Background: On January 1, 1997, the Rogue Valley experienced the worst flooding in over two decades. The Applegate River basin was significantly impacted. In the uplands of the Applegate AMA (Adaptive Management Area) soils became saturated, creeks overflowed, and the lakes were full to capacity. Most of the terrain on the Applegate District of the Rogue River National Forest is very steep and rugged. The soil types vary from the relatively stable granitic, clay- serpentine, and meta volcanic; to the slide prone mica schist, and sandy. The Applegate Valley also has a long history of human activity and disturbances. The delicate uplands of the area has seen a multitude of uses. These include the mining operations of the gold rush days through the clear-cut timber harvest practices of the 1970's and 80's. The heavy rains, a moderate snow pack, the soil types, the steep terrain, and the residual effects of human influence were the combined factors which contributed to the damage sustained in the Applegate AMA.
Findings: Several agencies dispatched staff to survey the flood damage once the rains ceased. The US Forest Service on the Siskiyou Zone organized into a Type III incident command management team. Forest Service reconnaissance began within two days of the disaster. All Applegate District employees received flood related assignments. The district was organized into six divisions. Employees equipped with maps and cameras assessed and recorded the damage. Many roads were so badly blocked by landslides that vehicles could not pass through until heavy machinery could open them. This resulted in abbreviated reports on some areas for up to three months. Surveys were conducted by hiking, on mountain bicycles, motorcycles, ATV's, and helicopters for staff to reach their destination. Some areas were only evaluated from the air.
The reports revealed that 100 percent of the road systems on the land administered by the Applegate Ranger District had received damage. On the Applegate side of the Siskiyou Zone, 73 percent of the known earth slides originated at the sites of human disturbances. The two main culprits causing slide damage on AMA land were fill slope road failures (40 percent) and clear-cut timber harvest areas (23 percent). Partial cut timber harvest units accounted for only 10 percent of landslide origins.
Applications: Several methods of landslide stabilization techniques were applied across extremely varied soils and terrains. These techniques consisted of securing erosion control blankets; planting native grass seed, shrubs, hardwoods, and conifers; constructing check dams; and in some cases yarding out log jams to clear the area for stabilization and safety. For example: An earth slide that began in an old (15-20 year) clear-cut in the upper Yale Creek drainage received several mats constructed of layers of straw bound with biodegradable materials called erosion control blankets (ECB). These mats were secured to the ground by inserting six inch metal staples, and placed along the head of the slide. Native grass seed grown at a local Forest Service nursery was distributed on top of the ECB to enhance spring germination. Hardwoods indigenous to the area were then planted in and around the slide to help establish root masses. In the spring, seven months later, the area was revisited to plant conifer seedlings replacing Douglas fir and ponderosa pine trees destroyed by the flood. On many other slides the primary damage occurred in the channels the flood waters followed. On those sites check dams were constructed to collect soil and debris where bedrock became exposed. The landslides in the Deer Creek area extended for over one mile in length which made it the largest area of watershed damage in the Rogue Valley. The Deer Creek slide project utilized all landslide stabilization techniques mentioned above.
In order to insure successful stabilization of the slide area projects monitoring has been ongoing. Grass seed germination has been successful. Established root masses will help prevent further erosion. The conifer trees planted show approximately a 70 percent survival rate. Willows planted along creeks and other wet areas also have shown good survival. Monitoring the repair progress of all the sites that have already received attention will be maintained using photographs, study, and written documentation for three to five years. Upland flood repair work for sites that have not yet received attention will be completed in the next one to two years.
Follow-up: Two years after the flood event, there are still dozens of service roads and several important Forest Service routes impassable. Ongoing monitoring and documentation of earth slide sites and repair work is essential for evaluating the success of AMA practices. The findings provide important information regarding the risks of fill slope roads and clear cutting to flood damage. For example, fill slope roads tallied as the highest percentage of vulnerable sites of landslide origination. Clear-cut timber harvests were tallied as the next most vulnerable sources of slide origination. Interestingly, partial cut timber harvest areas accounted for a minimal amount of landslide origins. Given this information, partial cuts are significantly less likely to render an area vulnerable to landslides. A perfect example of a partial cut success is the Fruit Fly timber sale on Scraggy Mountain. Fruit Fly was a well planned project producing good timber volume in a geographic location known for its steep and treacherous terrain. There were no known flood induced landslides within the sale area even though the soils include a large percentage of the slide prone mica schist.
The findings provide the opportunity for Forest Service road engineers and forest timber planners to choose appropriate practices and/or implement preventative stabilization techniques in human interface areas and along essential transportation routes. In the future, successful techniques can be refined and become common practice both preventively and in response to crisis events.
Submitted by: Tim Gonzales, Siskiyou Zone Fire and Life Sciences.