Research Issues for Riverine Bank Stability Analysis in the 21St Century

RESEARCH ISSUES FOR RIVERINE BANK STABILITY

ANALYSIS IN THE 21ST CENTURY

Submitted by: A.N. Papanicolaou, S. Dey, M. Rinaldi, and A. Mazumdar

IIHR Technical Report No. 457

IIHR-Hydroscience & Engineering College of Engineering The University of Iowa Iowa City, IA 52242

July 2006

TABLE OF CONTENTS

1. Introduction ...... 1

2. Literature review ...... 2

3. Sub-aerial processes ...... 3

4. Fluvial entrainment and erosion ...... 4

4.1.Erodibility parameters ...... 5

4.1.1 Granular loose sediment ...... 5

4.1.2 Granular partly packed or cemented sediment ...... 9

4.1.3 Fine-grained cohesive sediment ...... 9

4.1.4 Failed blocks of fine-grained cohesive sediment ...... 11

4.2 Near-bank shear stress ...... 12

5. Mass failures ...... 13

6. Interaction between fluvial erosion and mass failure ...... 15

7. Role of vegetation ...... 17

References ...... 33

LIST OF TABLES

1. Summary of methods of stability analysis applied to river banks ...... 30

2. Summary of effects of vegetation on river bank erosion processes ...... 32

LIST OF FIGURES

1. Summary of bibliographic review on riverbank erosion processes ...... 3

2. (a) Definition sketch; (b) Transverse Turbulent Redistribution Transport of Momentum ξ versus x ...... 6

3. A Laboratory flume equipped with a sediment box sampler for testing critical erosional strength ...... 11

Research Issues for Riverine Bank Stability Analysis in the 21st Century

A.N.Papanicolaou1, S.Dey2, M.Rinaldi3, A.Mazumdar4 1 IIHR, Hydroscience and Engineering, University of Iowa, USA 2 Indian Institute of Technology, Kharagpu, India 3 Department of Civil Engineering, University of Florence, Italy 4 School of Water Resources Engineering, Jadavpur University, India

Preface In July 2006 the Obermann Center, University of Iowa, funded a grant on “Research Issues for Riverine Bank Stability Analysis in the 21st Century”. This grant funded a visit of Prof.Dey and Prof.Rinaldi to the Obermann Center, of 1 month and 2 weeks respectively, with the main aim to review the problem of bank erosion modeling and to identify critical issues and needs for future collaborative research developments. In this report the preliminary results of this activity are reported. The collaboration was initiated by Prof. Papanicolaou of the University of Iowa, IIHR Hydroscience and Engineering.

1. Introduction Bank erosion is a key process in fluvial dynamics, affecting a wide range of physical, ecological and socio-economic issues in the fluvial environment. These include the establishment and evolution of river and floodplain morphology and their associated habitats (e.g. Hooke, 1980; Millar and Quick, 1993; Darby and Thorne, 1996a; Barker et al., 1997; Millar, 2000; Goodson et al., 2002), turbidity problems (e.g. Bull, 1997; Eaton et al., 2004), sediment, nutrient and contaminant dynamics (e.g. Reneau et al., 2004), loss of riparian lands (e.g. Amiri-Tokaldany et al., 2003), and associated threats to flood defense and transportation infrastructure (e.g. Simon, 1995). Moreover, recent studies have shown that the contribution of bank-derived sediments to catchment sediment budgets may be higher than previously thought. Walling et al. (1999) showed that bank sediments contribute up to 37% of the basin suspended sediment yield, even in the relatively low-energy catchments of the UK, with the contribution rising to values as high as 80% in some highly unstable, incised, channel systems (e.g. Simon and Darby, 2002). With such a significant fraction of material within the alluvial sedimentary system derived from river banks, it is evident that knowledge of the rates, patterns and controls on bank erosion events that release sediment to river systems is a pre-requisite for a complete understanding of the fluvial sediment transport regime. This report seeks to address two main objectives. First, a synthetic state of the art review on bank erosion processes is attempted, with particular focus on the recent progresses in modeling the two main bank erosion processes, namely fluvial erosion and mass failure and their interaction. Second, based on the critical review and on our own research experience in this field, we attempt to identify the main areas that need

1 further developments and future research and to propose possible approaches to deal with these aspects.

2 Literature review Naturally, much research has already been devoted to these issues. These contributions include a number of excellent reviews (Grissinger, 1982; Thorne, 1982; Lawler, 1993; Lawler et al., 1997b). A comprehensive literature review has been recently carried out (Rinaldi & Darby, in press). During this study, the previous review has been completed by including and revising papers published during the last two years. As result of this review, a comprehensive list of all papers dealing with river banks is reported in the Reference Table at the end of this report. A total of 194 papers are listed, and they are divided in some main categories of issues. Figure 1 outlines the available literature, by plotting the numbers of papers divided for 5 main categories as function of years of publication. Substantive developments have occurred since the last major review in 1997. As Figure 1 shows, there is a growing number of bank erosion investigations (38% of the publications appear since 1997) and a shift in the pattern of ‘hot’ topics in the discipline. Thus, new research has elucidated the role of riparian vegetation (e.g. Abernethy & Rutherfurd, 1998, 2000; Simon & Collison, 2002) and bank hydrology (e.g. Rinaldi & Casagli, 1999; Casagli et al., 1999; Rinaldi et al., 2004) as key controlling influences on bank stability. In contrast, few studies have been concerned with the process of fluvial erosion (i.e. the removal of bank sediments by the direct action of the flow), and little progress has been made in understanding fluvial bank erosion of cohesive sediments since the contributions of Arulanandan et al. (1980) and Grissinger (1982). Notable exceptions to this trend include some work that has sought to quantify entrainment thresholds and process rates (e.g. Lawler et al., 1997a; Simon et al., 2000; Dapporto, 2001). Also very little progress has been made in incorporating the knowledge that has gained in the area of turbulent flows and the interaction of three-dimensional flows with the bank soil (e.g. Nezu and Nakagawa 1993; Papanicolaou and Hilldale, 2002).

