§ 26-77. Method for stormwater design requirements.

(a)

Except when the Rational Method has been approved by the Erosion Control Administrator pursuant to Section 26-47(4), the requirements in this Section are the minimum standard methods and procedures to be used to comply with the stormwater design requirements of the Village. If an owner or his agent determines that different methods are necessary based on site-specific conditions, they must request approval from the Erosion Control Administrator (as defined in Section 26-21) to use other methods prior to submittal.

(b)

The design methods listed in this Subsection are readily available in a number of computer programs, including the Soil Conservation Service's TR 20 (SCS) and HIC - HMS (U.S. Army Corps of Engineers). Additionally, a simplified methodology which is based on the use of these methods is available in TR 55 (SCS, 1986). TR 55 can be applied using either manual computations or a computerized version.

(1)

Rainfall depth and intensity data. Use data for the County (Illinois State Water Survey, BUL-70/89, 1989) as presented in Table 1 and graphically in Figure 1 of this Section.

(2)

Storm event rainfall runoff. Use the SCS Runoff Curve Number Method to determine rainfall runoff depth. See Figure 2-1 and Tables 2-2a through 2-2c from TR 55. Soil type information is available from the SCS Peoria County Soil Survey, 1992.

(3)

Storm distribution (cumulative rainfall versus time). Use the SCS Type II storm distribution. See Table 3 and Figure 3.

(4)

Runoff hydrograph. Use the SCS dimensionless hydrograph. See SCS (1974) for information regarding this procedure. As a substitute for detailed hydrograph analysis, TR 55 (SCS, 1986) can be used, either manually or by computer program.

(5)

Storage routing (detention pond analysis). Use the continuity equation, also known as the Modified Puls and Storage Indication Methods. As a substitute for detailed storage routing of a hydrograph, TR 55 (SCS, 1986) (or TR 20) can be used, either manually or by computer program. If TR 55 is used and a detention basin with a two-stage outlet control structure including a rectangular weir and/or orifice outlet is included as a part of the control measures, use the detention basin outlet design worksheet to determine and present the structure design information.

_____

TABLE 1. RAINFALL DEPTH-DURATION-FREQUENCY DATA
FOR PEORIA COUNTY

	Rainfall Depth for Given Frequency (in inches)
Duration	2-Yr	5-Yr	10-Yr	25-Yr	50-Yr	100-Yr
5-min.	0.36	0.45	0.53	0.64	0.73	0.83
10-min.	0.66	0.83	0.98	1.17	1.34	1.52
15-min.	0.81	1.02	1.20	1.44	1.64	1.87
30-min.	1.12	1.39	1.64	1.97	2.25	2.56
1-hour	1.42	1.77	2.09	2.50	2.86	3.25
2-hour	1.78	2.22	2.62	3.14	3.59	4.08
3-hour	1.93	2.41	2.85	3.41	3.89	4.43
6-hour	2.26	2.82	3.33	3.99	4.56	5.19
12-hour	2.62	3.27	3.87	4.63	5.29	6.02
18-hour	2.75	3.46	4.09	4.90	5.59	6.37
24-hour	3.02	3.76	4.45	5.32	6.08	6.92
48-hour	3.38	4.19	4.86	5.78	6.62	7.51
72-hour	3.70	4.55	5.26	6.15	7.25	8.16
5-day	4.17	5.11	5.84	6.96	7.98	9.21
10-day	5.12	6.27	7.10	8.19	9.10	10.18

 

Source: ISWS/BUL-70/89

TABLE 2-2A. RUNOFF CURVE NUMBERS FOR URBAN AREAS

Cover Description					Curve Numbers for Hydrologic Soil Group
Cover Type and Hydrologic Condition				Average Percent Impervious Area	A	B	C	D
Fully developed urban areas (vegetation established):
	Open space (lawns, parks, golf courses, cemeteries, etc.):
		Poor condition (grass cover < 50%)			68	79	86	89
		Fair condition (grass cover 50% to 75%)			49	69	79	84
		Good condition (grass cover > 75%)			39	61	74	80
	Impervious areas:
		Paved parking lots, roofs, driveways, etc. (excluding right-of-way)			98	98	98	98
		Streets and roads:
			Paved; curbs and storm sewers (excluding right-of-way)		98	98	98	98
			Paved; open ditches (including right-of-way)		83	89	92	93
			Gravel (including right-of-way)		76	85	89	91
			Dirt (including right-of-way)		72	82	87	89
Urban districts:
	Commercial and business			85	89	92	94	95
	Industrial			72	81	88	91	93
Residential districts by average lot size:
	1/8 acre or less (townhouses)			65	77	85	90	92
	¼ acre			38	61	75	83	87
	1/3 acre			30	57	72	81	86
	½ acre			25	54	70	80	85
	1 acre			20	51	68	79	84
	2 acres			12	46	65	77	82
Developing urban areas:
	Newly graded areas (pervious areas only, no vegetation)				77	86	91	94

 

Note:

Average runoff condition and 1A = 0.2S.

