1. Anthony Alvarado, PE, CFM National Hydraulic Engineering Conference
Highways in Steep Canyons: SRH-2D Modeling and GIS-Automated Revetment Design
John Hunt, PE
Anthony Alvarado, PE, CFM
Will DeRosset, PE
Brian Varrella, PE, CFM
National Hydraulic Engineering Conference
Portland, Oregon
August 2016

2. Overview US 34 through Big Thompson Canyon Introduction and Background
Hydraulic Analysis using SRH-2D
Road Embankment Stability Design
Scour Evaluation in Support of Design
Summary of Findings

3. Big Thompson Watershed
78 mile long river with over 6,600 feet of fall
832 square mile watershed
Starts in Rocky Mountain National Park
Flows through Estes Park, Big Thompson Canyon, and Loveland

4. Flood of September 2013
Large frontal system, extended period of steady rainfall
Generally lower peak discharge rates than previous flood of record (half as much as flood)
Much longer duration of high flows than 1976
Peak flow roughly equal to 100-year flood in many locations
Q2013 = 15,500 cfs vs Q100=15,449 cfs

5. September 2013 Big Thompson Flood Damage

6. US 34 Permanent Repair - Hydraulic Design Challenges
100-year design flood
Floodway compliance
Road Embankment Stability
Access Bridges
US 34 Bridges

7. Achieving Required Road Embankment Stability
Challenges
Steep longitudinal profile of river
Combined with high flood discharges
Leads to extreme velocities and shear stresses
Large boulders were transported by the flood in some reaches
4-ft D50 riprap was removed from wall toes
Slope range 1.5% to 5%

8. Extreme Erosive Power of a Canyon Flood

9. Embankment Stability Design
Design Concepts
Shift the road onto bedrock
Used only in limited locations due to high cost
Walls or other vertical treatments
Where encroachment into floodplain is constrained
Armor the embankment slope
Primary treatment used

10. Extent of Design-Level 2D Modeling
Roughly 8 miles of detailed SRH-2D model
Drake
The Narrows
Total extent of design-level modeling = 8 miles
Divided into many segments for run time concerns

11. Design Use of Model Results
Riprap sizing for embankment protection
Scour evaluation for toe depth of revetment
Stability impacts of access bridge designs
Opposite bank stability impact assessment

12. Sample of Model Results

13. Sample of Model Results

14. Calculating Required Size of Loose Riprap
Contours of Required Riprap D50 from SMS Data Calculator
Red = 4 ft D50
Yellow = 3 ft D50
HEC-23 Design Guide 4 Eqn 4.1
Green = 2 ft D50
Blue = 1 ft D50 or less

15. Embankment Stability Design Loose Riprap

16. Embankment Stability Design Lift-Placed Matrix Riprap

17. Determining Toe Depth Requirements
Contraction scour
Bendway scour
Wall scour
Never less than 5 feet below channel thalweg
Bedrock considerations

18. Scour Calculations Semi-Automated with GIS
Using ArcView ModelMaker
Import solution shape files from SMS 2-D
Water surface elevation
Depth
Velocity
Use ModelMaker to develop properties across XSEC lines
Discharge
Hydraulic Depth
Average Water Surface
Average Velocity

19. Contraction Scour Automation
HEC-18 Live-Bed Contraction Scour Equation (Eqn. 6.2)

20. Bendway Scour Automation
HEC-23 Scour at Protected Bendways (Eq. 4.5)

21. Summary of Findings
River canyon environments pose major stability threat to parallel highways
Steep longitudinal slopes lead to extreme flood velocities and shear stresses
Two-dimensional modeling is justified
Innovative embankment protection methods are needed
Long canyons call for automation within SMS and GIS

22. Manager-River Engineering National Hydraulic Engineering Conference
THANK YOU !
John Hunt, PE
Manager-River Engineering
(mobile)
National Hydraulic Engineering Conference
Portland, Oregon
August 2016