18 16 interaction 14 others 12 vegetation 10 stability 8 erosion

Number of papers 4 2 0

1959 1961 1963 1965 1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 Figure 1 - Summary of the bibliographic review on riverbank erosion processes (from Rinaldi & Darby, in press, modified). Erosion: papers focused on fluvial entrainment; Stability: papers on mass failures and bank stability; Vegetation: papers focusing on the role of vegetation; Others: papers on other issues related to bank erosion (e.g. measurement of bank retreat, variables controlling rates of retreat, sediment delivery from bank processes, influence of bank processes on channel geometry, etc); Interaction: papers on modeling width adjustments and channel migration, and including to some extent the interaction between fluvial erosion and mass failures.

3. Sub-aerial processes Sub-aerial processes include a number of processes also known as weathering and weakening (Thorne, 1982). The role of weathering, in headwater reaches as a significant agent of erosion in its own right and, elsewhere, as an agent for enhancing bank erodibility and thereby promoting fluvial erosion, has started to be recognized (e.g. Lawler, 1993; Prosser et al., 2000; Couper & Maddock, 2001). Although subaerial processes are recognized to be important in preparing the bank material to be subject to erosion (they are also known as ‘preparation’ processes, Lawler, 1992), few studies have made attempted to quantify their impact on bank retreat. Within these few studies, Costard et al. (2003) proposed a one-dimensional model to estimate thermal erosion efficiency. In our conceptualization followed in the next sections, the role of sub-aerial processes is confined to providing a controlling influence on temporal variations in sediment erodibility and, partly, in shear strength properties. They are more significant in affecting erodibility because their impact is generally (but not exclusively) limited to the most superficial layer of sediment that is in fact subject to the fluvial action. However, some effect is also possible on shear strength parameters.

The freeze-thaw cycle is believed to affect the properties of bank soil such as soil porous structure, soil water content, soil composition, aggregate stability (Gatto, 2000). Quantifying the changes that the freeze-thaw cycle causes on the soil properties is of paramount importance as they are said to be related to bank stability. During freezing, the frost is more intense where water can easily migrate from deeper sections of the soil profile to the freezing front. Therefore, the amount of water will be higher in the freezing front than before freezing, and the freezing front will be more impermeable (Xiuqing and Flerchinger, 2001). The ice formation occurs in layers that are parallel to the bank surface, causing the upper soil layers to be displaced and moderately lifted (Carter and Bentley, 1991). In this way, the bulk density of the soil is also reduced (Gatto, 2000). The freezing front can protrude between particles and aggregates and can also reach the water adsorbed by the aggregates, causing disruption of particle bonding. Consequently, when thawing, the soil particles and aggregates have less cohesional strength (bonding) than before freezing (Bullock et al., 1988). According to Gatto (2000), this is especially true in newly thawed soils, were water content is fairly excessive and reduces particle interlocking and friction. Therefore, if rain or snowmelt occurs in this poorly stable soil, large amounts of bank erosion can take place. The reduced stability of the bank soil implies a reduction in the critical shear stress value. Limitations, problems and needs for future research 3.a - No studies have clearly tried to quantify the effects of sub-aerial processes on the reduction of erodibility and shear strength parameters. More effort should be made in future researches in this direction, possibly by both laboratory experiments and field tests.

4. Fluvial entrainment and erosion In general, fluvial erosion rates depend on the flow strength and physical characteristics (i.e. the erodibility) of the bank material. It is widely accepted that the rate of fluvial bank erosion can be quantified using an excess shear stress formula such as (Partheniades, 1965; Arulanandan et al., 1980):

a ε = kd (τ – τc) (1) where ε (m/s) is the fluvial bank erosion rate per unit time and unit bank area, τ (Pa) 2 is the boundary shear stress applied by the flow, kd (m s/kg) and τc (Pa) are erodibility parameters (erodibility coefficient, kd, and critical shear stress, τc) and a (dimensionless) is an empirically-derived exponent. Other models on bank erosion have been proposed. For example, the Ikeda et al. (1981) model expresses the bank erosion rate as the product of bank erodibility and the deviation of the near-bank velocity from the reach averaged velocity. In this case, bank erodibility accounts for both the parameters (erodibility coefficient and critical shear stress) in equation (1). The recent analytical model proposed by Duan (2005) defines the entrainment rate as product of a first term depending on bank material properties and lift coefficient, and a second term depending on the difference between friction velocity at the bank 4 surface and critical friction velocity for entrainment. The latter term can include the effects of cohesive forces, although it is not available an analytical expression of these forces, and they should be determined by experimental and field data. For the following discussion, we prefer the use of equation (1) because it allows for a better conceptualization of the process and of the parameters needed to define the erosion rate. According to this equation, we can divide the discussion in two main points: (1) determination of the erodibility parameters; (2) determination of the near- bank shear stresses.

