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.

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1 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

2 Chapter 3 Time of Concentration and Travel Time Technical Release 55 Urban Hydrology for Small Watersheds Sheet flow Sheet flow is flow over plane surfaces. It usually occurs in the headwater of streams. With sheet flow, the friction value (Manning s n) is an effective roughness coefficient that includes the effect of raindrop impact; drag over the plane surface; obstacles such as litter, crop ridges, and rocks; and erosion and transportation of sediment. These n values are for very shallow flow depths of about 0.1 foot or so. Table 3-1 gives Manning s n values for sheet flow for various surface conditions. Table 3-1 Roughness coefficients (Manning s n) for sheet flow Surface description n 1/ Smooth surfaces (concrete, asphalt, gravel, or bare soil) Fallow (no residue) Cultivated soils: Residue cover 20% Residue cover >20% Grass: Short grass prairie Dense grasses 2/ Bermudagrass Range (natural) Woods: 3/ Light underbrush Dense underbrush The n values are a composite of information compiled by Engman (1986). 2 Includes species such as weeping lovegrass, bluegrass, buffalo grass, blue grama grass, and native grass mixtures. 3 When selecting n, consider cover to a height of about 0.1 ft. This is the only part of the plant cover that will obstruct sheet flow. For sheet flow of less than 300 feet, use Manning s kinematic solution (Overtop and Meadows 1976) to compute T t : nl Tt = ( ).. ( P2 ) s where: [eq. 3-3] T t = travel time (hr), n = Manning s roughness coefficient (table 3-1) L = flow length (ft) P 2 = 2-year, 24-hour rainfall (in) s = slope of hydraulic grade line (land slope, ft/ft) This simplified form of the Manning s kinematic solution is based on the following: (1) shallow steady uniform flow, (2) constant intensity of rainfall excess (that part of a rain available for runoff), (3) rainfall duration of 24 hours, and (4) minor effect of infiltration on travel time. Rainfall depth can be obtained from appendix B. Shallow concentrated flow After a maximum of 300 feet, sheet flow usually becomes shallow concentrated flow. The average velocity for this flow can be determined from figure 3-1, in which average velocity is a function of watercourse slope and type of channel. For slopes less than ft/ft, use equations given in appendix F for figure 3-1. Tillage can affect the direction of shallow concentrated flow. Flow may not always be directly down the watershed slope if tillage runs across the slope. After determining average velocity in figure 3-1, use equation 3-1 to estimate travel time for the shallow concentrated flow segment. Open channels Open channels are assumed to begin where surveyed cross section information has been obtained, where channels are visible on aerial photographs, or where blue lines (indicating streams) appear on United States Geological Survey (USGS) quadrangle sheets. Manning s equation or water surface profile information can be used to estimate average flow velocity. Average flow velocity is usually determined for bankfull elevation. (210-VI-TR-55, Second Ed., June 1986) 3 3

4 TABLE 5.1 Pennsylvania Rainfall by County (For Use with Technical Release 55 - Urban Hydrology for Small Watersheds) NOT TO BE USED WITH THE RATIONAL EQUATION COUNTY 24 HR RAINFALL FOR VARIOUS FREQUENCIES 24 HR RAINFALL FOR VARIOUS FREQUENCIES COUNTY 1 yr. 2 yr. 5 yr. 10 yr. 25 yr. 50 yr. 100 yr. 1 yr. 2 yr. 5 yr. 10 yr. 25 yr. 50 yr. 100 yr. Adams Lackawanna Allegheny Lancaster Armstrong Lawrence Beaver Lebanon Bedford Lehigh Berks Luzerne Blair Lycoming Bradford McKean Bucks Mercer Butler Mifflin Cambria Monroe Cameron Montgomery Carbon Montour Centre Northampton Chester Northumberland Clarion Perry Clearfield Philadelphia Clinton Pike Columbia Potter Crawford Schuylkill Cumberland Snyder Dauphin Somerset Delaware Sullivan Elk Susquehanna Erie Tioga Fayette Union Forest Venango Franklin Warren Fulton Washington Greene Wayne Huntingdon Westmoreland Indiana Wyoming Jefferson York Juniata NWS - NOAA Atlas 14, Sept 25-29, 2008 NOTE: Data from this table may not be used for final design of E&S or PCSM BMPs / March 31, 2012 / Page 109

6 Chapter 3 Time of Concentration and Travel Time Technical Release 55 Urban Hydrology for Small Watersheds Figure 3-1 Average velocities for estimating travel time for shallow concentrated flow Watercourse slope (ft/ft) Unpaved Paved Average velocity (ft/sec) 3 2 (210-VI-TR-55, Second Ed., June 1986)

7 Figure 3-1 (Average velocities for estimating travel time for shallow concentrated flow) Where: Unpaved V = (s) 0.5 Paved V = (s) 0.5 V = velocity (fps) S = slope of hydraulic grade line (water course slope, ft/ft) These two equations are based upon a solution of Manning s Equation (Eq.3-4) with different assumptions on n (Manning s roughness coefficient) and r (hydraulic radius, ft). For unpaved areas, n is n = 0.05 and r = 0.4, and for paved areas, n is n = and r is r = 0.2.

8 Open channels Open channels are assumed to begin where surveyed cross section information has been obtained, where channels are visible on aerial photographs, or where blue lines (indicating streams) appear on United States Geological Survey (USGS) quadrangle sheets. Manning s equation or water surface profile information can be used to estimate average flow velocity. Average flow velocity is usually determined for bankfull elevation. Manning s equation is: V = (1.49 * r * s 0.5 ) / n [eq. 3-4] where: V = average velocity (ft/s) r = hydraulic radius (ft) and is equal to a/pw a = cross sectional flow area (ft2) pw = wetted perimeter (ft) s = slope of the hydraulic grade line (channel slope, ft/ft) n = Manning s roughness coefficient for open channel flow

9 Channels requiring protective liners should be either trapezoidal or parabolic in cross-section. V-shaped channels are not recommended for channels having fabric or geotextile liners due to the tendency for gaps to be left under the lining at the bottom of the channel. Where it is necessary to construct V-shaped channels with PennDOT PUB 72 9RC-73M standards should be strictly followed. - Flow capacity and velocity in open channels are typically computed by use of equation. Use of this equation (including derivative forms) is recommended by the Department: / 3 1/ 2 Q a r s n and / 3 1/ 2 V r s n Where: Q = V = n = a = P = r = s = Quantity of flow (cfs) Velocity (fps) Cross-sectional area of channel (sq. ft) Wetted perimeter of channel (ft) Hydraulic radius of channel (ft) = a/p Slope of channel bottom (ft/ft) TABLE 6.1 Geometric Elements of Channel Sections National Engineering Handbook, Section 5, ES / March 31, 2012 / Page 128

Drought. Montour County Multi-jurisdictional. General. History

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