Review of literature
2.4 Geochemical properties of bank materials as a facilitator of erosion
Bank failure block is a major source of sediment (Nasermoaddeli and Pasche, 2008), making sediment concentration increase in a short time and influencing river bed morphology at downstream. For example, the appearance of the peak sediment concentration in Yellow River has link to bank failure of the river canyon in Shanxi- Shaanxi province (Liu et al., 2013).
Studies on river bank stability analysis have accounted the effect of multiple horizontal layers of soil with different physical properties (Takaldany, 2003; EPA, 2003). Soil pH is the single most important chemical property of the soil. Soil pH influences most chemical and biological processes occurring in soil and some physical processes. These include supply and availability of essential elements, growth of soil organisms of all kinds, nitrification of ammonia, and rock weathering. For a high-silt soil, increased pH increases erodibility if the structure is very fine or fine granular. This is probably due to an effect on surface crusting. If the structure is medium or coarse granular, subangular, or angular, erodibility decreases with increased pH (Wischmeier and Mannering, 1969).
Sodium adsorption ratio (SAR) is the relative proportion of monovalent (Na+) to divalent (Ca2+ and Mg2+) cation concentrations in soil or sediment. Soils with high SAR are labeled sodic. Sodic soils present a serious management problem for agriculture, particularly when they contain high percentages of expansive smectite clays. Clay minerals absorb more water at high SAR, and expansion and dispersion of the minerals results in a soil with high porosity, low permeability and high erodibility (Rowell, 1994; Brady and Weil, 2002).
Organic content has long been recognized as a critical factor determining the erodibility of soils (Morgan, 2005). Wischmeier and Mannering (1969) postulated that organic matter content ranked next to particle size distribution as an indicator of erodibility. Robinson and Phillips (2001) showed that a high concentration of organic matter can facilitate aggregate stabilization in the topmost horizons. Soil with less than 2% organic content is considered erodible, and erodibility is negatively correlated with organic content from 0 to 10%
(Brady and Weil, 2002; Morgan, 2005).
Soil particle size and, particularly, the silt–clay content of the soil have long been recognized to influence fluvial erosion and mass failures (e.g. Wolman, 1959; Schumm, 1960; Walker et al., 1987). Dade et al. (1992) demonstrated that erosion threshold is positively correlated to particle size. While it is convenient to describe sediment by a mean particle size, most natural cohesive sediment is composed of a range of particle sizes, and the relative proportions of these different sized particles can substantially affect sediment
erodibility. Adding clay to a sand bed makes it more resistant to erosion, up to a maximum erosion threshold at 30–50% clay (Grabowski et al., 2011).
Clay minerals can be divided into three main groups based on their size and electro- chemical activity: (i) kaolinites; (ii) micas, e.g. illite; and (iii) smectites, e.g., montmorillonite and bentonite (Partheniades, 2007). Kaolinites have the largest particles and the lowest cation exchange capacity so are the least electro-chemically active mineral.
The clay sub units are strongly bound by van der Waals forces, so water and cations can only interact with the outer faces and edges of the particles. Smectite clays are considered to have the highest erodibility for soils, followed in decreasing order by micas and kaolinite (Morgan, 2005). The reason for high erodibility is their high CEC and expansive nature. When water infiltrates in between the clay units, they are pushed apart. The further they are pushed apart, the more inter-particle attraction is reduced, and the more erodible the clay becomes. Laboratory flume experiments by Torfs (1995) support this hypothesis for sediment; artificial sediment mixtures with montmorillonite erode at lower critical shear stresses than kaolinite at the same clay content. The small, thin, plate-like units of clays have high surface area to volume ratios, and their surfaces carry strong electro- chemical charges. The flat surfaces, or faces, of a clay particle generally carry permanent negative charges caused by the preferential adsorption of stabilizing anions and isomorphous substitutions in the clay mineral structure. Clay particles experience net repulsive forces due to their similarly charged surfaces. However, inter-particle attraction can occur if the electric double layer is reduced. Positively charged cations dissolved in the water neutralize surface charges on the clay minerals, reducing the thickness of the electric double layer, allowing the particles to get close enough to attract by van der Waals forces.
High valence cations, e.g., Ca2+, are more effective at reducing the thickness of the electric double layer, so are more effective at coagulating clay minerals than lower valence ones, e.g. Na+ (Partheniades, 2007).
The electro-chemical activity of a clay mineral, i.e., surface charge density, is linked to the cation exchange capacity and the particle size. CEC is a measure of the capacity of clay to adsorb cations in solution to the surface of the particles. A high CEC coupled with a small
particle size produces an electro-chemically active clay with a high charge density. In clay soils, the higher the Ca:Mg ratio, and the lower the Na%, the higher the likelihood of the soil being self-mulching. Self-mulching clays usually have Ca:Mg ratios of 2 to 4, and a Na% of 3% or less. Non-self-mulching clays generally have Ca:Mg ratios of about one, with the Na% usually exceeding 5%. Therefore, the Ca:Mg ratio and the exchangeable sodium percentage are important indicators of the structural stability of clay soils.
Metals can potentially alter the erodibility of cohesive sediment through several mechanisms as mentioned below; however, there is little empirical evidence.
(i) Soluble metals, particularly Fe, Al, and Mn can reduce the double layer thickness of clay particles, theoretically increasing cohesion and reducing sediment erodibility (Winterwerp and Kesteren, 2004). Soluble aluminum has been shown to increase the elastic strength of biofilms, and iron has been shown to increase their cohesive strength and resistance.
(ii) Divalent metal ions adsorbed to clays and organic matter can bind to metal ions on other particles, creating strong cation bridges (Dade et al., 1992).
(iii) Metals form insoluble complexes with oxides and carbonates, which act as cementing agents in soils (Brady and Weil, 2002). The compounds carry a net positive charge that strongly attracts two negatively charged clay particles (Partheniades, 2007).