4.1 Erodibility parameters Four main categories of bank sediments can be distinguished to discuss erodibility parameters. 4.1.1 Granular loose sediment. Granular loose material is typical of entirely non cohesive banks. The maximum slope angle should be equal to the angle of repose of the material, although in case of sandy sediment or gravel with a significant component of sand in the matrix, some apparent cohesion can develop during low flow stages, allowing for angle slopes exceeding the angle of repose. Granular loose sediment can also be present as lower layer of composite banks, when it has been recently deposited in channel bars, or just at the bank toe, when the lower layer of the composite bank is composed by packed or cemented gravel. In this former case, it creates a typical basal wedge of sediment with slope near to the angle of repose: its erosion has no direct effects on the stability of the upper cohesive layer (because it does not chance the slope angle of such layer) but acts as a protection, until it is removed, of the basal layer of packed granular sediment. Critical shear stress is modeled based on the same methods used to predict entrainment of bed sediments, albeit with modifications to take into account the effect of the bank angle on the downslope component of the particle weight. The most commonly used method for analyzing the threshold channel profile is the tractive force approach given by Glover and Florey (1951), which results in a cosine profile. Parker (1978), Ikeda et al. (1988), and Pizzuto (1990) also endorsed the cosine profile. On the other hand, Mironenko et al. (1984) proposed a parabolic profile and the exponential profile was put forward by Ikeda (1981), Diplas (1990), and Diplas and Vigilar (1992). More investigations on mobile-bed channels are reported elsewhere (Parker 1979; Ikeda and Izumi 1991; Cao and Knight 1997, 1998; Yu and Knight 1998). An experimental study was carried out by Stebbings (1963).

Herein a simplified approach for the computation of a bank profile of a self-formed straight threshold channel with noncohesive uniform sediment is included. An assumption on transverse momentum diffusion, being a function of the transverse distance from the center of the channel, simplifies the computational procedure considerably.

Model Equilibrium of Sediment Particles As the transverse bed slope of a channel is much greater than the longitudinal bed slope, the streamwise component of gravity force is ignored when analyzing the force acting on an individual sediment particle under a threshold condition. The forces acting on a sediment particle lying on the channel bed, shown in Figure 2 (a), are the drag force FD, lift force FL, and submerged weight of the particle FG. For a transverse bed slope θ, the threshold condition of a sediment particle can be defined by the equation proposed by Ikeda (1982)

Figure 2-- (a) Definition Sketch; (b) Transverse Turbulent Redistribution Transport of Momentum ξ versus x

−ημ 2 cosθ + μ 2 cos2 θ +η 2 μ 2 sin 2 θ − sin 2 θ τ~ = (1) (1 −ημ)μ ~ where τ = [τ]θ=θ/[τ]θ=0; [τ]θ=θ = threshold bed shear stress at θ = θ ; [τ]θ=0 = threshold bed shear stress at θ = 0 ; η = FL/FD; and µ = Coulomb static friction of sediment particles. The value of η is assumed to be 0.85, as in Diplas and Vigilar (1992) and Yu and Knight (1998). However, Yu and Knight (1998) proposed an empirical relationship of µ for uniform sediments ⎡0.302(logd )5 + 0.126(log d )4 −1.811(log d )3 − 0.57(log d ) 2 ⎤ μ = tan 50 50 50 50 ⎢+ + ⎥ ⎣ 5.952(log d50 ) 37.52 ⎦ (2) where d50 = mean sediment size (cm). The threshold shear stress [τ]θ=0 for uniform sediments can be obtained from the curve of Dey (1999) [also available in Dey et al.

(1999) and Dey and Debnath (2000)]. His curve for uniform sediment can be expressed as follows: τˆ = 0.142Dˆ −0.35 for Dˆ ≤ 1 (3a) τˆ = 0.148Dˆ −0.60 for 1 ≤ Dˆ ≤ 15 (3b) τˆ = 0.013Dˆ 0.32 for 15 ≤ Dˆ ≤ 50 (3c) τˆ = 0.0.045 for Dˆ > 50 (3d) 3 where τˆ = [τ]θ=0/(ρgΔd50); ρ = mass density of water (kg/m ); g = gravitational 2 2 1/3 constant (m/s ); Δ = s – 1; s = relative density of sediment; Dˆ = ks(Δg/υ ) ; ks = -6 2 equivalent roughness height (= 2d50); υ = kinematic viscosity of water (= 10 m /s).

Bed Shear Stress Based on the modified area method of Lundgren and Jonsson (1964), the following expression for the bed shear stress acting along the wetted perimeter dP is obtained considering the balance of downstream momentum for the area dA shown in Fig. 1(a): dA d ⎛ h ′ ′ ⎞ [τ ]θ =θ = ρgS + ⎜ ρu v dy⎟ (4) DP dP ⎝ ∫h− y ⎠ where S = longitudinal slope of free surface; h = flow depth (m); u′ and v′ = turbulent fluctuating velocities along longitudinal and transverse directions, respectively (m/s); and y = vertical distance from the midpoint of the channel bed (m). The second term of the right-hand side of (4) refers to the transverse momentum diffusion caused by the Reynolds stresses, due to turbulence. The transverse transport of momentum by turbulence acts to diffuse momentum from areas of high concentration (swifter flow) to areas of low concentration (slower flow). The integrated transport in the vertical direction is given by ξ(x) h ξ(x) = ρu′v′dy (5) ∫h− y where x = transverse distance from the midpoint of the channel (m). Ignoring secondary currents, the local momentum balance is obtained from (4) ⎡ dξ ⎤ [τ ]θ =θ = ρg(h − y)S + cosθ (6) ⎣⎢ dx ⎦⎥ where [τ]θ=0 = ρghS; (6) is expressed in normalized form ⎛ dξˆ ⎞ τˆ = ⎜ − ˆ + ⎟ θ ⎜1 y ⎟cos (7) ⎝ dxˆ ⎠ ˆ where yˆ = y/h; xˆ =x/h; and ξ = ξ/h[τ]θ=0. According to Parker (1979), as the bed shear varies monotonically with x, ξ(x) should have a variation as shown in Fig. 1(b). Hence, ξˆ is assumed as a function of x in the form of a power law given by ξ = Cxˆ m (8) where C and m = coefficient and exponent, respectively, are determined using experimental data (Stebbings 1963).

