Using Hydrologic Engineering Center River Analysis System
(HECRAS) To Evaluate Scour At Bridges
County of Orange
Presented to the Flood Division
August 13, 2001
by Nadeem Majaj
Note: This document is also available as a PowerPoint
presentation.
Approximately 575,000 bridges are built over waterways in the US.
The most common cause of bridge failure is due to bridge scour of the
foundation.
In 1993, the upper Mississippi flooding caused 23 bridge failures.
In 1994, flooding in Georgia (Alberto storm) 500 bridges were scour
damaged. 31 experienced 15-20 feet of scour.
Definition of Scour
Scour is the removal of sediment (soil and rocks) from stream beds
and stream banks caused by moving water
HEC18 - Evaluating Scour At Bridges
HEC18 was originally prepared by the FHWA in 1988. A fourth edition
was completed in May 2001 and released to the public in July 2001
HECRAS - Version 3.0.1
The Hydrologic Engineering Center recently released River Analysis
System (HECRAS) version 3.0.1 which includes significant new features,
most notably the Unsteady Flow and Bridge Scour options. The bridge
scour evaluation follows closely the HEC18 (4th Edition) methodology.
No reliable equations are available to predict
all hydraulic flow conditions that may be reasonably expected to occur.
Engineering judgment is required.
[picture of tank on top of collapsed bridge]
Hey HECRAS! Evaluate this!
Rate of Scour
Scour will reach its maximum depth in:
-
sand and gravel bed materials in hours;
-
cohesive bed materials in days;
-
glacial tills, sand stones and shales in months;
-
limestones in years and dense granites in centuries.
Interstate 90 crossing of Schoharie Creek near Amsterdam, NY on April
5, 1987
Bridge Failure Due to Scour, Glasgow, Missouri
Components of Scour
I - Long Term Aggradation or Degradation
+
II - Contraction Scour
+
III - Local Scour (Piers and Abutments)
=
Total Scour
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Piers and Abutments
Long-Term Aggradation or Degradation
Long-term aggradation or degradation is due to natural or man-made
induced causes which can affect the reach of river on which the bridge
is located. The challenge for the engineer is to estimate long-term
bed elevation changes that will occur during the life of the structure.
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Piers and Abutments
Contraction Scour
Involves removal of material from bed and banks across most of the
channel width.
May be "Live-bed Contraction Scour" or "Clear-water Contraction Scour"
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Piers and Abutments
Local Scour
At Piers: Pier scour occurs due to the acceleration of flow around
the pier and the formation of flow vortices. The "horseshoe vortices" remove
material from the base of the pier and creates a scour hole.
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Piers
Scour at a cylindrical pier
[diagram of Horseshoe and Wake Vortices around a Cylindrical Element]
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Piers and Abutments
Local Scour
At Abutments:The obstruction of the flow forms a horizontal vortex
starting at the upstream end of the abutment and running along the
toe of the abutment and forms a vertical wake vortex at the downstream
end of the abutment
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Abutments
Abutment scour
[diagram of Wake Vortex and Horizontal Vortex]
Contraction scour (somewhere) in Missouri during May and June of 1995.
Walnut Street Bridge (Harrisburg, PA) collapse--January 1996
[photo of collapsed bridge]
[photo of collapsed bridge]
This bridge (location unknown) failed due to scour at the base of the piers
caused by a turbulent horseshoe vortex system.
Bridge on the Enoree river in South Carolina which failed due to scour at
the base of the piers caused by a turbulent horseshoe vortex system.
