The document discusses the design of open channels and culverts for drainage. It covers topics such as transverse and longitudinal slopes to facilitate water movement, drainage channels and ditches, ditch shapes, flow velocities, Manning's formula for open channel design, culvert design basics, hydrologic and economic considerations, culvert design steps including determining design flow rates and allowable headwater depths, and types of culvert flow including inlet and outlet control.

1. 1
Design of Open Channels and
Culverts

2. 2
Transverse Slopes
 Removes water from pavement
surfaces in shortest amount of time
possible

3. 3
Longitudinal Slopes
 Gradient
longitudinal
direction of
highway to
facilitate
movement of
water along
roadway

4. 4
Drains
 Along ROW
 Collect surface water
A typical intercepting
drain placed in the
impervious zone
http://www.big-o.com/constr/hel-cor.htm

5. 5
Drainage Channels (Ditches)
 Design
 Adequate capacity
 Minimum hazard to traffic
 Hydraulic efficiency
 Ease of maintenance
 Desirable design (for safety): flat
slopes, broad bottom, and liberal
rounding

6. 6
Ditch Shape
 Trapezoidal – generally preferred
considering hydraulics, maintenance,
and safety
 V-shaped – less desirable from
safety point of view and
maintenance

7. 7

8. 8
Flow Velocity
• Depends on lining type
• Typically 1 to 5% slopes used
• Should be high enough to prevent deposit
of transported material (sedimentation)
• For most linings, problem if S < 1%
• Should be low enough to prevent erosion
(scour)
• For most types of linings, problem if S > 5%

9. 9
Use spillway or chute if Δelev is large

10. 10
Rip Rap for drainage over high slope

11. 11
Side Ditch/Open Channel Design-Basics
• Find expected Q at point of interest
(see previous lecture)
• Select a cross section for the slope,
and any erosion control needed
• Manning’s formula used for design
• Assume steady flow in a uniform
channel

12. 12
Manning’s Formula
V = R2/3*S1/2 (metric) V = 1.486 R2/3*S1/2
n n
where:
V = mean velocity (m/sec or ft/sec)
R = hydraulic radius (m, ft) = area of the cross section
of flow (m2, ft2) divided by wetted perimeter (m,f)
S = slope of channel
n = Manning’s roughness coefficient

13. 13
Side Ditch/Open Channel
Design-Basics
Q = VA
Q = discharge (ft3/sec, m3/sec)
A = area of flow cross section
(ft2, m2)
FHWA Hydraulic Design Charts
FHWA has developed charts to solve
Manning’s equation for different cross
sections

14. 14

15. 15
Open Channel Example
 Runoff = 340 ft3/sec (Q)
 Slope = 1%
 Manning’s # = 0.015
 Determine necessary cross-section to
handle estimated runoff
 Use rectangular channel 6-feet wide

16. 16
Open Channel Example
Q = 1.486 R2/3*S1/2
n
 Hydraulic radius, R = a/P
 a = area, P = wetted perimeter
P

17. 17
Open Channel Example
 Flow depth = d
 Area = 6 feet x d
 Wetted perimeter = 6 + 2d
6 feet
Flow depth (d)

18. 18
Example (continued)
Q = 1.486 a R2/3*S1/2
n
340 ft3/sec = 1.486 (6d) (6d)2/3 (0.01)1/2
(6 + 2d)
0.015
d  4 feet
Channel area needs to be at least 4’ x 6’

19. 19
Example (continued)
Find flow velocities.
V = 1.486 R2/3*S1/2
n
with R = a/P = 6 ft x 4 ft = 1.714
2(4ft) + 6ft
so, V = 1.486(1.714)2/3 (0.01)1/2 = 14.2 ft/sec
0.015
If you already know Q, simpler just to do
V=Q/A = 340/24 = 14.2)

20. 20

21. 21
Example (continued)
Find critical velocities.
From chart along critical curve, vc  13 ft/sec
Critical slope = 0.007
Find critical depth: yc = (q2/g)1/3
g = 32.2 ft/sec2
q = flow per foot of width
= 340 ft3/sec /6 feet = 56.67ft2/sec
yc = (56.672/32.2)1/3 = 4.64 feet > depth of 4’

22. 22
Check lining for max depth of flow …

23. 23

24. 24
Rounded

25. 25
A cut slope with ditch

26. 26
A fill slope

27. 27
Inlet or drain marker

28. 28
Ditch treatment near a bridge
US 30 – should pier be protected?

29. 29
A fill slope

30. 30

31. 31

32. 32
Median drain

33. 33
Culvert Design - Basics
 Top of culvert not used as pavement
surface (unlike bridge), usually less than 20
foot span
 > 20 feet use a bridge
 Three locations
 Bottom of Depression (no watercourse)
 Natural stream intersection with roadway
(Majority)
 Locations where side ditch surface drainage
must cross roadway

34. 34
Hydrologic and Economic Considerations
 Alignment and grade of culvert (with
respect to roadway) are important
 Similar to open channel
 Design flow rate based on storm with
acceptable return period (frequency)

35. 35
Culvert Design Steps
 Obtain site data and roadway cross
section at culvert crossing location
(with approximation of stream
elevation) – best is natural stream
location, alignment, and slope (may
be expensive though)
 Establish inlet/outlet elevations,
length, and slope of culvert

36. 36
Culvert Design Steps
 Determine allowable headwater depth
(and probable tailwater depth) during
design flood – control on design size –
f(topography and nearby land use)
 Select type and size of culvert
 Examine need for energy dissipaters

37. 37
Headwater Depth
 Constriction due to culvert creates
increase in depth of water just upstream
 Allowable level of headwater upstream
usually controls culvert size and inlet
geometry
 Allowable headwater depth depends on
topography and land use in immediate
vicinity

38. 38
Types of culvert flow
 Type of flow depends on total energy
available between inlet and outlet
 Inlet control
 Flow is controlled by headwater depth and inlet
geometry
 Usually occurs when slope of culvert is steep
and outlet is not submerged
 Supercritical, high v, low d
 Most typical
 Following methods ignore velocity head

39. 39

40. 40
Ans.
Example:
Design ElevHW = 230.5
Stream bed at inlet = 224.0
Drop = 6.5’
Flow = 250cfs
5x5 box
HW/D = 1.41
HW = 1.41x5 = 7.1’
Need 7.1’, have 6.5’
Drop box 0.6’ below stream - OK

41. 41
Types of culvert flow
 Outlet control
 When flow is governed by combination of
headwater depth, entrance geometry, tailwater
elevation, and slope, roughness, and length of
culvert
 Subcritical flow
 Frequently occur on flat slopes
 Concept is to find the required HW depth to
sustain Q flow
 Tail water depth often not known (need a
model), so may not be able to estimate for
outlet control conditions

42. 42

43. 43
Example:
Design ElevHW = 230.5
Flow = 250cfs
5x5 box
Stream elev at inlet = 240
200’ culvert
Outlet invert = 240-0.02x200
= 220.0’
Given tail water depth = 6.5’
Check critical depth = 4.3’
from fig. 17.23 (next page)
Depth to hydraulic grade line =
(dc+D)/2 = 4.7 < 6.5, use 6.5’
Head drop = 3.3’ (from chart)
220.0+6.5+3.3 = 229.8’<230.5
OK

44. 44