Understanding erosion is necessary as a basis for adequate control measures. Erosion is caused by rainfall, which displaces soil particles on inadequately protected areas, and by water running over soil, carrying some soil particles away in the process. The rate of soil particle removal is proportional to the intensity and duration of the rainfall and to the volume and characteristics of the water flow and soil properties. Deposition of water-borne sediment occurs when the velocity decreases and the transport capacity of the flowing water becomes insufficient to carry its entire sediment load.

Schematically, Figure 13-1 illustrates the typical forces involved in soil erosion.

It is usually not practical for the department to reduce erosion generated upstream of the highway. If possible, locations with high erosion potential should be avoided. In areas of considerable natural erosion and accelerated erosion, the quantity of sediment that reaches a stream before highway construction begins should be documented in a descriptive or qualitative way.

Damage that can occur on highway projects is not limited to the construction site. Sedimentation or degraded water quality may occur far downstream from the point where erosion occurs. The potential for damage exists because highways pass through watersheds, disrupting the natural drainage pattern. In addition, highway construction requires the removal of existing vegetation and the introduction of cuts and fills. This exposes large areas of disturbed soil, which increases the erosion hazard.

The potential for erosion is minimized by the following measures:

Erosion is a natural process that human activities often accelerate. Erosion and sedimentation are usually undesirable from an environmental standpoint. Technical competency in evaluating the severity of erosion problems and in planning and designing preventive and corrective measures is essential toward the goal of obtaining economical and environmentally satisfactory methods for erosion control.

Individuals involved in the process of controlling erosion and sedimentation include planners, designers, construction engineers, project inspectors, and contractors.

Effective and practical measures are available to minimize the erosion hazards and prevent sediment from reaching streams. Preventive measures taken during construction are more effective and economical than corrective measures. Erosion control involves the prevention of soil movement while sediment control deals with the interception of sediment-laden runoff and separation of soil particles already in motion or suspension. Erosion control at the source is the first consideration with sediment control the backup or last resort. Contact the DES-HYD for detailed information.

To deal adequately with the erosion and sediment problem, the erosion and sedimentation processes must be understood, erosion and sediment control plans must be developed, construction operations for erosion and sediment control must be scheduled, specific erosion and sediment control measures (when, where, and how) must be constructed, and water quality must be monitored and maintained.

The following general guidelines are considered BMPs: