Abstract—Subsurface erosion in river banks and its details, in spite of its occurrence in various parts of the world has rarely been paid attention by researchers. In this paper, quantitative concept of the subsurface bank erosion has been investigated for vertical banks. Vertical banks were simulated experimentally by considering a sandy erodible layer overlaid by clayey one under uniformly distributed constant overhead pressure. Results of the experiments are indicated that rate of sandy layer erosion is decreased by an increase in overburden; likewise, substituting 20% of coarse (3.5 mm) sand layer bed material by fine material (1.4 mm) may lead to a decrease in erosion rate by one-third. This signifies the importance of the bed material composition effect on sandy layers erosion due to subsurface erosion in river banks. Keywords—Subsurface Erosion, Vertical Banks, Bed Material Size I. INTRODUCTION ROSION of streambanks is a combination of: (1) lateral erosion of the bank toe by fluvial entrainment of in-situ bank materials, often tented fluvial erosion; and (2) mass failure of the upper part of the bank due to gravity. In one hand, streambank failure occurs when gravitational forces that tend to move soil down slope exceed the forces of friction and cohesion that resist movement. The risk of failure is usually expressed by a factor of safety (FS) representing the ratio of resisting-to-driving forces or moments. Banks may fail by four distinct types of failure mechanisms [1]: (1) planar failures, (2) rotational failures, (3) cantilever failures and (4) piping and sapping failures (Figure 1). Steep banks commonly fail along planar failure surfaces, with the failure block sliding downward and outward into the channel [2]. High, mildly sloped stream banks (bank angle less than 60°) usually fail along cursed surfaces. Cantilevered or overhanging banks are generated when erosion of an erodible layer in a stratified bank leads to undermining of overlying, erosion-resistant layers [3]. Streambanks may also fail by exfiltrating seepage and internal erosion known as piping and sapping [4].On the other hand, streambank erosion can occur at times and in places not consistent with common theories of tractive force erosion. Banks and shorelines may fail long after periods of high stage and in locations where deposition would be anticipated (e.g., on the convex or bar side of bends). Farhad Imanshoar, Ph. D. Candidate, Faculty of Civil Engineering, University of Tabriz, Tabriz, Iran (e-mail: imanshoar@tabrizu.ac.ir). Mohammad-reza Majdzadeh Tabatabai, Assistant Professor, Power and Water University of Technology, Tehran, Iran (e-mail: mrmtabatabai@pwut.ac.ir). Yousef Hassanzadeh, Professor, Faculty of Civil Engineering, University of Tabriz, Tabriz, Iran (e-mail: hassanzadeh@tabrizu.ac.ir). Mojtaba Rostamipoor, Master of Science, Power and Water University of Technology, Tehran, Iran. A major cause of such unanticipated erosion may be outflow of seepage, with attendant removal of soil particles in the exfiltration zone, and consequent instability of underlying strata located above the zone of soil loss. Figure 1-d shows a site where seepage flow out of a sandy layer carried sand out of the streambank, and the overlying more cohesive upper bank layer was undermined and collapsed [5]. According to figure 2, collapse of undercut soil layers may partially or totally obscure the exfiltration zone where the internal erosion was initiated [6]. Quite often, internal erosion of sandy soil creates approximately cylindrical conduits, or "pipes". Consequently, this form of erosion has been called "piping", defined by Mears in 1968 as "… subterranean erosion initiated by percolating waters which remove solid particles … to produce tubular underground conduits". Figure 3 shows an area of streambank in which multiple cavities were created by seepage outflow and where soil loss was extensive [7]. The detrimental effects of concentrated seepage outflow in cohesionless soils have long been recognized. Instability in soil embankments caused by seepage-related internal erosion was described by Casagrande and the importance of this erosion mechanism to the safety of dams has been demonstrated repeatedly by Terzaghi [4]. However, the significance of piping / sapping in bank and shoreline erosion has not been widely recognized [8]-[9].The important influence of antecedent moisture on the erodibility of soils has long been understood as has the role of pore-water pressure in slope stability [10]. Fig. 1 Bank failure mechanism: (a) Rotational; (b) Planar; (c) Cantilever and (d) Piping or sapping [5] Experimental Study of Subsurface Erosion in River Banks F. Imanshoar, M. R. M. Tabatabai, Y. Hassanzadeh and M. Rostamipoor E World Academy of Science, Engineering and Technology International Journal of Geological and Environmental Engineering Vol:6, No:1, 2012 27 International Scholarly and Scientific Research & Innovation 6(1) 2012 scholar.waset.org/1307-6892/138 International Science Index, Geological and Environmental Engineering Vol:6, No:1, 2012 waset.org/Publication/138