0.0. Pervious CN = 40. (Unconnected impervious) (Total impervious) Total impervious area (percent) Composite CN

Transcription

1 Figure 2-3 Composite CN with connected impervious area. 100 Composite CN Pervious CN = Connected impervious area (percent) Figure 2-4 Composite CN with unconnected impervious areas and total impervious area less than 30% 0.0 Pervious CN = (Unconnected impervious) (Total impervious) Composite CN Total impervious area (percent) 2 10 (210-VI-TR-55, Second Ed., June 1986)

2 Table 2-2a Runoff curve numbers for urban areas 1/ Curve numbers for Cover description hydrologic soil group Average percent Cover type and hydrologic condition impervious area 2/ A B C D Fully developed urban areas (vegetation established) Open space (lawns, parks, golf courses, cemeteries, etc.) 3/ : Poor condition (grass cover < 50%) Fair condition (grass cover 50% to 75%) Good condition (grass cover > 75%) Impervious areas: Paved parking lots, roofs, driveways, etc. (excluding right-of-way) Streets and roads: Paved; curbs and storm sewers (excluding right-of-way) Paved; open ditches (including right-of-way) Gravel (including right-of-way) Dirt (including right-of-way) Western desert urban areas: Natural desert landscaping (pervious areas only) 4/ Artificial desert landscaping (impervious weed barrier, desert shrub with 1- to 2-inch sand or gravel mulch and basin borders) Urban districts: Commercial and business Industrial Residential districts by average lot size: 1/8 acre or less (town houses) /4 acre /3 acre /2 acre acre acres Developing urban areas Newly graded areas (pervious areas only, no vegetation) 5/ Idle lands (CN s are determined using cover types similar to those in table 2-2c). 1 Average runoff condition, and I 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 CN s shown are equivalent to those of pasture. Composite CN s may be computed for other combinations of open space cover type. 4 Composite CN s for natural desert landscaping should be computed using figures 2-3 or 2-4 based on the impervious area percentage (CN = 98) and the pervious area CN. The pervious area CN s are assumed equivalent to desert shrub in poor hydrologic condition. 5 Composite CN s to use for the design of temporary measures during grading and construction should be computed using figure 2-3 or 2-4 based on the degree of development (impervious area percentage) and the CN s for the newly graded pervious areas. (210-VI-TR-55, Second Ed., June 1986) 2 5

3 Table 2-2b Runoff curve numbers for cultivated agricultural lands 1/ Curve numbers for Cover description hydrologic soil group Hydrologic Cover type Treatment 2/ condition 3/ A B C D Fallow Bare soil Crop residue cover (CR) Poor Good Row crops Straight row (SR) Poor Good SR + CR Poor Good Contoured (C) Poor Good C + CR Poor Good Contoured & terraced (C&T) Poor Good C&T+ CR Poor Good Small grain SR Poor Good SR + CR Poor Good C Poor Good C + CR Poor Good C&T Poor Good C&T+ CR Poor Good Close-seeded SR Poor or broadcast Good legumes or C Poor rotation Good meadow C&T Poor Good Average runoff condition, and I a =0.2S 2 Crop residue cover applies only if residue is on at least 5% of the surface throughout the year. 3 Hydraulic condition is based on combination 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, (d) percent of residue cover on the 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. 2 6 (210-VI-TR-55, Second Ed., June 1986)

4 Table 2-2c Runoff curve numbers for other agricultural lands 1/ Curve numbers for Cover description hydrologic soil group Hydrologic Cover type condition A B C D Pasture, grassland, or range continuous Poor forage for grazing. 2/ Fair Good Meadow continuous grass, protected from grazing and generally mowed for hay. Brush brush-weed-grass mixture with brush Poor the major element. 3/ Fair Good 30 4/ Woods grass combination (orchard Poor or tree farm). 5/ Fair Good Woods. 6/ Poor Fair Good 30 4/ Farmsteads buildings, lanes, driveways, and surrounding lots. 1 Average runoff condition, and I 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. (210-VI-TR-55, Second Ed., June 1986) 2 7

5 Worksheet 2: Runoff curve number and runoff Project By Date Location Checked Date Check one: Present Developed 1. Runoff curve number Soil name and hydrologic group (appendix A) Cover description (cover type, treatment, and hydrologic condition; percent impervious; unconnected/connected impervious area ratio) Table 2-2 CN Area Product of CN x area Figure Figure 2-4 acres mi 2 % 1 Use only one CN source per line Totals CN (weighted) = total product = = ; total area Use CN 2. Runoff Storm #1 Storm #2 Storm #3 Frequency... Rainfall, P (24-hour)... Runoff, Q... (Use P and CN with table 2-1, figure 2-1, or equations 2-3 and 2-4) yr in in D 2 (210-VI-TR-55, Second Ed., June 1986)

6 Worksheet 3: Time of Concentration (Tc) or travel time (Tt) Project By Date Location Checked Date Check one: Present Developed Check one: Tc Tt through subarea Notes: Space for as many as two segments per flow type can be used for each worksheet. Include a map, schematic, or description of flow segments. Sheet flow (Applicable to Tc only) 1. Surface description (table 3-1) Manning s roughness coefficient, n (table 3-1) Flow length, L (total L 300 ft)... ft 4. Two-year 24-hour rainfall, P 2... in 5. Land slope, s... ft/ft 6. T (nl) 0.8 t = P s 0.4 Shallow concentrated flow Segment ID Compute Tt... hr + = Segment ID 7. Surface description (paved or unpaved) Flow length, L...ft 9. Watercourse slope, s... ft/ft 10. Average velocity, V (figure 3-1)... ft/s 11. T L t = Compute Tt... hr 3600 V + = Channel flow Segment ID 12. Cross sectional flow area, a... ft Wetted perimeter, pw... ft a 14. Hydraulic radius, r= Compute r... ft p w 15 Channel slope, s... ft/ft 16. Manning s roughness coefficient, n V = 1.49 r 2/3 s 1/2 Compute V...ft/s n 18. Flow length, L... ft 19. T L t = Compute T t... hr + = 3600 V 20. Watershed or subarea T c or T t (add T t in steps 6, 11, and 19)... Hr (210-VI-TR-55, Second Ed., June 1986) D 3