The hydrologic impacts of roads in forested watersheds

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The hydrologic impacts of roads in forested
watersheds

• Dr. Laura C. Bowling
• Assistant Professor
• Department of Agronomy

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Outline

• What are hydrologic effects?
• Mechanisms of hydrologic change
• Downstream implications
• Local effects
• Watershed-scale effects
• Recommendations

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Watershed Hydrology
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Mechanisms of Hydrologic Effect

• Direct
• Vegetation removal evapotranspiration reduction
• Surface compaction - infiltration reduction
• Indirect
• Interception of subsurface flow
• Moisture redistribution

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Direct Effects
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Vegetation Removal

• On average for the U.S., evapotranspiration is
  approximately 2/3 of precipitation
• Local increase in available water of 200

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Infiltration Reduction

• Depending on road surface type, traffic, etc., 50
  to 100 of precipitation runs off directly.
• In a native forest almost 100 of precipitation
  infiltrates into the soil.

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Indirect Effects
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Subsurface Interception

• Subsurface water may be routed through surface
  networks
• Decrease in travel times
• Local increase in flow volumes

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Subsurface Interception

• Seepage from soil cutslope in forested watershed,
  Western Washington

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Moisture Redistribution

• By confining runoff to constructed channels and
  culverts, roads can redirect natural flow
  pathways
• Important in both low-gradient and high-gradient
  environments

Ponded water behind gravel road, Prudhoe Bay,
Alaska
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Spring 2000, Vermillion River, Minn.
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Simulated moisture redistributionWestern
Washington
downslope

• Drier areas downstream of roads that previously
  would receive subsurface flow
• Localized saturated areas below culverts and
  water bars

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Downstream Implications

• Local Effects
• Watershed Scale Effects

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Local Effects
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Concentrated Flow

• Sheetwash and rill erosion from road surfaces
• Gully erosion downslope of roads and culverts
• Road deterioration and increased sediment in
  streams

Rill Erosion on gravel forest road, Western
Washington
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Gully Erosion, Northern California
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Tippecanoe County
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Moisture Redistribution

• Road prism failure
• Slope failure road induced mass wasting

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Road Slump, Western Washington
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Debris Flow, Western Washignton
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Watershed Scale Effects
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Simulated streamflow w/ and w/o forest roads
17 increase in peaks
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Simulated response to road constructionSub-basins
of the Deschutes River, WA
Road densities between 5 and 8 mi/mi2
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Road Connectivity
Connected (gully) Connected (stream crossing) Not
connected
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Increase in Drainage Density
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Factors affecting road connectivity

• Related to
• Volume of water discharged
• Erosion resistance below the culvert

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Connectivity Predictors

• Field study in Western Washington
• Hillslope curvature
• Distance to stream

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Synchronization of Peak Flows
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Road effects on peak flows

• Road connectivity is most important factor in
  determining road effects at watershed scales
• Effects may therefore be mitigated by road design
  to reduce road connectivity to the channel
  network

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Recommendations

• Minimize local flow increase
• Minimize flow restriction and redistribution
• Reduce connectivity

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Indiana Logging and ForestryBest Management
Practices

• Minimize local flow increase
• Avoid or minimize disturbance to areas of high
  quality trees
• Construct only as much road as necessary
• Keep road width to the minimum necessary to
  operate safely

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Indiana Logging and ForestryBest Management
Practices

• Minimize flow restriction and redistribution
• Construct roads to drain at all times
• Install culverts or other breaks at specified
  intervals on steep grades, where inside ditches
  are required
• Install a properly sized culvert when necessary.
• Clear away even minor obstructions that may have
  accumulated in drainage structures.

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Indiana Logging and ForestryBest Management
Practices

• Reduce Connectivity
• Minimize the number of stream crossings
• Drain water diverting structures and road runoff
  onto the undisturbed forest floor away from
  stream channels
• Maintain buffers between roads and waterways
• Break road grades frequently to divert water from
  road surface

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Recommended Maximum Spacing
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Summary

• Roads locally increase water supply
• Reduction of ET
• Increased surface runoff
• Intercepted subsurface runoff
• Redistribution and concentration of runoff.
• Local increase in erosion and mobile sediment
  through sheetwash, rill and gully erosion

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Summary, cont.

• Roads may increase floods downstream
• Degree of connectivity to the natural drainage
  system determines the extent
• Can lead to net increase in basin sediment yield
• Increased channel erosion and
• Increased mass wasting and debris torrents.

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Summary, cont.

• Mitigation strategies include diffusing and
  redirecting stormwater runoff away from natural
  drainage systems.
• Increased frequency of ditch relief culverts and
  water bars to decrease potential flow volumes
• Avoid culvert discharge in natural hollows
• Minimize the necessity for road drainage in close
  proximity to natural streams

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Questions?