Wildland Road Erosion Workshop

by Jim Brobeck

This workshop was conducted on October 21, 2000 and was hosted by the Butte Creek Watershed Workshops and Field Tour Series.

Matt Quinn and Eric Ginney, both from CSU Chico, did extensive field exploration in the Scotts John Creek watershed (a tributary of Butte Creek) to find examples of road construction to illustrate the principles presented in their workshop handout. Greg Napper, USFS road engineer, joined us for the tour to add his professional observations and to describe some of the challenges that the under funded agency has in maintaining the extensive road system on our public lands.

We gathered at the junction of two graveled roads next to Scotts John Creek. Frost heave in the exposed soil showed us the first example of erosion potential associated with human activity. The soil is lifted by ice crystals and destabilized from the effects of gravity and precipitation. Frost heave is not a problem in areas that are insulated by forest duff. Cleared slopes adjacent to roads are susceptible to sediment movement either onto the road from upslope sites or downslope from water concentrated by road drainage.

One of the most important concepts emphasized by the experts is that chronic sedimentation of streams effects habitat more than the catastrophic sediment loads produced by extreme weather. Greg Napper agreed that funding is inadequate to properly maintain the existing network of roads on Forest Service land, and that most of the roads are in place to facilitate timber harvest.

An important factor in how much a road is going to affect water quality is its location on the landform. A road located on the ridge top is less likely to load the water with sediment. Midslope roads are less likely to dump into watercourses than inner gorge sites that typically have steep, unstable slopes. But midslope sites are likely to intersect tributaries. All of these locations are likely to have sections of roads that change in elevation and require thoughtful planning to deal with drainage. Another erosion exacerbation factor associated with timber harvest adjacent to roads is the accelerated snow runoff due to increased insolation (exposure to solar energy) and direct rain on snow. The increased volume and velocity of the water running over naked earth can add water runoff to road systems or increase damage from water running off roads.

One of the main principles of minimizing erosion damage is to avoid the concentration of water. Road drainage is achieved by the slope of the road. Outsloping is the preferred configuration; the road needs to be tilted toward the downslope and be connected to the cut bank on the upside so that water can sheet off the road. Many roads in the forest are tilted toward the upslope and an inslope drainage ditch collects water from both the road and the upslope flows. The resulting concentrated water moves sediment from the ditch and from wherever the ditch is relieved either through culverts or over the road due to ditch blockage. Inslope roads require more maintenance then outslope roads because the ditches are easily plugged by debris and sediment. Long runs of inboard ditches without culverts must be avoided.

Road design in the forest has been based on highway design. This is a problem because unpaved roads in the forest function differently then paved highways. The main difference is that they require much more maintenance and always receive much less. Of course some of the roads in the forest are paved, others are rocked and the majority are surfaced with native material. Surface resilience is somewhat dependent on the surface material.

Culverts are pipes designed to carry water under the road surface. Culverts are made of either corrugated metal pipe (most common) or black plastic pipe. The pipe is either round (in cross section), arch pipe (flattened bottom) or plate arch (a half pipe bolted to concrete anchors). The latter is appropriate when the culvert is carrying a stream because aquatic life can make use of the natural creek bottom for passage and other life functions. But fish need access to seasonal tributaries and the culvert must be placed properly to accommodate fish entree at the outflow.

Culvert size, placement and condition are factors of road design that effect erosion and watershed quality. Culverts must be large enough to carry water loads and debris that are likely to occur. The rust line inside of existing culverts is an indication of appropriate size. If the rust line is higher than 1/3 of the height of the pipe, then the pipe is too small to carry the water expected during flood events. A survey of the area drained by the culvert might provide information on the size of debris that might be entering the pipe. The angle that the pipe is placed should approximate the degree of the slope that holds the road to prevent a long waterfall at the down end of the culvert. The culvert inlet must be level with the ditch or stream to function properly. If the culvert is damaged by heavy equipment or debris it will not efficiently carry the water out of the ditch. Culvert failures are common and a rolling dip can be placed just down the road from the culvert to drain water off the road when the culvert is plugged instead of pouring down the road surface and eroding it. Culverts are prone to blockage and require frequent wet weather maintenance to protect both roads and habitat from erosion damage.

We visited the site of an undersized culvert failure located in a class one stream. Evidence of a major back up on the upstream side of the culvert is visible by debris deposits in the flood plain. The fill material that held the culvert in place and created the roadbed was carried by the water of Scotts John Creek and carried into Butte Creek during the 1997 high water event. The 2’ diameter pipe was broken in two and ended up 100 yards downstream. Ironically the blown out culvert is acting like downed large woody debris and has held back sediment and created fish habitat pools.

The term road prism refers to the triangular shape of the road cut or fill relative to the slope on which the road is placed. A full bench prism is cut into the slope and the material is hauled off site. The removal of cut material is very expensive. A partial bench cuts into the slope and uses the cut material to add to the adjacent downslope portion of the road. This is cost effective but the fill portion of the road is more prone to erosion because of inadequate compaction of soil. A full fill requires material to be hauled in and placed on the slope. This is both expensive and prone to erosion.

The road surface will be either insloped, outsloped, level or crowned. The inslope and crowned design require an inslope ditch that requires regular maintenance and still concentrates water and increases its sediment moving capacity. A level road indicates little planning and often results in a damaged road surface due to lack of drainage. The outsloped road design is usually preferred because it disperses the water evenly compared to the other designs.

Inadequate topographical survey data along road 26N11 is available from USGS maps that show only 4 streams crossing the road. There are 25 swales that show evidence of recent scour. Ground based surveys must be done prior to new road construction to locate ephemeral streams.

Eric and Matt estimate that 80% of the erosion in the watershed came from roads and that 65% of the total length of the roads are contributing water and sediment into the channel network. Most of the erosion resulted from the use of insloped roads with inboard ditches. They found that 30% of the watershed stream courses have diverted down the roadway. Although there were many problem areas, over 75% of the eroded volume from all crossings came from only two streams. The wild land road erosion workshop handout is a valuable guide to understanding some of the dynamics of road construction and road related watershed degradation.

From the Winter 2001 issue of the Environmental News.