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Roads can influence direct introduction of sediment from exposed cut bank, road fill, road surfaces, and ditch lines and intercept sub-surface runoff and increase available water for runoff (decreased evapo-transpiration, increased snowpack storage of water equivalent in the road prism, etc.). The resulting change in flow concentration times and/or intensity increases the drainage density of the landscape. Direct disturbance to streams often affects local channel base levels, upstream aggradation, and downstream degradation - especially due to contraction scour from culverts and bridges. Blockages or constrictions of the floodplain are common at stream crossings, a condition which reduces floodplain function and increass the risk of channel instability and adverse changes in sediment transport. This assessment step involves an inventory of roads that requires the following information:

a) Acres of sub-watershed (drainage above the study site)
b) Acres of surface disturbance of roads including road surface, cut, fill, and ditch line
c) Number of stream crossings in the drainage area
d) Slope position
e) Slope of road
f) Age of road
g) Mitigation such as road surfacing, ditch line surfacing, etc.
h) Vegetative cover of cut banks and road fills
i) Presence of unstable terrain associated with mass wasting processes

Data by 1st and 2nd order sub-drainages are summarized in Worksheet 7 (PDF, 59 kb, 1 p.). The risk analysis is performed by using the relations in Figures 82, 87 and 88 (below). The overall risk summary is accomplished using Figure 77.

The results are again summarized individually by sub-watershed in Worksheets 5a & 5b and for multiple sites/sub-drainages in Worksheet 6.

Worksheet 5a: Printable PDF (10 kb, 1 p.) | Excel Spreadsheet (27 kb)
Worksheet 5b: Printable PDF (9 kb, 1 p.) | Excel Spreadsheet (35 kb)
Worksheet 6: Printable PDF (8 kb, 1 p.) | Excel Spreadsheet (21 kb)

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A risk rating of moderate, indicating a disproportionate source of sediment or channel stability problems associated with a particular road system, would justify site-specific mitigation. The type of mitigation would typically be recommended to repair, stabilize, re-vegetate, change drainageway crossings, out-slope, or even 'hydrologically' close certain roads. Risk ratings of high or very high would require PLA analysis and eventually site-specific and process-specific mitigation.

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