The average percent impervious area shown was used to develop the composite CNs. Other assumptions are as follows: Impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. CNs for other combinations of conditions may be computed using Figure 2-3 or 2-4 in TR 55.

Source: SCS TR 55, 1986.

TABLE 2-2B. RUNOFF CURVE NUMBERS FOR AGRICULTURAL LANDS

Cover Description			Curve Numbers for Hydrologic Soil Group
Cover type	Treatment	Hydrologic Condition	A	B	C	D
Fallow	Bare soil	—	77	86	91	94
	Crop residue cover (CR)	Poor	76	85	90	93
		Good	74	83	88	90
Row crops	Straight row (SR)	Poor	72	81	88	91
		Good	67	78	85	89
	SR + CR	Poor	71	80	87	90
		Good	64	75	82	85
	Contoured (C)	Poor	70	79	84	88
		Good	65	75	82	86
	C + CR	Poor	69	78	83	87
		Good	64	74	81	85
	Contoured & terraced (C & T)	Poor	66	74	80	82
		Good	62	71	78	81
	C & T + CR	Poor	65	73	79	81
		Good	61	70	77	80
Small grain	SR	Poor	65	76	84	88
		Good	63	75	83	87
	SR + CR	Poor	64	75	83	86
		Good	60	72	80	84
	C	Poor	63	74	82	85
		Good	61	73	81	84
	C + CR	Poor	62	73	81	84
		Good	60	72	80	83
	C & T	Poor	61	72	79	82
		Good	59	70	78	81
	C & T + CR	Poor	60	71	78	81
		Good	58	69	77	80
Close-seeded or broadcast	SR	Poor to Good	66 to 58	77 to 72	85 to 81	89 to 85
Legumes or rotation	C	Poor to Good	64 to 55	75 to 69	83 to 78	85 to 83
Meadow	C & T	Poor to Good	63 to 51	73 to 67	80 to 76	83 to 80

 

Note:

Average runoff condition and 1a = 0.2s.

Crop residue cover applies only if residue is on at least five percent of the surface throughout the year.

Hydrologic condition is based on combination of factors that affect infiltration and runoff, including:

 a. Density and canopy of vegetative areas;

 b. Amount of year-round cover;

 c. Amount of grass or close-seeded legumes in rotations;

 d. Percent of residue cover on land surface (good ≥ 20 percent); and

 e. Degree of surface roughness.

Poor: Factors impair infiltration and tend to increase runoff.

Good: Factors encourage average and better than average infiltration and tend to decrease runoff.

Source: SCS TR 55, 1986.

TABLE 2-2C. RUNOFF CURVE NUMBERS FOR OTHER AGRICULTURAL LANDS

Cover Description		Curve Numbers for Hydrologic Soil Group
Cover type	Hydrologic Condition	A	B	C	D
Pasture, grassland, or range: continuous forage for grazing	Poor	68	79	86	89
	Fair	49	69	79	84
	Good	39	61	74	80
Meadow: continuous grass, protected from grazing and generally mowed for hay	Poor	30	58	71	78
Brush & brush-weed-grass mixture with brush the major element	Poor	48	67	77	83
	Fair	35	56	70	77
	Good	30	48	65	73
Woods & grass combination (orchard or tree farm)	Poor	57	73	82	86
	Fair	43	65	76	82
	Good	32	58	72	79
Woods	Poor	45	66	77	83
	Fair	36	60	73	79
	Good	30	55	70	77
Farmsteads: buildings, lanes, driveways, and surrounding lots	Poor	59	74	82	86

 

Note:

Average runoff condition and 1A = 0.2S.

Poor: < 50 percent ground cover or heavily grazed with no mulch.

  Fair: 50 to 75 percent ground cover and not heavily grazed.

  Good: > 75 percent ground cover and lightly or only occasionally grazed.

Poor: < 50 percent ground cover.

  Fair: 50 to 75 percent ground cover.

