Travel Time and Time of Concentration

Transcription

1 Methods in Stormwater Management Using HydroCAD ravel ime and ime of Concentration H05 ravel ime.pdf 1

2 opics 1. ime of Concentration Definition 2. Segmental Flow 3. Sheet Flow 4. Concentrated Flow 5. Channel and Pipe Flow 6. ravel ime Equations 7. Example: Echo Valley Watershed 2

3 Outcomes 1. Understand the concept of time of concentration. 2. Understand the differences between sheet flow, shallow concentrated flow and channel/pipe flow. 3. Be familiar with the NRCS and other travel time methods. 4. Be able to apply travel time equations in a segmental manner in order to compute a watershed time of concentration. 3

4 ime of Concentration Hydrograph analysis definition time from end of rainfall excess to point of inflection on receding limb of graph Flow (cfs) rainfall excess c 400 Lag inflection point ime (hours) 4

5 ime of Concentration Flow path analysis definition time required for runoff to travel from the most hydraulically remote point in the watershed to the design point of interest. sheet POI sheet In-channel concentrated sheet 5

6 Segmental Calculation of c Segmental Method Break the flow path into distinctly different flow types Select appropriate equation for each segment Compute travel time, t, for each segment Sum the travel times to get time of concentration, c 6

7 Segmental Flow ypes Overland Flow Sheet Concentrated (natural swale) In-channel Flow Designed swale Designed channel Natural stream Pipe (gravity flow) Different flow types require different mathematical models 7

8 Overland Sheet Flow Very shallow overland flow Surface resistance dominates flow Uniform depth across a plane surface Path lengths typically short Impervious surfaces: approx ft Pervious surfaces: approx ft Depth same order of magnitude as surface roughness 8

9 Overland Sheet Flow Paved sheet flow Grassed sheet flow Sheet transitioning to concentrated Depth and roughness similar 9

10 Shallow Concentrated Flow Overland flow with visible depth Depth not uniform but shallow compared to flow path width More dynamic flow than kinematic flow Depth significantly larger than surface roughness dimension Also known as swale flow Can be ditch flow if depth is shallow 10

11 Shallow Concentrated Flow Impervious (gutter) flow Pervious (grass) flow 11

12 In-channel Flow Flow in a well defined channel Flow depth significant Dynamic forces dominate flow Pipe flow or channel flow Manning s equation is typical default model 12

13 In-channel Flow Pipe flow storm sewers Vegetated channel Rip-rap channel 13

14 Equations and Methods Equation types can be categorized as Sheet flow Shallow concentrated flow Combined sheet and shallow concentrated flow In-channel flow Lumped flow Most methods developed for very specific situations Most methods cannot be verified directly Choose best method to match flow path situation (regime/type) to get reasonable results 14

15 Izzard (1946) sheet flow on roadway and turf surfaces iterative solution with i and (two unknowns) i S i C L 0.33 = time in minutes i = rainfall intensity (in/hr) C = retardance coefficient L = length of flow path (ft) S = slope of flow path (ft/ft) 15

16 Morgali and Linsley (1965) sheet flow kinematic wave on paved surfaces iterative solution (two unknowns) 0.94L 0.4 i S n 0.6 L = length of overland flow (ft) n = Manning roughness coefficient i = rainfall intensity (in/hr) S = average overland slope (ft/ft) 16

17 NRCS Sheet Flow Equation (1986) t 0.42 nl P S 0.8 P 2 = 2-yr, 24-hr rainfall depth (in) n = Manning s sheet flow roughness L = length of overland flow path (ft) S = average overland slope (ft/ft) t = travel time (minutes). 17

18 NRCS Sheet Flow Equation Roughness Coefficients Surface description n 1 Smooth surface (concrete, asphalt, gravel or bare soil) Fallow (no residue) 0.05 Cultivated soils: Residue cover < 20% Residue cover > 20% Grass: Short-grass prairie Dense grasses 2 Bermuda grass Range (natural) 0.13 Woods: 3 Light underbrush Dense underbrush NOES: 1. he Manning s equation n values are composite of information compiled by Engman (1986). 2. Includes species such as weeping love grass, bluegrass, buffalo grass, blue grama grass and native grass mixtures. 3. When selecting n consider cover to a height of about 0.1 ft. his is the only part of the plant cover that will obstruct sheet flow. 18

19 NRCS Sheet Flow Maximum Path Lengths (1995) l 100 S n l = maximum length of flow (ft) n = Manning s sheet flow roughness S = average overland slope (ft/ft) Cover ype n values Slope (ft/ft) Length (ft) Grass Woods Grass Woods Grass Woods

20 Kirpich (1940) overland sheet and concentrated flow agricultural basins in PA and N 3% to 10% slopes, 1.25 to 112 acre watershed K L S K = surface roughness adjustment factor L = length of flow path from headwater to outlet (ft) S = average slope of the flow path (ft/ft) 20

21 Kerby-Hathaway (1959) overland sheet and concentrated flow sheet flow dominated the flow path 1% or less slopes, 10 acre and smaller watersheds 0.83 nl 0.5 S 0.47 L = length of overland flow (ft) n = Manning roughness coefficient S = average overland slope (ft/ft) 21

22 FAA (1970) overland sheet and concentrated flow developed from airfield drainage data frequently used for overland flow in urban watersheds C S L 0.5 C = Rational method runoff coefficient L = length of flow path (ft) S = average overland slope (%) 22

