A suggested method for estimating setbacks from the crests of slopes on the Interior Plains in Alberta

Abstract

A method of estimating safe setbacks from the crests of river valley slopes in Alberta was evaluated for 39 slopes, of which eight are presented to illustrate the method and assess its reliability. The slope at each site is compared with angles of nearby stable, abandoned slopes with similar geology. An abandoned slope is a slope not being eroded by a river at present. The terrain behind the valley crest is often level. However, if the terrain rises or falls away from the crest, the setback should be corrected by a simply determined factor. Setbacks include an allowance for lateral river erosion, though this component is difficult to determine precisely. Ideally, site-specific rates of erosion should be determined and then applied to the model. In the eight cases discussed, no damage would have occurred had the structures been built behind the setback guideline. The suggested method for determining setbacks appears to be a prudent guideline for development along valley crests. The crests of slopes that have reached their ultimate angle are available for development with a small setback. For any slope that has not attained its ultimate angle, the setback represents land that is not available for development without detailed geotechnical analyses and remedial work. Key words : setback, slope, landslide, abandoned slope, angle of ultimate stability.