Zoneomics Logo
search icon

Peoria County Unincorporated
City Zoning Code

APPENDIX D

PEORIA COUNTY STANDARDS FOR STORMWATER DESIGN ANALYSES

The following are the minimum standard, methods and procedures to be used to comply with the stormwater design requirements of Sections 3.12 ("General Erosion and Sediment Control Permits"), 3.13 ("Erosion, Sediment, and Stormwater Control Permits"), and 7.13 ("Erosion, Sediment, and Stormwater Control"). If an applicant determines that different methods are necessary based on site specific conditions, the applicant must request approval from the erosion control administrator to use other methods prior to submittal.

The design methods listed below are readily available in a number of computer programs, including the Soil Conservation Service's TR 20 (SCS) and HEC-1 (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.

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

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 (attached) from TR 55. Soil type information is available from the SCS Peoria County Soil Survey, 1992.

Storm distribution (cumulative rainfall versus time). Use the SCS Type 11 storm distribution. See attached Table 3 and Figure 3.

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

Storage routing (detention pond analysis). Use the continuity equation, also known as the Modified-Pula and Storage indication methods. As a substitute for detailed storage routing of a hydrograph, TR 55 (SCS, 1986) can be used, either manually or 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 attached detention basin outlet work sheet to determine and present the structure design information.

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

Rainfall Depth (inches) for Given Frequency

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-hr. 1.42 1.77 2.09 2.50 2.86 3.25
2-hr. 1.78 2.22 2.62 3.14 3.59 4.08
3-hr. 1.93 2.41 2.85 3.41 3.89 4.43
6-hr. 2.26 2.82 3.33 3.99 4.56 5.19
12-hr. 2.62 3.27 3.87 4.63 5.29 6.02
18-hr. 2.75 3.46 4.09 4.90 5.59 6.37
24-hr. 3.02 3.76 4.45 4.32 6.08 6.92
48-hr. 3.38 4.19 4.86 5.78 6.62 7.51
72-hr. 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/BLTL-70/89

TABLE 2-2A
RUNOFF CURVE NUMBERS FOR URBAN AREAS1

Cover Description Curve Numbers for Hydrologic Soil Group
Cover type and hydrologic condition Average percent
impervious area 2 		A B C D
Fully Developed Urban Area (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:
⅛ acre or less (townhouses) 65 77 85 90 92
¼ acre 38 61 75 83 87
⅓ 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
1  Average runoff condition and 1 a = 0.2S
2  The average percent impervious area shown was used to develop the composite CN's. 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. CN's 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 LANDS1

Cover Description Curve Numbers for
Hydrologic Soil Group
Cover type Treatment 2 Hydrologic
Condition 3 		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 and terraced (C and T) Poor 66 74 80 82
Good 62 71 78 81
C and 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 and T Poor 61 72 79 82
Good 59 70 78 81
C and T + CR Poor 60 71 78 81
Good 58 69 77 80
Close-seeded or broadcast legumes or rotation meadow SR Poor 66 77 85 89
Good 58 72 81 85
C Poor 64 75 83 85
Good 55 69 78 83
C and T Poor 63 73 80 83
Good 51 67 76 80
1  Average runoff condition and 1 a = 0.2S.
2  Crop residue cover applies only if residue is on at least 5% of the surface throughout the year.
3  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%), 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 AGRICULTURAL LANDS1

Cover Description Curve Numbers for
Hydrologic Soil Group
Cover type Hydrologic
Condition 3 		A B C D
Pasture, grassland, or range—
Continuous forage for grazing 2 		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 3 Poor 48 67 77 83
Fair 35 56 70 77
Good 30 4 48 65 73
Woods—Grass combination (orchard or tree farm) 5 Poor 57 73 82 86
Fair 43 65 76 82
Good 32 58 72 79
Woods 6 Poor 45 66 77 83
Fair 36 60 73 79
Good 30 4 55 70 77
Farmsteads—Buildings, lanes, driveways, and surrounding lots Poor 59 74 82 86
1 Average runoff condition and 1 a = 0.2S.
2 Poor: <50% ground cover or heavily grazed with no mulch.
Fair: 50 to 75% ground cover and not heavily grazed.
Good: > 75% ground cover and lightly or only occasionally grazed.
3 Poor: <50% ground cover.
Fair: 50 to 75% ground cover.
Good: > 75% ground cover.
4 Actual curve number is less than 30; use CN = 30 for runoff computations.
5 CN's shown were computed for areas with 50% woods and 50% grass (pasture) cover. Other combinations of conditions may be computed from the CN's for woods and pasture.
6 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.

 

Computation sheet for 2-Stage Detention Basin Outlet Design
(to be used with TR 55 worksheet 6a or computer printout for storage routing)

Project Name: _____ Structure ID: _____ Date: _____

 

Note: attach TR 55 worksheet 6a or computer printout with basin routing information

FIRST STAGE

Maximum stage for two-year storm (E 2-yr ) = _____ ft

First stage control elevation (E 1 ) = _____ ft

Head on first stage structure (H 2-yr = E 2-yr - E 1 ) _____ ft

Allowable discharge for two-year storm (q 2-yr ) = _____ cfs

For rectangular weir outlet:

Required weir length L 1 = q 2-yr /(3.2* H 2-yr 1.5 ) _____/(3.2* _____ 1.5 ) = _____ ft

For orifice outlet:

Required orifice area A 1 = q 2-yr /(4.98* H 2-yr 0.5 ) = _____/(4.98* _____ 0.5 ) = _____ sq ft

SECOND STAGE

Maximum stage for twenty-five-year storm (E 25-yr ) = _____ ft

Second stage control elevation (E 2 ) = _____ ft

Head on second stage structure (H 25-yr = E 25-yr - E 2 ) _____ ft

Allowable discharge for twenty-five-year storm (q 25-yr ) = _____ cfs

Twenty-five-year storm discharge through first stage:

Weir: q′ = 3.2 * L 1 * (E 25-yr - E 1 ) 1.5 = 3.2* _____ *(_____ - _____) 1.5 = _____ cfs

Orifice: q′ = 4.98 * A 1 * (E 25-yr - E 1 ) 0.5 = 4.98* _____ *(_____ - _____) 0.5 = _____ cfs

Allowable discharge through second stage (q″) q 25-yr - q' = _____ cfs

For rectangular weir outlet:

Required weir length L 2 = q″/(3.2* H 25-yr 1.5 ) = _____/(3.2* _____ 1.5 ) = _____ ft

For orifice outlet:

Required orifice area A 2 = q″/(4.98* H 25-yr 0.5 ) = _____/(4.98* _____ 0.5 ) = _____ sq ft

TABLE 3
SCS TYPE II RAINFALL DISTRIBUTION

Time
Hour 		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