23 NRCS Average Velocity Method (1975, 2010) shallow concentrated flow velocity as a function of slope and surface cover reduced to two curves (paved and unpaved) by SCS when R was issued re-introduced by NRCS in May 2010 L 60V L = length of flow path (ft) S = watershed slope in percent V = average velocity 23

24 NRCS Concentrated Overland Flow (2010) V KS 0.5 t L 60V V = velocity (ft/s) K = surface coefficient S = watershed slope (ft/ft) L = length of path (ft) t = travel time (minutes). 24

25 L 1220S pavememt 0.5 L 968S grassed waterway 0.5 woodlands 0.5 nearly bare untilled L S L 526S cultivated row crops 0.5 L 418 S L 302S short-grass pasture 0.5 forest w/heavy ground litter and hay meadows 0.5 NRCS Shallow Concentrated Flow Equations L 151S L = length of path (ft) S = watershed slope (ft/ft) t = travel time (minutes). 25

26 NRCS R-55 (1986) Concentrated Flow L 1220S paved 0.5 L 968S unpaved 0.5 L = length of path (ft) S = watershed slope (ft/ft) t = travel time (minutes). 26

27 Manning s Equation (1890) in-channel flow compute velocity as a function of n, R h, and S V R S n h + t L 60V V = average channel velocity (ft/s) n = roughness coefficient R h = hydraulic radius (ft) = A/P A = flow area (ft 2 ) P = wetted perimeter of flow area (ft) S = average channel or pipe slope (ft/ft) t = travel time (minutes). L = length of flow path (ft) t 89.2 R nl h S 27

28 NRCS Lag Equation (1975) lumped watershed flow for upper reaches of the watershed adjust for channel improvements and impervious area L CN S 0.7 L = hydraulic length of watershed ; longest flow path (ft) CN = runoff curve number S = average watershed slope (ft/ft) 28

29 NRCS Segmental Method (2010) hree flow types sheet kinematic wave concentrated velocity method (Fig NEH 630 Ch. 15) in-channel Manning s equation 1. Sheet 0.42 nl P S Concentrated 3. In-channel nl 89.2 R h S 29

30 Exercise 1. Compute time of concentration for the Echo Valley watershed with the data provided on the following pages. 30

31 Echo Valley Watershed five flow segments 1. Sheet 2. Concentrated #1 3. Pipe 4. Concentrated #2 5. Channel 31

32 Echo Valley Watershed 1. Sheet Flow Segment L = 30 ft S = ft/ft Dense grass 2P 24 = 2.73 in (Atlas 14) 0.42 nl P S n = 0.24 (able 3.1 R-55) t = 6.0 minutes 32

33 Echo Valley Watershed 2. Concentrated Flow #1 L = 1080 ft S = ft/ft Wooded L 302 S woodland 0.5 t = 11.1 minutes 33

34 Echo Valley Watershed 3. Pipe Flow L = 520 ft S = ft/ft 18 Concrete Pipe (n = 0.013) Manning s channel flow nl 89.2 R h S nl 35.4 D circ. pipe S t = 1.1 minutes 34

35 Echo Valley Watershed 4. Concentrated Flow #2 L = 790 ft S = ft/ft Grass waterway L 968S grassed waterway 0.5 t = 5.8 minutes 35

36 Echo Valley Watershed 5. Channel Flow A = 3.0 ft 2 WP = 5.0 ft S = ft/ft n = L = 2500 ft 89.2 nl R S t = 5.4 minutes 36

37 NRCS Segmental Method Echo Valley Watershed c = c = c = 29.4 minutes 37

38 NRCS NEH Ch. 15 ime of Concentration Contains the latest time of concentration methodologies recommended by the NRCS. (May 2010) 38

39 Example: ime of Concentration for Echo Valley WS 39

40 Echo Valley WS travel time flow segments 1. Sheet Flow Segment L = 30 ft, S = ft/ft Dense grass, 2 P 24 = 2.68 in 2. Concentrated Flow #1 L = 1080 ft, S = ft/ft Wooded 3. Pipe Flow L = 520 ft, S = ft/ft 18 Concrete Pipe (n = 0.013) 4. Concentrated Flow #2 L = 790 ft, S = ft/ft Grass waterway 5. Channel Flow A = 3.0 ft 2, WP = 5.0 ft S = ft/ft, n = L = 2500 ft 40

41 ime of Concentration in HydroCAD for Echo Valley 1. Highlight WS Node 2. Under Nodes tab 3. Select Edit 4. Under c tab 5. Double Click on first row 6. Choose Sheet Flow. 7. Enter appropriate data 8. Click OK 9. Double click second row 10.Choose Shallow Concentrated Flow 11.Enter data, click OK 12.Repeat for other segments. 41

42 Summary opics 1. ime of Concentration Definition 2. Segmental Flow 3. Sheet Flow 4. Concentrated Flow 5. Channel and Pipe Flow 6. ravel ime Equations 7. Example: Echo Valley Watershed Outcomes 1. Understand the concept of time of concentration. 2. Understand the differences between sheet flow, shallow concentrated flow and channel/pipe flow. 3. Be familiar with the NRCS and other travel time methods. 4. Be able to apply travel time equations in a segmental manner in order to compute a watershed time of concentration. 42