§ 151.12 STANDARD FOR STORMWATER DESIGN ANALYSIS.

   The following are the minimum standard methods and procedures to be used to comply with this chapter. 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.

   (A)   Design methods. 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.

      (1)   Rainfall depth and intensity data. Use data for the county from the state’s water survey, BUL-70/89, 1989, as presented in division (B) below and graphically in division (C) below.

      (2)   Storm event rainfall runoff. Use the SCS Runoff Curve Number Method to determine rainfall runoff depth. See divisions (D) and (F) through (H) below from TR 55. Soil type information is available from the SCS county soil survey, 1992.

      (3)   Storm distribution (cumulative rainfall versus time). Use the SCS Type II storm distribution. See divisions (J) and (K) below.

      (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 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) 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 worksheet to determine and present the structure design information.

   (B)   Table 1, rainfall depth, duration, and frequency.

	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	5.32	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

 

   (C)   Figure 1, rainfall depth, duration, and frequency data.

   (D)   Figure 2.1, solution of runoff equation.

   (E)   Estimating runoff.

      (1)   SCS Runoff Curve Number Method.

         (a)   The SCS Runoff Curve Number (Cn) Method is described in detail in NEH-4 (SCS 1985). The SCS runoff equation is (Eq. 2-1):

Q= (P - Ia)2
(P - Ia) + S

         (b)   Where: Q= runoff (in.); P = rainfall (in.); S = potential maximum retention after runoff begins (in.); and Ia = initial abstraction (in.).

         (c)   Initial abstraction (Ia ) is all losses before runoff begins. It includes water retained in surface depressions, water intercepted by vegetation, evaporation, and infiltration. Ia is highly variable but generally is correlated with soil and cover parameters. Through studies of many small agricultural watersheds, Ia was found to be approximated by the following empirical equation (Eq. 2-2):

Ia = 0.2S

         (d)   By removing Ia as an independent parameter, this approximation allows use of a combination of S and P to produce a unique runoff amount. Substituting equation 2-2 into equation 2-1 gives (Eq. 2-3):

Q = (P - 0.2S)2
(P - 0.8S)

         (e)   S is related to the soil and cover conditions of the watershed through the CN. CN has a range of 0 to 100, and S is related to CN by (Eq. 2-4):

S= 1,000 - 10
   CN

         (f)   Division (D) above and table 2-1 solve equations 2-3 and 2-4 for a range of CNs and rainfall.

      (2)   Factors considered in determining runoff curve numbers.

         (a)   The major factors that determine CN are the hydrologic soil group (HSG), cover type, treatment, hydrologic condition, and antecedent runoff condition (ARC). Another factor considered is whether impervious areas outlet directly to the drainage system (connected) or whether the flow spreads over pervious areas before entering the drainage system (unconnected). Figure 2-2 is provided to aid in selecting the appropriate figure or table for determining curve numbers.

         (b)   CNs in divisions (F) through (H) represent average antecedent runoff condition for urban, cultivated agricultural, other agricultural, and arid and semiarid rangeland uses. Divisions (F) through (H) assumes impervious areas are directly connected. The following sections explain how to determine CNs and how to modify them for urban conditions.

      (3)   Hydrologic soil groups.

         (a)   Infiltration rates of soils vary widely and are affected by subsurface permeability as well as surface intake rates. Solid are classified into four HSGs (A, B, C, and D) according to their minimum infiltration rate, which is obtained for bare soil after prolonged wetting. This section defines the four groups and provides a list of most of the solid in the United States and their group classification. The soils in the area of interest may be identified from a soil survey report, which can be obtained from local SCS offices or soil and water conservation district offices.

         (b)   Most urban areas are only partially covered by impervious surfaces; the soil remains an important factor in runoff estimates. Urbanization has a greater effect on runoff in watersheds with solid having high infiltration rates (sands and gravels) than in watersheds predominantly of silts and clays, which generally have low infiltration rates.

         (c)   Any disturbance of a soil profile can significantly change its infiltration characteristics. With urbanization, native soil profiles may be mixed or removed or fill material from other areas may be introduced. Therefore, a method based on soil texture is given in Exhibit 2.2 for determining the HSG classification for disturbed soils.

      (4)   Cover type. Divisions (F) through (H) below address most cover types, such as vegetation, bare soil, and impervious surfaces. There are a number of methods for determining cover type. The most common are field reconnaissance, aerial photographs, and land use maps.

      (5)   Treatment. Treatment is a cover type modifier (used only in division (G) below) to describe the management of cultivated agricultural lands. It includes mechanical practices, such as contouring and terracing, and management practices, such as crop rotations and reduced or no tillage.

