PIPING SAPPING EROSION .1. BASIC CONSIDERATIONS

Erosion of streambanks and shorelines by exfiltrating seepage is called sapping or piping (because tubular cavities are produced). This mechanism is wide-spread in occurrence and is very significant to bank and shore stability, but is rarely recognized. The mechanism is complex and acts in concert with other processes of bank and shore erosion and deposition. Operation of those other mechanisms often masks the processes and products of the piping/sapping mechanism. Furthermore, failures caused by this mechanism may occur during periods of stream inactivity long after storm and/or flood events have ended. This paper describes and illustrates characteristic features of piping/sapping erosion and attendant instability. Situations typical of sites where piping/sapping is significant are illustrated. Conditions necessary (but not necessarily sufficient) to cause piping/sapping are presented. Diagnostic procedures are presented in a companion paper. The purpose of this paper is to present the fundamental aspects of this erosion mechanism and to identify those combinations of circumstances under which piping/sapping is likely to be an important process affecting bank or shoreline stability.