Peak Flows, Debris and Washout Prevention: Options for Mitigating the Risk of Road Failure

2. Peak Flows, Debris and Washout Prevention: Options for Mitigating the Risk of Road Failure Jeff Grizzel, HydrologistWashington Department of Natural Resources Western Forestry and Conservation Association Road Drainage Workshop April 22, 2010 Springfield, Oregon

3. Outline • What is a “Washout”? • Why Be Concerned? • State Regulatory Requirements • Causes of Culvert/Road Failure • Options for Mitigating Failure Risk

4. What is a “Washout”? • Partial or complete loss of road prism • Damage to, or loss of, culvert or other crossing structure • Erosion of road fillslope and/or fill around culvert • Road is generally impassable S. Slaughter

5. Why Be Concerned? • Impacts to publicly owned resources

6. Why Be Concerned? • Impacts to publicly owned resources • Loss of infrastructure

7. Why Be Concerned? • Impacts to publicly owned resources • Loss of infrastructure • Threats to public safety

8. Why Be Concerned? • Impacts to publicly owned resources • Loss of infrastructure • Threats to public safety • Legal liability

9. Why Be Concerned? • Impacts to publicly owned resources • Loss of infrastructure • Threats to public safety • Legal liability • Public perception

10. Regulatory Requirements • Oregon: • “Pass a peak flow that at least corresponds to the 50-year return interval” (OAR 629-625-0320(2)(a)) • Washington: • “All permanent culverts must be designed to pass the 100 year flood event with consideration for the passage of woody debris likely to be encountered” (WAC 222-24-040(3)(a))

11. Regulatory Requirements • Idaho: • “Design culverts for stream crossings to carry the fifty (50) year peak flow…” (IDAPA 20.02.01.040(2)(e)(ii)) • California: • “All permanent watercourse crossings that are constructed or reconstructed shall accommodate the estimated 100-year flood flow, including debris and sediment loads” (CCR 14.4.12.923,943,963)

12. Causes of Culvert Failure Wood = 79% n = 57 Source: Cafferata et al. 2004. Designing watercourse crossings for passage of 100-year flood flows, wood, and sediment. California Forestry Rep. No. 1, California Department of Forestry and Fire Protection, Sacramento, CA. 34 p.

13. Mitigating Failure Risk • Three Ingredients: • Ensure adequate culvert capacity • Minimize potential for inlet obstruction • Ensure fill stability

14. Mitigating Failure Risk • Ensuring adequate culvert capacity: • Estimate design flow (50- or 100-year flow) • Flood frequency equations • Q100 = a AaPb • http://water.usgs.gov/osw/programs/nss/pubs.html • Oregon: Adams et al. 1986, Research Bull. 55, Forest Research Lab, OSU R. Roames 0.5 USDOT Pub. No. FHWA-NHI-01-020

15. Mitigating Failure Risk • Ensuring adequate culvert capacity: • Required culvert diameter (D) is often 50% to 75% of bankfull channel width (BFW) • Diameter accommodates design flow, but does it adequately account for debris load? http://www.dnr.wa.gov/Publications/ fp_board_manual_section03.pdf

16. Mitigating Failure Risk • Ensuring adequate culvert capacity: • Woody debris pieces whose length <BFW are generally more mobile • These same pieces are most likely to obstruct culvert inlet • Bottom line? As D approaches BFW, failure risk decreases 0 50 100 Failure Probability (%) 0 50 100 D (% of BFW)

17. Mitigating Failure Risk • Ensuring adequate culvert capacity: • Other considerations: • Mitered inlet • Arch or “squash” pipe • Proper depth • Proper alignment J. Cornell R. Roames

18. Mitigating Failure Risk • Prevent/minimize inlet obstruction: • Regular fall/winter season storm patrol • Removal of debris/sediment through regular maintenance • Trash rack at upstream end of catch basin (for specific situations only) J. Cornell

19. Mitigating Failure Risk • Ensure fill stability: • Must consider channel and valley morphology • Low-gradient, unconfined channels • Generally lower potential for catastrophic failure • High-gradient, confined channels • Generally higher potential for catastrophic failure

20. Mitigating Failure Risk • Ensure fill stability: • Armor earth fills in unconfined, low-gradient channels • “Shot” rock fills in confined, high-gradient channels R. Roames

21. Rock Fill Schematic LIGHT LOOSE HEAVY LOOSE (minimum depth = 3 feet) Fill is to consist of mechanically placed (not end dumped) durable angular rock. Rock should be clean, free of soil, fines, and debris. The intent is for the fill to consist of various sizes of rock which will interlock to increase structural integrity. Ideally, the fill should consist of 40 to 90% ½ yd3 rock with the remainder (10 to 60%) 2+ ft3 material.

22. Mitigating Failure Risk • Ensure fill stability: • Minimize amount of fill atop culvert • “Dip” or “sag” the road grade atop the culvert R. Roames

23. R. Roames

24. Crossing Structure Matrix Bridge? Avoidance? Rock Fill Loose Moderate Tight Very Tight Channel Confinement Armored Earth Fill Earth Fill Low Moderate High Very High Channel Gradient

25. Summary • Multiple reasons for preventing washouts or road failures • Regulatory requirements vary slightly between PNW states • Woody debris is the primary culprit • Reducing the risk of road failure can be accomplished by: • Ensuring adequate culvert capacity • Minimizing the potential for inlet obstruction • Ensuring fill stability