Bank Profile Using cosθ = [1 + (dy/dx)2]-0.5 and equating (1) and (7), the following differential equation for the bank profile of a threshold channel is obtained:

dyˆ − η 2μ 2 −1 = μ [(1− yˆ + Cmxˆ m 1 )(1−ημ) +ημ]2 −1 (9) dxˆ Eq. (9) is a first-order differential equation, which is solved numerically by the fourth- order Runge-Kutta method to determine the variation of yˆ with xˆ , that is the nondimensional bank profile of a threshold channel. The values of C and m were taken as 0.0027 and 4.5, respectively, which were obtained using the experimental data of Stebbings (1963).

Discharge To determine the discharge Q, the following relationship is used:

= = ⎛ a / P ⎞ Q AV 2.5A g(A/ P)S ln⎜11 ⎟ (10) ⎝ d50 ⎠ where V = mean flow velocity (m/s). The above relationship was tested successfully with field and laboratory data (Diplas and Vigilar 1992).

Computational Scheme The equations developed in the preceding section can be implemented in a computer program that will provide a solution for the channel bank profile and discharge. As input data, the program requires the values of d50, S, and µ. If µ is not given, (2) can be used to determine it. The steps involved in the computation are as follows: 1. Compute Dˆ and then τˆ from (3).

2. Compute [τ]θ=0 using [τ]θ=0 = τˆ (ρgΔd50).

3. Compute h using h = [τ]θ=0/(ρgS). 4. Compute the variation of yˆ with xˆ from (9), using the fourth-order Runge- Kutta method. 5. Compute area A and top width T of flow numerically using the variation of y (= yˆ h) with x (= xˆ h). 6. Compute Q using (10).

Limitations, problems and needs for future research 4.1.a - Further than critical shear stress, the erodibility coefficient is needed in order to define the erosion rate. Are there models available? Are there tests possible to determine this? 4.1.b - Fluvial erosion of granular sediment at bank toe is not only a question of exceeding the incipient conditions, but it is also related to sediment transport and sediment delivered from the bank. Erosion occurs when rate of entrainment is greater than rate of deposition (Duan, 2005). Therefore application of models of incipient motion should limited to cases when it is assumed that the flow is competent to erode

8 and transport material from the bank, i.e. antecedent sediment loads are either minimal or can be ignored. 4.1.2 Granular partly packed or cemented sediment. This material is quite typical of composite riverbanks, where the basal gravel tends generally to exhibit some degree of packing and cementation due to the weight of the upper cohesive layer and to fluid circulation. In this case, this layer can be stable during low flow periods at an angle considerably higher than the angle of repose of the correspondent loose sediment. For this material, critical shear stress is not directly determinable by methods for incipient motion, except if some modification is made. A modification of the Lane criterion taking into account the case of packed sediment by an empirical coefficient has been recently proposed (Millar & Quick, 1993; Millar, 2000). Limitations, problems and needs for future research 4.1.c - In situ tests (like jet test) available for loose sediment are not suitable for this material. Some alternative in situ test is needed: an attempt could be to directly measure the shear stress at the conditions when the superficial layer is disrupted. However we are aware of the difficulties to measure shear stress in the field. Conversely, it could be very difficult (if not impossible) to bring in the laboratory undisturbed samples. 4.1.d – It is possible that packed and partially cemented material during a flow event can loose its cementation in the region at contact with water that becomes completely saturated. In such a case, its behavior is the same of granular loose material. This hypothesis needs to be investigated. 4.1.3 Fine-grained cohesive sediment. Determination of critical shear stress for cohesive materials is more complex, given that it is widely recognized that fluvial entrainment for cohesive sediments depends on several factors, including (amongst others) clay and organic content, and the composition of interstitial fluids (Arulanandan et al., 1980; Grissinger, 1982). Consequently the state-of-the-art is that methods for predicting the erodibility of cohesive banks remain poor. To address this issue, recent studies have deployed in situ jet-testing devices (e.g. Hanson, 1990; Hanson and Simon, 2001) to obtain direct measurements of bank erodibility (e.g. Dapporto, 2001). Limitations, problems and needs for future research 4.1.e - Very few theoretical or empirical methods for assessing erodibility of cohesive soils are available. Major efforts should be devoted to address this limitation. The main difficulty in describing the erodibility of cohesive soils stems from the fact that cohesive soil transport is governed not only by hydrodynamic forces (e.g., drag, lift), but also by electrochemical interparticle forces (e.g., van der Waals bonding, Coulombic repulsion, etc) along with biological forces (e.g. bacterial adhesiveness) 4.1.f – Given the limitations in assessing erodibility, one of the only viable ways is to measure in the field erodibility parameters. While jet-testers offer in situ sampling, their design makes their deployment unwieldy (and perhaps unreliable). The results seem to depend on the applied shear stresses (Dapporto, 2001). Furthermore, the

