Transcript Urban Drainage and Intersection Design

Design of Open Channels and
Culverts
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Transverse Slopes
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Removes water from pavement
surfaces in shortest amount of time
possible
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Longitudinal Slopes
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Gradient
longitudinal
direction of
highway to
facilitate
movement of
water along
roadway
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Drains
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Along ROW
Collect surface water
A typical intercepting
drain placed in the
impervious zone
http://www.big-o.com/constr/hel-cor.htm
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Drainage Channels (Ditches)
Design
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Adequate capacity
Minimum hazard to traffic
Hydraulic efficiency
Ease of maintenance
Desirable design (for safety): flat
slopes, broad bottom, and liberal
rounding
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Ditch Shape
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Trapezoidal – generally preferred
considering hydraulics, maintenance,
and safety
V-shaped – less desirable from
safety point of view and
maintenance
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Flow Velocity
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Depends on lining type
Typically 1 to 5% slopes used
Should be high enough to prevent deposit
of transported material (sedimentation)
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For most linings, problem if S < 1%
Should be low enough to prevent erosion
(scour)
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For most types of linings, problem if S > 5%
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Use spillway or chute if Δelev is large
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Rip Rap for drainage over high slope
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Side Ditch/Open Channel Design-Basics
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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
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Manning’s Formula
V = R2/3*S1/2
n
(metric)
V = 1.486 R2/3*S1/2
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
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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
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Open Channel Example
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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
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Open Channel Example
Q = 1.486 R2/3*S1/2
n
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Hydraulic radius, R = a/P
a = area, P = wetted perimeter
P
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Open Channel Example
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Flow depth = d
Area = 6 feet x d
Wetted perimeter = 6 + 2d
Flow depth (d)
6 feet
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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’
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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)
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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’
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Check lining for max depth of flow …
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Rounded
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A cut slope with ditch
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A fill slope
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Inlet or drain marker
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Ditch treatment near a bridge
US 30 – should pier be protected?
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A fill slope
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Median drain
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Culvert Design - Basics
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Top of culvert not used as pavement
surface (unlike bridge), usually less than 20
foot span
> 20 feet use a bridge
Three locations
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Bottom of Depression (no watercourse)
Natural stream intersection with roadway
(Majority)
Locations where side ditch surface drainage
must cross roadway
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Hydrologic and Economic Considerations
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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)
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Culvert Design Steps
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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
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Culvert Design Steps
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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
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Examine need for energy dissipaters
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Headwater Depth
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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
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Types of culvert flow
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Type of flow depends on total energy
available between inlet and outlet
Inlet control
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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
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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
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Types of culvert flow
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Outlet control
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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
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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
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