1. Variability in Scour Methods in the Western United States
Suzie Monk & Mikell Warms
WEST Consultants
20 minutes + 5 min for questions

2. Background Types of scour Lateral migration Long-term General
Contraction
Bend
Bedform
Local scour
Piers
Abutments
Sea wall
Lateral migration
Lobutcha Ck, USGS
Outline different types of scour that we could include in each analysis, when/where they’re appropriate (next slides)
Safety factor (some states require, others do not)

3. Data Needed Hydraulic data (model output) Planform data Soils data
Depth
Top width
Area
Velocity
Froude #
Planform data
Radius of curvature
Soils data
D50

4. Regulations Arizona California Oregon Local Flood Control District
Arizona DOT and DWR
FHWA (HEC-18)
California
California DOT
Oregon
Local restrictions
Oregon DOT
NOAA (SLOPES)
FHWA (HEC-18)
Background information for the site with pictures
Data from RAS
What types of scour do we need to include in the analysis?
Results for each type of scour
Total scour and how implemented in construction

5. Long-term Natural or man-induced influences
Aggradation: the deposition of material from upstream
Degradation: reach-wide lowering of the streambed
Deficit in upstream sediment supply
*aggradation does not contribute to total scour to be conservative
Alamogordo Canal

6. Long-term Scour Methods
Compare survey/historical data
Calculate equilibrium slope
SSCAFCA (New Mexico)- sediment-stable systems
Schocklitsch (USBR, 1984)- clear water sediment loading condition
Other more complex equations…
Nearby hard points
Method chosen is determined mostly by what data are available
Southern Sandoval County Arroyo and Flood Control Authority- simplified equation developed (assumptions include subcritical and uniform flow, very wide channel) because it is hard to get information for sediment load

7. Arizona Example Compare survey from 1989 and 2000 (same alignments)
Compare at thalweg and average XS elevation
Wash in Maricopa County
Used effective FEMA model and survey provided by FCDMC for this study because the scour was calculated for transmission lines that would be placed in the floodplain– need to be sure work is directly comparable with effective FEMA study
Historic survey data
0.1 ft erosion average over reach on average channel
0.0 ft erosion average over reach on thalweg
No deposition or degradation at this reach

8. California Example Method 1: Historic data
ft/yr; 4.7 ft over 50 years
Method 2: Horizontal slope
4.4 ft from grade control structure
Describe project (Grade control structure upstream, releases from dam upstream
Original ground before channel went in
ft/yr, so for a 50-year design life that’s 4.7 ft scour as long as releases from upstream stay the same (which they should because there is a dam upstream that controls releases)
**the weird dip downstream of the bridge is because of cattle grazing– stirs up sediment and such– but that’s a separate issue; field observations don’t suggest that it would move upstream because the channel is widening and such there (only reach that is active grazing land)

9. California Example Method 3: equilibrium slope
Schoklitsch Eqn: % or 4.2 ft scour
SSCAFCA Eqn: % or 4.3 ft scour
Schoklitsch Eqn: appropriate because the dam upstream limits the amount of sediment in the water
D = D50, B = channel bed width, Q = dominant discharge (10-year flood)
SSCAFCA Eqn: not really a stable system, but is appreaching stability because of the downstream grade control structure
Qd = dominant discharge, n = Manning, FD = width-depth ratio of flowing water, Fr is froude number (should be between 0.7 and 1)
**client with 4.4 ft from the horizontal projection

10. General Scour General lowering over short time periods
Contraction scour
Uniform or non-uniform
May be cyclic
Bend scour
Scour at outside of bend
Often occurs during a single event

11. General Scour Methods
USBR Abbott (1963)- ephemeral sandbed streams in the southwest
Lacey (1930)- natural river systems
Neill (1973)- channel constriction cases (bridge or structure)
Blench (1969)- hydraulic structure upstream reducing sediment inflow to the reach
USBR Mean Velocity (1984)- based on existing survey data
Competent Velocity (1984)- assumes scour will occur in channel cross-section until no bed material can be moved
Laursen (1960)- Live-bed or clear-water scenarios (contraction scour only)
HEC-18 suggests Laursen for contraction scour
Method chosen is based on what kind of system (some equations aren’t appropriate at all) and what all we need to account for (just bend? Just contraction? Both?)
- Abbott is a function of unit water discharge (cfs/ft)
Lacey takes into account curvature by means of a coefficient; function of design discharge, median diameter size, silt factor, mean depth
Neill takes into account curvature by means of a coefficient; average depth at bankfull, bankfull discharge (cfs/ft), design discharge (cfs/ft), coefficient representing gradation
Blench function of curvature coefficient, discharge (cfs/ft); zero bed factor (from plot, function of bed material D50)
USBR mean velocity function of multiplying factor and mean depth
Competent velocity function of depth at design flow, velocity at design flow, and competent velocity (from plot, function of bed material D50 and depth)
Laursen function of upstream depth, flow, width, and flow and width at the contracted section

12. Arizona Example
CDW or CW comparison of several methods

13. California Example Laursen’s method LA County methods
Sediment model (not used)
Plot based on velocity
Big Dry Creek use of live-bed equation from Laursen
Laursen is live bed scour because V > Vc; results in scour of 1.1 ft
LA plot method is 1.2 ft, but Laursen is more defensible