9 effects of apparent cohesion can play an important role: if the test is performed on the same position in different periods of the year, it is possible to get different results due to differences in hydrologic conditions. Because fluvial entrainment occurs in most cases (if not always) on saturated sediment, the tests should be performed on saturated soil (if the hypothesis that erodibility parameters depend on shear strength of material is proved). Instruments such as the Cohesive Strength Meter (Tolhurst et al., 1999) appear to offer advantages over conventional jet-testing devices. So far it has only been deployed in estuarine environments (Tolhurst et al., 1999), but the CSM appears to offer a potentially fruitful avenue of bank erosion research. 4.1.g – An alternative approach is to perform laboratory tests. This can be done on saturated soils, measuring soil concentration and shear stress and back calculating erodibility parameters (Papanicolaou and Hilldale, 2001). The problem can be the need to have undisturbed samples reproducing the actual field conditions. The Papanicolaou and Hilldale device is a water-recirculating, straight flume that has a useful length of 2 m and width of 0.15 m (figure 3). The tilting flume was modified by placing artificial roughness (fine sandpaper 240 grit) on the bed upstream and downstream of the test section (figure 3). Fine sandpaper represents closely the roughness of the soil (Aberle et al., 2004) and was glued atop of a plexiglas flat plate with thickness of 0.035 m (figure 3). The plate was screwed on the flume floor to prevent motion during testing. A honeycomb ensured rectilinear flow, flow rate was controlled by a gate valve, and an instream tail gate provided uniform flow control. The tilt of the flume was controlled by an electric motor, and slope was measured directly in percent using a digital level mounted on the flume. The maximum titling capacity of the flume was close to 5%. The test section was comprised of a sample tray built to fit into the flume and with outer and inner dimension given in figure 6 and a depth of 0.025 m. The sample tray itself was a removable plastic box that was easily removable from the test section of the flume for sample emplacement. Sandpaper roughness around the top edges of the test section box minimized the effects of the flow transition from the flume floor to the sediment surface. This device has been shown to provide reproducible results with an experimental error of less than 10%.

Figure 3- A laboratory flume equipped with a sediment box sampler for testing critical erosional strength (after Papanicolaou and Hilldale, 2001)

4.1.4 Failed blocks of fine-grained cohesive sediment. Blocks of cohesive sediments failed by mass failures can remain at the bank toe for some period. If the failure occurs during peak stage or high flow, it is likely that the failed block is transported directly by the flow or it is removed by the bank toe during the remaining portion of the event. However, if the failure occurs (as often) during the recessional phase of the hydrograph, it will probably remain on the bank toe until a next flow event will occur. In such a case, the failed block can protect the bank toe until it is not eroded by the flow. Critical shear stress can be modeled based on same methods used to predict entrainment of bed sediments, assuming the block as a single particle. Few studies have been done on this issue. Among these few studies, Wood et al. (2001) investigated the role of apparent cohesion in the entrainment of failed bank material. Limitations, problems and needs for future research 4.1.h – More research efforts should be done on this issue, both by field and laboratory experiments.

4.2 Near-bank shear stress Although the aforementioned describe recent developments in the quantification of bank erodibility, the ‘missing link’ in equation (1) remains the difficulty of characterizing the fluid stresses that are exerted on river banks during the large flows that typically drive erosion. Bank boundary shear stress is highly variable both in space and time, dependent as it is on such factors as the bank geometry (which is itself highly variable), cross-section size and shape, channel curvature, and flow stage. This variability presents a challenge for anyone seeking to characterize the shear stress distribution via direct measurement. Sampling strategies would need to capture this natural variability during the (hazardous) high flow conditions associated with bank erosion. It is, therefore, unsurprising that such investigations are lacking. If empirical data collection is impractical, the only viable alternative is to predict the shear stress values using hydraulic models. Different models are possible to calculate near-bank shear stress. The simplest way is to assume uniform flow conditions and to assume some near-bank shear stress distribution derived from laboratory channels experiments (i.e. Leutheusser, 1963; Kartha & Leutheusser, 1972; Simons & Senturk, 1977; Knight et al., 1984). However, these assumptions can only be applied with caution to natural rivers, as the bank and channel forms present in flumes with regular geometry represent the problem rather poorly. Another viable way is to use 2-D or 3-D hydrodynamic numerical modeling techniques as a substitute for empirical data in river flows that are difficult or impossible to measure. However, the application of 3D-CFD to near-bank flows remains novel and replication of near-bank flows would depend on (i) ensuring the discretized computational scheme accurately solves the underlying conservation equations; (ii) selecting an appropriate turbulence-closure model (TCM), and (iii) accurately defining the initial and boundary conditions. Limitations, problems and needs for future research 4.2.j – Measuring in-situ devices of shear stresses near natural river banks and during flow events are not available. An alternative way is to monitor superficial flow velocity by techniques based on video-camera and tracers (Papanicolaou) and to use then to calibrate 2-D or 3-D hydrodynamic models to calculate near-bank shear stresses. Recently, Fox et al. (2005) have employed the LSPIV is an image-based technology that provides free-surface velocity vectors for open-channel research applications. Measurements are obtained by measuring the displacement of floating fluid-markers (i.e. seeded material) between successive digital images. For the present application, use of the LSPIV system relies on the assumption that free- surface velocities justly approximate mean-flow characteristics near the bank. 4.2.k – Applications of 2-D or 3-D hydrodynamic numerical modeling techniques to river banks are still very limited and more examples can be very useful. 4.2.l – More efforts need to be made to include effects of channel curvature, turbulence and secondary flows.