March 10, 1995 - Interstate 5 near Coalinga, over the Arroyo Pasajero
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Piers and Abutments
Long-Term
Aggradation or Degradation
Procedures for estimating long-term aggradation and degradation at bridges
are presented in HEC20 (Stream Stability at Highway Structures) and are not
a part of this presentation
I - Long Term Aggradation or Degradation
II - Contraction Scour (Cases)
III - Local Scour at Piers and Abutments
Contraction Scour Cases
-
Case I - Overbank flow on a floodplain being forced back to the main
channel by the approaches to the bridge
-
Case II - Flow is confined to the main channel (no overbank flow). The
normal river channel width becomes narrower due to the bridge itself
or the bridge site is located at a narrowing reach of river
-
Case III - A relief bridge in the overbank area with little or no bed
material transport in the overbank area (clear water scour)
-
Case IV - A relief bridge over a secondary stream in the overbank area
with bed material transport (similar to case 1)
I - Long Term Aggradation or Degradation
II - Contraction Scour (Exhibits)
III - Local Scour at Piers
Case 1a - Abutments project into channel
[diagram of overbank flow on a floodplain being forced back to the main
channel by the approaches to the bridge]
I - Long Term Aggradation or Degradation
II - Contraction Scour (Exhibits)
III - Local Scour at Piers
Case 1b - Abutments at edge of channel
[diagram of overbank flow on a floodplain being forced back to the main
channel by the approaches to the bridge]
I - Long Term Aggradation or Degradation
II - Contraction Scour (Exhibits)
III - Local Scour at Piers
Case 1c - Abutments set back from channel
[diagram of overbank flow on a floodplain being forced back to the main
channel by the approaches to the bridge]
I - Long Term Aggradation or Degradation
II - Contraction Scour (Exhibits)
III - Local Scour at Piers
Case 2a - River narrows
[diagram of flow confined to the main channel (no overbank flow)]
I - Long Term Aggradation or Degradation
II - Contraction Scour (Exhibits)
III - Local Scour at Piers
Case 2b - Bridge abutments and or piers constrict flow
[diagram of flow confined to the main channel (no overbank flow)]
I - Long Term Aggradation or Degradation
II - Contraction Scour (Exhibits)
III - Local Scour at Piers
Case 3 - Relief bridge over floodplain
[diagram of a relief bridge in the overbank area with little or no bed material
transport in the overbank area (clear water scour)]
I - Long Term Aggradation or Degradation
II - Contraction Scour (Exhibits)
III - Local Scour at Piers
Case 4 - Relief bridge over secondary stream
[diagram of a relief bridge over a secondary stream in the overbank area
with bed material transport (similar to case 1)]
I - Long Term Aggradation or Degradation
II - Contraction Scour (Types)
III - Local Scour at Piers and Abutments
Contraction Scour Types
Live-bed Contraction Scour:
This occurs when bed material is already being transported into the contracted
bridge section from upstream of the approach section (before the Contraction
reach).
I - Long Term Aggradation or Degradation
II - Contraction Scour (Types)
III - Local Scour at Piers and Abutments
Contraction Scour Types
Clear-water Contraction Scour: This occurs when the bed material sediment
transport in the uncontracted approach section is negligible or less than
the carrying capacity of the flow.
I - Long Term Aggradation or Degradation
II - Contraction Scour (Type Determination)
III - Local Scour at Piers and Abutments
Live-bed or Clear-water Determination
Clear-water: Vc > mean velocity
Live-bed: Vc < mean velocity
where Vc = critical velocity for beginning of motion
I - Long Term Aggradation or Degradation
II - Contraction Scour (Determination)
III - Local Scour at Piers and Abutments
Live-bed or Clear-water Determination
Clear-water: Vc > mean velocity
Live-bed: Vc < mean velocity
Where:
Vc=10.95y11/6D501/3 (Laursen,
1963)
Y1 =depth of flow in the upstream of bridge
D50 = median diameter of bed material
I - Long Term Aggradation or Degradation
II - Contraction Scour (Live-bed)
III - Local Scour at Piers and Abutments
Live-bed Contraction Scour Determination
y2/y1 = [Q2/Q1]6/7[W1/W2]K1(n2/n1)K2 (Laursen,
1960)
And
ys = y2 - y0
Where:
Ys = Average depth of scour
Y0 = Average depth of flow in the contracted section before scour
Y1 = depth of flow in the upstream of bridge
Y2 = depth of flow in the contracted section
W1 = bottom width upstream of bridge
W2 = bottom width in the contracted section
Q1 = flow in the upstream of bridge transporting sediment
