Figure 3. — Downs lope 

 sediment movement 

 resulting from road 

 erosion above. 

 Sediment is easily 

 traced because of the 

 light color 3 coarse 

 texture 3 and uniform 

 gradation of the 

 materials. 



Sediment production during the first time period after construction was extremely 

 high on Watersheds 1 and 2 but decreased rapidly in subsequent periods. However, this 

 didn't occur on Watershed 3. A survey of sediment flow was conducted May 8, 1962, to 

 examine this anomalous behavior on Watershed 3. The downslope movement of sediment on 

 each watershed was mapped from its source to its downslope terminus. This is easily 

 done because of the light color, the coarse texture, and the uniform gradation of the 

 eroded material in this area (fig. 3). The cause of the limited sediment production on 

 Watershed 3 was readily apparent; a barrier of logs and debris in the drainage bottom 

 was catching the material en route (fig. 2). 



Sediment flow phenomena have been noted elsewhere and research has shown that 

 natural and/or artificial barriers delay and reduce coarse sediment movement downslope 

 (Trimble and Sartz 1957; Haupt 1959; Packer and Christensen 1964). 



Additional erratic behavior was noted on Watershed 3. A natural landslide scarp 

 existed on this watershed prior to road construction. This type of slide, classified 

 as a debris avalanche (National Research Council, Highway Research Board 1958), is 

 characterized by rapid downslope movement of soil and rock material having varying 



Figure 4. — View of the 

 lower jammer road in 

 Watershed 3 3 where the 

 debris avalanche 

 originated because of 

 a road fill failure. 



6 