5. Mass failures Mass failure is the collapse and movement of bank material under the action of gravity. Relative to fluvial erosion, mass failure is a discontinuous and large-scale detachment process. Mass failure occurs by any one of a large number of specific mechanisms (Thorne, 1982; Thorne, 1998). To model mass failures, a specific model is required for each mechanism. However, methods developed and used in literature have been concentrated only on a series of classical mechanisms, in particular slides (planar or rotational) and cantilever, because of the availability of existing methods to model such mechanisms. It has to be recognised that many other mechanisms can occur in specific situations, for which limit equilibrium method is not suitable to model them. For example, another mechanism occurring in sub aqueous slope failures of fine sand has been recognised for Mississippi riverbanks and originally assumed to be generated by sudden liquefaction and termed ‘flow slide’ by Torrey (1995). More recently, an alternative mechanism has been described by Van den Berg et al. (2002) and termed ‘breaching’, where retrogressive erosion is related to shear-dilatancy effect leading to negative pore pressures. The application of stability analyses is common in the bank erosion literature. The analysis of slide failures is typically performed using the Limit Equilibrium Method (LEM) to compute the factor of safety (F), defined as the ratio between stabilizing and destabilizing forces. Since the 1960s, specific methods of bank stability analysis have been progressively disseminated, with an increasing effort to define closed-form solutions representative of characteristic bank geometries (Table 1). It is evident that research has progressively sought to account for: (a) a more realistic bank geometry and the influence of tension cracks (Osman & Thorne, 1988); (b) positive pore water pressures and hydrostatic confining pressures (Simon et al., 1991; Darby & Thorne, 1996b); (c) the effects of negative pore water pressures in the unsaturated part of the bank (Rinaldi & Casagli, 1999; Casagli et al., 1999; Simon et al., 2000); and (d) the influence of riparian vegetation (Abernethy & Rutherfurd, 1998, 2000, 2001; Simon & Collison, 2002; Pollen et al., 2004; Van de Wiel & Darby, 2004). More recently, efforts have been made to apply commercial slope stability analysis software based on LEM to river bank problems. Some of these software codes combine features that overcome many of the previous limitations and they are now routinely utilised for river bank stability studies (Dapporto et al., 2001, 2003; Simon et al., 2002; Rinaldi et al., 2004). Changes in pore water content and pressures are recognised as one of the most important factors controlling the onset and timing of bank instability (i.e. Thorne, 1982; Springer et al., 1985) and the incorporation of these factors in bank process models is one of the major areas of recent progress. The actual mechanisms and timing of failure induced by pore water pressure effects are difficult to predict if their temporal changes, both at seasonal and intra- event time scales, are not accounted for (Rinaldi & Casagli, 1999; Casagli et al., 1999; Simon et al., 2000). For this reason, bank stability response at the intra-event time scale requires knowledge of the dynamics of saturated and unsaturated seepage flows.

Various studies (Dapporto et al., 2001, 2003; Rinaldi et al., 2001, 2004) have made use of the software Seep/w (Geo-Slope International Ltd) to perform two- dimensional, finite element seepage analyses. Limitations, problems and needs for future research Despite many recent advances, further progresses are still needed to better simulate mass failures. Various areas of possible progresses exist, as listed below. A. Some mechanisms of failure have not been modeled so far. 5.a - Only one (shear failure) of the three mechanisms of cantilever failures (Thorne & Tovey, 1981) is usually modeled. For the other two mechanisms (tensile and toppling), an analytical solution is also provided by the same authors (Thorne & Tovey, 1981), however its solution requires the tensile strength of a soil. This is a difficult parameter to determine, particularly for unsaturated conditions due to the effects of apparent cohesion. 5.b - Although toppling (or slab-type failure) is a common mechanism of bank failures, it is very difficult to model because it needs to calculate the gravity center of the failing block. 5.c - Other mechanisms of failure, such as piping and pop-out, dry or wet flows, have not been usually considered because of much higher difficulties to model the problem and because the occurrence of these mechanisms is generally limited. 5.d - An important mechanism for riverbanks composed of fine sand appears to be the ‘breaching’ described by Van den Berg et al. (2002). To model this mechanism, additional research is needed. Flume experiments could be a suitable approach, reproducing a bank of fine sand in scale 1:1 and performing experiments where all the main factors controlling the process (i.e. pore water pressures, water content, flow velocity, etc.) can be measured. 5.e - Other more sophisticated types of methods used for landslide analysis, i.e. stress- deformation analysis, have not been employed specifically for riverbanks, due to some main reasons: 1) stress-deformation analyses are particularly data-demanding and complex to use; 2) riverbank failures are typically fast, while stress-deformation analyses have typical applications on slow landslides and/or progressive failures on large slopes. An example of such type of analysis was conducted by Collison (2003) to analyze gully head retreat. A similar approach could be used for riverbanks in order to better understand the possible role of stress release and bank deformation due to basal fluvial erosion. B. Limitations and problems with seepage analysis. 5.f – Although positive pore water pressures are now considered in bank stability analysis, seepage forces are usually not explicitly included in the analysis. Pore water pressures are accounted only as a factor reducing the effective stress and therefore the shear strength of the soil. 5.g - Improvements in accounting some of the hydrological processes occurring on the bank and in the riparian zone, and their integration with seepage flow modeling are possible. Overland flow on the top of the bank can occur when surface runoff

14 from the upland field exceeds the infiltration capacity of the riparian soil. These effects can also be accounted. During high intensity storms, the available water may exceed the infiltration rate. In such conditions, infiltration can be simulated using the Green-Ampt - type equation or its modifications. 5.h – Evapotranspiration effects on pore water pressures related to climatic conditions and plants can be taken into account by specific software codes (i.e. Vadose by Geo- Slope International). These processes however are effective mainly during low-flow conditions, when the bank is typically stable. C. Three dimensionality of the problem. 5.i - Three-dimensional seepage and stability analysis has not been attempted so far. Saturated and unsaturated flow can be modeled, while bank stability is more difficult to model in three dimensions. 5.j - The mechanisms and factors determining the longitudinal extension of a bank failure are not very clear. The presence of bar sediments on the opposite side is definitively a factor, but some other factors related to bank mechanics and probably to flow variability can be also important. We are not aware about studies on morphological characterization of the longitudinal extent and shape of single bank failures and their relations with morphological, geotechnical and hydraulic factors. This could be in some extent addressed by flume experiments. D. Problems with parameterization. 5.k - Conductivity and water content functions are difficult to define, particularly the first one that needs to perform some measurements of unsaturated conductivity. 5.l - Shear strength parameters for unsaturated soils are also difficult to obtain. It is possible to get an estimation of the angle φb (angle of resistance in terms of matric suction) combining triaxial tests with in situ tests carried out on the unsaturated soil and measuring matric suction (Rinaldi & Casagli, 1999). However the assumption that the angle φb is constant is a simplification, because it tends to vary as function of pore water pressure (Fredlund & Rahardjo, 1993).

