Table 4 (online only).Projected climate change effects and adaptation options in the context of road management at Olympic National Forest and Olympic National Park. Projected effects on physical watershed processes are based on projected climate change effects on Olympic Peninsula, including increased winter air temperatures/fluctuation above and below freezing; more precipitation falling as rain rather than snow; increased winter and spring streamflows in some types of watersheds; decreased summer streamflows in some types of watersheds; increased winter precipitation and runoff; increased storm intensity; increased flood frequency and magnitude in some types of watersheds; and elevation shifts in transition (rain on snow) zones.

Program / Project / Projected effects on physical watershed processes / Current and expected sensitivities / Adaptation management options and strategies
Road maintenance / Planning /
  • Increased flow volume
  • Increased mass wasting and avalanches
  • Increased sediment delivery
  • Increased floodplain inundation
  • Increased channel migration
/
  • Culvert capacity
  • Water diversion
  • Fill slope failures
  • Stream-adjacent road failure
/
  • Prioritize road treatment by watershed risk and road risk (the roads with the most sensitivities and that are most connected to streams)

Implementation /
  • Increased flow volume
  • Increased mass wasting/debris flows
  • Increased sediment delivery to culvert inlets and ditches
  • Increased rate and volume of water delivery to channels
/
  • Culvert capacity
  • Culvert plugging
  • Water diversion
  • Fill slope failures
  • Extension of channel network (development of first order channels)
/
  • Increased maintenance (preparation and response), including:
  • culvert cleaning
  • armoring outlets and ditches
  • installing water bars and drivable dips

Road operations / Design
- water crossing
- fish passage /
  • Increased flow volume
  • Increased sediment and wood transport
  • Increased channel migration
/
  • Culvert capacity
  • Foundation scour
  • Lateral channel adjustments
/
  • Design more resilient structures (e.g., design resilient bridges and larger structures)

Design
- bank protection /
  • Increased flow volume
  • Increased erosion and scour
  • Increased channel migration
  • Increased mass wasting and/or avalanches
/
  • Stream-adjacent road failure
  • Cut-bank failures on montane and subalpine roads
/
  • Either relocate road segment, decommission road segment, or accept higher maintenance costs
  • Install retaining walls or other cut-bank stabilization

Design
- reconstruction
- drainage upgrades
- stabilization
- storm-proofing /
  • Increased flow volume
  • Increased rate and volume of water delivery to channels
  • Increased sediment and wood transport
  • Increased mass wasting/debris flows
  • Increased sediment delivery to culvert inlets and ditches
/
  • Culvert capacity
  • Culvert plugging; water diversion
  • Fill slope failures
  • Extension of channel network (development of first order channels
/
  • Implement more conservative design elements (more intensive treatments such as larger diameter culverts, closer spacing between ditch relief culverts and/or waterbars
  • Increase maintenance frequency of drainage features