Design Criteria

Table B-1

Point Precipitation Frequency Estimates from NOAA Atlas 14

Reading WB City, Pennsylvania (36-7318) 40.3333° N, 75.9667° W, 298 feet

Source: “Precipitation-Frequency Atlas of the United States” NOAA Atlas 14, Volume 2, Version 3 G.M. Bonnin, D. Martin, B. Lin, T. Parzybok, M. Yekta, and D. Riley NOAA, National Weather Service, Silver Spring, Maryland, 2004 Extracted: Fri Jun 13 2008.

Precipitation Frequency Estimates (inches)
ARI* (years)	5 min	10 min	15 min	30 min	60 min	120 min	3 hr	6 hr	12 hr	24 hr	48 hr	4 day	7 day	10 day	20 day	30 day	45 day	60 day
1	0.34	0.53	0.66	0.90	1.12	1.36	1.49	1.87	2.29	2.65	3.08	3.43	4.03	4.62	6.24	7.77	9.80	11.76
2	0.40	0.63	0.79	1.09	1.36	1.65	1.81	2.25	2.77	3.19	3.72	4.13	4.83	5.51	7.40	9.15	11.50	13.77
5	0.47	0.74	0.94	1.32	1.69	2.05	2.26	2.81	3.46	4.00	4.68	5.14	5.96	6.71	8.81	10.65	13.21	15.70
10	0.52	0.82	1.03	1.49	1.93	2.37	2.60	3.25	4.04	4.68	5.46	5.97	6.89	7.67	9.93	11.82	14.50	17.13
25	0.57	0.90	1.14	1.68	2.22	2.79	3.08	3.89	4.89	5.67	6.57	7.16	8.23	9.03	11.44	13.36	16.14	18.95
50	0.61	0.96	1.22	1.81	2.45	3.14	3.46	4.41	5.61	6.50	7.49	8.14	9.34	10.12	12.63	14.54	17.36	20.28
100	0.65	1.02	1.29	1.95	2.68	3.49	3.85	4.97	6.39	7.41	8.49	9.18	10.51	11.26	13.83	15.70	18.52	21.52
200	0.68	1.07	1.35	2.08	2.90	3.86	4.26	5.57	7.25	8.40	9.55	10.29	11.78	12.45	15.05	16.86	19.64	22.70
500	0.72	1.13	1.42	2.23	3.18	4.37	4.83	6.42	8.52	9.84	11.07	11.88	13.58	1.116	16.71	18.37	21.03	24.15
1,000	0.75	1.17	1.46	2.34	3.39	4.77	5.28	7.12	9.59	11.04	12.33	13.18	15.06	15.45	17.99	19.50	22.04	25.19

 

* These precipitation frequency estimates are based on a partial duration series. ARI is the “average recurrence interval.”

Precipitation Intensity Estimates (inches/hour)
ARI* (years)	5 min	10 min	15 min	30 min	60 min	120 min	3 hr	6 hr	12 hr	24 hr	48 hr	4 day	7 day	10 day	20 day	30 day	45 day	60 day
1	4.02	3.19	2.65	1.81	1.12	0.68	0.50	0.31	0.19	0.11	0.06	0.04	0.02	0.02	0.01	0.01	0.01	0.01
2	4.78	3.80	3.17	2.18	1.36	0.82	0.60	0.38	0.23	0.13	0.08	0.04	0.03	0.02	0.02	0.01	0.01	0.01
5	5.59	4.45	3.74	2.64	1.69	1.02	0.75	0.47	0.29	0.17	0.10	0.05	0.04	0.03	0.02	0.01	0.01	0.01
10	6.20	4.91	4.13	2.97	1.93	1.18	0.87	0.54	0.34	0.20	0.11	0.06	0.04	0.03	0.02	0.02	0.01	0.01
25	6.86	5.42	4.56	3.35	2.22	1.40	1.03	0.65	0.41	0.24	0.14	0.07	0.05	0.04	0.02	0.02	0.01	0.01
50	7.32	5.77	4.86	3.63	2.45	1.57	1.15	0.74	0.47	0.27	0.16	0.08	0.06	0.04	0.03	0.02	0.02	0.01
100	7.78	6.12	5.15	3.90	2.68	1.75	1.28	0.83	0.53	0.31	0.18	0.10	0.06	0.05	0.03	0.02	0.02	0.01
200	8.21	6.43	5.39	4.15	2.90	1.93	1.42	0.93	0.60	0.35	0.20	0.11	0.07	0.05	0.03	0.02	0.02	0.02
500	8.66	6.77	5.67	4.46	3.18	2.18	1.61	1.07	0.71	0.41	0.23	0.12	0.08	0.06	0.03	0.03	0.02	0.02
1,000	8.99	7.00	5.85	4.67	3.39	2.38	1.76	1.19	0.80	0.46	0.26	0.14	0.09	0.06	0.04	0.03	0.02	0.02

 

* These precipitation frequency estimates are based on a partial duration series. ARI is the “average recurrence interval.”