Q2 = flow in the contracted section
n1 = Manning's "n" for the upstream of bridge
n2 = Manning's "n" for the contracted section
K1 and K2 = Exponents depending upon the mode of bed
material transport
| V*/w |
K1 |
K2 |
Mode of Bed Material Transport |
| <0.50 |
0.59 |
0.066 |
Mostly contact bed material |
| 0.50 to 2.0 |
0.64 |
0.21 |
Some suspended bed material discharge |
| >2.0 |
0.69 |
0.37 |
Mostly suspended bed material discharge |
I - Long Term Aggradation or Degradation
II - Contraction Scour (Live-bed)
III - Local Scour at Piers and Abutments
Live-bed Contraction Scour Determination
| V*/w |
K1 |
K2 |
Mode of Bed Material Transport |
| <0.50 |
0.59 |
0.066 |
Mostly contact bed material |
| 0.50 to 2.0 |
0.64 |
0.21 |
Some suspended bed material discharge |
| >2.0 |
0.69 |
0.37 |
Mostly suspended bed material discharge |
V* = (t/r)1/2 =
(gy1S1)1/2
Where:
V* = Shear Velocity in the upstream section
w = Fall velocity of bed material
t = Shear stress on the bed
r = Density of water
g = Acceleration of gravity
S1 = Slope of the energy grade line of main channel
I - Long Term Aggradation or Degradation
II - Contraction Scour (Live-bed)
III - Local Scour at Piers
[graph of fall velocity of sand-sized particles with specific gravity of
2.65 in metric]
I - Long Term Aggradation or Degradation
II - Contraction Scour (Live-bed)
III - Local Scour at Piers and Abutments
Live-bed Contraction Scour Determination
y2/y1 = [Q2/Q1]6/7[W1/W2]K1 Modified
(Laursen, 1960)
And
ys = y2 - y0
Where:
Ys = Average depth of scour
Y0 = Average depth of flow in the contracted section before scour
Y1 = depth of flow in the upstream of bridge
Y2 = depth of flow in the contracted section
W1 = bottom width upstream of bridge
W2 = bottom width in the contracted section
Q1 = flow in the upstream of bridge transporting sediment
Q2 = flow in the contracted section
K1 = Exponents depending upon the mode of bed material transport
| V*/w |
K1 |
Mode of Bed Material Transport |
| <0.50 |
0.59 |
Mostly contact bed material |
| 0.50 to 2.0 |
0.64 |
Some suspended bed material discharge |
| >2.0 |
0.69 |
Mostly suspended bed material discharge |
I - Long Term Aggradation or Degradation
II - Contraction Scour (Live-bed)
III - Local Scour at Piers and Abutments
Live-bed Contraction Scour Determination
| V*/w |
K1 |
Mode of Bed Material Transport |
| <0.50 |
0.59 |
Mostly contact bed material |
| 0.50 to 2.0 |
0.64 |
Some suspended bed material discharge |
| >2.0 |
0.69 |
Mostly suspended bed material discharge |
V* = (t/r)1/2 =
(gy1S1)1/2
Where:
V* = Shear Velocity in the upstream section
w = Fall velocity of bed material
t = Shear stress on the bed
r = Density of water
g = Acceleration of gravity
S1 = Slope of the energy grade line of main channel
I - Long Term Aggradation or Degradation
II - Contraction Scour (Clear-water)
III - Local Scour at Piers and Abutments
Clear-water Contraction Scour Determination
ys/y1 = 0.13[Q/Dm1/3y17/6W]6/7 -
1 (Laursen, 1963)
Where Dm is the effective mean diameter of the bed material (1.25
D50)
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour (at Piers)
Local Scour
at Piers
Pier scour occurs due to the acceleration of flow around the pier and the
formation of flow vortices. The "horseshoe vortices) remove material from
the base of the pier and creates a scour hole.
[photo of telephone pole in snow]
This is erosion caused by the formation of a horseshoe vortex system at
the base of a telephone pole. This occurred during the blizzard of '96 in
the northeast.
[picture of snowed-in van]
This is erosion due to the formation of a horseshoe vortex around a van.
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour (at Piers)
Pier Scour Factors
-
The greater the velocity upstream of the pier the deeper the scour
-
An increase in flow depth can have a significant influence on the scour
depth. It can be as much as twice.
-
As the width of the pier increases, so does the scour depth
-
If pier is skewed to the flow, the length can have an influence on the
scour depth. When doubling the length, the scour depth increased by 30-60%
depending upon angle of attack.
-
Size and gradation of the bed material generally will not have an effect
on the scour depth. What differs is the time it takes to achieve the
maximum scour.
-
Shape of the pier plays an important part in the scour depth.
-
Formation of debris can increase the width of the pier, change its shape
or change its projected length.
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Piers
Live-bed and Clear-water Scour Determination
by CSU (Richardson 1990 eq.)