6. Interaction between fluvial erosion and mass failure The two groups of processes described in the previous sections (fluvial erosion and mass failure) are likely to act in combination. Particularly in the middle and lower reaches of drainage basins, retreat is likely to be driven by a combination of the hydraulic forces of the flow, and mass failures driven by gravity. An analytical approach to calculate rate of bank erosion that accounts for both basal erosion due to fluvial hydraulic forces and bank failures has been recently developed by Duan (2005). Although some simplifications and limitations are included in the analysis, it represents a worth attempt to accounts for both processes and to include a statistical approach for calculating the frequency of bank failures. In particular, the main limitation in trying to solve analytically the problem is due to the fact that generally a single representative river stage (usually the peak stage of the hydrograph) is considered in the analysis. This masks the complex interactions due to

15 rapid, transient changes in driving factors (pore water pressures, confining river pressures, etc.) occurring within the flow hydrograph. To fully capture these complex interactions between the two groups of processes, it is therefore necessary to investigate bank changes at the intra-event scale. Logistical and safety concerns usually limit the frequency of monitoring to relatively coarse timescales, at best perhaps resolving individual flow events. This is problematic because the pre- versus post- flow event ‘window’ is not the same thing as the bank erosion event window, such that it is not usually possible to resolve process thresholds, timing and rates (Lawler, 2005). A numerical modeling approach provides an alternative way to solve the problem. Few attempts have been made to investigate bank erosion dynamics combining fluvial erosion, pore water pressure changes, and mass bank stability into a single, integrated, modeling approach. Simon et al. (2003) used three models (Seep/w in combination with the ARS Toe-Erosion and ARS Bank-Stability models) to simulate bank response to flow events, employing a series of artificial, rectangular-shaped, hydrographs of specified height and duration. Alternative examples of numerical simulations of river bank retreat in which fluvial erosion, seepage, and mass failure models are fully integrated are those of recent studies on the Sieve River and Cecina River in Italy (Dapporto & Rinaldi, 2003; Rinaldi & Darby, in press; Darby et al., in press). Of particular interest is that these simulation results are qualitatively distinct from conceptual models of bank sediment delivery processes that are founded on event-scale analyses. Previous studies have tended to emphasise mass-failure as a quasi-catastrophic event, typically timed to occur on the falling limb of event hydrographs. In contrast, some of these simulations suggest that mass failures occur as a series of erosion episodes, timed at frequent intervals as progressive fluvial erosion undermines the bank and trigger failures throughout the flow event. These simulations demonstrate how modeling the interactions between hydraulic and geotechnical processes, predictions with qualitatively different outcomes (in terms of the nature of the onset and timing of bank sediment delivery to the alluvial sedimentary system) can be obtained compared to the results derived from existing models that treat these processes in isolation. Limitations, problems and needs for future research 6.a – There is a need to extent previous simulations to a wider number of study cases in different contexts and for different flow hydrographs. This should be based on field monitoring and numerical modeling of bank retreat in different sites. It is also possible to consider the possibility of some flume experiments, with a bank at scale 1:1 composed by fine-grained material (fine sand), running different flow hydrographs with various characteristics (peak, duration, hydrograph shape) and measuring all controlling parameters (flow parameters, pore water pressures, etc.). 6.b - Most of the previous recommendations made for the two single processes are applicable also in the case of the interaction to improve the simulations and to take into account additional factors.

6.c – The 3-D nature of the problem appears to be of particular interest. Coupling 3-D (or 2-D depth averaged) hydrodynamic numerical models with 3-D seepage and stability analyses offer an exciting perspective for future progresses in understanding bank retreat. 6.d - Vertical bed changes have not been considered, but they can play an important role, with a direct impact on cohesive banks (bed lowering causes an increase in bank height and therefore a reduction in stability). Sedimentation at the toe can also induce changes in flow characteristics. It is possible to address this by coupling a morphodynamic numerical model simulating bed changes.

7. Role of vegetation The effects of vegetation on river bank processes are many and complex, and most are difficult to quantify. A comprehensive review of such effects is beyond the immediate scope of this report, and so we provide a brief discussion on aspects (Table 2) related to the fluvial erosion and mass failure processes considered herein.

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188 Weigel T.A. & Hagerty D.J. (1983) – Riverbank change – Sixmile 1983 Other Island, Ohio River, USA. Engrg.Geol., 19, 119-132. processes & Mass failure 189 Wiele S.M. & Paola C. (1989) – Calculation of bed stress and bank 1989 Fluvial erosion erosion in a straight channel. EOS: Trans.Am.Geophys.Union, 70, 329-339. 190 Williams D.R., Romeril P.M. & Mitchell R.J. (1979) – Riverbank 1979 All processes erosion and recession in the Ottawa area. Canadian Geotechnical Journal, 16(4), 641-650. 191 Wolman, M.G. 1959. ‘Factors influencing the erosion of cohesive 1959 All processes river banks’, American Journal of Science, 257, 204-216. 192 Wood A.L., Simon A., Downs P.W. & Thorne C.R. (2001) – Bank- 2001 Fluvial erosion toe processes in incised channels: the role of apparent cohesion in the entrainment of failed bank materials. Hydrological Processes, 15, 39-61. 193 Wynn T.M. & Mostaghimi S. (2006) – Effects of riparian vegetation 2006 Weathering & on stream bank subaerial processes in southwestern Virginia, USA. Vegetation Earth Surface Processes and Landforms, 31, 399-413. 194 Xia J., Yuan X. & Wang G. (2000) – Preliminary simulation of 2000 Modeling channel lateral widening in degradation of alluvial rivers. Journal of width Sediment Research, 6, 16-24. adjustments