Figure B-1
NRCS (SCS) Type II Rainfall Distribution – S Curve

Table B-2
Runoff Curve Numbers
(From NRCS (SCS) TR-55)

Land Use Description		Hydrologic Soil Group
		A		B	C	D
Open space		44	65	77	82
Meadow/orchard		30	58	71	78
Agricultural		59	71	79	83
Forest		36	60	73	79
Commercial	(85% impervious)	89	92	94	95
Industrial	(72% impervious)	81	88	91	93
Institutional	(50% impervious)	71	82	88	90
Residential
Average lot size	% impervious
1/8 acre or less*	65	77	85	90	92
1/8 – 1/3 acre	34	59	74	82	87
1/3 – 1 acre	23	53	69	80	85
1-4 acres	12	46	66	78	82
Farmstead		59	74	82	86
Smooth surfaces (concrete, asphalt, gravel or bare compacted soil)		98	98	98	98
Water		98	98	98	98
Mining/newly graded areas		77	86	91	94
(Pervious areas only)

* Includes multifamily housing unless justified lower density can be provided.

NOTE: Existing site conditions of bare earth or follow ground shall be considered as meadow when choosing a CN value.

Table B-3
Rational Runoff Coefficients

By Hydrologic Soils Group and Overland Slope (%)

Land Use	A			B			C			D
	0-2%	2-6%	6%+	0-2%	2-6%	6%+	0-2%	2-6%	6%+	0-2%	2-6%	6%+
Cultivated land	0.08a 0.14b	0.13 0.18	0.16 0.22	0.11 0.16	0.15 0.21	0.21 0.28	0.14 0.20	0.19 0.25	0.26 0.34	0.18 0.24	0.23 0.29	0.31 0.41
Pasture	0.12 0.15	0.20 0.25	0.30 0.37	0.18 0.23	0.28 0.34	0.37 0.45	0.24 0.30	0.34 0.42	0.44 0.52	0.30 0.37	0.40 0.50	0.50 0.62
Meadow	0.10 0.14	0.16 0.22	0.25 0.30	0.14 0.20	0.22 0.28	0.30 0.37	0.20 0.26	0.28 0.35	0.36 0.44	0.24 0.30	0.30 0.40	0.40 0.50
Forest	0.05 0.08	0.08 0.11	0.11 0.14	0.08 0.10	0.11 0.14	0.14 0.18	0.10 0.12	0.13 0.16	0.16 0.20	0.12 0.15	0.16 0.20	0.20 0.25
Residential
Lot size 1/8 acre	0.25 0.33	0.28 0.37	0.31 0.40	0.27 0.35	0.30 0.39	0.25 0.44	0.30 0.38	0.33 0.42	0.38 0.49	0.33 0.41	0.36 0.45	0.42 0.54
Lot size 1/4 acre	0.22 0.30	0.26 0.34	0.29 0.37	0.24 0.33	0.29 0.37	0.33 0.42	0.27 0.36	0.31 0.40	0.36 0.47	0.30 0.38	0.34 0.42	0.40 0.52
Lot size 1/3 acre	0.19 0.28	0.23 0.32	0.26 0.35	0.22 0.30	0.26 0.35	0.30 0.39	0.25 0.33	0.29 0.38	0.34 0.45	0.28 0.36	0.32 0.40	0.39 0.50
Lot size 1/2 acre	0.16 0.25	0.20 0.29	0.24 0.32	0.19 0.28	0.23 0.32	0.28 0.36	0.22 0.31	0.27 0.35	0.32 0.42	0.26 0.34	0.30 0.38	0.37 0.48
Lot size 1 acre	0.14 0.22	0.19 0.26	0.22 0.29	0.17 0.24	0.21 0.28	0.26 0.34	0.20 0.28	0.25 0.32	0.31 0.40	0.24 0.31	0.29 0.35	0.35 0.46
Industrial	0.67 0.85	0.68 0.85	0.68 0.86	0.68 0.85	0.68 0.86	0.69 0.86	0.68 0.86	0.69 0.86	0.69 0.87	0.69 0.86	0.69 0.86	0.70 0.88
Commercial	0.71 0.88	0.71 0.88	0.72 0.89	0.71 0.89	0.72 0.89	0.72 0.89	0.72 0.89	0.72 0.89	0.72 0.90	0.72 0.89	0.72 0.89	0.72 0.90
Streets	0.70 0.76	0.71 0.77	0.71 0.79	0.71 0.80	0.72 0.82	0.74 0.84	0.72 0.84	0.73 0.85	0.76 0.89	0.73 0.89	0.75 0.91	0.78 0.95
Open space	0.05 0.11	0.10 0.16	0.14 0.20	0.08 0.14	0.13 0.19	0.19 0.26	0.12 0.18	0.17 0.23	0.24 0.32	0.16 0.22	0.21 0.27	0.28 0.39
Parking	0.85 0.95	0.86 0.96	0.87 0.97	0.85 0.95	0.86 0.96	0.87 0.97	0.85 0.95	0.86 0.96	0.87 0.97	0.85 0.95	0.86 0.96	0.87 0.97

a Runoff coefficients for storm recurrence intervals less than 25 years.

