Postfire Stability Problems 



Following the wildfire, it was apparent that accelerated sur- 

 face soil erosion (by rilling and rain splash) was occurring on the 

 clearcut watershed. This was corroborated by studies con- 

 ducted by Megahan and Molitor (1975). The erosion was entire- 

 ly postfire: there was no evidence of surface erosion on the 

 clearcut watershed before the fire. The clearcutting had a two- 

 fold effect: it removed protective canopy cover (an important 

 source of postfire litter): and it increased surface fuel loading 

 that led to a more intense burn on the clearcut watershed. 



Mass erosion also occurred in 1 974 and 1 975 after the fire. It 

 was manifested by progressive failures of cut slopes above the 

 haul road that traversed the two watersheds (see fig. 9). Other 

 landslides also occurred within the 2,200-acre (890-ha) Pine 

 Creek burn (table 2). It is interesting that only 50 percent of 

 these slides were road associated (most were rotational slumps 

 in road cuts). The remainder were shallow, debris slides or flows 

 with an average depth of 26 inches (66 cm). These slides 

 occurred on natural slopes with an average gradient of 73 

 percent (36 degrees). Typical examples of both erosion and 

 slope stability problems in the study watersheds are shown in 

 figures 1 and 1 1 . 



Table 2. — Landslide study on the Boise National Forest (1975) 



Slides in the Pine Creek Burn 



Slide 



identification Slide 



number type Length Width Depth Volume Gradient Soil depth Cause 



-Feet- 



Cubic yard Percent 



Inches 



BO-613-75 

 80-614-75 



80-615-75 

 80-616-75 

 80-617-75 



BO-618-75 



80-619-75 

 80-620-75 

 80-621-75 



80-622-75 

 80-623-75 



80-624-75 

 80-625-75 



80-626-75 



80-627-75 



80-628-75 



80-629-75 



80-630-75 



Debris slide 

 Debris slide 



Debris slide or flow 

 Debris flow(?) 

 Debris torrent flow 



Debris avalanche 



Rotational slump 

 Rotational slump 

 Rotational slump and 

 debris slide 

 Rotational slump 

 Debris avalanche 



Rotational slump 

 Debris flow 



Debris flow 



Debris flow 



Debris flow 



Debris flow 



Debris flow 



24 

 12 



Missing 

 15 

 200 



20 



15 

 35 



30 

 7 



62 



54 

 150 



75 



25 



90 



40 



65 



37 

 15 



Missing 

 15 

 10 



10 



30 

 57 



110 

 45 

 10 



11 

 20 



30 



8 



10 



17 



22 



3 



1.5 



Missing 

 6 

 1 



1 

 1 



2 

 1 

 1 



2 



2.5 

 2 

 3 

 2 

 2 

 2 



50 

 7-10 



9 



10-12 

 200 



40 



60 

 220 



350 

 30 

 160 



150 

 550 



240 



30 



140 



55 



125 



60 

 60-70 



Missing 

 75 

 65 



12 

 Missing 



Missing 

 18 

 16 



Missing Fill failure 



100 

 80 



70 

 70 



173 



80 

 80 



85 



70 



70 



80 



70 



22 

 Missing 



14 

 35 



Missing 



Missing 

 15 



23 



16 



15 



30 



16 



Road cut 

 Not road 



associated 

 Road cut 

 Road cut 

 Not road 



associated 

 Road fill 



(culvert) 

 Road cut 

 Road cut 



Road cut 

 Road cut 

 Not road 



associated 

 Road cut 

 Not road 



associated 

 Not road 



associated 

 Not road 



associated 

 Not road 



associated 

 Not road 



associated 

 Not road 



associated 



5 



