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Hillslope Processes (Introduced Sediment Supply)

This step applies methods for assessing sediment delivery from roads, surface erosion, and mass wasting.

Potential Sediment Delivery – Roads

Follow the procedure below to determine potential sediment delivery from roads, and enter data into Worksheet 26a (PDF, 21 kb, 1 p.). If there appears to be high risk of mass wasting, the user should go to the mass wasting procedure outlined in WRENSS (USEPA, 1980) (PDF, 8.9 mb, 54 p.).

  1. Divide watershed into 300-1000 acre sub-watersheds.

  2. Locate roads on topographic map and/or aerial photos

  3. Calculate acres of road length.

  4. Count number of stream crossings.

  5. Determine dominant slope position (most representative) mid/upper 1/3 or lower 1/3 slope.

  6. Calculate sediment delivered by road impact index (RRI),
    RRI =    acres of road   
    acres of sub-drainage
     X  number of stream crossings

  7. Obtain sediment yield (tons/acre of road) by use of Equations V-2 or V-3 compared with Figure 122 (PDF, 8 kb, 1 p.).

    (eq. V-2)
    Y = 1.7 + 40 X (Lower 1/3 position)

    (eq. V-3)
    Y = -0.1595 + 3.09X (Mid to upper 1/3 slope position)
    Y = Sediment yield (tons/acres of road)
    X = Road Impact Index


  8. Decrease delivered sediment based on quality of construction (road surfacing, ditch line surfacing, out-sloping, etc.) or special mitigation to reduce road fill erosion from entering streams. In many instances, extensive road sediment can be effectively reduced with proper designs and stabilization measures.

  9. Calculate recovery potential using Megahan's (1974) negative exponential recovery relation, Equation V-4

    (eq. V-4)
    Where: = Erosion Rate Recovery (tons/mi2/day)
         e = natural logarithm
         t = elapsed time since disturbance

    See Figure 123. Since this empirical relation calculates erosion rate recovery for an Idaho site, to use this equation for recovery, a conversion of the elapsed time - erosion rate reduction needs to be converted to percent reduction. This percent reduction can then be applied to the initial sediment yield as calculated from the Road Impact Index.

    Figure 123. Erosion rate recovery over time (Megahan, 1974).

  10. Adjustments. Use 95% reduction in delivered sediment from calculated rates if roads are surfaced with stable ditch and/or out-sloped road grade.
    1. Adjustments - If road fills are less than 200 feet from stream, treat as surface erosion. Decrease sediment yields if mitigation by surfacing, out-sloping, etc., use with sediment delivery ratio relations.
    2. If road is involved in mass wasting or debris torrent, use mass wasting procedure from WRENSS (USEPA, 1980).
    3. If a road encroaches on the stream and subsequently alters dimension pattern and/or profile, use channel process analysis to quantify impacts.

Surface Erosion

The assumption must be met prior to using any surface erosion model that the site has the potential for overland flow. Prediction of surface erosion is recommended using the Revised Universal Soil Loss Equation procedure, RUSLE Exit EPA Disclaimer , or Watershed Erosion Prediction Project, WEPP Exit EPA Disclaimer , for forested watersheds. The USDA manual of soil loss prediction uses RUSLE from the Agricultural Handbook No. 703 (USDA,1997), which has been adapted to the RUSLE computer program.

Sediment delivery ratios, that convert surface erosion rates into potential sediment supply, can be determined using the procedure (Figure 124) in WRENSS (EPA,1980), or adjusted by Williams (1975). WEPP (Laflen et al, 1997) is also used for roads and forested environments.

Figure 124. Stiff diagram for estimating sediment delivery (EPA, 1980).

Mass Wasting

Follow procedure in WRENSS (EPA, 1980)
Useful information for Mass Wasting assessment
  • Obtain soil type maps
  • Topographic maps
  • Aerial photography, during runoff periods if possible
  • Geologic maps
  • Geologic hazard maps
  • Slope shape and gradient

Summary: Introduced Sediment from Hillslope Processes

This output from the hillslope processes of mass wasting, surface erosion and roads will be used in several upcoming steps of PLA. The tons/year of potential sediment supply will be compared to the total annual sediment yield (Step 28), and baseline (Step 29) and consequence of sediment yield (Step 30). Potential aggradation or excess deposition can occur based on the size and magnitude of introduced sediment. The entrainment calculations (Step 22) will address the sediment size. The POWERSED, Bagnold or similar bedload transport models will simulate the increased coarse-grained sediment supply in terms of consequence of annual load (potential stream channel aggradation).

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