      (6)   Hydrologic condition. Hydrologic condition indicates the effect of cover type and treatment on infiltration and runoff and is generally estimated from density of plan and residue cover on sample areas. Good hydrologic condition indicates that the soil usually has a low runoff potential for that specific hydrologic soil group, cover type, and treatment. Some factors to consider in estimating the effect of cover on infiltration and runoff are canopy or density of lawns, crops, or other vegetative areas; amount of year-round cover; amount of grass or close-seeded legumes in rotations; percent of residue cover; and degree of surface roughness.

      (7)   Antecedent runoff condition. The index of runoff potential before a storm event is the antecedent runoff condition (ARC). ARC is an attempt to account for the variation in CN at a site from storm to storm. CN for the average ARC at a site is the median value as taken from sample rainfall and runoff date. The CNs in divisions (F) through (H) are for the average ARC, which is primarily used for design applications. See the SCS NEH-4 and Rallison and Miller for more detailed discussion of storm to storm variation and a demonstration of upper and lower enveloping curves.

      (8)   Urban impervious area modifications. Several factors, such as the percentage of impervious area and the means of conveying runoff from impervious areas to the drainage system, should be considered in computing CN for urban areas. For example, do the impervious areas connect directly to the drainage system, or do they outlet onto lawns or other pervious areas where infiltration can occur?

      (9)   Connected impervious areas.

         (a)   An impervious area is considered connected if runoff from it flows directly into the drainage system. It is also considered connected if runoff from it occurs as concentrated shallow flow that runs over a pervious area and then into a drainage system.

         (b)   1.   Urban CNs, division (F) below, were developed for typical land use relationships based on specific assumed percentages of impervious area.

            2.   These CN values were developed on the assumptions that pervious urban areas are equivalent to pasture in good hydrologic condition and impervious areas have a CN of 98 and are directly connected to the drainage system.

            3.   Some assumed percentages of impervious area are shown in division (F) below.

   (F)   Runoff curve numbers for urban areas1.

Cover Description		Curve Numbers for Hydrologic Soil Group
Cover type and hydrologic condition	Average percent impervious area2	A	B	C	D
Fully developed urban areas (vegetation established):
Open space (lawns, parks, golf courses, cemeteries, and the like):
Poor condition (grass cover < 50%)		68	79	86	89
Fair condition (grass cover 50% to 75%)		49	69	79	84
Good condition (grass over > 75%)		39	61	74	80
Impervious areas:
Paved parking lots, roofs, driveways, and the like (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
1/4 acre	38	61	75	83	87
1/3 acre	30	57	72	81	86
1/2 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 Ia = 0.2S.
2. 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.

 

   (G)   Runoff curve numbers for agricultural lands1.

Cover Description			Curve Numbers for Hydrologic Soil Group
Cover type	Treatment2	Hydrologic Condition3	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&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	66	77	85	89
		Good	58	72	81	85
Legumes or rotation	C	Poor	64	75	83	85
		Good	55	69	78	83
Meadow	C&T	Poor	63	73	80	83
		Good	51	67	76	80
1. Average runoff condition and Ia = 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 a combination of factors that affect infiltration and runoff, including: density and canopy of vegetative areas; amount of year-round cover; amount of grass or close-seeded legumes in rotations; percent of residue cover on land surface (good > = 20%); and degree of surface roughness. Poor factors impair infiltration and lend to increase runoff. Good factors encourage average and better than average infiltration and tend to decrease runoff.

 

   (H)   Runoff curve numbers for other agricultural lands1.

Cover Description		Curve Numbers for Hydrologic Soil Group
Cover Type	Hydrologic Condition3	A	B	C	D
Brush - brush-weed-grass mixture with brush the major element3	Poor Fair Good	48 35 30 4	67 56 48	77 70 65	83 77 73
Farmsteads - buildings, lanes, driveways, and surrounding lots	Poor	59	74	82	86
Meadow - continuous grass, protected from grazing and generally mowed for hay	Poor	30	58	71	78
Pasture, grassland, or range - continuous forage for grazing2	Poor Fair Good	68 49 39	79 69 61	86 79 74	89 84 80
Woods - grass combination (orchard or tree farm)5	Poor Fair Good	57 43 32	73 65 58	82 76 72	86 82 79
Woods6	Poor Fair Good	45 36 30 4	66 60 55	77 73 70	83 79 77
1. Average runoff condition and la = 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. CNs shown were computed for areas with 50% woods and 50% grass (pasture) cover. Other combinations of conditions may be computed from the CNs 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.

 

   (I)   Computation sheet.

   (J)   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

 

   (K)   Figure 3, SCS Type II storm distribution.

(Prior Code, 8 TCC 3-Appendix A)