  Good: > 75 percent ground cover.

Actual curve number is less than 30; use CN = 30 for runoff computations.

CNs shown were computed for areas with 50 percent woods and 50 percent grass (pasture) cover. Other combinations of conditions may be computed from the CNs for woods and pasture.

Poor: Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning.

  Fair: Woods are grazed but not burned, and some forest litter covers the soil.

  Good: Woods are protected from grazing, and litter and brush adequately cover the soil.

Source: SCS TR 55, 1986.

DETENTION BASIN OUTLET DESIGN WORKSHEET

Project Name: ____________ Structure ID: ____________ Date: ____________ / ____________ / ____________

FIRST STAGE

Maximum stage for two-year storm (E2-yr) = ____________ ft

First stage control elevation (E1) = ____________ ft

Head on first stage structure (H2-yr = E2-yr - E1) ____________ ft

Allowable discharge for two-year storm (q2-yr) = ____________ cfs

For rectangular weir outlet:

Required weir length L1 = q2-yr/(3.2* H2-yr 1.5) ____________ /(3.2* ____________ 1.5)

= ____________ ft

For orifice outlet:

Required orifice area A1 = q2-yr/(4.98* H2-yr 0.5) = ____________ /(4.98* ____________ 0.5)

= ____________ sq ft

SECOND STAGE

Maximum stage for twenty-five-year storm (E25-yr) = ____________ ft

Second stage control elevation (E2) = ____________ ft

Head on second stage structure (H25-yr = E25-yr - E2) ____________ ft

Allowable discharge for twenty-five-year storm (q25-yr) = ____________ cfs

Twenty-five-year storm discharge through first stage:

Weir: q' = 3.2* L1 * (E25-yr - E1)1.5 = 3.2* ____________ * ( ____________ - ____________ )1.5

= ____________ cfs

Orifice: q' = 4.98* A1 * (E25-yr - E1)0.5 = 4.98* ____________ * ( ____________ - ____________ )0.5

= ____________ cfs

Allowable discharge through second stage (q") q25-yr - q' = ____________ cfs

For rectangular weir outlet:

Required weir length L2 = q"/(3.2* H25-yr 1.5) = ____________ /(3.2* ____________ 1.5)

= ____________ ft

For orifice outlet:

Required orifice area A2 = q"/(4.98* H25-yr 0.5) = ____________ /(4.98* ____________ 0.5)

= ____________ sq ft

TABLE 3. SCS TYPE II RAINFALL DISTRIBUTION

Time (in hours)	Fraction of Total	Fraction of Total Rainfall
0.50	0.021	0.005
1.00	0.042	0.011
1.50	0.063	0.017
2.00	0.083	0.023
2.50	0.104	0.029
3.00	0.125	0.035
3.50	0.146	0.042
4.00	0.167	0.049
4.50	0.188	0.056
5.00	0.208	0.064
5.50	0.229	0.072
6.00	0.250	0.080
6.50	0.271	0.090
7.00	0.292	0.100
7.50	0.313	0.110
8.00	0.333	0.120
8.50	0.354	0.134
9.00	0.375	0.147
9.50	0.396	0.163
10.00	0.417	0.181
10.50	0.438	0.204
11.00	0.458	0.235
11.25	0.468	0.260
11.50	0.479	0.300
11.75	0.490	0.420
12.00	0.500	0.663
12.25	0.510	0.710
12.50	0.521	0.735
13.00	0.542	0.772
13.50	0.563	0.799
14.00	0.583	0.820
14.50	0.604	0.835
15.00	0.625	0.850
15.50	0.646	0.865
16.00	0.667	0.880
16.50	0.688	0.889
17.00	0.708	0.898
17.50	0.729	0.907
18.00	0.750	0.916
18.50	0.771	0.925
19.00	0.792	0.934
19.50	0.813	0.943
20.00	0.833	0.952
20.50	0.854	0.958
21.00	0.875	0.964
21.50	0.896	0.970
22.00	0.917	0.976
22.50	0.938	0.982
23.00	0.958	0.988
23.50	0.979	0.994
24.00	1.000	1.000

 

See Figure 3: SCS Type II Storm Distribution

Figure 1. Rainfall Depth-Duration-Frequency Data for Peoria County

Figure 2-1. Solution of Runoff Equation

Figure 3. SCS Type II Storm Distribution

(Ord. No. 1552, § 10, 2-14-2008; Ord. No. 1756, § 2.C., 1-9-2017)