Ys/Y1 = 2.0K1K2K3K4(a/Y1)0.65 Fr10.43
where:
Ys = Scour depth
Y1 = Flow depth directly upstream of the pier
K1 = Correction factor for pier nose shape
K2 = Correction factor for angle of attack of flow
K3 = Correction factor for bed condition
K4 = Correction factor for armoring by bed material size
a = Pier width
Fr1 = Froude number directly upstream of pier
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Piers
Common Pier Shapes
To be used for determining the K1 (Pier Nose Shape correction
factor) in equation:
Ys/Y1 = 2.0K1K2K3K4(a/Y1)0.65 Fr10.43
[diagram of Pier Nose shapes]
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Piers
K1 is the Pier Nose Shape correction factor in equation: Ys/Y1 =
2.0K1K2K3K4(a/Y1)0.65 Fr10.43
Table 6.1 Correction Factor, K1,
for Pier Nose Shape. |
| Shape of Pier Nose |
K1 |
| (a) Square nose |
1.1 |
| (b) Round nose |
1.0 |
| (c) Circular cylinder |
1.0 |
| (d) Group of cylinders |
1.0 |
| (e) Sharp nose |
0.9 |
For angle of attack < 5 deg. For greater angles, K1=1.0 and
K2 dominates
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Piers
K2 = (cos q + (L/a)sin q)0.65
K2 is the Angle of Attack correction factor in equation: Ys/Y1 =
2.0K1K2K3K4(a/Y1)0.65 Fr10.43
Table 6.2 Correction Factor, K2,
for Angle of Attack, q, of the Flow. |
| Angle |
L/a=4 |
L/a=8 |
L/a=12 |
| 0 |
1.0 |
1.0 |
1.0 |
| 15 |
1.5 |
2.0 |
2.5 |
| 30 |
2.0 |
2.75 |
3.5 |
| 45 |
2.3 |
3.3 |
4.3 |
| 90 |
2.5 |
3.9 |
5.0 |
Angle = skew angle of flow
L = length of pier, m |
Notes: K2 should only be applied when the
entire length is subjected to the attack of flow
K2 max = 5.0
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Piers
K3 is the Bed Condition correction factor in equation: Ys/Y1 =
2.0K1K2K3K4(a/Y1)0.65 Fr10.43
Table 6.3 Increase in Equilibrium
Pier Scour Depths, K3, for Bed Condition. |
| Bed Condition |
Dune Height m |
K3 |
| Clean-Water Scour |
N/A |
1.1 |
| Plane bed and Antidune flow |
N/A |
1.1 |
| Small Dunes |
3 > H >= 0.6 |
1.1 |
| Medium Dunes |
9 > H >= 3 |
1.2 to 1.1 |
| Large Dunes |
H >= 9 |
1.3 |
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Piers
K4 is the Correction Factor for armoring by bed-material size
in equation: Ys/Y1 = 2.0K1K2K3K4(a/Y1)0.65 Fr10.43
K4 min = 0.4
If D50 < 2mm or D95 < 20mm, then K4 =
1.0
If D50 >= 2mm and D95 >= 20mm then
K4 = (VR)0.15
where VR = [(V1 - VicD50)
/ (VcD50 - VicD95)] > 0
VicDx = 0.645(Dx/a)0.053 VcDx
VcDx = KuY11/6Dx1/3
VicDx = Approach velocity required to initiate scour at the pier
for grain size Dx
VcDx = critical velocity for incipient motion for grain size
Dx
y1 = Depth of flow just upstream of the pier, excluding local
scour, m (ft)
V1 = Velocity of the approach flow just upstream of the pier,
m/s (ft/s)
Dx = Grain size for which x percent of the bed material is finer,
m (ft)
a = Pier width (ft)
Ku = 6.19 SI Units
Ku = 11.17 English Units
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Abutments
Local Scour
at Abutments
Local scour occurs at abutments when the abutment and embankment obstruct
the flow. The obstruction of the flow forms a horizontal vortex starting
at the upstream end of the abutment and running along the toe of the abutment
and forms a vertical wake vortex at the downstream end of the abutment
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour (at Abutments)
Abutment failure Causes
-
Overtopping of abutments or approach embankments
-
Lateral channel migration or stream widening processes
-
Contraction scour
-
Local scour at one or both abutments
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Abutments
Abutment Shapes
[diagram of abutment shapes]
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour (at Abutments)
Abutment Scour Factors
-
Velocity of the flow just upstream of
the abutment
-
Depth of flow
-
Length of the abutment if skewed to the
flow.