b Runoff coefficients for storm recurrence intervals 25 years or more.

Source: Rawls, W.J., S.L. Wong and R.H. McCuen, 1981, “Comparison of Urban Flood Frequency Procedures,” Preliminary Draft, U.S. Department of Agriculture, Soil Conservation Service, Baltimore, MD.

Table B-4
Manning Roughness Coefficients
Roughness Coefficients (Manning’s “n”) for Overland Flow

(U.S. Army Corps of Engineers, HEC-1 User’s Manual)

Surface Description	N
Dense growth	0.4. – 0.5
Pasture	0.3 – 0.4
Lawns	0.2 – 0.3
Bluegrass sod	0.2 – 0.5
Short grass prairie	0.1 – 0.2
Sparse vegetation	0.05 – 0.13
Bare clay-loam soil (eroded)	0.01 – 0.03
Concrete/asphalt
Very shallow depths (less than 1/4 inch)	0.10 – 0.15
Small depths (1/4 inch to several inches)	0.05 – 0.10

Roughness Coefficients (Manning’s “n”) for Channel Flow

Reach Description	N
Natural stream, clean, straight, no rifts or pools	0.03
Natural stream, clean, winding, some pools or shoals	0.04
Natural stream, winding, pools, shoals, stony with some weeds	0.05
Natural stream, sluggish deep pools and weeds	0.07
Natural stream or swale, very weedy or with timber underbrush	0.10
Concrete pipe, culvert or channel	0.012
Corrugated metal pipe	0.012-0.027(1)
High-density polyethylene (HDPE) pipe
Corrugated	0.021-0.029(2)
Smooth-lined	0.012-0.020(2)

   NOTES:

(1) Depending upon type, coating and diameter.

(2) Values recommended by the American Concrete Pipe Association, check manufacturer’s recommended value.

Figure B-2
Recommendation Chart for Infiltration Stormwater Management BMPs in Carbonate Bedrock

 

Site Risk Factors	Geology Type	Carbonate Bedrock
	Effective Soil Thickness	Less than 2 Feet	2 to 4 Feet									Over 4 Feet to 8 Feet									Over 8 Feet
	Special Geologic Features*	Low/Medium/ High Buffer	Low Buffer			Medium Buffer			High Buffer			Low Buffer			Medium Buffer			High Buffer			Low Buffer			Medium Buffer			High Buffer
Site Investigation Recommended		(Unacceptable)	Preliminary			Preliminary			Preliminary			Preliminary			Preliminary			Preliminary			Preliminary			Preliminary			Preliminary
Design Factors	Infiltration Loading Rates (% increase)**	(Unacceptable)	0- 100 %	100- 300 %	300- 500 %	0- 100 %	100- 300%	300-500%	0-100%	100-300%	300- 500 %	0- 100 %	100- 300 %	300- 500 %	0- 100 %	100- 300 %	300- 500 %	0- 100 %	100- 300 %	300- 500 %	0- 100 %	100- 300 %	300- 500 %	0- 100 %	100- 300 %	300- 500 %	0- 100 %	100- 300 %	300- 500 %
Program Summary Ordinance***						1	1				1	2								1	2					1

 

Recommended

 

 

Not Recommended

 

*   Special geologic feature buffer widths are as follows: Low buffer is less than 50 feet.

Medium buffer is 50 feet to 100 feet.

High buffer is greater than 100 feet.

**   Rates greater than 500% not recommended.

*** Assumes adequately permeable soils and lack of natural constraints as required for all infiltration systems.

1 Infiltration systems may be allowed at the determination of the engineer and/or geologist, provided that a detailed site investigation is undertaken which confirms nature of rock, location of special geologic features, and adequacy of the buffer between the SGF and the proposed stormwater system(s).

2 In these special geologic features low buffer situations, infiltration systems may be allowed at the determination of the engineer and/or geologist, provided that a detailed site investigation is undertaken and a twenty- five-foot buffer from SGFs is maintained.

Source: Little Lehigh Creek Watershed ACT 167 – Stormwater Management Ordinance, May 2004.