Coweeta Research on Road Design

The design and construction of, and soil loss from, forest roads have been researched by Coweeta Hydrologic Laboratory since the 1930s.

Roadbank Stabilization Tests: 1934-1958

Early roadbank stabilization tests at Coweeta were aimed at using natural materials and labor-intensive methods to stabilize eroding slopes. These tests involved trials using mulch, poles, planted grass, or natural vegetation to slow erosion on exposed areas. Results showed that shrubs and trees provided relatively permanent stabilization (Hursh 1945, Swift 1988).

Exploitative Logging Demonstrations: 1941-1956

In 1940, Coweeta demonstrated the effects of exploitive logging and poor land management practices. Most of the roads constructed for this demonstration were adjacent to streambeds, skid trails and spur roads were often steep, logs were skidded downslope, and little effort was made to divert storm waters off the roads or to vegetate disturbed soil. These practices severely degraded water quality. About 408 m³ of soil were lost from each kilometer of road length (860 cubic yards/mile) (Lieberman and Hoover 1948). Sediment concentrations peaked at 5700 ppm during a storm in 1947. Observers concluded that watershed damage had little to do with the poor silviculture of exploitative logging, but was principally due to road design and methods used to remove logs from the woods (Swift 1988).

Integrated Forest and Watershed Management Demonstration: 1954-1955

In the mid-50s, Coweeta began to demonstrate road-building techniques that could be used to protect water quality. In this demonstration, skid trails were not permitted, logs were winched to roads, and downhill skidding was discouraged. Contour roads crossed streams at right angles, streams were carried through corrugated metal pipe, and open-topped culverts or narrow water bars were used for surface drainage. Roads were relatively narrow (10 feet/3 m wide) and slightly outsloped without an inside ditch.  Roads were seeded with grass after logging was completed. This road system met water-quality goals, but high maintenance costs and high initial costs discouraged acceptance by managers and loggers (Swift 1988).

Management Tests: 1956-1960

The cost and practicality of the Coweeta road design were tested on National Forest ranger districts. In one district, loggers found the road costs acceptable because savings in equipment maintenance and higher work efficiency compensated for the higher initial construction investment (Swift 1988).

Multi-Resource Management Demonstrations: 1962-1964

In the 60s, Coweeta began studies to demonstrate the concept of multiple-use management in which all resources in the watershed were to be made available. Long-term access to the entire basin was needed. Goals were: (1) to improve earlier designs so that maintenance requirements such as frequent cleaning of narrow-based water bars could be reduced, and (2) to demonstrate that timber access roads are permanent investments and not temporary expedients. A solution was the broadbased dip, a design feature that has become a part of nearly every set of forest road guidelines in the eastern United States. The broad-based dip is a gentle roll in the centerline profile of a level or climbing road. Where the roadbed can be drained by outsloping and broadbased dips, the problems associated with inside ditches can be avoided. Also, dips permit use of vertical banks, which are less expensive because less right-of-way clearing is required, less soil is moved, and smaller fills are created. At Coweeta, vertical cuts up to 6 1/2 feet (2 m) high have stabilized naturally on moist, fertile sites (Swift 1988).

Transportation Planning

One outgrowth of the multiresource management demonstration at Coweeta was a realization that long-range planning of a forest transportation system should include intermittent-use or local roads along with fully engineered forest development roads. With the recognition that even the lowest class of road could be a permanent capital investment came the understanding that planning was necessary to assure that each mile of road was constructed at the best possible location (Swift 1988).

Best Management Practices

The knowledge of road design accumulated by years of studies at Coweeta contributed strongly to the development of Best Management Practices (BMPs). Although BMPs deal with chemical pollution and increased water temperature, the greatest effort is directed at erosion control caused by roading, logging, and site preparation. Almost without exception, BMP guidelines for forest access roads include design features based on Coweeta experience (Swift 1988).