Categories of Issues 1. Weathering processes: it includes papers focussed on sub-aerial (weathering and weakening) processes. 2. Fluvial Erosion: it includes studies on the erodibility of bank material (assuming that erodibility is mainly referred to fluvial entrainment). 3. Other erosion processes: it includes piping and other processes. 4. Mass failure: papers focussed on processes of mass failure and geotechnical stability. 5. All processes: papers concerning some aspect of various processes. 6. Measurement of bank retreat & sediment yield: it also includes rates of bank retreat and their relationships with some controlling factors, distribution of bank retreat in the fluvial system, contribution of bank erosion in terms of delivered sediments. 7. Modeling: it includes modeling of channel width adjustments (where bank processes are taken into account) or channel migration, and effects of bank processes on channel geometry. 8. Vegetation: it includes papers mainly focussed on vegetation effects. 9. Interaction of processes: specific papers on the interaction between fluvial erosion and mass failure.

Table 1 – Summary of methods of stability analysis applied to river banks (from Rinaldi & Darby, in press).

Analysis Mechanism of New capabilities Main limitations Typical Applications Main references failure and bank (compared to previous geometry methods) ‘Culmann’ Planar failure, Simple to use Simplified geometry Massive silt or clay, incised Thorne et al. (1981); uniform bank slope (no tension crack); rivers of southeastern – Thorne (1982) failure surface passing midwestern U.S. from the bank toe; only “dry” conditions Thorne & Cantilever failure First method specific Data required Composite banks Thorne & Tovey (1981); Tovey for cantilever failure (cantilever block Thorne (1982) geometry, tensile strength) not easily available Osman & Planar failure with More realistic geometry Failure surface passing Homogeneous cohesive Osman & Thorne (1988) Thorne tension crack; bank including effects of from the bank toe; only banks (O&T) profile taking into basal erosion “dry” conditions account basal erosion and relic tension crack Simon et al. Planar failure, Failure surface not Simplified bank Homogeneous cohesive Simon et al. (1991) uniform bank slope necessarily passing geometry (no tension banks from the bank toe; crack); positive pore pressures are incorporated by a pore pressure ratio; confining pressures are taken into account Darby & Planar failure with Failure surface not Unsaturated conditions Homogeneous cohesive Darby & Thorne (1996b) Thorne vertical tension necessarily passing are not considered banks crack, O&T from the bank toe;

geometry positive pore pressures and confining pressures are incorporated Rinaldi & Planar failure with Negative pore water Simplified bank River banks formed in Rinaldi & Casagli (1999) Casagli vertical tension pressures (further than geometry (no basal partially saturated soils; rivers crack, uniform bank positive and confining erosion and relic with relatively rapid slope pressures) are taken tension crack); based drawdown into account on the assumption that water table during drawdown remains at the level of the peak river stage Casagli et al. Planar failure with More realistic geometry Homogeneous Homogeneous cohesive river Casagli et al. (1999), vertical tension material; Relation river banks formed in partially Rinaldi & Casagli (1999) crack, O&T stage – water table saturated soils geometry needs to be specified Simon et al. Planar failure with Layered bank materials Relation river stage – Layered cohesive river banks Simon et al. (2000) vertical tension water table needs to be formed in partially saturated crack, O&T specified soils geometry ARS bank Planar (wedge-type) Incorporates soil Simplified bank Vegetated river banks Simon & Collison (2002) stability failure reinforcement and geometry model surcharge due to vegetation Various Slides (planar, Negative pore water Generally more data- When pore water pressure Dapporto et al. (2001, software rotational, and confining pressures demanding; requires changes at the intra-event 2003); Rinaldi et al. (2004) packages composite); generic are taken into account; expertise scale need to be accounted; bank geometry generic bank geometry rotational or other non planar and failure surfaces; failure surfaces and generic possible to account for bank geometry main vegetative mechanical effects

Table 2 – Summary of effects of vegetation on river bank erosion processes (from Rinaldi & Darby, in press).

Processes Specific effects Impacts Key references Fluvial Increasing flow resistance Flow velocity / shear stress - Kouwen et al. (1969); Kouwen & Li (1980); erosion Kouwen (1988) Local scour at isolated trees Flow velocity / shear stress + Thorne (1990) Root reinforcement Soil erodibility - Thorne (1990); Millar (2000) Mass failure Surcharge Shear forces +; Resisting Gray (1978); Selby (1982) forces (friction) + Root reinforcement Shear strength + Gray (1978); Wu et al. (1979); Greenway (1987); Abernethy & Rutherfurd (2000); Gray & Barker (2004); Pollen et al. (2004) Anchoring, Buttressing and Resisting forces + Gray (1978); Coppin & Richards (1990) Arching Wind Shear forces + Hsi & Nath (1970); Greenway (1987) Interception Pore water pressures – (+) Greenway (1987); Simon & Collison (2002) Infiltration Pore water pressures + Greenway (1987); Simon & Collison (2002) Transpiration Pore water pressures - Brenner (1973); Greenway (1987); Simon & Collison (2002)

Additional References

Nezu, I., and Nakagawa, H. (1993). Turbulence in Open Channels, A. A. Balkema, Brookfield.

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