14. Bedform Scour Sand-bed streams Troughs between crests of bedforms
Dunes- lower regime flow
Antidunes- transitional or upper regime flow
Simons and Senturk (1992)
Mostly in the southwest
Usually fairly small (a couple of tenths of a foot)
Function of hydraulic depth

15. Low-flow Incisement Occurs during low-flow conditions after flood
Mostly dry systems
Min. 1 ft, possibly greater than 2 ft (FCDMC)
In ‘washes’; usually estimate as 1 ft and move on (other scour estimates are conservative too)
I haven’t seen this used anywhere other than the southwest, and mostly in Maricopa County and in counties that adopt FCDMC standards

16. Local Scour
Local scour involves removal of material from around piers, abutments, spurs, embankments, etc.
It is caused by an acceleration of flow and resulting vortices induced by obstructions to the flow
occurs at bridge piers, abutments, embankments, transmission line foundations, and other structures that
obstruct flow

17. Arizona/California Example
HEC-18 (5th Ed.) version of CSU scour eqn
Correction factors for pier shape, angle of attack, and bed condition
a = pier width
Y1 = upstream depth
Fr1 = upstream froude number (function of velocity)
Ys = scour depth
Example of values from Arizona; used in California too (but on an actual bridge pier)

18. Oregon Example Arch culvert scour (HEC-18) With wingwall
Without wingwall
HEC-18 equation for arch culvert– includes contraction and local scour
Ymax is depth at entrance corner including contraction and local scour
Qbi is flow blocked by road embankment on one side
Q is discharge through culvert
Wc is width
D50 is median diameter
Ku is a constant
Total scour should use the max scour calculated (don’t vary on either side in the total estimate)

19. Lateral Migration
May affect the stability of piers, erode abutments, or approach the roadway
Factors that affect lateral stream movement:
Geomorphology of the stream,
Location of the crossing on the stream
Flood characteristics
Bed and bank materials
Natural process that occurs usually within the floodplain (braided channels are a great example, but most streams move at least a little bit)

20. Lateral Migration Methods
Arizona State Standards
Simplified method for watersheds <30 mi2
Otherwise:
Historic data
FEMA Lateral Migration Setback Analysis (1999)
D is depth to the lowest point in the cross section and Zt is total scour for the 100-year event
FEMA equation developed for AZ but may be used in other areas; setback measured from bankline

21. Arizona Example
Transmission line scour– very conservative approach (FCDMC)
4(D+Zt)

22. California Example Historic data- 0.011 and 0.512 feet/year
FEMA setbacks: 26.5 ft on each side
1991, 1998, 2003, 2013 from aerials
Lots of possible reasons for widening– local scour (pier and abutment), general degradation, mass wasting from slip-plane geotechnical failure on the edges due to wetting processes, livestock impact
More widening just downstream of the bridge than anywhere else in the system– remember this is a constructed channel– probably because of scour processes at the bridge, but also hindered by the bridge itself (so it doesn’t travel upstream)
At current rate over 50-year design life, results in 2063 lines; assumes releases will be same as historic releases and no active maintenance of the channel
FEMA setbacks (for straight channel, function of Q100) result in ‘calculated’ bank lines– worst case scenario

23. Sea Wall Scour Due to wave action against a structure
Cyclic with the tide
FHWA: Highways in the Coastal Environment (2008)
USACE: Coastal Engineering Manual (2006)
USACE: Design of Coastal Revetments, Seawalls, and Bulkheads (1995)

24. California Example High tide Water level at base of sea wall Low tide
Tide conditions
Sheltered location on Shelter Island, San Diego Bay
Previous Sea Wall Failed due to Wave Scour
Design New Sea Wall to withstand Wave Scour
High tide
Water level at base of sea wall
Low tide
Water level 50 feet from sea wall

25. California Example Sheltered Location  Depth-Limited Sea State
Depth is shallow immediately offshore
Large “breaker” waves not anticipated
Depth-Limited Design Sea Level (d)
d = Extreme High Tide year Sea Level Rise + (wave setup) [feet above MLLW]
Maximum Design Wave Height (Hmax) [FHWA]
Wave setup ~= 0.5 – 1 ft for San Diego Area (from local references)
All values references to Mean Low Low Water (MLLW)
- Equation is only suitable because it is a sheltered area, so can neglect storm surcharge and turbulence and use depth-limited sea state
- Scour depth is a function of the maximum design wave height
- Design wave height (USACE 1995) is depth-limited design sea level (sum(extreme high tide, sea level rise, freeboard)) minus (the surveyed ground level)
- Conservative estimate of sea wall toe scour depth
- Therefore, Sea Wall Toe needs to be built approximately 6 feet below surveyed ground level
*2.53 denotes surveyed ground level at current sea wall (feet above MLLW)

26. Takeaways
Most calculations are standard across the west (federal standards)
Exceptions:
Bedform (sand bed)
Low-flow incisement (desert hydrology)
Coastal
There may be preferred methods in some areas
Lateral migration
Longterm
Local
Safety factor in AZ

27. Questions?