I - Long Term Aggradation or Degradation
II - Contraction Scour (Clear-water)
III - Local Scour at Abutments
Live-bed and Clear-water
Scour Determination
I - Froelich's Live-bed Abutment Scour Equation
(when the ratio of the length of the abutment (normal to flow) to flow depth <=
25)
II - Hire Live-bed Abutment Scour Equation
(when the ratio of the length of the abutment (normal to flow) to flow depth > 25)
I - Long Term Aggradation or Degradation
II - Contraction Scour (Clear-water)
III - Local Scour at Abutments
I - Froelich's (1989) Live-bed Abutment Scour Equation
(ratio of the length of the abutment (normal to flow) to flow depth <=
25)
Ys/Ya = 2.27 K1K2(L')0.43ya0.57Fr10.61 +
1
Where
K1 = Coefficient for abutment shape
K2 = Coefficient for angle of embankment to flow
K2 = (q/90)0.13
q < 90 if embankment points downstream
q > 90 if embankment points upstream
L' = Length of active flow obstructed by the embankment
Ae = Flow area of the approach cross section obstructed by the
embankment
Fr = Froude Number of approach flow upstream of the abutment = Ve/(gya)1/2
Ve = Qe/Ae
Qe = Flow obstructed by the abutment and approach embankment
Ya = Average depth of flow on the floodplain (Ae/L)
L = Length of embankment projected normal to the flow
Ys = Scour depth
I - Long Term Aggradation or Degradation
II - Contraction Scour (Clear-water)
III - Local Scour at Abutments - Froelich
Abutment Coefficients
K1
| Description |
K1 |
| Vertical-wall abutment |
1.00 |
| Vertical-wall abutment with wing walls |
0.82 |
| Spill-through abutment |
0.55 |
K2
K2 = Coefficient for angle of embankment to flow
K2 = (q/90)0.13
q < 90 if embankment points downstream
q > 90 if embankment points upstream
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour at Abutments - Froelich
Abutment Skew
[diagram of abutment skew]
For abutments angles upstream, the depth of scour increases
I - Long Term Aggradation or Degradation
II - Contraction Scour (Clear-water)
III - Local Scour at Abutments - HIRE
II - HIRE (Richardson 1990) Live-bed Abutment Scour Equation
(Recommended when the ratio of the length of the abutment (normal to flow)
to flow depth > 25)
Ys/Y1 = 4[K1/0.55]K2Fr10.33
K1 = Coefficient for abutment shape
K2 = Coefficient for angle of embankment to flow as calculated
for Froelich's equation
Fr = Froude Number based upon the velocity and depth adjacent to and upstream
of the abutment
Y1 = Depth of flow at the abutment on the overbank or in the
main channel.
Ys = Scour depth
I - Long Term Aggradation or Degradation
II - Contraction Scour
III - Local Scour (at Abutments)
Suggested design approach
- No reliable equations are available to predict all hydraulic flow conditions
that may be reasonably expected to occur. Engineering judgment is required.
- Place piers & abutment on scour resistant foundation such as rock
or deep foundation.
- Pilings should be driven below the elevation of long-term degradation
and contraction scour.
- Need to consider the potential for lateral channel instability.
- Spread footings should be placed below the elevation of total scour.
General Design Procedure
-
Select flood event
-
Develop water surface profiles
-
Estimate total scour
-
Plot total scour depth
-
Evaluate answers to above
-
Evaluate the bridge type, size and
location
-
Perform bridge foundation analysis
-
Repeat the above procedure and calculate
the scour for a super flood (500-year recommended). If hydrology
for this flood is unavailable, use 1.7xQ100.
HECRAS EXAMPLE
[scan of page titled Interim Procedure for Estimating Pier Scour with
Debris]
[scan of page titled Topwidth of Scour Holes]
[untitled diagram]
Mohammad Salim and Sterling Jones published "Scour Around Exposed
Pile Foundations" in 1996 which more accurately estimates this case.
However, more studies are needed for verification.
[diagram of scour components for a complex pier]
Pressure Flow Scour
WS is > than the LC and plunges flow downward.
[diagram of pressure flow scour]