Landowner Road Inventory
Analysis Process
Measurements from the Landowner Road Inventory forms are evaluated in order to recommend priorities for road treatment to minimize sediment. As described below, relative priorities are first established within each site type: Road Crossings, Road Drainage Erosion Sites, Road Fill Failure Sites, and Fish Passage Sites. Each site is color coded by site type and relative priority on topographic maps. Relative priorities should then be modified by considering the following geographic and economic factors:
Location with respect to fish source areas
Interactions with other sites that increase the consequences of failure
(eg. plugged pipe at a road crossing causing a diversion to road fill failure site)
Location on the landscape
Debris flow runout potential
Presence of other roads (potential for cascading effects)
Grouping of high priority sites for cost-effective treatment.
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Road Crossings - All streams with evidence of annual scour
Priorities are based on
Condition (Life Rating),
Recurrence intervals of flows that exceed pipe capacity and road fill height
Plug potential - subjective judgement based on channel activity, substrate, mobile wood,
inlet basin width (future check pipe diam/bankfull >75%, see Flannigan)
Fill volume
Diversion distance
At some crossings, culvert capacity will be exceeded at less than a 50 year flow, but the water will pond and not overtop the fill until a greater than 50 year flow. Where the crossing has no other evidence of fill failure, this condition is judged to be a lower priority than a crossing with lower recurrence intervals for overtopping and diversion. Crossings with culvert capacity exceeded at a flow that is greater than the overtopping flow are low priority unless the plug potential is high.
Methods:
Note that Life Rating = 0 for crossings with culverts washed out or buried/not found.
Use fill geometry to estimate volume.
Draw watershed area for each crossing on map at scale of 1:12,000 (7.5 minute topo map enlarged 2x). Measure watershed area using a planimeter. Export database fields to roadcalcs.123 (spreadsheet) for culvert capacity calculations:
Use Piehl et al equations for culverts based on diameter and headwater height, varying by inlet type. Reduce diameter by the percent of inlet open. Estimate flow at each pipe that
1) exceeds pipe capacity (pipe fail) and
2) exceeds the height of the road fill (fill fail)
Use Oregon Dept of Forestry maps to estimate flow at 50 year recurrence interval [West Lobster 400 cfs/sq mi, East Lobster 500 cfs/sq mi.]. Use Andrus et al curves to estimate flows corresponding to lesser recurrence intervals (5 year increments). Compare flow at pipe fail and fill fail to get recurrence interval using lookup table
Estimate the diameter of the culvert required to pass the 50 year flow by backsolving for headwater/diameter and then manually adjusting the diameter until Hw/D<1. Check to determine if fixing the inlet (100% open) is enough to meet the 50 year flow.
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Road Drainage Erosion Sites
Priorities are based on
Average gully depth at drainage outlet
Road surface gullies, bypassing ditch relief pipes
Ditch drainage distance and road grade
Additional water contributed by cutbank springs, seeps, ponding
Swale without a drain
Methods:
determine ditch length for each ditch relief culvert, look for gully correlations
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Road Fill Failure Sites
Priorities based on
Fill Volume with potential to move
Probability of Failure
Delivery Potential
Methods:
Use fill dimensions with table of values from Redwood National Park to estimate max volume (assuming max distance from edge of fill is somewhat representative of volume requiring pullback). Reclassify into volume classes from 1-6, 0-500cy, 500-1000cy, etc.
Assign probability of failure in classes 1-6, based on vertical displacement of scarp (4), exposed mineral soil indicating recent movement (3), presence of water (2), leaning/bowed conifers (2), and presence of prior failure (2).
Assign delivery potential based on distance to channel, natural slope gradient and shape
Sites with low delivery potential were not inventoried with separate forms.
Notes on recommended treatments:
Identify fills failing where road is located on a deep-seated landslide (evidence is seepage, scarps, or leaning/bowed conifers upslope). Pullback treatment is not appropriate for these sites. Need to assess whether road is contributing drainage that accelerates the rate of failure. If so, appropriate treatments may include additional ditch relief or deeper interception of drainage (eg french drain).
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Fish Barriers
not completed -
Propose to check designs for crossings on Type F streams. [Are there any?] Design criteria provided by “Interim Fish Passage Culvert/Bridge Sizing Guidance for Road Crossings” - June 27, 1997 letter from George Robison, Forest Practices Hydrologist.